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[1]  arXiv:1407.5619 [pdf, other]
Title: The Starfish Diagram: Visualising Data Within the Context of Survey Samples
Authors: Iraklis S. Konstantopoulos (Australian Astronomical Observatory)
Comments: Submitted to Astronomy & Computing. Posted to arXiv ahead of acceptance to coincide with the early data release of the SAMI survey, in which the presented code features heavily. Code available from this https URL See also ascl.net/1407.001
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

As astronomy becomes increasingly invested in large surveys, the ample representation of an individual target becomes a significant challenge. Tabulations of basic properties can convey the message in an absolute sense, but not within the context of the sample from which the individual is drawn. We present a novel but simple plot that simultaneously visualises the properties of the sample and the individual. Numbers and characters are kept at an absolute minimum to enable the stacking of such plots without introducing too much verbal information. Once the user becomes accustomed to their appearance, a set of 'starfish diagrams' provide a direct representation of the individual within a sample, or between various samples. The utility and versatility of the plot is demonstrated through its application to astrophysical data and sports statistics. We provide a brief description of the concept and the source code, which is simple to adapt to any statistical dataset, be it descriptive of physics, demographics, finance, and more.

[2]  arXiv:1407.5621 [pdf, ps, other]
Title: The influence of the environmental history on quenching star formation in a $Λ$CDM universe
Comments: 24 pages, 14 figures, submitted to MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

We present a detailed analysis of the influence of the environment and of the environmental history on quenching star formation in central and satellite galaxies in the local Universe. We take advantage of publicly available galaxy catalogues obtained from applying a galaxy formation model to the Millennium simulation. In addition to halo mass, we consider the local density of galaxies within various fixed scales. Comparing our model predictions to observational data (SDSS), we demonstrate that the models are failing to reproduce the observed density dependence of the quiescent galaxy fraction in several aspects: for most of the stellar mass ranges and densities explored, models cannot reproduce the observed similar behaviour of centrals and satellites, they slightly under-estimate the quiescent fraction of centrals and significantly over-estimate that of satellites. We show that in the models, the density dependence of the quiescent central galaxies is caused by a fraction of "backsplash" centrals which have been satellites in the past (and were thus suffering from environmental processes). Turning to satellite galaxies, the density dependence of their quiescent fractions reflects a dependence on the time spent orbiting within a parent halo of a particular mass, correlating strongly with halo mass and distance from the halo centre. Comparisons with observational estimates suggest relatively long gas consumption time scales of roughly 5 Gyr in low mass satellite galaxies. The quenching time scales decrease with increasing satellite stellar mass. Overall, a change in modelling both internal processes (star formation and feedback) and environmental processes (e.g. making them dependent on dynamical friction time-scales and preventing the re-accretion of gas onto backsplash galaxies) is required for improving currently used galaxy formation models.

[3]  arXiv:1407.5622 [pdf, ps, other]
Title: Constraints on Core Collapse from the Black Hole Mass Function
Authors: C.S. Kochanek (Department of Astronomy, The Ohio State University)
Comments: 11 pages, 9 figures, submitted to MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We model the observed black hole mass function under the assumption that black hole formation is controlled by the compactness of the stellar core at the time of collapse. Low compactness stars are more likely to explode as supernovae and produce neutron stars, while high compactness stars are more likely to be failed supernovae that produce black holes with the mass of the helium core of the star. Using three sequences of stellar models and marginalizing over a model for the completeness of the black hole mass function, we find that the compactness xi(2.5) above which 50% of core collapses produce black holes is xi(2.5)=0.24 (0.15 < xi(2.5) < 0.37) at 90% confidence). While models with a sharp transition between successful and failed explosions are always the most likely, the width of the transition between the minimum compactness for black hole formation and the compactness above which all core collapses produce black holes is not well constrained. The models also predict that f=0.18 (0.09 < f < 0.39) of core collapses fail assuming a minimum mass for core collapse of 8Msun. We tested four other criteria for black hole formation based on xi(2.0) and xi(3.0), the compactnesses at enclosed masses of 2.0 or 3.0 rather than 2.5Msun, the mass of the iron core, and the mass inside the oxygen burning shell. We found that xi(2.0) works as well as xi(2.5), while the compactness xi(3.0) works significantly worse, as does using the iron core mass or the mass enclosed by the oxygen burning shell. As expected from the high compactness of 20-25Msun stars, black hole formation in this mass range provides a natural explanation of the red supergiant problem.

[4]  arXiv:1407.5623 [pdf, other]
Title: Mapping the Integrated Sachs-Wolfe Effect
Comments: 10 pages, 9 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

On large scales, the anisotropies in the cosmic microwave background (CMB) reflect not only the primordial field but also the energy gain when photons traverse decaying gravitational potentials of large scales structure, the Integrated Sachs-Wolfe (ISW) effect. Decomposing the anisotropy signal into a primordial piece and an ISW component is more urgent than ever as cosmologists strive to understand the Universe on the largest of scales. Here we present a likelihood technique for extracting the ISW signal from measurements of the CMB, the distribution of galaxies, and maps of gravitational lensing. We test this technique first to simulated data and then we apply it to the combination of temperature anisotropies, the lensing map made by the Planck satellite, and the NVSS galaxy survey. We also show projections for upcoming surveys.

[5]  arXiv:1407.5625 [pdf, other]
Title: Challenges in Explaining the Galactic Center Gamma-Ray Excess with Millisecond Pulsars
Comments: 5pages, 2 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR); High Energy Physics - Phenomenology (hep-ph)

Millisecond pulsars have been discussed as a possible source of the gamma-ray excess observed from the region surrounding the Galactic Center. With this in mind, we use the observed population of bright low-mass X-ray binaries to estimate the number of millisecond pulsars in the Inner Galaxy. This calculation suggests that only ~1-5% of the excess is produced by millisecond pulsars. We also use the luminosity function derived from local measurements of millisecond pulsars, along with the number of point sources resolved by Fermi, to calculate an upper limit for the diffuse emission from such a population. While this limit is compatible with the millisecond pulsar population implied by the number of low-mass X-ray binaries, it strongly excludes the possibility that most of the excess originates from such objects.

[6]  arXiv:1407.5626 [pdf, other]
Title: Inverse cascade of non-helical magnetic turbulence in a relativistic fluid
Authors: Jonathan Zrake
Comments: Submitted to ApJ Letters
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The free decay of non-helical relativistic magnetohydrodynamic turbulence is studied numerically, and found to exhibit cascading of magnetic energy toward large scales. Evolution of the magnetic energy spectrum $P_M(k,t)$ is self-similar in time and well modeled by a broken power law with sub-inertial and inertial range indices very close to $7/2$ and $-2$ respectively. We propose that non-helical inverse cascading proceeds by the hierarchical coalescence of similarly oriented magnetic flux loops. That relativistic MHD turbulence attains increasing spatial coherence over time may explain optical polarization of gamma-raw burst (GRB) afterglow emission. No bursty or explosive energy loss is observed in relativistic MHD turbulence having modest magnetization, which constrains magnetic reconnection models for rapid time variability of GRB prompt emission, blazars and the Crab nebula.

[7]  arXiv:1407.5627 [pdf, other]
Title: Surface photometry of BCGs and intracluster stars in Lambda-CDM
Authors: Andrew P. Cooper (1), Liang Gao (1 and 2), Qi Guo (1), Carlos S. Frenk (2), Adrian Jenkins (2), Volker Springel (3 and 4), Simon D.M. White (5) ((1) NAOC, Beijing (2) ICC Durham, UK (3) HITS, Heidelberg (4) ZAH, Heidelberg (5) MPA Garching)
Comments: Submitted to MNRAS; 17 pages, 16 figures (appendix 2 pages, 3 figures)
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We simulate the phase-space distribution of stellar mass in 9 massive Lambda-CDM galaxy clusters by applying the semi-analytic particle tagging method of Cooper et al. to the Phoenix suite of high-resolution N-body simulations (M200 = 7.5 to 33 x 10^14 Msol). The resulting surface brightness (SB) profiles of brightest cluster galaxies (BCGs) match well to observations. On average, stars formed in galaxies accreted by the BCG account for ~90 per cent of its total mass (the remainder is formed in situ). In circular BCG-centred apertures, the superposition of multiple debris clouds (each ~10 per cent of the total BCG mass) from different progenitors can result in an extensive outer diffuse component, qualitatively similar to a 'cD envelope'. These clouds typically originate from tidal stripping at z < 1 and comprise both streams and the extended envelopes of other massive galaxies in the cluster. The faint regions of the BCG contribute a significant part of the total cluster stellar mass budget: in the central 1 Mpc^2 of a z ~ 0.15 cluster imaged at SDSS-like resolution, our fiducial model predicts 80-95 per cent of stellar mass below a SB of mu_V = 26.5 mag arcsec^2 is associated with accreted stars in the envelope of the BCG.The ratio of BCG stellar mass to total cluster stellar mass is ~30 per cent.

[8]  arXiv:1407.5632 [pdf, ps, other]
Title: Three-dimensional Boltzmann-Hydro code for core-collapse in massive stars I. special relativistic treatments
Comments: 25 pages, 22 figures, submitted to ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We propose a novel numerical method for solving multi-dimensional, special relativistic Boltzmann equations for neutrinos coupled to hydrodynamics equations. It is meant to be applied to simulations of core-collapse supernovae. We handle special relativity in a non-conventional way, taking account of all orders of v/c. Consistent treatment of advection and collision terms in the Boltzmann equations is the source of difficulties, which we overcome by employing two different energy grids: Lagrangian remapped and laboratory fixed grids. We conduct a series of basic tests and perform a one-dimensional simulation of core-collapse, bounce and shock-stall for a 15M_{sun} progenitor model with a minimum but essential set of microphysics. We demonstrate in the latter simulation that our new code is capable of handling all phases in core-collapse supernova. For comparison, a non-relativistic simulation is also conducted with the same code, and we show that they produce qualitatively wrong results in neutrino transfer. Finally, we discuss a possible incorporation of general relativistic effects in our method.

[9]  arXiv:1407.5633 [pdf, other]
Title: The stellar initial mass function of early type galaxies from low to high stellar velocity dispersion: homogeneous analysis of ATLAS$^{\rm 3D}$ and Sloan Lens ACS galaxies
Authors: Silvia Posacki (1), Michele Cappellari (2), Tommaso Treu (3), Silvia Pellegrini (1), Luca Ciotti (1) ((1) University of Bologna, (2) Oxford University, (3) University of California)
Comments: 15 pages, 8 figures. Submitted to MNRAS. Comments are welcome
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present an investigation about the shape of the initial mass function (IMF) of early-type galaxies (ETGs), based on a joint lensing and dynamical analysis, and on stellar population synthesis models, for a sample of 55 lens ETGs identified by the Sloan Lens ACS (SLACS) Survey. We construct axisymmetric dynamical models based on the Jeans equations which allow for orbital anisotropy and include a dark matter halo. The models reproduce in detail the observed HST photometry and are constrained by the total projected mass within the Einstein radius and the stellar velocity dispersion ($\sigma$) within the SDSS fibers. Comparing the dynamically-derived stellar mass-to-light ratios $(M/L)_{\rm dyn}$ to the stellar population ones $(M/L)_{\rm pop}$, derived from full-spectrum fitting and assuming a Salpeter IMF, we infer the mass normalization of the IMF. Our results confirm the previous analysis by the SLACS team that the mass normalization of the IMF of high $\sigma$ galaxies is consistent on average with a Salpeter slope. Our study allows for a fully consistent study of the trend between IMF and $\sigma$ for both the SLACS and ATLAS$^{\rm 3D}$ samples, which explore quite different $\sigma$ ranges. The two samples are highly complementary, the first being essentially $\sigma$ selected, and the latter volume-limited and nearly mass selected. We find that the two samples merge smoothly into a single trend of the form $\log\alpha =(0.38\pm0.04)\times\log(\sigma_e/200\, km s^{-1})+(-0.06\pm0.01)$, where $\alpha=(M/L)_{\rm dyn}/(M/L)_{\rm pop}$ and $\sigma_e$ is the luminosity averaged $\sigma$ within one effective radius $R_e$. This is consistent with a systematic variation of the IMF normalization from Kroupa to Salpeter in the interval $\sigma_e\approx 90-270\,km s^{-1}$.

[10]  arXiv:1407.5639 [pdf, other]
Title: The Role of Cold Flows and Reservoirs in Galaxy Formation With Strong Feedback
Comments: 10 pages, 7 figures. Accepted for publication in MNRAS
Journal-ref: Monthly Notices of the Royal Astronomical Society, 2014, 442 (1): 732-740
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We examine gas accretion and subsequent star formation in representative galaxies from the McMaster Unbiased Galaxy Simulations (Stinson et al. 2010). Accreted gas is bimodal with a natural temperature division at $10^5$ K, near the peak of the cooling curve. Cold-mode accretion dominates inflows at early times, creating a peak in total accretion at redshift z=2-4 and declining exponentially below z$\sim$2. Hot-mode accretion peaks near z=1-2 and declines gradually. Hot-mode exceeds cold-mode accretion at z$\sim$1.8 for all four galaxies rather than when the galaxy reaches a characteristic mass. Cold-mode accretion can fuel immediate star formation, while hot-mode accretion preferentially builds a large, hot gas reservoir in the halo. Late-time star formation relies on reservoir gas accreted 2-8 Gyr prior. Thus, the reservoir allows the star formation rate to surpass the current overall gas accretion rate. Stellar feedback cycles gas from the interstellar medium back into the hot reservoir. Stronger feedback results in more gas cycling, gas removal in a galactic outflow and less star formation overall, enabling simulations to match the observed star formation history. For lower mass galaxies in particular, strong feedback can delay the star formation peak to z=1-2 from the accretion peak at z=2-4.

[11]  arXiv:1407.5644 [pdf, ps, other]
Title: Evidence of a Mira-like tail and bow shock about the semi-regular variable V CVn from four decades of polarization measurements
Comments: 8 pages, 5 figures, 1 table. Accepted for publication in A&A
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Polarization is a powerful tool for understanding stellar atmospheres and circumstellar environments. Mira and semi-regular variable stars have been observed for decades and some are known to be polarimetrically variable, however, the semi-regular variable V Canes Venatici displays an unusually large, unexplained amount of polarization. We present ten years of optical polarization observations obtained with the HPOL instrument, supplemented by published observations spanning a total interval of about forty years for V CVn. We find that V CVn shows large polarization variations ranging from 1 - 6%. We also find that for the past forty years the position angle measured for V CVn has been virtually constant suggesting a long-term, stable, asymmetric structure about the star. We suggest that this asymmetry is caused by the presence of a stellar wind bow shock and tail, consistent with the star's large space velocity.

[12]  arXiv:1407.5650 [pdf, ps, other]
Title: Energy-dependent evolution in IC10 X-1: hard evidence for an extended corona, and implications
Comments: Accepted for publication in ApJ; 10 pages, 4 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We have analyzed a ~130 ks XMM-Newton observation of the dynamically confirmed black hole + Wolf-Rayet (BH+WR) X-ray binary (XB) IC10 X-1, covering ~1 orbital cycle. This system experiences periodic intensity dips every ~35 hours. We find that energy-independent evolution is rejected at a >5 sigma level. The spectral and timing evolution of IC10 X-1 are best explained by a compact disk blackbody and an extended Comptonized component, where the thermal component is completely absorbed and the Comptonized component is partially covered during the dip. We consider three possibilities for the absorber: cold material in the outer accretion disk, as is well documented for Galactic neutron star (NS) XBs at high inclination; a stream of stellar wind that is enhanced by traveling through the L1 point; and a spherical wind. We estimated the corona radius (r_ADC) for IC10 X-1 from the dip ingress to be ~1 E+6 km, assuming absorption from the outer disk, and found it to be consistent with the relation between r_m ADC and 1--30 keV luminosity observed in Galactic NS XBs that spans 2 orders of magnitude. For the other two scenarios, the corona would be larger. Prior BH mass (M_BH) estimates range over 23--38 M_Sun, depending on the inclination and WR mass. For disk absorption, the inclination, i, is likely to be ~60--80 degrees, with M_BH ~24--41 M_Sun. Alternatively, the L1-enhanced wind requires i ~80 degrees, suggesting ~24--33 M_Sun. For a spherical absorber, i ~ 40 degrees, and M_BH ~50--65 M_Sun.

[13]  arXiv:1407.5653 [pdf]
Title: Ecliptic proximity and clustering of fast radio bursts
Comments: 8 pages
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

It is pointed out that the positional distribution of fast radio bursts shows proximity and clustering around the ecliptic rather than the celestial or the galactic equator in the corresponding coordinate systems. The result is interpreted as indicating a role, more significant than credited so far, the solar system plays in their occurrence. FRBs considered lie inside a region of 1.97 steradian. The probability, assuming a uniform distribution, that six of these FRBs, which are considered to be cleaner cosmological candidates, lie inside a region of 1.97 steradian, is about 0.00005. This result rules out a uniform distribution of galactic/cosmological origin. Their dispersion measure has a weak correlation with the magnitude of the ecliptic latitude. The so called Perytons, however, show no significant proximity to any one of these three planes although they are closer to the galactic plane.

[14]  arXiv:1407.5655 [pdf, other]
Title: A framework for modeling the detailed optical response of thick, multiple segment, large format sensors for precision astronomy applications
Comments: 13 pages, 8 figures
Journal-ref: Proc. SPIE 9150-41 (2014)
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

Near-future astronomical survey experiments, such as LSST, possess system requirements of unprecedented fidelity that span photometry, astrometry and shape transfer. Some of these requirements flow directly to the array of science imaging sensors at the focal plane. Availability of high quality characterization data acquired in the course of our sensor development program has given us an opportunity to develop and test a framework for simulation and modeling that is based on a limited set of physical and geometric effects. In this paper we describe those models, provide quantitative comparisons between data and modeled response, and extrapolate the response model to predict imaging array response to astronomical exposure. The emergent picture departs from the notion of a fixed, rectilinear grid that maps photo-conversions to the potential well of the channel. In place of that, we have a situation where structures from device fabrication, local silicon bulk resistivity variations and photo-converted carrier patterns still accumulating at the channel, together influence and distort positions within the photosensitive volume that map to pixel boundaries. Strategies for efficient extraction of modeling parameters from routinely acquired characterization data are described. Methods for high fidelity illumination/image distribution parameter retrieval, in the presence of such distortions, are also discussed.

[15]  arXiv:1407.5664 [pdf, ps, other]
Title: Cosmic-ray pressure driven magnetic field amplification: dimensional, radiative and field orientation effects
Comments: 24 pages, 17 figures. Accepted for publication in MNRAS
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Observations of non-thermal emission from several supernova remnants suggest that magnetic fields close to the blastwave are much stronger than would be naively expected from simple shock compression of the field permeating the interstellar medium (ISM). We investigate in some detail a simple model based on turbulence generation by cosmic-ray pressure gradients. Previously this model was investigated using 2D MHD simulations.
Motivated by the well-known qualitative differences between 2D and 3D turbulence, we further our investigations of this model using both 2D and 3D simulations to study the influence of the dimensionality of the simulations on the field amplification achieved. Further, since the model implies the formation of shocks which can, in principle, be efficiently cooled by collisional cooling we include such cooling in our simulations to ascertain whether it could increase the field amplification achieved. Finally, we examine the influence of different orientations of the magnetic field with respect to the normal of the blastwave.
We find that dimensionality has a slight influence on the overall amplification achieved, but a significant impact on the morphology of the amplified field. Collisional cooling has surprisingly little impact, primarily due to the short time which any element of the ISM resides in the precursor region for supernova blastwaves. Even allowing for a wide range of orientations of the magnetic field, we find that the magnetic field can be expected to be amplified by, on average, at least an order of magnitude in the precursors of supernova blastwaves.

[16]  arXiv:1407.5668 [pdf, other]
Title: The power spectrum and bispectrum of SDSS DR11 BOSS galaxies I: bias and gravity
Comments: 43 pages, 22 figures, Submitted to MNRAS
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA)

We analyse the anisotropic clustering of the Baryon Oscillation Spectroscopic Survey (BOSS) CMASS Data Release 11 sample, which consists of 690827 galaxies in the redshift range 0.43<z<0.70 and has a sky coverage of 8498 deg$^2$ corresponding to an effective volume of $\sim6\,\rm{Gpc}^3$. We fit the Fourier space statistics, the power spectrum and bispectrum monopoles to measure the linear and quadratic bias parameters, $b_1$ and $b_2$, for a non-linear non-local bias model, the growth of structure parameter $f$ and the amplitude of dark matter density fluctuations parametrised by $\sigma_8$. We obtain $b_1(z_{\rm eff})^{1.40}\sigma_8(z_{\rm eff})=1.672\pm 0.060$ and $b_2^{0.30}(z_{\rm eff})\sigma_8(z_{\rm eff})=0.579\pm0.082$ at the effective redshift of the survey, $z_{\rm eff}=0.57$. The main cosmological result is the constraint on the combination $f^{0.43}(z_{\rm eff})\sigma_8(z_{\rm eff})=0.582\pm0.084$, which is complementary to $f\sigma_8$ constraints obtained from 2-point redshift space distortion analyses. A less conservative analysis yields $f^{0.43}(z_{\rm eff})\sigma_8(z_{\rm eff})=0.584\pm0.051$. We ensure that our result is robust by performing detailed systematic tests using a large suite of survey galaxy mock catalogs and N-body simulations. The constraints on $f^{0.43}\sigma_8$ are useful for setting additional constrains on neutrino mass, gravity, curvature as well as the number of neutrino species from galaxy surveys analyses (as presented in a companion paper).

[17]  arXiv:1407.5671 [pdf, other]
Title: Ultra-high Energy Cosmic Rays and Neutrinos from Gamma-Ray Bursts, Hypernovae and Galactic Shocks
Authors: P. Mészáros
Comments: 13 pages, 10 figures. Based on a talk given at the "Cosmic Ray Origins: Beyond the Standard Models" international conference in San Vito di Cadore, Dolomites, Italy, 16-22 March 2014. To be published in the conference proceedings, in Nucl. Phys. B; Editors: O. Tibolla, L. Drury, M. Persic, S. Kaufmann, H. Voelk, K. Mannheim and A. de Angelis
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

I review gamma-ray burst models (GRBs) and observations, and discuss the possible production of ultra-high energy cosmic rays and neutrinos in both the standard internal shock models and the newer generation of photospheric and hadronic GRB models, in the light of current constraints imposed by IceCube, Auger and TA observations. I then discuss models that have been proposed to explain the recent astrophysical PeV neutrino observations, including star-forming and star-burst galaxies, hypernovae and galaxy accretion and merger shocks.

[18]  arXiv:1407.5676 [pdf, ps, other]
Title: Giant Outbursts in Be/X-ray Binaries
Comments: Accepted for publication in ApJ Letters
Journal-ref: 2014, ApJ, 790, L34
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Be/X-ray binary systems exhibit both periodic (Type I) X-ray outbursts and giant (Type II) outbursts, whose origin has remained elusive. We suggest that Type II X-ray outbursts occur when a highly misaligned decretion disk around the Be star becomes eccentric, allowing the compact object companion to capture a large amount of material at periastron. Using 3D smoothed particle hydrodynamics simulations we model the long term evolution of a representative Be/X-ray binary system. We find that periodic (Type I) X-ray outbursts occur when the neutron star is close to periastron for all disk inclinations. Type II outbursts occur for large misalignment angles and are associated with eccentricity growth that occurs on a timescale of about 10 orbital periods. Mass capture from the eccentric decretion disk results in an accretion disk around the neutron star whose estimated viscous time is long enough to explain the extended duration of Type II outbursts. Previous studies suggested that the outbursts are caused by a warped disk but our results suggest that this is not sufficient, the disk must be both highly misaligned and eccentric to initiate a Type II accretion event.

[19]  arXiv:1407.5680 [pdf, ps, other]
Title: Magnetic fields near the peripheries of galactic discs
Comments: 8 pages, 5 figures
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Magnetic fields are observed beyond the peripheries of optically detected galactic discs, while numerical models of their origin and the typical magnitudes are still absent. Previously, studies of galactic dynamo have avoided considering the peripheries of galactic discs because of the very limited (though gradually growing) knowledge about the local properties of the interstellar medium. Here we investigate the possibility that magnetic fields can be generated in the outskirts of discs, taking the Milky Way as an example. We consider a simple evolving galactic dynamo model in the "no-z" formulation, applicable to peripheral regions of galaxies, for various assumptions about the radial and vertical profiles of the ionized gas disc. The magnetic field may grow as galaxies evolve, even in the more remote parts of the galactic disc, out to radii of 15 to 30 kpc, becoming substantial after times of about 10 Gyr. This result depends weakly on the adopted distributions of the half thickness and surface density of the ionized gas component. The model is robust to changes in the amplitude of the initial field and the position of its maximum strength. The magnetic field in the remote parts of the galactic disc could be generated in situ from a seed field by local dynamo action. Another possibility is field production in the central regions of a galaxy, followed by transport to the disc's periphery by the joint action of the dynamo and turbulent diffusivity. Our results demonstrate the possibilities for the appearance and strengthening of magnetic fields at the peripheries of disc galaxies and emphasize the need for observational tests with new and anticipated radio telescopes (LOFAR, MWA, and SKA).

[20]  arXiv:1407.5686 [pdf, other]
Title: The Distortion of the Cosmic Microwave Background by the Milky Way
Comments: 4 pages, 3 Figures, Brief Report in Physical Review D, accepted for publication
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

The Milky Way can act as a large-scale weak gravitational lens of the cosmic microwave background (CMB). We study this effect using a photon ray-tracing code and a Galactic mass distribution with disk, bulge and halo components. For an observer at the Sun's coordinates in the Galaxy, the bending of CMB photon paths is limited to less than one arcsecond, and only for rays that pass within a few degrees of the Galactic Center. However, the entire sky is affected, resulting in global distortions of the CMB on large angular scales. These distortions can cause the low-order multipoles of a spherical harmonic expansion of the CMB sky temperature to leak into higher-order modes. Thus the component of the CMB dipole that results from the Local Group's motion relative to the local cosmic frame of rest contributes to higher-order moments for an observer in the solar system. With our ray-tracing code we show that the phenomenon is not sensitive to the specific choice of Galactic potential. We also quantitatively rule it out as a contributor to CMB anomalies such as power asymmetry or correlated alignment of low-order multipole moments.

[21]  arXiv:1407.5693 [pdf, ps, other]
Title: Pulsar timing arrays and the challenge of massive black hole binary astrophysics
Authors: Alberto Sesana (Albert Einstein Institute)
Comments: 18 pages, 4 figures. To appear in the Proceedings of the 2014 Sant Cugat Forum on Astrophysics. Astrophysics and Space Science Proceedings, ed. C.Sopuerta (Berlin: Springer-Verlag)
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE)

Pulsar timing arrays (PTAs) are designed to detect gravitational waves (GWs) at nHz frequencies. The expected dominant signal is given by the superposition of all waves emitted by the cosmological population of supermassive black hole (SMBH) binaries. Such superposition creates an incoherent stochastic background, on top of which particularly bright or nearby sources might be individually resolved. In this contribution I describe the properties of the expected GW signal, highlighting its dependence on the overall binary population, the relation between SMBHs and their hosts, and their coupling with the stellar and gaseous environment. I describe the status of current PTA efforts, and prospect of future detection and SMBH binary astrophysics.

[22]  arXiv:1407.5723 [pdf, ps, other]
Title: Diffuse Atomic and Molecular Gas in the Interstellar Medium of M82 toward SN 2014J
Authors: Adam M. Ritchey (1), Daniel E. Welty (2), Julie A. Dahlstrom (3), Donald G. York (2) ((1) Univ. of Washington, (2) Univ. of Chicago, (3) Carthage College)
Comments: 30 pages, 6 figures, submitted to ApJ (comments welcome)
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present a comprehensive analysis of interstellar absorption lines seen in moderately-high resolution, high S/N ratio optical spectra of SN 2014J in M82. Our observations were acquired over the course of six nights, covering the period from ~7 days before to ~29 days after the supernova reached its maximum V-band brightness. Complex interstellar absorption is observed from Na I, Ca II, K I, Ca I, CH+, CH, and CN, much of which arises from gas in the interstellar medium of M82, although absorption features associated with the Galactic disk and halo are also observed. We detect Li I absorption over a range in velocity consistent with that exhibited by the strongest Na I and K I components associated with M82; this is the first detection of interstellar Li in a galaxy outside of the Local Group. There are no significant temporal variations in the absorption-line profiles over the 37 days sampled by our observations. The relative abundances of the observed atomic and molecular species reveal that the ISM of M82 probed by SN 2014J is complex, consisting of a mixture of diffuse atomic and molecular clouds characterized by a wide range of physical and environmental conditions. Decreasing N(Na I)/N(Ca II) ratios and increasing N(Ca I)/N(K I) ratios with increasing velocity are indicative of reduced depletion in the higher-velocity material. Significant component-to-component scatter in the N(Na I)/N(Ca II) and N(Ca I)/N(Ca II) ratios may be due to variations in the local ionization conditions. An apparent anti-correlation between the N(CH+)/N(CH) and N(Ca I)/N(Ca II) ratios can be understood in terms of an opposite dependence on gas density and radiation field strength. We find evidence for an enhanced abundance of Li in the ISM of M82, which supports the conclusions of recent gamma-ray emission studies that the cosmic-ray acceleration processes are greatly enhanced in this starburst galaxy.

[23]  arXiv:1407.5727 [pdf, ps, other]
Title: Spectral atlases of the Sun from 3980 to 7100 Å at the center and at the limb
Comments: Accepted for publication in Ap&SS journal. 11 pages, 10 figures. The full atlas can be accessed via this http URL
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

In this work, we present digital and graphical atlases of spectra of both the solar disk-center and of the limb near the Solar poles using data taken at the UTS-IAP & RIAAM (the University of Tabriz Siderostat, telescope and spectrograph jointly developed with the Institut d'Astrophysique de Paris and Research Institute for Astronomy and Astrophysics of Maragha). High resolution and high signal-to-noise ratio (SNR)CCD-slit spectra of the sun for 2 different parts of the disk, namely for $\mu$~=~1.0 (solar center) \& for $\mu$~=~ 0.3 solar limb) are provided and discussed. While there are several spectral atlases of the solar disk-center, this is the first spectral atlas ever produced for the solar limb at this spectral range. The resolution of the spectra is about \emph{R}~$\sim$~70 000 ($\Delta\lambda$~$\sim$~0.09 {\AA} with the signal-to-noise ratio (SNR) of 400$-$600. The full atlas covers the 3980 to 7100 {\AA} spectral regions and contains 44 pages with three partial spectra of the solar spectrum put on each page to make it compact. The difference spectrum of the normalized solar disk-center and the solar limb is also included in the graphic presentation of the atlas to show the difference of line profiles, including far wings. The identification of the most significant solar lines is included in the graphic presentation of the atlas. Telluric lines are producing a definite signature on the difference spectra which is easy to notice. At the end of this paper we present only two sample pages of the whole atlas while the graphic presentation of the whole atlas along with its ASCII file can be accessed via the ftp server of the CDS in Strasbourg via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via this link \footnote{\url{this http URL}}.

[24]  arXiv:1407.5729 [pdf, other]
Title: Metal distribution in the ICM - a comprehensive numerical study of twelve galaxy clusters
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

We present a simulation setup for studying the dynamical and chemical evolution of the intracluster medium (ICM) and analyze a sample of 12 galaxy clusters that are diverse both kinetically (pre-merger, merging, virialized) and in total mass (M vir = 1.17 x 10^14 - 1.06 x 10^15 M). We analyzed the metal mass fraction in the ICM as a function of redshift and discuss radial trends as well as projected 2D metallicity maps. The setup combines high mass resolution N-body simulations with the semi-analytical galaxy formation model Galacticus for consistent treatment of the subgrid physics (such as galactic winds and ram-pressure stripping) in the cosmological hydrodynamical simulations. The interface between Galacticus and the hydro simulation of the ICM with FLASH is discussed with respect to observations of star formation rate histories, radial star formation trends in galaxy clusters, and the metallicity at different redshifts. As a test for the robustness of the wind model, we compare three prescriptions from different approaches. For the wind model directly taken from Galacticus, we find mean ICM metallicities between 0.2 - 0.8Z within the inner 1Mpc at z = 0. The main contribution to the metal mass fraction comes from galactic winds. The outflows are efficiently mixed in the ICM, leading to a steady homogenization of metallicities until ram-pressure stripping becomes effective at low redshifts. We find a very peculiar and yet common drop in metal mass fractions within the inner ~200kpc of the cool cores, which is due to a combination of wind suppression by outer pressure within our model and a lack of mixing after the formation of these dense regions.

[25]  arXiv:1407.5735 [pdf, ps, other]
Title: On the Long-Term Modulation of Solar Differential Rotation
Authors: Miyoshi Suzuki
Comments: 12pages,6figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Long-term modulation of solar differential rotation was studied with data from Mt. Wilson and our original observations during Solar Cycles 16 through 23. The results are: i) The global B-value (i.e. latitudinal gradient of differential rotation), is modulated in a period of about six or seven solar cycles. ii) The B-values of the northern and southern hemispheres are also modulated with a similar period to the global one, but iii) they show quasi-oscillatory behavior with a phase shift between them. We examined the yearly fluctuations of the B-values in every solar cycle with reference to the phase of the sunspot cycle and found that the B-values show high values over the full-cycle years, when the cycle-averaged B-values are high. We discuss the independent long-term behavior of solar differential rotation between the northern and southern solar hemispheres and its implication for the solar dynamo.

[26]  arXiv:1407.5738 [pdf]
Title: The EPN-TAP protocol for the Planetary Science Virtual Observatory
Comments: 21 pages. Submitted to Astronomy & Computing, S.I. Virtual Observatory
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

A Data Access Protocol has been set up to search and retrieve Planetary Science data in general. This protocol will allow the user to select a subset of data from an archive in a standard way, based on the IVOA Table Access Protocol (TAP). The TAP mechanism is completed by an underlying Data Model and reference dictionaries. This paper describes the principle of the EPN- TAP protocol and interfaces, underlines the choices that have been made, and discusses possible evolutions.

[27]  arXiv:1407.5741 [pdf, ps, other]
Title: The study of triple systems V819 Her, V2388 Oph, and V1031 Ori
Comments: 18 print pages, 8 figures, 3 tables, 2 long data tables, published in: 2014AcA....64..125Z
Journal-ref: 2014AcA....64..125Z
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The systems V819 Her, V2388 Oph, and V1031 Ori are triples comprised of an eclipsing binary orbiting with a distant visual component on a longer orbit. A detailed analysis of these interesting systems, combining the two observational techniques: interferometry and apparent period variation, was performed. The interferometric data for these three systems obtained during the last century determine the visual orbits of the distant components in the systems. The combined analysis of the positional measurements together with the analysis of apparent period changes of the eclipsing binary (using the minima timings) can be used to study these systems in a combined approach, resulting in a set of parameters otherwise unobtainable without the radial velocities. The main advantage of the technique presented here is the fact that one needs no spectroscopic monitoring of the visual orbits, which have rather long periods: 5.5 yr for V819 Her, 9.0 yr for V2388 Oph, and 31.3 yr for V1031 Ori, respectively. The eccentricities of the outer orbits are 0.69, 0.33, and 0.92, respectively. Moreover, the use of minima timings of the eclipsing pairs help us to derive the orientation of the orbit in space with no ambiguity around the celestial plane. And finally, the combined analysis yielded also an independent determination of the distance of V819 Her (68.7 +/- 1.8 pc), and V2388 Oph (70.6 +/- 8.9 pc). We also present a list of similar systems, which would be suitable for a combined analysis like this one.

[28]  arXiv:1407.5784 [pdf, other]
Title: Exact Bound-Bound Gaunt Factor Values for Quantum Levels up to n=2000
Comments: published in MNRAS, 6 pages, 3 figures, online data can be found at: this http URL
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Comparison of observations of radio recombination lines in the interstellar medium with theoretical models can be used to constrain electron temperature and density of the gas. An important component of the models is spontaneous transition rates between bound levels. Calculating these rates relies on accurate bound-bound oscillator strengths, which can be cast in terms of the Gaunt factor. The Gaunt factor contains terminating hypergeometric functions that cannot be calculated with sufficient accuracy for high quantum levels ($n \gtrsim 50$) by standard machine-precision methods. Methods to overcome the accuracy problem have been developed, which include asymptotic expansions and recursion relations. These methods, used in astrophysical models to calculate oscillator strengths, can introduce errors, sometimes up to as much as $\sim 8$ per cent. Detections of radio recombination lines with the new Low Frequency Array (LOFAR) has prompted an examination of theoretical models of the interstellar medium. We revisit the calculation of the Gaunt factor, employing modern arbitrary-precision computational methods to tabulate the Gaunt factor for transitions up to quantum level $n=2000$, sufficient to model low frequency Carbon radio recombination lines. The calculations provide a relative error of $\sim3\times 10^{-4}$ when compared to more detailed calculations including relativistic corrections. Our values for the Gaunt factor are provided for download in a tabular format to be used for a wide range of applications.

[29]  arXiv:1407.5791 [pdf, ps, other]
Title: The bow shock, cold fronts and disintegrating cool core in the merging galaxy group RXJ0751.3+5012
Comments: 14 pages, 12 figures, accepted by MNRAS
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA)

We present a new Chandra X-ray observation of the off-axis galaxy group merger RXJ0751.3+5012. The hot atmospheres of the two colliding groups appear highly distorted by the merger. The images reveal arc-like cold fronts around each group core, produced by the motion through the ambient medium, and the first detection of a group merger shock front. We detect a clear density and temperature jump associated with a bow shock of Mach number M=1.9+/-0.4 ahead of the northern group. Using galaxy redshifts and the shock velocity of 1100+/-300 km/s, we estimate that the merger axis is only 10deg from the plane of the sky. From the projected group separation of 90 kpc, this corresponds to a time since closest approach of 0.1 Gyr. The northern group hosts a dense, cool core with a ram pressure stripped tail of gas extending 100 kpc. The sheared sides of this tail appear distorted and broadened by Kelvin-Helmholtz instabilities. We use the presence of this substructure to place an upper limit on the magnetic field strength and, for Spitzer-like viscosity, show that the development of these structures is consistent with the critical perturbation length above which instabilities can grow in the intragroup medium. The northern group core also hosts a galaxy pair, UGC4052, with a surrounding IR and near-UV ring 40 kpc in diameter. The ring may have been produced by tidal stripping of a smaller galaxy by UGC4052 or it may be a collisional ring generated by a close encounter between the two large galaxies.

[30]  arXiv:1407.5793 [pdf, ps, other]
Title: New constraints on dust emission and UV attenuation of z=6.5-7.5 galaxies from IRAM and ALMA observations
Comments: 10 pages, 8 figures. Submitted to A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We have targeted two recently discovered Lyman break galaxies (LBGs) to search for dust continuum and [CII] 158 micron line emission. The strongly lensed z~6.8 LBG A1703-zD1 behind the galaxy cluster Abell 1703, and the spectroscopically confirmed z=7.508 LBG z8-GND-5296 in the GOODS-N field have been observed with the Plateau de Bure interferometer (PdBI) at 1.2mm. These observations have been combined with those of three z>6.5 Lya emitters (named HCM6A, Himiko, and IOK-1), for which deep measurements were recently obtained with the PdBI and ALMA. [CII] is undetected in both galaxies, providing a deep upper limit for Abell1703-zD1, comparable to recent ALMA non-detections. Dust continuum emission from Abell1703-zD1 and z8-GND-5296 is not detected with an rms of 0.12 and 0.16 mJy/beam. From these non-detections we derive upper limits on their IR luminosity and star formation rate, dust mass, and UV attenuation. Thanks to strong gravitational lensing the limit for Abell1703-zD1 is probing the sub-LIRG regime ($L_{IR} <8.1 \times 10^{10}$ Lsun) and very low dust masses ($M_d<1.6 \times 10^7$ Msun). We find that all five galaxies are compatible with the Calzetti IRX-$\beta$ relation, their UV attenuation is compatible with several indirect estimates from other methods (the UV slope, extrapolation of the attenuation measured from the IR/UV ratio at lower redshift, and SED fits), and the dust-to-stellar mass ratio is not incompatible with that of galaxies from z=0 to 3. For their stellar mass the high-z galaxies studied here have an attenuation below the one expected from the mean relation of low redshift (z<1.5) galaxies. More and deeper (sub)-mm data are clearly needed to directly determine the UV attenuation and dust content of the dominant population of high-z star-forming galaxies and to establish more firmly their dependence on stellar mass, redshift, and other properties.

[31]  arXiv:1407.5797 [pdf, ps, other]
Title: A simple chemical evolution model for the Milky Way disc with radial gas flows and stellar migration
Comments: Submitted to MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We introduce a simple treatment of stellar migration in a detailed chemical evolution model for the thin disc of the Milky Way that already includes gas radial flows and reproduces several observational constraints for the solar vicinity, as well as the whole disc. We find that stellar migration has a negligible effect on the G-dwarf metallicity distribution in the solar neighbourhood, even in presence of a significant drift from the innermost regions. Therefore we conclude that the G-dwarf metallicity distribution hardly gives any information to be used to quantify the extent of migration. On the other hand, a large fraction of the spread observed in the age-metallicity relation of solar neighbourhood stars can be explained by the presence of stars that originated at different Galactocentric distances, though part of the observed spread could still be due to errors in the determination of stellar ages. Finally, we show that a substantial stellar migration can significantly affect the observed distribution of stars along the disk, so that the stellar surface density seems to be another important constraint to stellar migration models. In conclusion, our simulations suggest that, while stellar migration should be present at some extent, its amount has been probably overestimated in previous works.

[32]  arXiv:1407.5800 [pdf, ps, other]
Title: Low-metallicity stellar halo populations as tracers of dark matter haloes
Authors: Patricia B. Tissera (UNAB, Chile, Conicet, Argentina), Cecilia Scannapieco (IAP, Germany)
Comments: 5 pages,3 figures, MNRAS Letters accepted
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We analyse the density profiles of the stellar halo populations in eight Milky-Way mass galaxies, simulated within the $\Lambda$-Cold Dark Matter scenario. We find that accreted stars can be well-fitted by an Einasto profile, as well as any subsample defined according to metallicity. We detect a clear correlation between the Einasto fitting parameters of the low-metallicity stellar populations and those of the dark matter haloes. The correlations for stars with [Fe/H]$<-3$ allow us to predict the shape of the dark matter profiles within residuals of $\sim 10 $ per cent, in case the contribution from in situ stars remains small. Using Einasto parameters estimated for the stellar halo of the Milky Way and assuming the later formed with significant contributions from accreted low-mass satellite, our simulations predict $\alpha \sim 0.15 $ and $r_2 \sim 15$ kpc for its dark matter profile. These values, combined with observed estimations of the local dark matter density, yield an enclosed dark matter mass at $\sim 8$ kpc in the range $3.9 - 6.7 \times 10^{10}$ M$_{\odot}$, in agreement with recent observational results. These findings suggest that low-metallicity stellar haloes could store relevant information on the DM haloes. Forthcoming observations would help us to further constrain our models and predictions.

[33]  arXiv:1407.5804 [pdf, other]
Title: Dust properties inside molecular clouds from coreshine modeling and observations
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Context. Using observations to deduce dust properties, grain size distribution, and physical conditions in molecular clouds is a highly degenerate problem. Aims. The coreshine phenomenon, a scattering process at 3.6 and 4.5 $\mu$m that dominates absorption, has revealed its ability to explore the densest parts of clouds. We want to use this effect to constrain the dust parameters. The goal is to investigate to what extent grain growth (at constant dust mass) inside molecular clouds is able to explain the coreshine observations. We aim to find dust models that can explain a sample of Spitzer coreshine data. We also look at the consistency with near-infrared data we obtained for a few clouds. Methods. We selected four regions with a very high occurrence of coreshine cases: Taurus-Perseus, Cepheus, Chameleon and L183/L134. We built a grid of dust models and investigated the key parameters to reproduce the general trend of surface bright- nesses and intensity ratios of both coreshine and near-infrared observations with the help of a 3D Monte-Carlo radiative transfer code. The grid parameters allow to investigate the effect of coagulation upon spherical grains up to 5 $\mu$m in size derived from the DustEm diffuse interstellar medium grains. Fluffiness (porosity or fractal degree), ices, and a handful of classical grain size distributions were also tested. We used the near- and mostly mid-infrared intensity ratios as strong discriminants between dust models. Results. The determination of the background field intensity at each wavelength is a key issue. In particular, an especially strong background field explains why we do not see coreshine in the Galactic plane at 3.6 and 4.5 $\mu$m. For starless cores, where detected, the observed 4.5 $\mu$m / 3.6 $\mu$m coreshine intensity ratio is always lower than $\sim$0.5 which is also what we find in the models for the Taurus-Perseus and L183 directions. Embedded sources can lead to higher fluxes (up to four times greater than the strongest starless core fluxes) and higher coreshine ratios (from 0.5 to 1.1 in our selected sample). Normal interstellar radiation field conditions are sufficient to find suitable grain models at all wavelengths for starless cores. The standard interstellar grains are not able to reproduce observations and, due to the multi-wavelength approach, only a few grain types meet the criteria set by the data. Porosity does not affect the flux ratios while the fractal dimension helps to explain coreshine ratios but does not seem able to reproduce near-infrared observations without a mix of other grain types. Conclusions. Combined near- and mid-infrared wavelengths confirm the potential to reveal the nature and size distribution of dust grains. Careful assessment of the environmental parameters (interstellar and background fields, embedded or nearby reddened sources) is required to validate this new diagnostic.

[34]  arXiv:1407.5816 [pdf, other]
Title: Scalar suppression on large scales in open inflation
Comments: 11 pages, 13 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)

We consider two toy models of open inflation and investigate their ability to give a suppression of scalar power on large scales whilst also satisfying observational constraints on the spatial curvature of the universe. Qualitatively we find that both models are indeed capable of fulfilling these two requirements, but we also see that effects of the quantum tunnelling must be carefully taken into account if we wish to make quantitative predictions.

[35]  arXiv:1407.5817 [pdf, ps, other]
Title: ARES+MOOG - a practical overview of an EW method to derive stellar parameters
Authors: Sérgio G. Sousa
Comments: 15 pages, 4 figures, accepted for publication as a chapter in "Determination of Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E. Niemczura, B. Smalley, W. Pych
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The goal of this document is to describe the important practical aspects in the use of an Equivalent Width (EW) method for the derivation of spectroscopic stellar parameters. A general description of the fundamental steps composing any EW method is given, together with possible differences that may be found in different methods used in the literature. Then ARES+MOOG is then used as an example where each step of the method is described in detail. A special focus is given for the specific steps of this method, namely the use of a differential analysis to define the atomic data for the adopted line list, the automatic EW determinations, and the way to find the best parameters at the end of the procedure. Finally, a practical tutorial is given, where we focus on simple exercises useful to illustrate and explain the dependence of the abundances with the assumed stellar parameters. The interdependences are described and a clear procedure is given to find the "final" stellar parameters.

[36]  arXiv:1407.5829 [pdf, ps, other]
Title: Near Infrared Spectroscopy of M Dwarfs. I. CO Molecule as an Abundance Indicator of Carbon
Comments: 33 pages, 17 figures, 13 tables + 1 table for electroni version only, minor changes in abstract, accepted by PASJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Based on the near infrared spectra of 42 M dwarfs, carbon abundances are determined from the ro-vibrational lines of CO 2-0 band. We apply Teff values based on the angular diameters if available or use the Teff values in a logTeff - M3.4 (the absolute magnitude at 3.4 micron based on the WISE W1 flux and the Hipparcos parallax) relation to estimate Teff values of objects for which angular diameters are unknown. Also, we discuss briefly the HR diagram of low mass stars. On the observed spectrum of M dwarf, the continuum is depressed by the numerous weak lines of H2O and only the depressed continuum or the pseudo- continuum can be seen. On the theoretical spectrum of M dwarfs, we find that the pseudo-continuum can be evaluated accurately thanks to the recent H2O line database. Then quantitative analysis of the spectrum of M dwarf can be done by referring to the pseudo-continua both on the observed and theoretical spectra. Since the basic principle of the spectroscopic analysis should be the same whether the true- or pseudo-continuum is referred to, the difficulty related to the continuum in cool stars can in principle be overcome. Then, the numerous CO lines can be excellent abundance indicators of carbon, since almost all the carbon atoms are in stable CO molecules which remain almost unchanged for the changes of physical condition in the photosphere and, somewhat unexpectedly, carbon abundances in late-type stars can best be determined in M dwarfs rather than in solar type stars. The resulting C/Fe ratios for most M dwarfs are nearly constant at about the solar value based on the classical high carbon abundance rather than on the recently revised lower value. This result implies that the solar carbon abundance is atypical for its metallicity among the stellar objects in the solar neighborhood if the downward revised solar carbon abundance is correct.

[37]  arXiv:1407.5836 [pdf, other]
Title: A common column density threshold for scattering at 3.6 mum and water-ice in molecular clouds
Comments: Accepted in A&A Letters
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Context: Observations of scattered light in the 1-5 $\mu$m range have revealed dust grains in molecular cores with sizes larger than commonly inferred for the diffuse interstellar medium. It is currently unclear whether these grains are grown within the molecular cores or are an ubiquitous component of the interstellar medium. Aims: We investigate whether the large grains necessary for efficient scattering at 1-5 mum are associated with the abundance of water-ice within molecular clouds and cores. Methods: We combined water-ice abundance measurements for sight lines through the Lupus IV molecular cloud complex with measurements of the scattered light at 3.6 mum for the same sight lines. Results: We find that there is a similar threshold for the cores in emission in scattered light at 3.6 mum (tau_9.7=0.15pm0.05, A_K=0.4pm0.2 as water-ice (tau_9.7=0.11pm0.01, A_K=0.19pm0.04) and that the scattering efficiency increases as the relative water-ice abundance increases. The ice layer increases the average grain size, which again strongly increases the albedo. Conclusions: The higher scattering efficiency is partly due to layering of ice on the dust grains. Although the layer can be relatively thin it can enhance the scattering substantially.

[38]  arXiv:1407.5837 [pdf, other]
Title: Initiation of CME event observed on November 3, 2010: Multi-wavelength Perspective
Comments: 17 pages, 9 figures, Accepted for Publication in The Astrophysical Journal
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

One of the major unsolved problems in Solar Physics is that of CME initiation. In this paper, we have studied the initiation of a flare associated CME which occurred on 2010 November 03 using multi-wavelength observations recorded by Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory (SDO) and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). We report an observation of an inflow structure initially in 304~{\AA} and in 1600~{\AA} images, a few seconds later. This inflow strucure was detected as one of the legs of the CME. We also observed a non-thermal compact source concurrent and near co-spatial with the brightening and movement of the inflow structure. The appearance of this compact non-thermal source, brightening and movement of the inflow structure and the subsequent outward movement of the CME structure in the corona led us to conclude that the CME initiation was caused by magnetic reconnection.

[39]  arXiv:1407.5842 [pdf]
Title: Status Update and Closed-Loop Performance of the Magellan Adaptive Optics VisAO Camera
Journal-ref: 2012 Proc. SPIE 8447, Adaptive Optics Systems III, 84473D
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

We present laboratory results of the closed-loop performance of the Magellan Adaptive Optics (AO) Adaptive Secondary Mirror (ASM), pyramid wavefront sensor (PWFS), and VisAO visible adaptive optics camera. The Magellan AO system is a 585-actuator low-emissivity high-throughput system scheduled for first light on the 6.5 meter Magellan Clay telescope in November 2012. Using a dichroic beamsplitter near the telescope focal plane, the AO system will be able to simultaneously perform visible (500-1000 nm) AO science with our VisAO camera and either 10 micron or 3-5 micron science using either the BLINC/MIRAC4 or CLIO cameras, respectively. The ASM, PWS, and VisAO camera have undergone final system tests in the solar test tower at the Arcetri Institute in Florence, Italy, reaching Strehls of 37% in i'-band with 400 modes and simulated turbulence of 14 cm ro at v-band. We present images and test results of the assembled VisAO system, which includes our prototype advanced Atmospheric Dispersion Corrector (ADC), prototype calcite Wollaston prisms for SDI imaging, and a suite of beamsplitters, filters, and other optics. Our advanced ADC performs in the lab as designed and is a 58% improvement over conventional ADC designs. We also present images and results of our unique Calibration Return Optic (CRO) test system and the ASM, which has successfully run in closed- loop at 1kHz. The CRO test is a retro reflecting optical test that allows us to test the ASM off-sky in close-loop using an artificial star formed by a fiber source.

[40]  arXiv:1407.5846 [pdf, other]
Title: Focal-plane wavefront sensing with high-order adaptive optics systems
Comments: 11 pages, 7 figures, published in SPIE proceedings
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Optics (physics.optics)

We investigate methods to calibrate the non-common path aberrations at an adaptive optics system having a wavefront-correcting device working at an extremely high resolution (larger than 150x150). We use focal-plane images collected successively, the corresponding phase-diversity information and numerically efficient algorithms to calculate the required wavefront updates. The wavefront correction is applied iteratively until the algorithms converge. Different approaches are studied. In addition of the standard Gerchberg-Saxton algorithm, we test the extension of the Fast & Furious algorithm that uses three images and creates an estimate of the pupil amplitudes. We also test recently proposed phase-retrieval methods based on convex optimisation. The results indicate that in the framework we consider, the calibration task is easiest with algorithms similar to the Fast & Furious.

[41]  arXiv:1407.5851 [pdf, ps, other]
Title: Parametric decay of parallel and oblique Alfven waves in the expanding solar wind
Comments: 20 pages, 9 figures, accepted for publication in Joural of Plasma Physics
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Space Physics (physics.space-ph)

The long-term evolution of large-amplitude Alfven waves propagating in the solar wind is investigated by performing two-dimensional MHD simulations within the expanding box model. The linear and nonlinear phases of the parametric decay instability are studied for both circularly polarized waves in parallel propagation and for arc-polarized waves in oblique propagation. The non-monochromatic case is also considered. In the oblique case, the direct excitation of daughter modes transverse to the local background field is found for the first time in an expanding environment, and this transverse cascade seems to be favored for monochromatic mother waves. The expansion effect reduces the instability growth rate, and it can even suppress its onset for the lowest frequency modes considered here, possibly explaining the persistence of these outgoing waves in the solar wind.

[42]  arXiv:1407.5853 [pdf]
Title: Laboratory Results and Status Update for Pathfinder at LBT, The LINC-NIRVANA NGS Ground- Layer AO Subsystem
Comments: 2013 AO4ELT Proceedings
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

The full LINC-NIRVANA instrument will be one of the most complex ground-based astronomical systems ever built. It will consist of multiple subsystems, including two multi-conjugate ground layer AO systems (MCAO) that drive the LBT adaptive secondaries, two mid-high layer AO systems with their own Xynetics 349 actuator DM's , a fringe tracker, a beam combiner, and the NIR science camera. In order to mitigate risk, we take a modular approach to instrument testing and commissioning by decoupling these subsystems individually. The first subsystem tested on-sky will be one of the ground-layer AO systems, part of a test-bed known as the Pathfinder. The Pathfinder consists of a 12-star pyramid wavefront sensor (PWFS) that drives one of the LBT's adaptive secondaries, a support structure known as "The Foot," and the infrared test camera (IRTC), which is used for acquisition and alignment. The 12 natural guide stars are acquired by moveable arms called "star enlargers," each of which contains its own optical path. The Pathfinder was shipped from MPIA in Heidelberg, Germany to the LBT mountain lab on Mt. Graham, Arizona in February 2013. The system was unpacked, assembled in the LBT clean room, and internally optically aligned. We present the results of our system tests, including star enlarger alignment and system alignment. We also present our immediate plans for on-sky closed loop tests on the LBT scheduled for late Fall. Because plans for all ELTs call for ground layer correction, the Pathfinder provides valuable preliminary information not only for the full LINC-NIRVANA system, but also for future advanced MCAO systems.

[43]  arXiv:1407.5855 [pdf]
Title: Pathfinder first light: alignment, calibration, and commissioning of the LINC-NIRVANA ground-layer adaptive optics subsystem
Journal-ref: Proceedings of the SPIE, Astronomical Instrumentation 2014
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

We present descriptions of the alignment and calibration tests of the Pathfinder, which achieved first light during our 2013 commissioning campaign at the LBT. The full LINC-NIRVANA instrument is a Fizeau interferometric imager with fringe tracking and 2-layer natural guide star multi-conjugate adaptive optics (MCAO) systems on each eye of the LBT. The MCAO correction for each side is achieved using a ground layer wavefront sensor that drives the LBT adaptive secondary mirror and a mid-high layer wavefront sensor that drives a Xinetics 349 actuator DM conjugated to an altitude of 7.1 km. When the LINC-NIRVANA MCAO system is commissioned, it will be one of only two such systems on an 8-meter telescope and the only such system in the northern hemisphere. In order to mitigate risk, we take a modular approach to commissioning by decoupling and testing the LINC-NIRVANA subsystems individually. The Pathfinder is the ground-layer wavefront sensor for the DX eye of the LBT. It uses 12 pyramid wavefront sensors to optically co-add light from natural guide stars in order to make four pupil images that sense ground layer turbulence. Pathfinder is now the first LINC-NIRVANA subsystem to be fully integrated with the telescope and commissioned on sky. Our 2013 commissioning campaign consisted of 7 runs at the LBT with the tasks of assembly, integration and communication with the LBT telescope control system, alignment to the telescope optical axis, off-sky closed loop AO calibration, and finally closed loop on-sky AO. We present the programmatics of this campaign, along with the novel designs of our alignment scheme and our off-sky calibration test, which lead to the Pathfinder's first on-sky closed loop images.

[44]  arXiv:1407.5859 [pdf, ps, other]
Title: Abundance analysis of red clump stars in the old, inner disc, open cluster NGC 4337: a twin of NGC 752?
Authors: Giovanni Carraro (ESO-Chile), Lorenzo Monaco (ESO-Chile), Sandro Villanova (Concepcion)
Comments: 7 pages, 8 eps figures, in press in Astronomy and Astrophysics. arXiv admin note: text overlap with arXiv:1401.1563
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

Open star clusters older than ~ 1 Gyr are rare in the inner Galactic disc. Still, they are objects that hold crucial information for probing the chemical evolution of these regions of the Milky Way. We aim at increasing the number of old open clusters in the inner disc for which high-resolution metal abundances are available. Here we report on NGC 4337, which was recently discovered to be an old, inner disc open cluster. We present the very first high-resolution spectroscopy of seven clump stars that are all cluster members. We performed a detailed abundance analysis for them. We find that NGC 4337 is marginally more metal-rich than the Sun, with [Fe/H]=+0.12$\pm$0.05. The abundance ratios of $\alpha$-elements are generally solar. At odds with recent studies on intermediate-age and old open clusters in the Galactic disc, Ba is under-abundant in NGC 4337 compared with the Sun. Our analysis of the iron-peak elements (Cr and Ni) does not reveal anything anomalous. Based on these results, we estimate the cluster age to be 1.6$^{+0.1}_{-0.1}$ Gyr, and derive a reddening E(B-V)=0.23$\pm$0.05, and an apparent distance modulus $(m-M)_{V}=12.80\pm0.15$. Its distance to the Galactic centre is 7.6 kpc. With this distance and metallicity, NGC 4337 fits the metallicity gradient for the inner Galactic disc fairly well. The age and metallicity we measured make NGC 4337 a twin of the well-known old open cluster NGC 752. The red clumps of these two clusters bear an amazing resemblance. But the main sequence of NGC 752 is significantly more depleted in stars than that of NGC 4337. This would mean that NGC 752 is in a much more advanced dynamical stage, being on the verge of dissolving into the general Galactic field. Our results make NGC 4337 an extremely interesting object for further studies of stellar evolution in the critical turn-off mass range 1.1-1.4 solar masses.

[45]  arXiv:1407.5881 [pdf, other]
Title: Magnetar activity mediated by plastic deformations of neutron star crust
Authors: Maxim Lyutikov (Purdue)
Comments: 26 pages, 3 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We advance a "Solar flare" model of magnetar activity, whereas a slow evolution of the magnetic field in the upper crust, driven by electron MHD (EMHD) flows, twists the external magnetic flux tubes, producing persistent emission, bursts and flares. At the same time the neutron star crust plastically relieves the imposed magnetic field stress, limiting the strain $ \epsilon_t $ to values well below the critical strain $ \epsilon_{crit}$ of a brittle fracture, $ \epsilon_t \sim 10^{-2}\epsilon_{crit} $.
Magnetar-like behavior, occurring near the magnetic equator, takes place in all neutron stars, but to a different extent. The persistent luminosity is proportional to cubic power of the magnetic field (at a given age), and hence is hardly observable in most rotationally powered neutron stars. Giant flares can occur only if the magnetic field exceeds some threshold value, while smaller bursts and flares may take place in relatively small magnetic fields.
Bursts and flares are magnetospheric reconnection events that launch Alfven shocks which convert into high frequency whistlers upon hitting the neutron star surface. The resulting whistler pulse induces a strain that increases with depth both due to the increasing electron density (and the resulting slowing of the waves), and due to the increasing coherence of a whistler pulse with depth. The whistler pulse is dissipated on a time scale of approximately a day at shallow depths corresponding to $\rho \sim 10^{10} {\rm g cm}^{-3}$; this energy is detected as enhanced post-flare surface emission.

[46]  arXiv:1407.5892 [pdf, other]
Title: Large-area Reflective Infrared Filters for Millimeter/sub-mm Telescopes
Comments: 6 pages, 5 figures. Presented at the 15th International Workshop on Low Temperature Detectors
Journal-ref: Journal of Low Temperature Physics, September 2014, Volume 176, Issue 5-6, pp 835-840
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

Ground-based millimeter and sub-millimeter telescopes are attempting to image the sky with ever-larger cryogenically-cooled bolometer arrays, but face challenges in mitigating the infrared loading accompanying large apertures. Absorptive infrared filters supported by mechanical coolers scale insufficiently with aperture size. Reflective metal-mesh filters placed behind the telescope window provide a scalable solution in principle, but have been limited by photolithography constraints to diameters under 300 mm. We present laser etching as an alternate technique to photolithography for fabrication of large-area reflective filters, and show results from lab tests of 500 mm-diameter filters. Filters with up to 700 mm diameter can be fabricated using laser etching with existing capability.

[47]  arXiv:1407.5895 [pdf]
Title: Sunspot Count Periodicities in Different Zurich Sunspot Group Classes since 1986
Comments: Accepted Solar Physics, 2014
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

In this study, we used two methods to investigate the periodic behavior of sunspot counts in four categories for the time period January 1986-October 2013. These categories include the counts from simple (A and B), medium (C), large (D, E, and F), and final (H) sunspot groups. We used: i) the Multi-taper Method with red noise approximation, and ii) the Morlet wavelet transform for periodicity analysis. Our main findings are: (1) the solar rotation periodicity of about 25 to 37 days, which is of obvious significance, is found in all groups with at least a 95% significance level; (2) the periodic behavior of a cycle is strongly related to its amplitude and group distribution during the cycle; (3) the appearance of periods follow the amplitude of the investigated solar cycles, (4) meaningful periods do not appear during the minimum phases of the investigated cycles. We would like to underline that the cyclic behavior of all categories is not completely the same; there are some differences between these groups. This result can provide a clue for the better understanding of solar cycles.

[48]  arXiv:1407.5897 [pdf, other]
Title: The properties of single WO stars
Comments: 2 pages, 1 figure, Proceedings IAU Symposium No. 307, 2014, 'New windows on massive stars: asteroseismology, interferometry, and spectropolarimetry'
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The enigmatic oxygen sequence Wolf-Rayet (WO) stars represent a very late stage in massive star evolution, although their exact nature is still under debate. The spectra of most of the WO stars have never been analysed through detailed modelling with a non-local thermodynamic equilibrium expanding atmosphere code. Here we present preliminary results of the first homogeneous analysis of the (apparently) single WOs.

[49]  arXiv:1407.5923 [pdf, other]
Title: A Pilot Search for Evidence of Extrasolar Earth-analog Plate Tectonics
Comments: ApJ Letters, accepted
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Relative to calcium, both strontium and barium are markedly enriched in Earth's continental crust compared to the basaltic crusts of other differentiated rocky bodies within the solar system. Here, we both re-examine available archived Keck spectra to place upper bounds on n(Ba)/n(Ca) and revisit published results for n(Sr)/n(Ca) in two white dwarfs that have accreted rocky planetesimals. We find that at most only a small fraction of the pollution is from crustal material that has experienced the distinctive elemental enhancements induced by Earth-analog plate tectonics. In view of the intense theoretical interest in the physical structure of extrasolar rocky planets, this search should be extended to additional targets.

[50]  arXiv:1407.5928 [pdf, other]
Title: BICEP3: a 95 GHz refracting telescope for degree-scale CMB polarization
Comments: 12 pages, 5 figures. Presented at SPIE Astronomical Telescopes and Instrumentation 2014: Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII. To be published in Proceedings of SPIE Volume 9153
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

BICEP3 is a 550 mm-aperture refracting telescope for polarimetry of radiation in the cosmic microwave background at 95 GHz. It adopts the methodology of BICEP1, BICEP2 and the Keck Array experiments - it possesses sufficient resolution to search for signatures of the inflation-induced cosmic gravitational-wave background while utilizing a compact design for ease of construction and to facilitate the characterization and mitigation of systematics. However, BICEP3 represents a significant breakthrough in per-receiver sensitivity, with a focal plane area 5$\times$ larger than a BICEP2/Keck Array receiver and faster optics ($f/1.6$ vs. $f/2.4$). Large-aperture infrared-reflective metal-mesh filters and infrared-absorptive cold alumina filters and lenses were developed and implemented for its optics. The camera consists of 1280 dual-polarization pixels; each is a pair of orthogonal antenna arrays coupled to transition-edge sensor bolometers and read out by multiplexed SQUIDs. Upon deployment at the South Pole during the 2014-15 season, BICEP3 will have survey speed comparable to Keck Array 150 GHz (2013), and will significantly enhance spectral separation of primordial B-mode power from that of possible galactic dust contamination in the BICEP2 observation patch.

[51]  arXiv:1407.5940 [pdf, other]
Title: Explaining Jupiter's magnetic field and equatorial jet dynamic
Comments: 7 pages, 4 figures, accepted for publication in GRL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Geophysics (physics.geo-ph)

Spacecraft data reveal a very Earth-like Jovian magnetic field. This is surprising since numerical simulations have shown that the vastly different interiors of terrestrial and gas planets can strongly affect the internal dynamo process. Here we present the first numerical dynamo that manages to match the structure and strength of the observed magnetic field by embracing the newest models for Jupiter's interior. Simulated dynamo action primarily occurs in the deep high electrical conductivity region while zonal flows are dynamically constrained to a strong equatorial jet in the outer envelope of low conductivity. Our model reproduces the structure and strength of the observed global magnetic field and predicts that secondary dynamo action associated to the equatorial jet produces banded magnetic features likely observable by the Juno mission. Secular variation in our model scales to about 2000 nT per year and should also be observable during the one year nominal mission duration.

[52]  arXiv:1407.5941 [pdf, ps, other]
Title: Suprathermal Electrons in the Solar Corona: Can Nonlocal Transport Explain Heliospheric Charge States?
Authors: Steven R. Cranmer (CfA)
Comments: Accepted for publication in Astrophysical Journal Letters. 5 pages (emulateapj style), 3 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

There have been several ideas proposed to explain how the Sun's corona is heated and how the solar wind is accelerated. Some models assume that open magnetic field lines are heated by Alfven waves driven by photospheric motions and dissipated after undergoing a turbulent cascade. Other models posit that much of the solar wind's mass and energy is injected via magnetic reconnection from closed coronal loops. The latter idea is motivated by observations of reconnecting jets and also by similarities of ion composition between closed loops and the slow wind. Wave/turbulence models have also succeeded in reproducing observed trends in ion composition signatures versus wind speed. However, the absolute values of the charge-state ratios predicted by those models tended to be too low in comparison with observations. This letter refines these predictions by taking better account of weak Coulomb collisions for coronal electrons, whose thermodynamic properties determine the ion charge states in the low corona. A perturbative description of nonlocal electron transport is applied to an existing set of wave/turbulence models. The resulting electron velocity distributions in the low corona exhibit mild suprathermal tails characterized by "kappa" exponents between 10 and 25. These suprathermal electrons are found to be sufficiently energetic to enhance the charge states of oxygen ions, while maintaining the same relative trend with wind speed that was found when the distribution was assumed to be Maxwellian. The updated wave/turbulence models are in excellent agreement with solar wind ion composition measurements.

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[54]  arXiv:1407.5967 [pdf, ps, other]
Title: The RCB star V854 Cen is surrounded by a hot dusty shell
Authors: Olivier Chesneau (LAGRANGE), Florentin Millour (LAGRANGE), Orsola De Marco, S.N. Bright (LAGRANGE), Alain Spang (LAGRANGE), Eric Lagadec (LAGRANGE), Djamel Mékarnia (LAGRANGE), W. J. de Wit (ESO)
Comments: Astronomy and Astrophysics (2014) Will be set by the publisher
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Aims. The hydrogen-deficient supergiants known as R Coronae Borealis Stars might be the result of a double degenerate merger of two white dwarfs (WDs), or a final helium shell flash in a PN central star. In this context, any information on the geometry of their circumstellar environment and, in particular, the potential detection of elongated structures is of great importance. Methods. We obtained near-IR observations of \object{V854\,Cen} with the {{\sc AMBER}} recombiner located at the Very Large Telescope Interferometer ({{\sc VLTI}}) array with the compact array (B$\leq$35m) in 2013 and the long array (B$\leq$140m) in 2014. At each time, \object{V854\,Cen} was at maximum light. The H- and K-band continua were investigated by means of spectrally-dependant geometric models. These data are supplemented with mid-IR {{\sc VISIR}}/VLT images. Results. A dusty slightly elongated over-density is discovered both in the H and K-band images. With the compact array, the central star is unresolved ($\Theta\leq2.5$\,mas), but a flattened dusty environment of $8 \times 11$ mas is discovered whose flux raises from about $\sim$20\% in the H band to reach about $\sim$50\% at 2.3\micron, indicative of the presence of hot (T$\sim$1500\,K) dust in the close vicinity of the star. The major axis is oriented at a position angle (P.A.) of 126$\pm$29\deg. Adding the long array configuration dataset provides tighter constraints on the star diameter ($\Theta\leq1.0$\,mas), a slight increase of the over-density representing $12 \times 15$ mas and a consistent P.A. of 133$\pm$49\deg. The closure phases, sensitive to asymmetries, are null and compatible with a centro-symmetric, unperturbed environment excluding point sources at the level of 3\% of the total flux in 2013 and 2014. The VISIR images exhibit at larger distances ($\sim$1\arcsec) a flattened aspect ratio at the 15-20\% level with a position angle of 92$\pm$19\deg, marginally consistent with the interferometric observations. Conclusions. This is the first time that a moderately elongated structure has been observed around an RCB star. These observations confirm the numerous suggestions for this star for a bipolar structure proposed in the literature, mainly based on polarimetric and spectroscopic observations.