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J.P. Magué & B. Ménard



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[1]  arXiv:1404.4364 [pdf, other]
Title: Nonsingular bouncing cosmologies in light of BICEP2
Comments: 8 pages, 3 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)

We confront various nonsingular bouncing cosmologies with the recently released BICEP2 data and investigate the observational constraints on their parameter space. In particular, within the context of the effective field approach, we analyze the constraints on the matter bounce curvaton scenario with a light scalar field, and the new matter bounce cosmology model in which the universe successively experiences a period of matter contraction and an ekpyrotic phase. Additionally, we consider three nonsingular bouncing cosmologies obtained in the framework of modified gravity theories, namely the Ho\v{r}ava-Lifshitz bounce model, the $f(T)$ bounce model, and loop quantum cosmology.

[2]  arXiv:1404.4365 [pdf, ps, other]
Title: Stars Get Dizzy After Lunch
Comments: 8 pages, 4 figures, to be published in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Exoplanet searches have discovered a large number of 'hot Jupiters'--high-mass planets orbiting very close to their parent stars in nearly circular orbits. A number of these planets are sufficiently massive and close-in to be significantly affected by tidal dissipation in the parent star, to a degree parametrized by the tidal quality factor $Q_*$. This process speeds up their stars' rotation rate while reducing the planets' semimajor axis. In this paper, we investigate the tidal destruction of hot Jupiters. Because the orbital angular momenta of these planets are a significant fraction of their stars' rotational angular momenta, they spin up their stars significantly while spiralling to their deaths. Using Monte Carlo simulation, we predict that for $Q_* = 10^6$, $3.9\times 10^{-6}$ of stars with the Kepler Target Catalog's mass distribution should have a rotation period shorter than 1/3 day (8 h) due to accreting a planet. Exoplanet surveys such as SuperWASP, HATnet, HATsouth, and KELT have already produced light curves of millions of stars. These two facts suggest that it may be possible to search for tidally-destroyed planets by looking for stars with extremely short rotational periods, then looking for remnant planet cores around those candidates, anomalies in the metal distribution, or other signatures of the recent accretion of the planet.

[3]  arXiv:1404.4368 [pdf, other]
Title: Formation, tidal evolution and habitability of the Kepler-186 system
Comments: Paper submitted to ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

The Kepler-186 system consists of five planets orbiting an early-M dwarf. The planets have physical radii of 1.0-1.50 R$_\oplus$ and orbital periods of 4 to 130 days. The $1.1~$R$_\oplus$ Kepler-186f with a period of 130 days is of particular interest. Its insolation of roughly $0.32~S_\odot$places it within the liquid water habitable zone. We present a multi-faceted study of the Kepler-186 system. First, we show that the distribution of planet masses can be roughly reproduced if the planets accreted from a high-surface density disk presumably sculpted by an earlier phase of migration. However, our simulations predict the existence of 1-2 undetected planets between planets e and f. Next, we present a dynamical analysis of the system including the effect of tides. The timescale for tidal evolution is short enough that the four inner planets must have small obliquities and near-synchronous rotation rates. Tidal evolution of Kepler-186f is slow enough that its current spin state depends on a combination of its dissipation rate and the stellar age. Finally, we study the habitability of Kepler-186f with a 1-D climate model. The planet's surface temperature can be raised above 273 K with 0.5-5 bars of CO$_2$, depending on the amount of N$_2$ present. Kepler-186f represents a case study of an Earth-sized planet in the cooler regions of the habitable zone of a cool star.

[4]  arXiv:1404.4371 [pdf, ps, other]
Title: Molecular Gas Content of HI Monsters and Implications to Cold Gas Content Evolution in Galaxies
Authors: Cheoljong Lee (1,2), Aeree Chung (1,3), Min S. Yun (4), Ryan Cybulski (4), G. Narayanan (4), N. Erickson (4) (1-Yonsei University, 2-University of Virginia, 3-Yonsei University Observatory, 4-University of Massachusetts)
Comments: 18 pages, 11 figures, 2 tables; Accepted for publication in MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present 12CO (J=1-0) observations of a sample of local galaxies (0.04<z<0.08) with a large neutral hydrogen reservoir, or "HI monsters". The data were obtained using the Redshift Search Receiver on the FCRAO 14 m telescope. The sample consists of 20 HI-massive galaxies with M(HI)>3e10Msun from the ALFALFA survey and 8 LSBs with a comparable M(HI) (>1.5e10Msun). Our sample selection is purely based on the amount of neutral hydrogen, thereby providing a chance to study how atomic and molecular gas relate to each other in these HI-massive systems. We have detected CO in 15 out of 20 ALFALFA selected galaxies and 4 out of 8 LSBs with molecular gas mass M(H2) of (1-11)e9Msun. Their total cold gas masses of (2-7e10Msun make them some of the most gas-massive galaxies identified to date in the Local Universe. Observed trends associated with HI, H2, and stellar properties of the HI massive galaxies and the field comparison sample are analyzed in the context of theoretical models of galaxy cold gas content and evolution, and the importance of total gas content and improved recipes for handling spatially differentiated behaviors of disk and halo gas are identified as potential areas of improvement for the modeling.

[5]  arXiv:1404.4372 [pdf, other]
Title: The Epoch of Reionization Window: II. Statistical Methods for Foreground Wedge Reduction
Comments: 19 pages, 7 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Instrumentation and Methods for Astrophysics (astro-ph.IM)

For there to be a successful measurement of the 21 cm Epoch of Reionization (EoR) power spectrum, it is crucial that foreground contaminants be robustly suppressed. These foregrounds come from a variety of sources (such as Galactic synchrotron emission and extragalactic point sources), but almost all share the property of being spectrally smooth. When considered with the chromatic response of an interferometric measurement, spectrally smooth foregrounds contaminate a signature "wedge" region in cylindrical $k_\perp k_\parallel$ Fourier space. The complement of the foreground wedge is termed the "EoR window", and is expected to be mostly foreground-free, allowing clean measurements of the power spectrum. This paper is a sequel to a previous paper that established a rigorous mathematical framework for describing the foreground wedge and the EoR window. Here, we explore statistical methods by which the EoR window can be enlarged, thereby increasing power spectrum sensitivity. We adapt the Feldman-Kaiser-Peacock approximation (commonly used in galaxy surveys) for 21 cm cosmology, and also compare the optimal quadratic estimator to basic estimators (where Fourier modes are simply squared and binned without regard for the covariance between modes). The optimal quadratic estimator uses both spatial and spectral information to reduce errors, and (compared to the basic estimators) suppresses foregrounds by a factor of $\sim 10^5$ in power at the peripheries of the EoR window. If numerical issues can be finessed, decorrelation techniques allow the EoR window to be further enlarged, enabling measurements to be made deep within the foreground wedge. These techniques do not assume that foreground are Gaussian-distributed, and we additionally prove that a final round of foreground subtraction can be performed after decorrelation in a way that is guaranteed to have no cosmological signal loss.

[6]  arXiv:1404.4376 [pdf, other]
Title: A 3D radiative transfer framework: XI. multi-level NLTE
Comments: 20 pages, 14 figures, A&A, in press
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Multi-level non-local thermodynamic equilibrium (NLTE) radiation transfer calculations have become standard throughout the stellar atmospheres community and are applied to all types of stars as well as dynamical systems such as novae and supernovae. Even today spherically symmetric 1D calculations with full physics are computationally intensive. We show that full NLTE calculations can be done with fully 3 dimensional (3D) radiative transfer. With modern computational techniques and current massive parallel computational resources, full detailed solution of the multi-level NLTE problem coupled to the solution of the radiative transfer scattering problem can be solved without sacrificing the micro physics description. We extend the use of a rate operator developed to solve the coupled NLTE problem in spherically symmetric 1D systems. In order to spread memory among processors we have implemented the NLTE/3D module with a hierarchical domain decomposition method that distributes the NLTE levels, radiative rates, and rate operator data over a group of processes so that each process only holds the data for a fraction of the voxels. Each process in a group holds all the relevant data to participate in the solution of the 3DRT problem so that the 3DRT solution is parallelized within a domain decomposition group. We solve a spherically symmetric system in 3D spherical coordinates in order to directly compare our well-tested 1D code to the 3D case. We compare three levels of tests: a) a simple H+He test calculation, b) H+He+CNO+Mg, c) H+He+Fe. The last test is computationally large and shows that realistic astrophysical problems are solvable now, but they do require significant computational resources. With presently available computational resources it is possible to solve the full 3D multi-level problem with the same detailed micro-physics as included in 1D modeling.

[7]  arXiv:1404.4379 [pdf, ps, other]
Title: KOI-3278: A Self-Lensing Binary Star System
Comments: Published in Science. Main text 6 pages, 3 figures. Supplement 16 pages, 7 figures. Code used for analysis is available at this http URL . Animation showing ingress of the lensing event at this https URL . Executive summary for non-astronomers available at this http URL
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Over 40% of Sun-like stars are bound in binary or multistar systems. Stellar remnants in edge-on binary systems can gravitationally magnify their companions, as predicted 40 years ago. By using data from the Kepler spacecraft, we report the detection of such a "self-lensing" system, in which a 5-hour pulse of 0.1% amplitude occurs every orbital period. The white dwarf stellar remnant and its Sun-like companion orbit one another every 88.18 days, a long period for a white dwarf-eclipsing binary. By modeling the pulse as gravitational magnification (microlensing) along with Kepler's laws and stellar models, we constrain the mass of the white dwarf to be ~63% of the mass of our Sun. Further study of this system, and any others discovered like it, will help to constrain the physics of white dwarfs and binary star evolution.

[8]  arXiv:1404.4381 [pdf, ps, other]
Title: A Consistent Orbital Stability Analysis for the GJ 581 System
Comments: 26 pages, 8 figures, 6 tables, accepted for publication in the Astrophysical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

We apply a combination of N-body modeling techniques and automated data fitting with Monte Carlo Markov Chain uncertainty analysis of Keplerian orbital models to radial velocity data to determine long term stability of the planetary system GJ 581. We find that while there are stability concerns with the 4-planet model as published by Forveille et al. (2011), when uncertainties in the system are accounted for, particularly stellar jitter, the hypothesis that the 4-planet model is gravitationally unstable is not statistically significant. Additionally, the system including proposed planet g by Vogt et al. (2012) also shows some stability concerns when eccentricities are allowed to float in the orbital fit, yet when uncertainties are included in the analysis the system including planet g also can not be proven to be unstable. We present revised reduced chi-squared values for Keplerian astrocentric orbital fits assuming 4-planet and 5-planet models for GJ~581 under the condition that best fits must be stable, and find no distinguishable difference by including planet g in the model. Additionally we present revised orbital element estimates for each assuming uncertainties due to stellar jitter under the constraint of the system being gravitationally stable.

[9]  arXiv:1404.4390 [pdf, ps, other]
Title: The Light Curve Shapes as a Key to Resolving the Origin of Long Secondary Periods in Red Giant Stars
Comments: 7 pages, 7 figures, accepted for publication in ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We present a study of OGLE light curves of red giant stars exhibiting long secondary periods (LSPs) - an enigmatic phenomenon commonly observed in stars on the upper red giant branch and asymptotic giant branch. We show that the light curves of LSP stars are essentially identical to those of the spotted variables with one dark spot on their photospheres. Such a behavior can be explained by a presence of a dusty cloud orbiting the red giant together with a low-mass companion in a close, circular orbit. We argue that the binary scenario is in agreement with most of the observational properties of LSP variables, including non-sinusoidal shapes of their radial velocity curves.

[10]  arXiv:1404.4393 [pdf]
Title: The Orbit of Transneptunian Binary Manwë and Thorondor and their Upcoming Mutual Events
Comments: 15 pages, 3 figures, 5 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

A new Hubble Space Telescope observation of the 7:4 resonant transneptunian binary system (385446) Manw\"e has shown that, of two previously reported solutions for the orbit of its satellite Thorondor, the prograde one is correct. The orbit has a period of 110.18 $\pm$ 0.02 days, semimajor axis of 6670 $\pm$ 40 km, and an eccentricity of 0.563 $\pm$ 0.007. It will be viewable edge-on from the inner solar system during 2015-2017, presenting opportunities to observe mutual occultation and eclipse events. However, the number of observable events will be small, owing to the long orbital period and expected small sizes of the bodies relative to their separation. This paper presents predictions for events observable from Earth-based telescopes and discusses the associated uncertainties and challenges.

[11]  arXiv:1404.4404 [pdf, other]
Title: The formation of the W43 complex: constraining its atomic-to-molecular transition and searching for colliding clouds
Comments: 12 pages, 11 figures, accepted by A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Numerical simulations have explored the possibility to form molecular clouds through either a quasi-static, self-gravitating mechanism or the collision of gas streams or lower-density clouds. They also quantitatively predict the distribution of matter at the transition from atomic to molecular gases. We aim to observationally test these models by studying the environment of W43, a molecular cloud complex near the tip of the Galactic long bar. Using Galaxy-wide HI and 12CO surveys we searched for gas flowing toward the W43 molecular cloud complex. We also estimated the HI and H2 mass surface densities to constrain the transition from atomic to molecular gas around and within W43. We found 3 cloud ensembles within the position-velocity diagrams of 12CO and HI gases. They are separated by 20km/s along the line of sight and extend into the 13CO velocity structure of W43. Since their velocity gradients are consistent with free-fall, they could be nearby clouds attracted by, and streaming toward, the W43 10^7Msun potential well. We show that the HI surface density, Sigma_HI=45-85Msun/pc2, does not reach any threshold level but increases when entering the 130pc-wide molecular complex previously defined. This suggests that an equilibrium between H2 formation and photodissociation has not yet been reached. The H2-to-HI ratio measured over the W43 region and its surroundings, R_H2~3.5, is high, indicating that most of the gas is already in molecular form in W43 and in structures several hundreds of parsecs downstream along the Scutum-Centaurus arm. The W43 molecular cloud complex may have formed, and in fact may still be accreting mass from the agglomeration of clouds. Already in the molecular-dominated regime, most of these clouds are streaming from the Scutum-Centaurus arm. This is in clear disagreement with quasi-static and steady-state models of molecular cloud formation.

[12]  arXiv:1404.4411 [pdf, ps, other]
Title: Calibrating the updated overshoot mixing model on eclipsing binary stars: HY Vir, YZ Cas, X2 Hya & VV Crv
Authors: Y. Meng, Q.S. Zhang
Comments: 7 pages, 7 figures, accepted for publication in ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The detached eclipsing binary stars with convective cores provide a good tool to investigate the convective core overshoot. It has been performed on some binary stars to restrict the classical overshoot model which simply extends the boundary of fully mixed region. However, the classical overshoot model is physically unreasonable and inconsistent with the helioseismic investigation. An updated model of the overshoot mixing was established recently. There is a key parameter in the model. In this paper, we use the observations of four eclipsing binary stars, i.e., HY Vir, YZ Cas, $\rm{\chi^2}$ Hya and VV Crv, to investigate the suitable value for the parameter. It is found that the suggested value by the calibrations on eclipsing binary stars is same as the recommended value by other ways. And we have studied the effects of the updated overshoot model on the stellar structure. The diffusion coefficient of the convective/overshoot mixing is very high in the convection zone, then quickly decreases near the convective boundary, and exponentially decreases in the overshoot region. The low value of the diffusion coefficient in the overshoot region leads to weak mixing and the partially mixed overshoot region. The semi-convection, which appears in the standard stellar models of low-mass stars with convective core, is removed by the partial overshoot mixing.

[13]  arXiv:1404.4417 [pdf, ps, other]
Title: HAT-P-54b: A hot jupiter transiting a 0.64 Msun star in field 0 of the K2 mission
Authors: G.Á.Bakos (1,9,10), J.D.Hartman (1), W.Bhatti (1), A.Bieryla (2), M.de Val-Borro (1), D.W.Latham (2), L.A.Buchhave (2,3), Z.Csubry (1), K.Penev (1), G.Kovács (4,5), B.Béky (2), E.Falco (2), T.Kovács (4), A.W.Howard (6), J.A.Johnson (2,9,10), H.Isaacson (7), G.W.Marcy (7), G.Torres (2), R.W.Noyes (2), P.Berlind (2), M.L.Calkins (2), G.A.Esquerdo (2), J.Lázár (8), I.Papp (8), P.Sári (8) ((1) Princeton University, (2) Harvard-Smithsonian Center for Astrophysics (3) University of Copenhagen, (4) Konkoly Observatory, (5) University of North Dakota, (6) IfA, Hawaii, (7) UCB, (8) Hungarian Astronomical Association, (9) Sloan Fellow, (10) Packard Fellow)
Comments: Submitted to AJ 2014 April 16
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

We report the discovery of HAT-P-54b, a planet transiting a late K dwarf star in field 0 of the NASA K2 mission. We combine ground-based photometric light curves with radial velocity measurements to determine the physical parameters of the system. HAT-P-54b has a mass of 0.760 $\pm$ 0.032 $M_J$, a radius of 0.944 $\pm$ 0.028 $R_J$, and an orbital period of 3.7998 d. The star has V = 13.505 $\pm$ 0.060, a mass of 0.645 $\pm$ 0.020 $M_{\odot}$, a radius of 0.617 $\pm$ 0.013 $R_{\odot}$, an effective temperature of Teff = 4390 $\pm$ 50K, and a subsolar metallicity of [Fe/H] = -0.127 $\pm$ 0.080. HAT-P-54b has a radius that is smaller than 92% of the known transiting planets with masses greater than that of Saturn, while HAT-P-54 is one of the lowest-mass stars known to host a hot Jupiter. Follow-up high-precision photometric observations by the K2 mission promise to make this a well-studied planetary system.

[14]  arXiv:1404.4418 [pdf, ps, other]
Title: Robustness of $H_0$ determination at intermediate redshifts
Comments: 6 pages, 4 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc)

The most recent Hubble constant $(H_0)$ estimates from local methods ($z<<1$), $H_0=73.8\pm 2.4$ km s$^{-1}$ Mpc$^{-1}$, and the one from high redshits (Planck Collaboration 2013), $H_0=67.3\pm 1.2$ km s$^{-1}$ Mpc$^{-1}$, are discrepant at $2.4 \sigma$ confidence level. Within this context, Lima & Cunha (LC) (ApJL 781, 38, 2014) derived a new determination of $H_0$ using four cosmic probes at intermediate redshifts ($0.1<z<1.8$) based on the so-called flat $\Lambda$CDM model. They obtained $H_0=74.1\pm 2.2$ km s$^{-1}$ Mpc$^{-1}$, in full agreement with local measurements. In this letter, we explore the robustness of the LC result searching for systematic errors and its dependence from the cosmological model used. We find that the $H_0$ value from this joint analysis is very weakly dependent on the underlying cosmological model, but the morphology adopted to infer the distance to galaxy clusters changes the result sizeably, being the main source of systematic errors.

[15]  arXiv:1404.4422 [pdf, ps, other]
Title: How gravitational lensing helps gamma-ray photons avoid $γ- γ$ absorption
Authors: Anna Barnacka (CfA), Markus Bottcher (NWU), Iurii Sushch (NWU)
Comments: Submitted to ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We investigate potential $\gamma-\gamma$ absorption of gamma-ray emission from blazars arising from inhomogeneities along the line of sight, beyond the diffuse Extragalactic Background Light (EBL). As plausible sources of excess $\gamma-\gamma$ opacity, we consider (1) foreground galaxies, including cases in which this configuration leads to strong gravitational lensing, (2) individual stars within these foreground galaxies, and (3) individual stars within our own galaxy, which may act as lenses for microlensing events. We found that intervening galaxies close to the line-of-sight are unlikely to lead to significant excess $\gamma-\gamma$ absorption. This opens up the prospect of detecting lensed gamma-ray blazars at energies above 10 GeV with their gamma-ray spectra effectively only affected by the EBL. The most luminous stars located either in intervening galaxy or in our galaxy provides an environment in which these gamma-rays could, in principle, be significantly absorbed. However, despite a large microlensing probability due to stars located in intervening galaxies, gamma-rays avoid absorption by being deflected by the gravitational potentials of such intervening stars to projected distances ("impact parameters"') where the resulting $\gamma-\gamma$ opacities are negligible. Thus, neither of the intervening excess photon fields considered here, provide a substantial source of excess $\gamma-\gamma$ opacity beyond the EBL, even in the case of very close alignments between the background blazar and a foreground star or galaxy.

[16]  arXiv:1404.4424 [pdf, ps, other]
Title: Dark matter relic density in Gauss-Bonnet braneworld cosmology
Comments: 8 pages, 3 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

The relic density of symmetric and asymmetric dark matter in the Gauss-Bonnet braneworld cosmology is investigated. The reduced expansion rate in this scenario delays particle freeze-out, leading to relic abundances which are suppressed by up to $\mathcal{O}(10^{-3})$. In this case the annihilation cross section must be reduced by up to two orders of magnitude below the canonical value $\langle\sigma v\rangle \approx 2\times 10^{-26}$ cm$^3$s$^{-1}$ to reconcile the predicted dark matter density with observation. We use the latest observational bound $\Omega_{DM}h^2 = 0.1187 \pm 0.0017$ to constrain the various model parameters and discuss the implications for direct/indirect dark matter detection experiments as well as dark matter particle models.

[17]  arXiv:1404.4430 [pdf, other]
Title: Companions of Stars: From Other Stars to Brown Dwarfs to Planets: The Discovery of the First Methane Brown Dwarf
Comments: 31 pages, 13 figures
Journal-ref: Chapter from "50 Years of Brown Dwarfs," V. Joergens, ed., Astrophysics and Space Science Library, Volume 401, (Zurich: Springer) 2014
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The discovery of the first methane brown dwarf provides a framework for describing the important advances in both fundamental physics and astrophysics that are due to the study of companions of stars. I present a few highlights of the history of this subject along with details of the discovery of the brown dwarf Gliese 229B. The nature of companions of stars is discussed with an attempt to avoid biases induced by anthropocentric nomenclature. With the newer types of remote reconnaissance of nearby stars and their systems of companions, an exciting and perhaps even more profound set of contributions to science is within reach in the near future. This includes an exploration of the diversity of planets in the universe and perhaps soon the first solid evidence for biological activity outside our Solar System.

[18]  arXiv:1404.4432 [pdf, ps, other]
Title: Linear Growth of the Kelvin-Helmholtz Instability with an Adiabatic Cosmic-ray Gas
Comments: 12 pages, 8 figures, accepted for publication in ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We investigate effects of cosmic-rays on the linear growth of the Kelvin-Helmholtz instability. Cosmic-rays are treated as an adiabatic gas and allowed to diffuse along magnetic field lines. We calculated the dispersion relation of the instability for various sets of two free parameters, the ratio of the cosmic-ray pressure to the thermal gas pressure and the diffusion coefficient. Including cosmic-ray effects, a shear layer is more destabilized and the growth rates can be enhanced in comparison with the ideal magnetohydrodynamical case. Whether the growth rate is effectively enhanced or not depends on the diffusion coefficient of cosmic-rays. We obtain the criterion for effective enhancement by comparing the growing time scale of the instability with the diffusion time scale of cosmic-rays. These results can be applied to various astrophysical phenomena where a velocity shear is present, such as outflows from star-forming galaxies, AGN jet, channel flows resulting from the nonlinear development of the magnetorotational instability, and galactic disks.

[19]  arXiv:1404.4434 [pdf, ps, other]
Title: Perspectives of current-layer diagnostics in solar flares
Journal-ref: Astronomy & Astrophysics, 558, A16, (2013)
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

A reconnecting current layer is a `heart' of a solar flare, because it is a place of magnetic-field energy release. However there are no direct observations of these layers. The aim of our work is to understand why we actually do not directly observe current layers and what we need to do it in the future. The method is based on a simple mathematical model of a super-hot (T ~ 1E8 K) turbulent-current layer (SHTCL) and a model of plasma heating by the layer. The models allow us to study a correspondence between the main characteristics of the layer, such as temperature and dimensions, and the observational features, such as differential and integral emission measure of heated plasma, intensity of spectral lines Fe XXVI (1.78 and 1.51A) and Ni XXVII (1.59 A). This method provides a theoretical basis for determining parameters of the current layer from observations. Observations of SHTCLs are difficult, because the spectral line intensities are faint, but it is theoretically possible in the future. Observations in X-ray range 1.5--1.8 A with high spectral resolution (better than 0.01 A) and high temporal resolution (seconds) are needed. It is also very important to interpret the observations using a multi-temperature approach instead of the usual single or double temperature method.

[20]  arXiv:1404.4444 [pdf, ps, other]
Title: New 2MASS Near-infrared Photometry for Globular Clusters in M31
Authors: Song Wang (1,2), Jun Ma (1), Zhenyu Wu (1), Xu Zhou (1) ((1) Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, (2) University of Chinese Academy of Sciences)
Comments: Accepted for Publication in AJ, 14 pages, 17 figures and 6 tables
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present 2MASS $JHK_{\rm s}$ photometry for 913 star clusters and candidates in the field of M31, which are selected from the latest Revised Bologna Catalog of M31 globular clusters (GCs) and candidates. The photometric measurements in this paper supplement this catalog, and provide a most comprehensive and homogeneous photometric catalog for M31 GCs in the $JHK_{\rm s}$ bandpasses. In general, our photometry is consistent with previous measurements. The globular cluster luminosity function (GCLF) peaks for the confirmed GCs derived by fitting a $t_5$ distribution using maximum likelihood method are: $J_0 = 15.348_{-0.208}^{+0.206}$, $H_0 = 14.703_{-0.180}^{+0.176}$, and ${K_{\rm s}}_0 = 14.534_{-0.146}^{+0.142}$, all of which agree well with previous studies. The GCLFs are different between metal-rich (MR) and metal-poor (MP), inner and outer subpopulations, as that MP clusters are fainter than their MR counterparts, and the inner clusters are brighter than the outer ones, which confirm previous results. The NIR colors of the GC candidates are on average redder than those of the confirmed GCs, which lead to an obscure bimodal distribution of the color indices. The relation of $(V-K_{\rm s})_0$ and metallicity shows a notable departure from linearity, with a shallower slope towards the redder end. The color-magnitude diagram (CMD) and color-color diagram show that many GC candidates are located out of the evolutionary tracks, suggesting that some of them may be false M31 GC candidates. The CMD also shows that the initial mass function of M31 GCs covers a large range, and the majority of the clusters have initial masses between $10^3$ and $10^6$ $M_{\odot}$.

[21]  arXiv:1404.4446 [pdf, ps, other]
Title: Constraints on the winds of hot subdwarf stars from X-ray observations of two sdB binaries with compact companions: CD -30 11223 and PG 1232-136
Comments: Accepted for publication on MNRAS
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR)

Little observational data are available on the weak stellar winds of hot subdwarf stars of B spectral type (sdB). Close binary systems composed of an sdB star and a compact object (white dwarf, neutron star or black hole) could be detected as accretion-powered X-ray sources. The study of their X-ray emission can probe the properties of line-driven winds of sdB stars that can not be derived directly from spectroscopy because of the low luminosity of these stars. Here we report on the first sensitive X-ray observations of two sdB binaries with compact companions. CD -30 11223 is the sdB binary with the shortest known orbital period (1.2 h) and its companion is certainly a white dwarf. PG 1232-136 is an sdB binary considered the best candidate to host a black hole companion. We observed these stars with XMM-Newton in August 2013 for 50 ks and in July 2009 for 36 ks, respectively. None of them was detected and we derived luminosity upper limits of about 1.5x10^29 erg/s for CD -30 11223 5x10^29 erg/s for PG 1232-136. The corresponding mass loss rate for PG 1232-136 is poorly constrained, owing to the unknown efficiency for black hole accretion. On the other hand, in the case of CD -30 11223 we could derive, under reasonable assumptions, an upper limit of about 3x10^-13 solar masses/yr on the wind mass loss rate from the sdB star. This is one of the few observational constraints on the weak winds expected in this class of low mass hot stars. We also report the results on the X-ray emission from a cluster of galaxies serendipitously discovered in the field of CD -30 11223.

[22]  arXiv:1404.4450 [pdf, ps, other]
Title: Next generation population synthesis of accreting white dwarfs: I. Hybrid calculations using BSE + MESA
Comments: 12 pages,10 figures, submitted to MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); High Energy Astrophysical Phenomena (astro-ph.HE)

Accreting, nuclear-burning white dwarfs have been deemed to be candidate progenitors of SNe Ia, and to account for supersoft X-ray sources, novae, etc. We have carried out a binary population synthesis (BPS) study of hydrogen-accreting WDs. First, we use the BPS code \textsf{BSE} as a baseline for the commonly used "rapid" approach. Second, we apply a "hybrid" approach: we use \textsf{BSE} to generate a population of WDs with non-degenerate companions on the verge of mass transfer. We then follow their evolution using the detailed stellar evolutionary code \textsf{MESA}. We investigate the evolution of the number of rapidly accreting white dwarfs (RAWDs), stably nuclear-burning white dwarfs (SNBWDs), and the SNe Ia rate produced by "single-degenerate" systems (SD). The two algorithms differ significantly in the predicted numbers of SNBWDs at early and late times, and also in the delay time distribution (DTD) of SD SNe Ia. The differences in the treatment of mass transfer may partially account for differences in the SNe Ia rate and DTD found by different groups. Using our hybrid algorithm, we found the number of RAWDs to be $\simeq (160 - 180)$ and $\simeq (2250 - 2500)$ in $10^{11}\,M_{\odot}$ elliptical and spiral-like galaxies, respectively. The numbers of SNBWDs found are $\simeq (750 - 1900)$ and $\simeq (4550 - 6550)$, respectively, larger than in previous studies. For 100% retention of accumulated helium, our calculations produce a SD SN Ia rate in a Milky-Way-like galaxy of $2.0\times10^{-4}\rm{yr}^{-1}$, an order of magnitude below that observed. In agreement with previous studies, our calculated SD DTD is inconsistent with observations. Subsequent articles will be devoted to the electromagnetic output from accreting WDs, and comparison of populations in varying accretion states with observations. (abridged)

[23]  arXiv:1404.4460 [pdf]
Title: Earth-like Habitats in Planetary Systems
Comments: 36 Pages, 6 figures, 2014, Special Issue in Planetary and Space Science on the Helmholtz Research Alliance on Planetary Evolution and Life
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Understanding the concept of habitability is related to an evolutionary knowledge of the particular planet-in-question. Additional indications so-called "systemic aspects" of the planetary system as a whole governs a particular planet's claim on habitability. Here we focus on such systemic aspects and discuss their relevance to the formation of an 'Earth-like' habitable planet. We summarize our results obtained by lunar sample work and numerical models within the framework of the Research Alliance "Planetary Evolution and Life". We consider various scenarios which simulate the dynamical evolution of the Solar System and discuss the likelihood of forming an Earth-like world orbiting another star. Our model approach is constrained by observations of the modern Solar System and the knowledge of its history. Results suggest that the long-term presence of terrestrial planets is jeopardized due to gravitational interactions if giant planets are present. But habitability of inner rocky planets may be supported in those planetary systems hosting giant planets.
Gravitational interactions within a complex multiple-body structure including giant planets may supply terrestrial planets with materials which formed in the colder region of the proto-planetary disk. During these processes, water, the prime requisite for habitability, is delivered to the inner system. This may occur either during the main accretion phase of terrestrial planets or via impacts during a post-accretion bombardment. Results for both processes are summarized and discussed with reference to the lunar crater record.
Starting from a scenario involving migration of the giant planets this contribution discusses the delivery of water to Earth, the modification of atmospheres by impacts in a planetary system context and the likelihood of the existence of extrasolar Earth-like habitable worlds.

[24]  arXiv:1404.4463 [pdf, ps, other]
Title: VLBI Astrometry of AGB Variables with VERA -- A Mira Type Variable T Lepus
Comments: 13 pages, 6 figures, accepted for publication in PASJ
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We conducted phase referencing VLBI observations of the Mira variable T~Lepus (T~Lep) using VERA, from 2003 to 2006. The distance to the source was determined from its annual parallax which was measured to be 3.06$\pm$0.04 mas, corresponding to a distance of 327$\pm$4\,pc. Our observations revealed the distribution and internal kinematics of H$_2$O masers in T~Lep, and we derived a source systemic motions of 14.60$\pm$0.50 mas yr$^{-1}$ and $-$35.43$\pm$0.79 mas yr$^{-1}$ in right ascension and declination, respectively. We also determined a LSR velocity of $V_\mathrm{LSR}^{\ast} = -$27.63 km s$^{-1}$. Comparison of our result with an image recently obtained from the VLTI infrared interferometer reveals a linear scale picture of the circumstellar structure of T~Lep. Analysis of the source systemic motion in the Galacto-centric coordinate frame indicates a large peculiar motion, which is consistent with the general characteristics of AGB stars. This source makes a contribution to the calibration of the period-luminosity relation of Galactic Mira variables. From the compilation of data for nearby Mira variables found in the literature, whose distances were derived from astrometric VLBI observations, we have calibrated the Galactic Mira period-luminosity relation to a high degree of accuracy.

[25]  arXiv:1404.4493 [pdf, ps, other]
Title: The Effect of Planetary Illumination on Climate Modelling of Earthlike Exomoons
Comments: 11 pages, 16 figures, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

From analytical studies of tidal heating, eclipses and planetary illumination, it is clear that the exomoon habitable zone (EHZ) - the set of moon and host planet orbits that permit liquid water on an Earthlike moon's surface - is a manifold of higher dimension than the planetary HZ.
This paper outlines the first attempt to produce climate models of exomoons which possess all the above sources and sinks of energy. We expand on our previous 1D latitudinal energy balance models (LEBMs), which follow the evolution of the temperature on an Earthlike moon orbiting a Jupiterlike planet, by adding planetary illumination.
We investigate the EHZ in four dimensions, running two separate suites of simulations. The first investigates the EHZ by varying the planet's orbit, keeping the moon's orbit fixed, to compare the EHZ with planetary habitable zones. In general, planetary illumination pushes EHZs slightly further away from the star.
Secondly, we fix the planet's orbit and vary the moon's orbit, to investigate the circumplanetary inner habitable edge. We demonstrate that an outer edge can exist due to eclipses (rather than merely orbital stability), but this edge may be pushed outwards when the effect of the carbonate-silicate cycle is taken into account.

[26]  arXiv:1404.4514 [pdf, ps, other]
Title: On the origin of stars with and without planets. Tc trends and clues to Galactic evolution
Comments: 7 pages, 7 figures, 1 table
Journal-ref: A&A, 564, L15, 2014
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

We explore a sample of 148 solar-like stars to search for a possible correlation between the slopes of the abundance trends versus condensation temperature (known as the Tc slope) with stellar parameters and Galactic orbital parameters in order to understand the nature of the peculiar chemical signatures of these stars and the possible connection with planet formation. We find that the Tc slope significantly correlates (at more than 4sigma) with the stellar age and the stellar surface gravity. We also find tentative evidence that the Tc slope correlates with the mean galactocentric distance of the stars (Rmean), suggesting that those stars that originated in the inner Galaxy have fewer refractory elements relative to the volatiles. While the average Tc slope for planet-hosting solar analogs is steeper than that of their counterparts without planets, this difference probably reflects the difference in their age and Rmean. We conclude that the age and probably the Galactic birth place are determinant to establish the star's chemical properties. Old stars (and stars with inner disk origin) have a lower refractory-to-volatile ratio.

[27]  arXiv:1404.4521 [pdf, other]
Title: Detection of Galaxies with Gaia
Comments: 6 figures, 22 pages
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Besides its major objective tuned to the detection of the stellar galactic population the Gaia mission experiment will also observe a large number of galaxies. In this work we intend to evaluate the number and the characteristics of the galaxies that will effectively pass the onboard selection algorithm of Gaia. The detection of objects in Gaia will be performed in a section of the focal plane known as the Sky Mapper. Taking into account the Video Processing Algorithm criterion of detection and considering the known light profiles of discs and bulges galaxies we assess the number and the type of extra-galactic objects that will be observed by Gaia. We show that the stellar disk population of galaxies will be very difficult to be observed. On the contrary the spheroidal component of elliptical galaxies and bulges having higher central surface brightness and steeper brightness profile will be more easy to be detected. We estimate that most of the 20 000 elliptical population of nearby galaxies inside the local region up to 170 Mpc are in condition to be observed by Gaia. A similar number of bulges could also be observed although the low luminosity bulges should escape detection. About two thirds of the more distant objects up to 600 Mpc could also be detected increasing the total sample to half a million objects including ellipticals and bulges. The angular size of the detected objects will never exceed 4.72 arcsec which is the size of the largest transmitted windows. An heterogeneous population of elliptical galaxies and bulges will be observable by Gaia. This nearby Universe sample of galaxies should constitute a very rich and interesting sample to study their structural properties and their distribution.

[28]  arXiv:1404.4569 [pdf, ps, other]
Title: The C IV Forest as a Probe of Baryon Acoustic Oscillations
Authors: Matthew M. Pieri
Comments: 6 pages, 3 figures. Submitted to MNRAS letters
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

In light of recent successes in measuring baryon acoustic oscillations in quasar absorption using the Lyman-alpha (Ly-alpha) transition, I explore the possibility of using the 1548 Ang transition of triply-ionized carbon (C IV) as a tracer. While the Ly-alpha forest is a more sensitive tracer of intergalactic gas, it is limited by the fact that it can only be measured in the optical window at redshifts z > 2. Quasars are challenging to identify and observe at these high-redshifts, but the C IV forest can be probed down to redshifts z = 1.3, taking full advantage of the peak in the redshift distribution of quasars that can be targeted with high efficiency.
I explore the strength of the C IV absorption signal and show that the absorbing population on the red side of the Ly-alpha emission line is dominated by C IV. As a consequence, I argue that forthcoming surveys will have a sufficient increase in quasar number density to offset the lower sensitivity of the C IV forest and provide competitive precision using both the C IV autocorrelation and the C IV-quasar cross correlation at <z> = 1.6.

[29]  arXiv:1404.4570 [pdf, ps, other]
Title: The orbital evolution of a passive high-orbit fragment with large surface area
Comments: 4 pages, 3 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

The observation data for artificial celestial body 43096, which had been obtained during 2006-2012 within the framework of international project "The Scientific Network of Optical Instruments for Astrometric and Photometric Observations" - International Scientific Optical Network (ISON), were processed. The Keplerian elements and state vector as of 24 November 2006 01:55:50.76 UTC were determined. The numerical integration of the motion equations was performed accounting for the perturbations due to the polar flattening of the Earth, Moon and Sun, as well as the solar radiation pressure. Based on the numerical model of a motion in the near-Earth space that accounts for only the most powerful perturbations, a new method for de-orbiting artificial celestial bodies from high altitudes is suggested. For the first time such a considerable amount of data over long time intervals was gathered for the objects with high area-to-mass ratio that enabled to determine their specific characteristics.

[30]  arXiv:1404.4577 [pdf, ps, other]
Title: Spectral evolution in gamma-ray bursts: predictions of the internal shock model and comparison to observations
Authors: Z. Bosnjak (1,2,3), F. Daigne (2) ((1) CEA-Saclay, (2) UPMC-CNRS, Institut d'Astrophysique de Paris, (3) University of Rijeka)
Comments: 21 pages, 18 figures, accepted for publication in A&A
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Several trends have been identified in the prompt gamma-ray burst (GRB) emission: e.g. hard-to-soft evolution, pulse width evolution with energy, time lags, hardness-intensity/-fluence correlations. Recently Fermi has significantly extended the spectral coverage of GRB observations and improved the characterization of this spectral evolution. We study how internal shocks can reproduce these observations. In this model the emission comes from the synchrotron radiation of shock accelerated electrons, and the spectral evolution is governed by the evolution of the physical conditions in the shocked regions. We present a comprehensive set of simulations of a single pulse and investigate the impact of the model parameters, related to the shock microphysics and to the initial conditions in the ejecta.
We find a general qualitative agreement between the model and the various observations used for the comparison. All these properties or relations are governed by the evolution of the peak energy and photon indices of the spectrum. In addition, we identify the conditions for a quantitative agreement. We find that the best agreement is obtained for (i) steep electron slopes (p>~2.7), (ii) microphysics parameters varying with shock conditions so that more electrons are accelerated in stronger shocks, (iii) steep variations of the initial Lorentz factor in the ejecta. When simulating short GRBs by contracting all timescales, all other parameters being unchanged, we show that the hardness-duration correlation is reproduced, as well as the evolution with duration of the pulse properties. Finally, we investigate the signature at high energy of these different scenarios and find distinct properties - delayed onset, longer emission, and flat spectrum in some cases - suggesting that internal shocks could have a significant contribution to the prompt LAT emission. [abridged]

[31]  arXiv:1404.4614 [pdf, other]
Title: Negative running prevents eternal inflation
Authors: William H. Kinney (Univ. at Buffalo, SUNY), Katherine Freese (Univ. of Michigan)
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Current data from the Planck satellite and the BICEP2 telescope favor, at around the $2 \sigma$ level, negative running of the spectral index of curvature perturbations from inflation. We show that for negative running $\alpha < 0$, the curvature perturbation amplitude has a maximum on scales larger than our current horizon size. A condition for the absence of eternal inflation is that the curvature perturbation amplitude always remain below unity on superhorizon scales. For current bounds on $n_{\rm S}$ from Planck, this corresponds to an upper bound of the running $\alpha < - 4 \times 10^{-5}$, so that even tiny running of the scalar spectral index is sufficient to prevent eternal inflation from occurring, as long as the running remains negative on scales outside the horizon. In single-field inflation models, negative running is associated with a finite duration of inflation: we show that eternal inflation may not occur even in cases where inflation lasts as long as $10^4$ e-folds.

[32]  arXiv:1404.4615 [pdf, ps, other]
Title: Laboratory gamma-ray pulsar
Authors: Andrei Gruzinov
Comments: 2 pages
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The mechanism by which gamma-ray pulsars shine might be reproducible in a laboratory. This claim is supported by three observations: (i) properly focusing a few PW optical laser gives an electromagnetic field in the so-called Aristotelian regime, when a test electron is radiation-overdamped; (ii) the Goldreich-Julian number density of this electromagnetic field (the number density of elementary charges needed for a nearly full conversion of optical power into gamma-rays) is of order the electron number density in a solid; (iii) above about $50$PW, the external source of electrons is not needed -- charges will be created by a pair production avalanche.