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[1]  arXiv:1706.07047 [pdf, other]
Title: High-Energy Gamma Rays and Neutrinos from Nearby Radio Galaxies
Comments: 28 pages, 17 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA); High Energy Physics - Phenomenology (hep-ph)

Multi-messenger data suggest that radio galaxies (i.e. non-blazar active galaxies) are perhaps the most likely class of sources for the diffuse flux of high-energy neutrinos reported by the IceCube Collaboration. In this study, we consider the gamma-ray spectrum observed from four nearby radio galaxies (Centaurus A, PKS 0625-35, NGC 1275 and IC 310) and constrain the intensity and spectral shape of the emission injected from these sources, accounting for the effects of attenuation and contributions from electromagnetic cascades (initiated both within the radio galaxy itself and during extragalactic propagation). Assuming that this gamma-ray emission is generated primarily through the interactions of cosmic-ray protons with gas, we calculate the neutrino flux predicted from each of these sources. Although this scenario is consistent with the constraints published by the IceCube and ANTARES Collaborations, the predicted fluxes consistently fall within an order of magnitude of the current point source sensitivity. The prospects appear very encouraging for the future detection of neutrino emission from the nearest radio galaxies.

[2]  arXiv:1706.07048 [pdf, other]
Title: Disc truncation in embedded star clusters: Dynamical encounters versus face-on accretion
Comments: Accepted for publication in A&A, 14 pages, 8 figures, 1 table
Subjects: Astrophysics of Galaxies (astro-ph.GA); Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

Observations indicate that the dispersal of protoplanetary discs in star clusters occurs on time scales of about 5 Myr. Several processes are thought to be responsible for this disc dispersal. Here we compare two of these processes: dynamical encounters and interaction with the interstellar medium, which includes face-on accretion and ram pressure stripping. We perform simulations of embedded star clusters with parameterisations for both processes to determine the environment in which either of these processes is dominant. We find that face-on accretion, including ram pressure stripping, is the dominant disc truncation process if the fraction of the total cluster mass in stars is $\lesssim 30\,\%$ regardless of the cluster mass and radius. Dynamical encounters require stellar densities $\gtrsim 10^4$ pc$^{-3}$ combined with a mass fraction in stars of $\approx 90\,\%$ to become the dominant process. Our results show that during the embedded phase of the cluster, the truncation of the discs is dominated by face-on accretion and dynamical encounters become dominant when the intra-cluster gas has been expelled. As a result of face-on accretion the protoplanetary discs become compact and their surface density increases. In contrast, dynamical encounters lead to discs that are less massive and remain larger.

[3]  arXiv:1706.07050 [pdf, other]
Title: Detection and Implications of Laser-Induced Raman Scattering at Astronomical Observatories
Comments: 17 pages, 11 figures, accepted for publication in Physical Review X
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Atmospheric and Oceanic Physics (physics.ao-ph)

(Abr.) Laser guide stars employed at astronomical observatories provide artificial wavefront reference sources to help correct (in part) the impact of atmospheric turbulence on astrophysical observations. Following the recent commissioning of the 4 Laser Guide Star Facility (4LGSF) on UT4 at the VLT, we characterize the spectral signature of the uplink beams from the 22W lasers to assess the impact of laser scattering from the 4LGSF on science observations. We use the MUSE optical integral field spectrograph to acquire spectra at a resolution of R~3000 of the uplink laser beams over the wavelength range of 4750\AA\ to 9350\AA. We report the first detection of laser-induced Raman scattering by N2, O2, CO2, H2O and (tentatively) CH4 molecules in the atmosphere above the astronomical observatory of Cerro Paranal. In particular, our observations reveal the characteristic spectral signature of laser photons -- but 480\AA\ to 2210\AA\ redder than the original laser wavelength of 5889.959\AA\ -- landing on the 8.2m primary mirror of UT4 after being Raman-scattered on their way up to the sodium layer. Laser-induced Raman scattering is not unique to the observatory of Cerro Paranal, but common to any astronomical telescope employing a laser-guide-star (LGS) system. It is thus essential for any optical spectrograph coupled to a LGS system to handle thoroughly the possibility of a Raman spectral contamination via a proper baffling of the instrument and suitable calibrations procedures. These considerations are particularly applicable for the HARMONI optical spectrograph on the upcoming Extremely Large Telescope. At sites hosting multiple telescopes, laser collision prediction tools also ought to account for the presence of Raman emission from the uplink laser beam(s) to avoid the unintentional contamination of observations acquired with telescopes in the vicinity of a LGS system.

[4]  arXiv:1706.07051 [pdf, ps, other]
Title: Imprints of the super-Eddington accretion on the quasar clustering
Authors: Taira Oogi (1 and 2), Motohiro Enoki (3), Tomoaki Ishiyama (4), Masakazu A. R. Kobayashi (5), Ryu Makiya (1 and 6), Masahiro Nagashima (2), Takashi Okamoto (7), Hikari Shirakata (7) ((1) Kavli IPMU, (2) Bunkyo University, (3) Tokyo Keizai University, (4) Chiba University, (5) Kure College, (6) MPA, (7) Hokkaido University)
Comments: 6 pages, 4 figures, to appear in MNRAS Letters
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Super-Eddington mass accretion has been suggested as an efficient mechanism to grow supermassive black holes (SMBHs). We investigate the imprint left by the radiative efficiency of the super-Eddington accretion process on the clustering of quasars using a new semi-analytic model of galaxy and quasar formation based on large-volume cosmological $N$-body simulations. Our model includes a simple model for the radiative efficiency of a quasar, which imitates the effect of photon trapping for a high mass accretion rate. We find that the model of radiative efficiency affects the relation between the quasar luminosity and the quasar host halo mass. The quasar host halo mass has only weak dependence on quasar luminosity when there is no upper limit for quasar luminosity. On the other hand, it has significant dependence on quasar luminosity when the quasar luminosity is limited by its Eddington luminosity. In the latter case, the quasar bias also depends on the quasar luminosity, and the quasar bias of bright quasars is in agreement with observations. Our results suggest that the quasar clustering studies can provide a constraint on the accretion disc model.

[5]  arXiv:1706.07053 [pdf, other]
Title: GW170104 and the origin of heavy, low-spin binary black holes via classical isolated binary evolution
Comments: to be submitted to A&A (comments welcome)
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The Advanced LIGO has observed four BH-BH mergers thus far. The effective spin of GW170104 has an 82 percent probability of being negative, which would indicate spin-orbit misalignment. It has been suggested that LIGO's detections favor a population of binaries with frequent and significant spin-orbit misalignment, hence supporting dynamical formation over classical isolated binary evolution. However, it is also well known that massive stars can have efficient transport of angular momentum within the star and have strong winds that carry away substantial angular momentum. The progenitors of the heaviest stellar-mass BHs (M>30Msun) are expected to efficiently reduce their angular momentum, producing BHs with low spin (consistent with existing LIGO constraints). A stellar evolution model that incorporates rotation is used to determine the natal spins of the BHs produced by core collapse. These natal spins are incorporated into our population synthesis code which follows the evolution of Population I/II stars across cosmic time and predicts the BH-BH population in the universe. Our classical isolated binary evolution model produces a BH-BH population that is consistent with the observed LIGO population. In particular, it produces systems that are consistent with the measured properties of GW170104; total mass, mass ratio, effective spin and the overall BH-BH merger rate. Neither strong natal BH kicks nor dynamical interactions are required to recover GW170104. LIGO's observations do not provide evidence for dynamical formation over classical isolated binary formation and it is not yet possible to establish the evolutionary channel through which GW170104 was formed.

[6]  arXiv:1706.07054 [pdf, other]
Title: Predicting the locations of possible long-lived low-mass first stars: Importance of satellite dwarf galaxies
Comments: 16 pages, 11 figures, 1 table, submitted to MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

The search for metal-free stars has so far been unsuccessful, proving that if there are surviving stars from the first generation, they are rare, they have been polluted, or we have been looking in the wrong place. To predict the likely location of Population~III (Pop~III) survivors, we semi-analytically model early star formation in progenitors of Milky Way-like galaxies and their environments. We base our model on merger trees from the high-resolution dark matter only simulation suite \textit{Caterpillar}. Radiative and chemical feedback are taken into account self-consistently, based on the spatial distribution of the haloes. Our results are consistent with the non-detection of Pop III survivors in the Milky Way today. We find that possible surviving Population III stars are more common in Milky Way satellites than in the main Galaxy. In particular, low mass Milky Way satellites contain a much larger fraction of Pop~III stars than the Milky Way. Such nearby, low mass Milky Way satellites are promising targets for future attempts to find Pop~III survivors, especially for high-resolution, high signal-to-noise spectroscopic observations. We provide the probabilities to find a Pop~III survivor in the red giant branch phase for all known Milky Way satellites to guide future observations.

[7]  arXiv:1706.07055 [pdf, ps, other]
Title: Stellar dynamics in the strong-lensing central galaxy of Abell 1201: A low stellar mass-to-light ratio, a large central compact mass, and a standard dark matter halo
Comments: MNRAS accepted
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We analyse the stellar kinematics of the z=0.169 brightest cluster galaxy (BCG) in Abell 1201, using integral field observations with VLT/MUSE. This galaxy has a gravitationally-lensed arc located at unusually small radius ($\sim$5 kpc), allowing us to constrain the mass distribution using lensing and stellar dynamical information over the same radial range. We measure a velocity dispersion profile which is nearly flat at $\sigma$ $\approx$ 285 km/s in the inner $\sim$5 kpc, and then rises steadily to $\sigma$ $\approx$ 360 km/s at $\sim$30 kpc. We analyse the kinematics using axisymmetric Jeans models, finding that the data require both a significant dark matter halo (to fit the rising outer profile) and a compact central component, with mass $M_{\rm cen}$ $\approx$ 2.5$\times$10$^{10}$ $M_\odot$ (to fit the flat {\sigma} in the inner regions). The latter component could represent a super-massive black hole, in which case it would be among the largest known to date. Alternatively $M_{\rm cen}$ could describe excess mass associated with a gradient in the stellar mass-to-light ratio. Imposing a standard NFW dark matter density profile, we recover a stellar mass-to-light ratio $\Upsilon$ which is consistent with a Milky-Way-like initial mass function (IMF). By anchoring the models using the lensing mass constraint, we break the degeneracy between $\Upsilon$ and the inner slope $\gamma$ of the dark matter profile, finding $\gamma$=1.0$\pm$0.1, consistent with the NFW form. We show that our results are quite sensitive to the treatment of the central mass in the models. Neglecting $M_{\rm cen}$ biases the results towards both a heavier-than-Salpeter IMF and a shallower-than-NFW dark matter slope ($\gamma$ $\approx$ 0.5).

[8]  arXiv:1706.07059 [pdf, ps, other]
Title: The Main Sequence relation in the HST Frontier Fields
Comments: Submitted to ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We investigate the relation between the star formation rate (SFR) and the stellar mass, i.e. the Main Sequence (MS) relation of star-forming galaxies, at 1.3 <= z < 6 in the first four HST Frontier Fields, based on rest-frame UV observations. Gravitational lensing combined with deep HST observations allows us to extend the analysis of the MS down to stellar masses as low as logM/Msun~7.5 at z<~4 and logM/Msun~8 at higher redshifts, a factor of ~10 below most previous results. We perform an accurate simulation to take into account the effect of observational uncertainties on the MS and correct for the Eddington bias. This step allows us to reliably measure the MS and in particular its slope. While the normalization increases with redshift, we fit an unevolving and approximately linear slope. We nicely extend to lower masses the results of brighter surveys. Thanks to the large dynamic range in mass for this galaxy sample and by making use of the simulation, we analyzed any possible dependence of the dispersion around the MS on the stellar mass. We find tentative evidence that the scatter decreases with increasing stellar masses, suggesting a larger variety of star formation histories in low mass galaxies. This trend agrees with the predictions of theoretical models of galaxy evolution, and is explained as either a consequence of the smaller number of progenitors of low mass galaxies in a hierarchical scenario and/or of the efficient but intermittent stellar feedback processes in low mass halos. Finally, we observe an increase in the sSFR with redshift milder than predicted by theoretical models, implying a still incomplete theoretical understanding of the processes responsible for galaxy growth.

[9]  arXiv:1706.07060 [pdf, ps, other]
Title: The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
Authors: C. S. Kochanek (1), B. J. Shappee (2), K. Z. Stanek (1), T. W.-S. Holoien (1), Todd A. Thompson (1), J.-L. Prieto (3), Subo Dong (4), J. V. Shields (1), D. Will (1), C. Britt (1), D. Perzanowski (1), G. Pojmanski (5) ((1) Department of Astronomy, The Ohio State University, (2) Carnegie Observatories, (3) Nucleo de Astronomia de la Facultad de Ingenieria y Ciencias, Universidad Diego Portales, (4) Kavli Institute for Astronomy and Astrophysics, Peking University, (5) Warsaw University Observatory)
Comments: 8 pages, 9 figures, submitted to PASP
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Instrumentation and Methods for Astrophysics (astro-ph.IM)

The All-Sky Automated Survey for Supernovae (ASAS-SN) is working towards imaging the entire visible sky every night to a depth of V~17 mag. The present data covers the sky and spans ~2-5~years with ~100-400 epochs of observation. The data should contain some ~1 million variable sources, and the ultimate goal is to have a database of these observations publicly accessible. We describe here a first step, a simple but unprecedented web interface https://asas-sn.osu.edu/ that provides an up to date aperture photometry light curve for any user-selected sky coordinate. Because the light curves are produced in real time, this web tool is relatively slow and can only be used for small samples of objects. However, it also imposes no selection bias on the part of the ASAS-SN team, allowing the user to obtain a light curve for any point on the celestial sphere. We present the tool, describe its capabilities, limitations, and known issues, and provide a few illustrative examples.

[10]  arXiv:1706.07061 [pdf, ps, other]
Title: The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440
Comments: 8 pages, 6 figures. Accepted for publication in ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We used a combination of deep optical and Halpha images of the Galactic globular cluster NGC 6440, acquired with the Hubble Space Telescope, to identify the optical counterpart to the accreting millisecond X-ray pulsar SAX J1748.9-2021during quiescence. A strong Halpha emission has been detected from a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15" from the nominal position of the X-ray source. The position of the star also agrees with the optical counterpart found by Verbunt et al. (2000) during an outburst. We propose this star as the most likely optical counterpart to the binary system. By direct comparison with isochrones, we estimated that COM-SAX J1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsun and a superficial temperature of 5250pm80 K. These parameters combined with the orbital characteristics of the binary suggest that the system is observed at a very low inclination angle (~8 deg -14 deg) and that the star is filling or even overflowing its Roche Lobe. This, together with the equivalent width of the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The possibile presence of such a on-going mass transfer during a quiescence state also suggests that the radio pulsar is not active yet and thus this system, despite its similarity with the class of redback millisecond pulsars, is not a transitional millisecond pulsar.

[11]  arXiv:1706.07062 [pdf, ps, other]
Title: GRay2: A General Purpose Geodesic Integrator for Kerr Spacetimes
Authors: Chi-kwan Chan (1, 2), Lia Medeiros (1, 3, 2), Feryal Ozel (1, 2), Dimitrios Psaltis (1, 4, 2) ((1) University of Arizona, (2) Black Hole Initiative, (3) UCSB, (4) Radcliffe Institute for Advanced Study)
Comments: 11 pages, 9 figures, 3 tables; submitted to ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Fast and accurate integration of geodesics in Kerr spacetimes is an important tool in modeling the orbits of stars and the transport of radiation in the vicinities of black holes. Most existing integration algorithms employ Boyer-Lindquist coordinates, which have coordinate singularities at the event horizon and along the poles. Handling the singularities requires special numerical treatment in these regions, often slows down the calculations, and may lead to inaccurate geodesics. We present here a new general-purpose geodesic integrator, GRay2, that overcomes these issues by employing the Cartesian form of Kerr-Schild coordinates. By performing particular mathematical manipulations of the geodesic equations and several optimizations, we develop an implementation of the Cartesian Kerr-Schild coordinates that outperforms calculations that use the seemingly simpler equations in Boyer-Lindquist coordinates. We also employ the OpenCL framework, which allows GRay2 to run on multi-core CPUs as well as on a wide range of GPU hardware accelerators, making the algorithm more versatile. We report numerous convergence tests and benchmark results for GRay2 for both time-like (particle) and null (photon) geodesics.

[12]  arXiv:1706.07063 [pdf, other]
Title: The State-of-the-Art HST Astro-Photometric Analysis of the core of ωCentauri. III. The Main Sequence's Multiple Populations Galore
Authors: A. Bellini (1), A. P. Milone (2), J. Anderson (1), A. F. Marino (2), G. Piotto (3,4), R. P. van der Marel (1), L. R. Bedin (3), I. R. King (5) ((1) STScI, (2) ANU, (3) INAF-OAPd, (4) UNIPD, (5) Univ. Washington)
Comments: 22 pages, 17 figures (most in lower res), 2 tables, accepted for publication in ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics of Galaxies (astro-ph.GA)

We take advantage of the exquisite quality of the Hubble Space Telescope 26-filter astro-photometric catalog of the core of Omega Cen presented in the first paper of this series and the empirical differential-reddening correction presented in the second paper in order to distill the main sequence into its constituent populations. To this end, we restrict ourselves to the five most useful filters: the magic "trio" of F275W, F336W, and F438W, along with F606W and F814W. We develop a strategy for identifying color systems where different populations stand out most distinctly, then we isolate those populations and examine them in other filters where their sub-populations also come to light. In this way, we have identified at least 15 sub-populations, each of which has a distinctive fiducial curve through our 5-dimensional photometric space. We confirm the MSa to be split into two subcomponents, and find that both the bMS and the rMS are split into three subcomponents. Moreover, we have discovered two additional MS groups: the MSd (which has three subcomponents) shares similar properties with the bMS, and the MSe (which has four subcomponents), has properties more similar to those of the rMS. We examine the fiducial curves together and use synthetic spectra to infer relative heavy-element, light-element, and Helium abundances for the populations. Our findings show that the stellar populations and star formation history of Omega Cen are even more complex than inferred previously. Finally, we provide as a supplement to the original catalog a list that identifies for each star which population it most likely is associated with.

[13]  arXiv:1706.07073 [pdf, other]
Title: Pulsed Accretion in the T Tauri Binary TWA 3A
Comments: 6 pages, 4 figures
Journal-ref: ApJL, 842, L12 (2017)
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

TWA 3A is the most recent addition to a small group of young binary systems that both actively accrete from a circumbinary disk and have spectroscopic orbital solutions. As such, it provides a unique opportunity to test binary accretion theory in a well-constrained setting. To examine TWA 3A's time-variable accretion behavior, we have conducted a two-year, optical photometric monitoring campaign, obtaining dense orbital phase coverage (~20 observations per orbit) for ~15 orbital periods. From U-band measurements we derive the time-dependent binary mass accretion rate, finding bursts of accretion near each periastron passage. On average, these enhanced accretion events evolve over orbital phases 0.85 to 1.05, reaching their peak at periastron. The specific accretion rate increases above the quiescent value by a factor of ~4 on average but the peak can be as high as an order of magnitude in a given orbit. The phase dependence and amplitude of TWA 3A accretion is in good agreement with numerical simulations of binary accretion with similar orbital parameters. In these simulations, periastron accretion bursts are fueled by periodic streams of material from the circumbinary disk that are driven by the binary orbit. We find that TWA 3A's average accretion behavior is remarkably similar to DQ Tau, another T Tauri binary with similar orbital parameters, but with significantly less variability from orbit to orbit. This is only the second clear case of orbital-phase-dependent accretion in a T Tauri binary.

[14]  arXiv:1706.07095 [pdf, other]
Title: The Multiplicity of M-Dwarfs in Young Moving Groups
Comments: 25 pages
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We image 104 newly identified low-mass (mostly M-dwarf) pre-main sequence members of nearby young moving groups with Magellan Adaptive Optics (MagAO) and identify 27 binaries with instantaneous projected separation as small as 40 mas. 15 were previously unknown. The total number of multiple systems in this sample including spectroscopic and visual binaries from the literature is 36, giving a raw multiplicity rate of at least $35^{+5}_{-4}\%$ for this population. In the separation range of roughly 1 - 300 AU in which infrared AO imaging is most sensitive, the raw multiplicity rate is at least $24^{+5}_{-4}\%$ for binaries resolved by the MagAO infrared camera (Clio). The M-star sub-sample of 87 stars yields a raw multiplicity of at least $30^{+5}_{-4}\%$ over all separations, $21^{+5}_{-4}\%$ for secondary companions resolved by Clio from 1 to 300 AU ($23^{+5}_{-4}\%$ for all known binaries in this separation range). A combined analysis with binaries discovered by the Search for Associations Containing Young stars shows that multiplicity fraction as a function of mass and age over the range of 0.2 to 1.2 $M_\odot$ and 10 - 200 Myr appears to be linearly flat in both parameters and across YMGs. This suggests that multiplicity rates are largely set by 100 Myr without appreciable evolution thereafter. After bias corrections are applied, the multiplicity fraction of low-mass YMG members ($< 0.6 M_\odot$) is in excess of the field.

[15]  arXiv:1706.07099 [pdf, ps, other]
Title: Large-scale environmental dependence of chemical abundances in dwarf galaxies and implications for connecting star formation and halo mass
Comments: 18 pages, 14 figures, submitted to ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We study how the void environment affects the chemical evolution of galaxies in the universe by comparing the oxygen and nitrogen abundances of dwarf galaxies in voids with dwarf galaxies in denser regions. Using spectroscopic observations from SDSS DR7, we estimate the oxygen and nitrogen abundances of 993 void dwarf galaxies and 759 dwarf galaxies in denser regions. We use the Direct Te method for calculating the gas-phase chemical abundances in the dwarf galaxies because it is best suited for low metallicity, low mass (dwarf) galaxies. A substitute for the [OII] 3727 doublet is developed, permitting oxygen abundance estimates of SDSS dwarf galaxies at all redshifts with the Direct Te method. We find that void dwarf galaxies have slightly higher oxygen abundances than dwarf galaxies in denser environments. The opposite trend is seen in both the nitrogen abundance and N/O ratio: void dwarf galaxies have slightly lower nitrogen abundances and lower N/O ratios than dwarf galaxies in denser regions. Our mass-N/O relationship shows that the secondary production of nitrogen commences at a lower stellar mass in void dwarf galaxies than in dwarf galaxies in denser environments. The lower N/O ratios and smaller stellar mass for secondary nitrogen production seen in void dwarf galaxies may indicate both delayed star formation and a dependence of cosmic downsizing on the large-scale environment. The shift toward higher oxygen abundances in void dwarf galaxies might be evidence of larger ratios of dark matter halo mass to stellar mass in voids than in denser regions.

[16]  arXiv:1706.07102 [pdf, ps, other]
Title: Magnetism and activity of planet hosting stars
Comments: 10 pp, 4 figures. From "Chapter 9: Flares and star-planet interaction"; Solar and Stellar Flares and Their Effects on Planets, Proceedings IAU Symposium No. 320, 2015, A.G. Kosovichev, S.L. Hawley & P. Heinzel, eds., From the 2015 IAU General Assembly in Honolulu, HI, USA, Focus Meeting 13: Brightness Variations of the Sun and Sun-like Stars, 5-6 August 2015
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The magnetic activity levels of planet host stars may differ from that of stars not known to host planets in several ways. Hot Jupiters may induce activity in their hosts through magnetic interactions, or through tidal interactions by affecting their host's rotation or convection. Measurements of photospheric, chromospheric, or coronal activity might then be abnormally high or low compared to control stars that do not host hot Jupiters, or might be modulated at the planet's orbital period. Such detections are complicated by the small amplitude of the expected signal, by the fact that the signals may be transient, and by the difficulty of constructing control samples due to exoplanet detection biases and the uncertainty of field star ages. We review these issues, and discuss avenues for future progress in the field.

[17]  arXiv:1706.07103 [pdf, other]
Title: A hybrid supervised/unsupervised machine learning approach to solar flare prediction
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Learning (cs.LG)

We introduce a hybrid approach to solar flare prediction, whereby a supervised regularization method is used to realize feature importance and an unsupervised clustering method is used to realize the binary flare/no-flare decision. The approach is validated against NOAA SWPC data.

[18]  arXiv:1706.07106 [pdf, ps, other]
Title: The new semianalytic code GalICS 2.0 - Reproducing the galaxy stellar mass function and the Tully-Fisher relation simultaneously
Comments: 31 pages, 19 figures, 1 table, submitted to MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

GalICS 2.0 is a new semianalytic code to model the formation and evolution of galaxies in a cosmological context. N-body simulations based on a Planck cosmology are used to construct halo merger trees, track subhaloes, compute spins and measure concentrations. The accretion of gas onto galaxies and the morphological evolution of galaxies are modelled with prescriptions derived from hydrodynamic simulations. Star formation and stellar feedback are described with phenomenological models (as in other semianalytic codes). GalICS 2.0 computes rotation speeds from the gravitational potential of the dark matter, the disc and the central bulge. As the rotation speed depends not only on the virial velocity but also on the ratio of baryons to dark matter within a galaxy, our calculation predicts a different Tully-Fisher relation from models in which the rotation speed is proportional to the virial velocity. This is why GalICS 2.0 is able to reproduce the galaxy stellar mass function and the Tully-Fisher relation simultaneously. Our results are also in agreement with halo masses from weak lensing and satellite kinematics, gas fractions, the relation between star formation rate (SFR) and stellar mass, the evolution of the cosmic SFR density, bulge-to-disc ratios, disc sizes and the Faber-Jackson relation.

[19]  arXiv:1706.07109 [pdf, ps, other]
Title: The gas-to-extinction ratio and the gas distribution in the Galaxy
Comments: 32 pages, 10 figures, 7 tables; accepted for publication in MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We investigate the relation between the optical extinction ($A_V$) and the hydrogen column density ($N_H$) determined from X-ray observations of a large sample of Galactic sightlines toward 35 supernova remnants, 6 planetary nebulae, and 70 X-ray binaries for which $N_H$ was determined in the literature with solar abundances. We derive an average ratio of ${N_H}/{A_V}=(2.08\pm0.02)\times10^{21}{\rm H\, cm^{-2}\, mag^{-1}}$ for the whole Galaxy. We find no correlation between ${N_H}/{A_V}$ and the number density of hydrogen, the distance away from the Galactic centre, and the distance above or below the Galactic plane. The ${N_H}/{A_V}$ ratio is generally invariant across the Galaxy, with ${N_H}/{A_V}=(2.04\pm0.05)\times10^{21}{\rm H\, cm^{-2}\, mag^{-1}}$ for the 1st and 4th Galactic quadrants and ${N_H}/{A_V}=(2.09\pm0.03)\times10^{21}{\rm H\, cm^{-2}\, mag^{-1}}$ for the 2nd and 3rd Galactic quadrants. We also explore the distribution of hydrogen in the Galaxy by enlarging our sample with additional 74 supernova remnants for which both $N_H$ and distances are known. We find that, between the Galactic radius of 2 kpc to 10 kpc, the vertical distribution of hydrogen can be roughly described by a Gaussian function with a scale height of $h=75.5\pm12.4\,{\rm pc}$ and a mid-plane density of $n_{H}(0)=1.11\pm0.15\,{\rm cm^{-3}}$, corresponding to a total gas surface density of ${\sum}_{gas}{\sim}7.0\,{M_{\bigodot}}\,{\rm pc^{-2}}$. We also compile $N_H$ from 19 supernova remnants and 29 X-ray binaries for which $N_H$ was determined with subsolar abundances. We obtain ${N_H}/{A_V}=(2.47\pm0.04)\times10^{21}{\rm H\, cm^{-2}\, mag^{-1}}$ which exceeds that derived with solar abundances by $\sim$20%. We suggest that in future studies one may simply scale $N_H$ derived from subsolar abundances by a factor of $\sim$1.2 when converting to $N_H$ of solar abundances.

[20]  arXiv:1706.07120 [pdf, other]
Title: Possible correlations between the emission properties of short GRBs and their offsets from the host galaxies
Comments: 5 pages, 2 figures, Accepted for publication in ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Short Gamma-Ray Bursts(SGRBs) are widely believed to be from mergers of binary compact objects involving at least one neutron star and hence have a broad range of spatial offsets from their host galaxies. In this work we search for possible correlations between the emission properties of 18 SGRBs and their offsets from the host galaxies. The SGRBs with and without extended emission do not show significant difference between their offset distribution, in agreement with some previous works. There are however possible correlations between the optical and X-ray afterglow emission and the offsets. The underlying physical origins are examined.

[21]  arXiv:1706.07131 [pdf, other]
Title: Gap and rings carved by vortices in protoplanetary dust
Comments: 11 pages, 11 figures, accepted for publication in A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Large-scale vortices in protoplanetary disks are thought to form and survive for long periods of time. Hence, they can significantly change the global disk evolution and particularly the distribution of the solid particles embedded in the gas, possibly explaining asymmetries and dust concentrations recently observed at sub-millimeter and millimeter wavelengths. We investigate the spatial distribution of dust grains using a simple model of protoplanetary disk hosted by a giant gaseous vortex. We explore the dependence of the results on grain size and deduce possible consequences and predictions for observations of the dust thermal emission at sub-millimeter and millimeter wavelengths. Global 2D simulations with a bi-fluid code are used to follow the evolution of a single population of solid particles aerodynamically coupled to the gas. Possible observational signatures of the dust thermal emission are obtained using simulators of ALMA and ngVLA observations. We find that a giant vortex not only captures dust grains with Stokes number St < 1 but can also affect the distribution of larger grains (with St '~' 1) carving a gap associated to a ring composed of incompletely trapped particles. The results are presented for different particle size and associated to their possible signatures in disk observations. Gap clearing in the dust spatial distribution could be due to the interaction with a giant gaseous vortex and their associated spiral waves, without the gravitational assistance of a planet. Hence, strong dust concentrations at short sub-mm wavelengths associated with a gap and an irregular ring at longer mm and cm wavelengths could indicate the presence of an unseen gaseous vortex.

[22]  arXiv:1706.07153 [pdf, ps, other]
Title: Magnetohydrodynamic Simulations for Studying Solar Flare Trigger Mechanism
Comments: 28 pages, 10 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

In order to understand the flare trigger mechanism, we conducted three-dimensional magnetohydrodynamic simulations using a coronal magnetic field model derived from data observed by the Hinode satellite. Several types of magnetic bipoles were imposed into the photospheric boundary of the Non-linear Force-Free Field (NLFFF) model of Active Region NOAA 10930 on 2006 December 13 to investigate what kind of magnetic disturbance may trigger the flare. As a result, we confirm that certain small bipole fields, which emerge into the highly sheared global magnetic field of an active region, can effectively trigger a flare. These bipole fields can be classified into two groups based on their orientation relative to the polarity inversion line: the so called opposite polarity (OP) and reversed shear (RS) structures as it was suggested by Kusano et al. (2012). We also investigated the structure of the footpoints of reconnected field lines. By comparing the distribution of reconstructed field lines and the observed flare ribbons, the trigger structure of the flare can be inferred. Our simulation suggests that the data-constrained simulation taking into account both the large-scale magnetic structure and the small-scale magnetic disturbance such as emerging fluxes is a good way to find out a flare productive active region for space weather prediction.

[23]  arXiv:1706.07158 [pdf, ps, other]
Title: Sunspot Light Walls Suppressed by Nearby Brightenings
Comments: 6 pages, 5 figures, accepted for publication in ApJL
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Light walls, as ensembles of oscillating bright structures rooted in sunspot light bridges, have not been well studied, although they are important for understanding sunspot properties. Using the Interface Region Imaging Spectrograph and Solar Dynamics Observatory observations, here we study the evolution of two oscillating light walls each within its own active region (AR). The emission of each light wall decays greatly after the appearance of adjacent brightenings. For the first light wall, rooted within AR 12565, the average height, amplitude, and oscillation period significantly decrease from 3.5 Mm, 1.7 Mm, and 8.5 min to 1.6 Mm, 0.4 Mm, and 3.0 min, respectively. For the second light wall, rooted within AR 12597, the mean height, amplitude, and oscillation period of the light wall decrease from 2.1 Mm, 0.5 Mm, and 3.0 min to 1.5 Mm, 0.2 Mm, and 2.1 min, respectively. Particularly, a part of the second light wall becomes even invisible after the influence of nearby brightening. These results reveal that the light walls are suppressed by nearby brightenings. Considering the complex magnetic topology in light bridges, we conjecture that the fading of light walls may be caused by a drop in the magnetic pressure, where flux is cancelled by magnetic reconnection at the site of the nearby brightening. Another hypothesis is that the wall fading is due to the suppression of driver source (p-mode oscillation), resulting from the nearby avalanche of downward particles along reconnected brightening loops.

[24]  arXiv:1706.07164 [pdf, ps, other]
Title: CO observations of the molecular gas in the galactic HII region Sh2-48; Evidence for cloud-cloud collision as a trigger of high-mass star formation
Comments: 15 pages, 13 figures
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Understanding the mechanism of high-mass star formation is one of the top-priority issues in contemporary astrophysics. Sh2-48 is a galactic HII region located at 3.8kpc from the Sun. It harbors an O9.5-type star at the center of the HII region which is extended for ~10 arcsin. As a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) project, we obtained the CO J=1-0 dataset for a large area of Sh2-48 at a spatial resolution of 21 arcsec, which corresponds to ~0.4pc at 3.8kpc. The CO data revealed that the molecular gas having a total molecular mass of 8.5x10^4 M_sun is associated with Sh2-48, which shows a characteristic line-symmetric velocity gradient over ~4km/s. Such a velocity gradient cannot be formed by a spherical expansion the HII region. In this paper, we discuss a cloud-cloud collision scenario to interpret the observed signatures including the velocity gradient. By comparing between the observations and simulations, we found that this line-symmetric velocity gradient is an expected outcome of a collision between a cylindrical cloud and a large spherical cloud, and we concluded that the high-mass star formation in Sh2-48 was triggered by the collision. Our results reinforce the importance of cloud-cloud collision for high-mass star formation in the Milky Way.

[25]  arXiv:1706.07183 [pdf, other]
Title: Intraday Optical Variability of BL Lacertae
Comments: 9 pages, 7 figures, accepted by MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA); High Energy Astrophysical Phenomena (astro-ph.HE)

We monitored BL Lacertae simultaneously in the optical B, V, R and I bands for 13 nights during the period 2012-2016. The variations were well correlated in all bands and the source showed significant intraday variability (IDV). We also studied its optical flux and colour behaviour, and searched for inter-band time lags. A strong bluer-when-brighter chromatism was found on the intra-night time-scale. The spectral changes are not sensitive to the host galaxy contribution. Cross-correlation analysis revealed possible time delay of about 10 min between variations in the V and R bands. We interpreted the observed flares in terms of the model consisting of individual synchrotron pulses.

[26]  arXiv:1706.07186 [pdf, other]
Title: Design of an adaptable Stokes polarimeter for exploring chromospheric magnetism
Comments: Accepted for publication in Advances in Space Research, 26 pages, 9 Figures, 1 Table
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The chromosphere is a highly complex and dynamic layer of the Sun, that serves as a conduit for mass and energy supply between two, very distinct regions of the solar atmosphere, namely, the photosphere and corona. Inferring magnetic fields in the chromosphere, has thus become an important topic, that can be addressed with large-aperture solar telescopes to carry out highly sensitive polarimetric measurements. In this article, we present a design of a polarimeter for investigating the chromospheric magnetic field. The instrument consists of a number of lenses, two ferro-electric liquid crystals, a Wollaston prism, and a CCD camera. The optical design is similar to that of a commercial zoom lens which allows a variable f-number while maintaining focus and aberrations well within the Airy disc. The optical design of the Adaptable ChRomOspheric POLarimeter (ACROPOL) makes use of off-the-shelf components and is described for the 70 cm Vacuum Tower Telescope and the 1.5 m GREGOR telescope at Observatorio del Teide, Tenerife, Spain. Our design shows that the optical train can be separated into two units where the first unit, consisting of a single lens, has to be changed while going from the VTT to the GREGOR configuration. We also discuss the tolerances within which, diffraction limited performance can be achieved with our design.

[27]  arXiv:1706.07199 [pdf, ps, other]
Title: Orbital variations in intensity and spectral properties of the highly obscured sgHMXB IGR J16318-4848
Comments: 10 pages, 14 Figures, 3 tables, Accepted for publication in MNRAS
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

IGR J16318-4848 is an X-ray binary with the highest known line of sight absorption column density among all known X-ray binary systems in our galaxy. In order to investigate the reason behind such a large absorption column, we looked at the variations in the X-ray intensity and spectral parameters as a function of the tentatively discovered $\sim$ 80 day orbit of this source. The orbital period is firmly confirmed in the long term ($\sim$ 12 year) Swift BAT lightcurve. Two peaks about half an orbit apart, one narrow and small, and the other broad and large are seen in the orbital intensity profile. We find that while most orbits show enhanced emissions at these two peaks, the larger peak in the folded longterm lightcurve is more a result of randomly occurring large flares spread over $\sim$ 0.2 orbital phase. As opposed to this, the smaller peak is seen in every orbit as a regular increase in intensity. Using archival data spread over different phases of the orbit and the geometry of the system as known from previously published infrared observations, we present a possible scenario which explains the orbital intensity profile, bursting characteristics and large column density of this X-ray binary.

[28]  arXiv:1706.07219 [pdf, other]
Title: Cloud formation in metal-rich atmospheres of hot super-Earths like 55 Cnc e and CoRot7b
Authors: G.Mahapatra (1), Ch.Helling (1), Y. Miguel (2) ((1) - Centre for Exoplanet Science, University of St Andrews, (2) - Laboratoire Lagrange, Observatoire de la Côte d'Azur)
Comments: 19 pages, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

Clouds form in the atmospheres of planets where they can determine the observable spectra, the albedo and phase curves. Cloud properties are determined by the local thermodynamical and chemical conditions of an atmospheric gas. A retrieval of gas abundances requires a comprehension of the cloud formation mechanisms under varying chemical conditions. With the aim of studying cloud formation in metal rich atmospheres, we explore the possibility of clouds in evaporating exoplanets like CoRoT-7b and 55 Cnc e in comparison to a generic set of solar abundances and the metal-rich gas giant HD149026b. We assess the impact of metal-rich, non-solar element abundances on the gas-phase chemistry, and apply our kinetic, non-equilibrium cloud formation model to study cloud structures and their details. We provide an overview of global cloud properties in terms of material compositions, maximum particle formation rates, and average cloud particle sizes for various sets of rocky element abundances. Our results suggest that the conditions on 55 Cnc e and HD149026b should allow the formation of mineral clouds in their atmosphere. The high temperatures on some hot-rocky super-Earths (e.g. the day-side of Corot-7b) result in an ionised atmospheric gas and they prevent gas condensation, making cloud formation unlikely on its day-side.

[29]  arXiv:1706.07225 [pdf, other]
Title: Neutron-capture element abundances in the planetary nebula NGC 5315 from deep optical and near-infrared spectrophotometry
Comments: 31 pages, 6 figures, 15 tables. Accepted for publication in MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics of Galaxies (astro-ph.GA)

We analyze the chemical composition of the planetary nebula (PN) NGC 5315, through high-resolution (R = 40000) optical spectroscopy with UVES at the Very Large Telescope, and medium-resolution (R = 4800) near-infrared spectroscopy with FIRE at Magellan Baade Telescope, covering a wide spectral range from 0.31 to 2.50 micron. The main aim of this work is to investigate neutron (n)-capture element abundances to study the operation of the slow n-capture ("s-process") in the AGB progenitor of NGC 5315. We detect more than 700 emission lines, including ions of the n-capture elements Se, Kr, Xe, and possibly Br. We compute physical conditions from a large number of diagnostic line ratios, and derive ionic abundances for species with available atomic data. The total abundances are computed using recent ionization correction factors (ICFs) or by summing ionic abundances. Total abundances of common elements are in good agreement with previous work on this object. Based on our abundance analysis of NGC 5315, including the lack of s-process enrichment, we speculate that the most probable scenario is that the progenitor star is in a binary system as hinted at by radial velocity studies, and interactions with its companion truncated the AGB before s-process enrichment could occur. However there are other two possible scenarios for its evolution, that cannot be ruled out: i) the progenitor is a low-mass single star that did not undergo third dredge-up; ii) the progenitor star of NGC 5315 had an initial mass of 4--6 solar masses, and any s-process enhancements were heavily diluted by the massive envelope during the AGB phase.

[30]  arXiv:1706.07228 [pdf, other]
Title: Prospects for the detection of high-energy (E>25 GeV) Fermi pulsars with the Cherenkov Telescope Array
Comments: 17 pages, 7 figures, 7 tables
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Around 160 gamma-ray pulsars were discovered by the Fermi Large Area Telescope (LAT) since 2008. The most energetic of them, 12 objects with emission above 25 GeV, are suitable candidates for the detection with the current and future Imaging Atmospheric Cherenkov Telescopes above few tens of GeV. We perform an analysis of the Fermi-LAT data of these high-energy pulsars in order to determine if such objects can be detected with the Cherenkov Telescope Array (CTA). Our goal is to forecast the significance of their point source detection with CTA. We analyze 5 years of the Fermi-LAT data fitting the spectra of each pulsar at energies E>10 GeV with a power-law function. Assuming no spectral cut-off, we extrapolate the resulting spectra to the very high energy range (VHE, E>0.1 TeV) and simulate CTA observations of all 12 pulsars with the ctools software package. Using different analysis tools, individual CTA sensitivity curves are independently calculated for each pulsar and cross-checked with the ctools results. Our simulations result in significant CTA detections of up to 8 pulsars in 50 h. Observations of the most energetic Fermi pulsars with CTA will shed light on the nature of the high-energy emission of pulsars, clarifying whether the VHE emission detected in the Crab pulsar spectrum is present also in other gamma-ray pulsars.

[31]  arXiv:1706.07236 [pdf, other]
Title: Apparent cosmic acceleration from type Ia supernovae
Comments: 17 pages, 6 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc)

Parameters that quantify the acceleration of cosmic expansion are conventionally determined within the standard Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) model, which fixes spatial curvature to be homogeneous. Generic averages of Einstein's equations in inhomogeneous cosmology lead to models with non-rigidly evolving average spatial curvature, and different parameterizations of apparent cosmic acceleration. The timescape cosmology is a viable example of such a model without dark energy. Using the largest available supernova data set, the JLA catalogue, we find that the timescape model fits the luminosity distance-redshift data with a likelihood that is statistically indistinguishable from the standard spatially flat $\Lambda$CDM cosmology by Bayesian comparison. In the timescape case cosmic acceleration is non-zero but has a marginal amplitude, with best fit apparent deceleration parameter, $q_0=-0.042^{+0.04}_{-0.01}$. Systematic issues regarding standardization of supernova light curves are analysed. Cuts of data at the statistical homogeneity scale affect light curve parameter fits independent of cosmology. A cosmological model dependency of empirical changes to the mean colour parameter is also found. Irrespective of which model ultimately fits better, we argue that as a competitive model with a non-FLRW expansion history, the timescape model may prove a useful diagnostic tool for disentangling selection effects and astrophysical systematics from the underlying expansion history.

[32]  arXiv:1706.07262 [pdf, other]
Title: MOVES I. The evolving magnetic field of the planet-hosting star HD189733
Comments: 14 pages, 6 figures, accepted for publication in MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

HD189733 is an active K dwarf that is, with its transiting hot Jupiter, among the most studied exoplanetary systems. In this first paper of the Multiwavelength Observations of an eVaporating Exoplanet and its Star (MOVES) program, we present a 2-year monitoring of the large-scale magnetic field of HD189733. The magnetic maps are reconstructed for five epochs of observations, namely June-July 2013, August 2013, September 2013, September 2014, and July 2015, using Zeeman-Doppler Imaging. We show that the field evolves along the five epochs, with mean values of the total magnetic field of 36, 41, 42, 32 and 37 G, respectively. All epochs show a toroidally-dominated field. Using previously published data of Moutou et al. 2007 and Fares et al. 2010, we are able to study the evolution of the magnetic field over 9 years, one of the longest monitoring campaign for a given star. While the field evolved during the observed epochs, no polarity switch of the poles was observed. We calculate the stellar magnetic field value at the position of the planet using the Potential Field Source Surface extrapolation technique. We show that the planetary magnetic environment is not homogeneous over the orbit, and that it varies between observing epochs, due to the evolution of the stellar magnetic field. This result underlines the importance of contemporaneous multi-wavelength observations to characterise exoplanetary systems. Our reconstructed maps are a crucial input for the interpretation and modelling of our MOVES multi-wavelength observations.

[33]  arXiv:1706.07312 [pdf, other]
Title: Large amplitude tip/tilt estimation by geometric diversity for multiple-aperture telescopes
Comments: Accepted for publication in Journal of the Optical Society of America A (JOSAA)
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

A novel method nicknamed ELASTIC is proposed for the alignment of multiple-aperture telescopes, in particular segmented telescopes. It only needs the acquisition of two diversity images of an unresolved source, and is based on the computation of a modified, frequency-shifted, cross-spectrum. It provides a polychromatic large range tip/tilt estimation with the existing hardware and an inexpensive noniterative unsupervised algorithm. Its performance is studied and optimized by means of simulations. They show that with 5000 photo-electrons/sub-aperture/frame and 1024x1024 pixel images, residues are within the capture range of interferometric phasing algorithms such as phase diversity. The closed-loop alignment of a 6 sub-aperture mirror provides an experimental demonstration of the effectiveness of the method.

[34]  arXiv:1706.07316 [pdf, other]
Title: Image-Optimized Coronal Magnetic Field Models
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We have reported previously on a new method we are developing for using image-based information to improve global coronal magnetic field models. In that work we presented early tests of the method which proved its capability to improve global models based on flawed synoptic magnetograms, given excellent constraints on the field in the model volume. In this follow-up paper we present the results of similar tests given field constraints of a nature that could realistically be obtained from quality white-light coronagraph images of the lower corona. We pay particular attention to difficulties associated with the line-of-sight projection of features outside of the assumed coronagraph image plane, and the effect on the outcome of the optimization of errors in localization of constraints. We find that substantial improvement in the model field can be achieved with this type of constraints, even when magnetic features in the images are located outside of the image plane.

[35]  arXiv:1706.07321 [pdf, ps, other]
Title: Cosmological constraints on the hot gas fraction in galaxy clusters
Comments: 5 pages, 1 figure, 1 table
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

The evolution of the X-ray emitting gas mass fraction in massive galaxy clusters can be used as an independent cosmological tool to probe the expansion history of the Universe. Its use, however, depends upon a crucial quantity, i.e., the depletion factor $\gamma$, which corresponds to the ratio by which the X-ray emitting gas fraction in galaxy clusters is depleted with respect to the universal baryonic mean. Since this quantity is not directly observed, assumptions about the cosmology need to be made and usually hydrodynamical cosmological simulations are used to calibrate it. In this letter, we obtain for the first time self-consistent observational constraints on the gas depletion factor combining 40 X-ray emitting gas mass fraction measurements and 580 distance measurements from type Ia supernovae. Using non-parametric methods to reconstruct a possible redshift evolution of $\gamma$, we find no evidence for such evolution, which confirms the current results from hydrodynamical simulations.

[36]  arXiv:1706.07344 [pdf, ps, other]
Title: Nuclei of dwarf spheroidal galaxies KKs3 and ESO269-66 and their counterparts in our Galaxy
Comments: Accepted for publication in MNRAS, 14 pages, 8 figures, 10 tables
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present the analysis of medium-resolution spectra obtained at the Southern African Large Telescope (SALT) for nuclear globular clusters (GCs) in two dwarf spheroidal galaxies (dSphs). The galaxies have similar star formation histories, but they are situated in completely different environments. ESO269-66 is a close neighbour of the giant S0 NGC5128. KKs3 is one of the few truly isolated dSphs within 10 Mpc. We estimate the helium abundance $Y=0.3$, $\rm age=12.6\pm1$ Gyr, $[Fe/H]=-1.5,-1.55\pm0.2$ dex, and abundances of C, N, Mg, Ca, Ti, and Cr for the nuclei of ESO269-66 and KKs3. Our surface photometry results using HST images yield the half-light radius of the cluster in KKs3, $\rm r_h=4.8\pm0.2$ pc. We demonstrate the similarities of medium-resolution spectra, ages, chemical compositions, and structure for GCs in ESO269-66 and KKs3 and for several massive Galactic GCs with $[Fe/H]\sim-1.6$ dex. All Galactic GCs posses Extended Blue Horizontal Branches and multiple stellar populations. Five of the selected Galactic objects are iron-complex GCs. Our results indicate that the sample GCs observed now in different environments had similar conditions of their formation $\sim$1 Gyr after the Big Bang.

[37]  arXiv:1706.07377 [pdf, other]
Title: Spectral Linear Dark Field Control: Stabilizing Deep Contrast for Exoplanet Imaging Using out-of-band Speckle Field
Authors: Olivier Guyon (1,2,3,4), Kelsey Miller (2), Jared Males (2), Ruslan Belikov (4), Brian Kern (5) ((1) NINS Astrobiology Center, (2) University of Arizona, (3) Subaru Telescope, (4) NASA Ames Research Center, (5) Jet Propulsion Laboratory)
Comments: 12 pages, 8 figures, submitted to PASP
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

Wavefront stabilization is a fundamental challenge to high contrast imaging of exoplanets. For both space and ground observations, wavefront control performance is ultimately limited by the finite amount of starlight available for sensing, so wavefront measurements must be as efficient as possible. To meet this challenge, we propose to sense residual errors using bright focal-plane speckles at wavelengths outside the high contrast spectral bandwidth. We show that a linear relationship exists between the intensity of the bright out-of-band speckles and residual wavefront aberrations. An efficient linear control loop can exploit this relationship. The proposed scheme, referred to as Spectral Linear Dark Field Control (spectral LDFC), is more sensitive than conventional approaches for ultra-high contrast imaging. Spectral LDFC is closely related to, and can be combined with, the recently proposed spatial LDFC which uses light at the observation wavelength but located outside of the high contrast area in the focal plane image. Both LDFC techniques do not require starlight to be mixed with the high contrast speckle field, so full-sensitivity uninterrupted high contrast observations can be conducted simultaneously with wavefront correction iterations. We also show that LDFC is robust against deformable mirror calibration errors and drifts, as it relies on detector response stability instead of deformable mirror stability. LDFC is particularly advantageous when science acquisition is performed at a non-optimal wavefront sensing wavelength, such as nearIR observations of planets around solar-type stars, for which visible-light speckle sensing is ideal. We describe the approach at a fundamental level and provide an algorithm for its implementation. We demonstrate, through numerical simulation, that spectral LDFC is well-suited for picometer-level cophasing of a large segmented space telescope.

[38]  arXiv:1706.07385 [pdf, ps, other]
Title: Trigger of successive filament eruptions observed by SDO and STEREO
Comments: 25 pages, 14 figures, accepted to Solar Physics Journal
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Using multi-wavelength observation from SDO and STEREO, we investigated the mechanism of two successive eruptions (F1 & F2) of a filament in active region NOAA 11444 on 27 march, 2012. The filament was inverse `J' shaped and lying along a quasi-circular polarity inversion line (PIL). The first part of the filament (F1) erupted at ~2:30UT on 27 March 2012, the second part of the filament (F2) erupted at around 4:20 UT on the same day. A precursor/pre-flare brightening was observed below filament's main axis about 30 min prior to F1. The brightening was followed by a jet-like ejection below filament, which triggered the eruption. Before the eruption of F2, the filament seems to be trapped within the overlying arcade loops almost for $\sim$1.5~hr before its successful eruption. Interestingly, we observed simultaneously contraction (~12km/s) and expansion (~20km/s) of arcade loops in the active region before F2. HMI magnetograms show the converging motion of the opposite polarities resulting in flux cancellation near PIL. We suggest that flux cancellation at PIL resulted jet-like ejection below filament's main axis, which triggered the eruption F1 similar to tether-cutting process. The eruption F2 was triggered by removal of the overlying arcade loops via reconnection process. Both filament eruptions produced high speed (~1000km/s) CMEs.

[39]  arXiv:1706.07400 [pdf, other]
Title: Is Flat Fielding Safe for Precision CCD Astronomy?
Comments: 9 pages, 11 figures, submitted to PASP. Code available at this https URL
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

The ambitious goals of precision cosmology with wide-field optical surveys such as the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope (LSST) demand, as their foundation, precision CCD astronomy. This in turn requires an understanding of previously uncharacterized sources of systematic error in CCD sensors, many of which manifest themselves as static effective variations in pixel area. Such variation renders a critical assumption behind the traditional procedure of flat fielding--that a sensor's pixels comprise a uniform grid--invalid. In this work, we present a method to infer a curl-free model of a sensor's underlying pixel grid from flat field images, incorporating the superposition of all electrostatic sensor effects--both known and unknown--present in flat field data. We use these pixel grid models to estimate the overall impact of sensor systematics on photometry, astrometry, and PSF shape measurements in a representative sensor from the Dark Energy Camera (DECam) and a prototype LSST sensor. Applying the method to DECam data recovers known significant sensor effects for which corrections are currently being developed within DES. For an LSST prototype CCD with pixel-response non-uniformity (PRNU) of 0.4%, we find the impact of "improper" flat-fielding on these observables is negligible in nominal .7" seeing conditions. These errors scale linearly with the PRNU, so for future LSST production sensors, which may have larger PRNU, our method provides a way to assess whether pixel-level calibration beyond flat fielding will be required.

[40]  arXiv:1706.07404 [pdf, other]
Title: Using eigenmode-mixing to measure or constrain the Sun's interior B-field
Authors: Curt Cutler
Comments: 12 pages, 1 figure, submitted to ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Understanding the generation and distribution of the Sun's interior magnetic (B-) field is a longstanding challenge. Here we describe how measurements of the Sun's oscillation eigenfunctions might be used to measure the Sun's interior B-field. The B-field induces mode-mode couplings, causing the angular patterns of the eigenfunctions to differ from simple Y_{lm}'s We concentrate on the magnetic coupling between modes with the same (n,l) values and different but nearby $m$-values, since these non-axisymmetric couplings clearly cannot be due to the Sun's axisymmetric rotation and since for these cases, mode mixing is enhanced by the near-degeneracy of the mode frequencies. We analyze magnetically-induced mode mixing in two stages of increasing complexity: first neglecting mode damping, and then incorporating realistic damping rates. We introduce a novel detection statistic that tests for the presence of non-axisymmetric mode-mixing in Solar Doppler data. We show that our detection statistic is naturally robust against spatial aliasing. We estimate our statistic's signal-to-noise ratio (SNR) as a function of the mode-mixing amplitude. While B-induced mode-mixing is probably not detectable in a single mode pair, we argue that the phase of the B-induced mixing should be approximately the same across a wide range of modes. The total SNR then grows roughly as Np^{1/2}, where Np is the number of mode pairs. We conclude that B-induced mode-mixing should be detectable for a fairly wide range of B-field magnitudes and geometries.

[41]  arXiv:1706.07410 [pdf, other]
Title: Imaging Spectroscopy of Type U and J Solar Radio Bursts with LOFAR
Comments: 9 pages, 7 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Radio U-bursts and J-bursts are signatures of electron beams propagating along magnetic loops confined to the corona. The more commonly observed type III radio bursts are signatures of electron beams propagating along magnetic loops that extend into interplanetary space. Given the prevalence of solar magnetic flux to be closed in the corona, it is an outstanding question why type III bursts are more frequently observed than U-bursts or J-bursts. We use LOFAR imaging spectroscopy between 30-80 MHz of low-frequency U-bursts and J-bursts, for the first time, to understand why electron beams travelling along coronal loops produce radio emission less often. The different radio source positions were used to model the spatial structure of the guiding magnetic flux tube and then deduce the energy range of the exciting electron beams without the assumption of a standard density model. The radio sources infer a magnetic loop 1 solar radius in altitude, with the highest frequency sources starting around 0.6 solar radii. Electron velocities were found between 0.13 c and 0.24 c, with the front of the electron beam travelling faster than the back of the electron beam. The velocities correspond to energy ranges within the beam from 0.7-11 keV to 0.7-43 keV. The density along the loop is higher than typical coronal density models and the density gradient is smaller. We found that a more restrictive range of accelerated beam and background plasma parameters can result in U-bursts or J-bursts, causing type III bursts to be more frequently observed. The large instability distances required before Langmuir waves are produced by some electron beams, and the small magnitude of the background density gradients make closed loops less facilitating for radio emission than loops that extend into interplanetary space.

[42]  arXiv:1706.07425 [pdf, other]
Title: A mid-infrared statistical investigation of clumpy torus model predictions
Comments: 23 pages, 17 figures, 5 tables, Accepted for publication in MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present new calculations of the CAT3D clumpy torus models, which now include a more physical dust sublimation model as well as AGN anisotropic emission. These new models allow graphite grains to persist at temperatures higher than the silicate dust sublimation temperature. This produces stronger near-infrared emission and bluer mid-infrared (MIR) spectral slopes. We make a statistical comparison of the CAT3D model MIR predictions with a compilation of sub-arcsecond resolution ground-based MIR spectroscopy of 52 nearby Seyfert galaxies (median distance of 36 Mpc) and 10 quasars. We focus on the AGN MIR spectral index $\alpha_{MIR}$ and the strength of the 9.7 $\mu$m silicate feature $S_{Sil}$. As with other clumpy torus models, the new CAT3D models do not reproduce the Seyfert galaxies with deep silicate absorption ($S_{Sil}<-1$). Excluding those, we conclude that the new CAT3D models are in better agreement with the observed $\alpha_{MIR}$ and $S_{Sil}$ of Seyfert galaxies and quasars. We find that Seyfert 2 are reproduced with models with low photon escape probabilities, while the quasars and the Seyfert 1-1.5 require generally models with higher photon escape probabilities. Quasars and Seyfert 1-1.5 tend to show steeper radial cloud distributions and fewer clouds along an equatorial line-of-sight than Seyfert 2. Introducing AGN anisotropic emission besides the more physical dust sublimation models alleviates the problem of requiring inverted radial cloud distributions (i.e., more clouds towards the outer parts of the torus) to explain the MIR spectral indices of type 2 Seyferts.