EDP Sciences

Vol. 606
In section 6. Galactic structure, stellar clusters and population

Effects of the selection function on metallicity trends in spectroscopic surveys of the Milky Way

by G. Nandakumar, M. Schultheis, M. Hayden, A. Rojas-Arriagada, G. Kordopatis, and M. Haywood, A&A 606, A97


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Large spectroscopic Galactic surveys all undergo a selection process for choosing their targets. This selection function can alter the final form of the metallicity distribution function and the vertical metallicity gradient differently for different surveys. This paper investigates the effects of the selection functions on metallicity trends in the common fields between the APOGEE, LAMOST, RAVE, and Gaia-ESO surveys. The authors use the stellar population synthesis models GALAXIA and TRILEGAL to create mock fields for each survey and apply the selection function in the form of colour and magnitude cuts to replicate the observed sample and investigate the effects of the different selection functions. They find that the selection function has a negligible effect on the metallicity distribution function or the vertical metallicity gradients for the APOGEE, LAMOST, and RAVE surveys. The corrected vertical metallicity gradients between the four surveys are consistent within 1-σ. It is, therefore, possible to combine common fields of these different surveys provided that their metallicities are on the same scale.

Vol. 606
In section 6. Interstellar and circumstellar matter

Three-dimensional mapping of the local interstellar medium with composite data

by L. Capitanio, R. Lallement, J.L. Vergely, M. Elyajouri, and A. Monreal-Ibero, A&A 606, A65


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Obtaining a detailed picture of the 3-D structure of even the local interstellar medium remains a major challenge. The fine structure, averaged out on larger scales, has thus far been the only unambiguous view we have of how the gas and dust are structured on scales of tens of parsecs. The ambitious, successful aim of the present study is to cover lines of sight of up to a few kiloparsecs using stellar parallax and proper motions of background light sources with high accuracy, using the first Gaia data release. The authors do this by combining different measures for the two diffuse components, using a near infrared absorption band (DIB) to obtain extinction (reddening) estimates. They use a novel Bayesian inversion method to reconstruct the fine structure from tens of thousands of DIB measurements of profiles and velocities, with the constraint of maximum extinction in any direction. The structures thus obtained, both clouds and vacancies, can then be compared with multiwavelength data from the UV and XR for further refinement.

Vol. 606
In section 3. Cosmology

Mapping the hot gas temperature in galaxy clusters using X-ray and Sunyaev-Zel'dovich imaging

by R. Adam, M. Arnaud, I. Bartalucci, et al. A&A 606, A64


Temperature is one of the key properties of the hot ionised gas in the intra-cluster medium (ICM) traditionally obtained from X-ray spectroscopic observations. The thermal Sunyaev-Zel'dovich (tSZ) effect can provide an alternative measurement of the ICM temperature if it is combined with X-rays that measure the gas density. In this paper, the authors use this combination to map the temperature distribution of the ICM in the merging cluster MACS J0717.5+3745 taking advantage of deep and resolved tSZ observations from the New IRAM KIDs Array (NIKA) instrument, and from XMM-Newton. The authors demonstrate the capabilities of their approach and find a good morphological agreement between the temperature maps derived from X-ray data only and their map obtained from the tSZ+X-ray combination.

Vol. 606
In section 3. Cosmology

Measurement of the EBL spectral energy distribution using the VHE gamma-ray spectra of H.E.S.S. blazars

by H.E.S.S. Collaboration, A&A 606, A59


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The low-energy spectrum of extragalactic background light (EBL) has already been inferred from measurements at low energy, but this study presents a novel recovery of the ELB using very high energy observations in the range of 0.2 to nearly 20 TeV using the H.E.S.S. imaging atmospheric Cherenkov telescope array. Using a single array provides better control of systematic uncertainties, that are extensvely discussed. The authors use a set of nine blazars, including those displaying different activity states, spanning almost a decade in redshift, 0.03 < z 0.29. Systematic deviations in the (relatively simple and monotonic) intrinsic spectral shape are produced by the interaction of the VHE source photons and the cosmologically ambient light. Although a model function is required for the redshift evolution, the recovered shape is not based on a spectral model. The authors extract the restframe spectrum, in four optical and infrared bands between 0.5 and 100 microns (Fig. 5), and the redshift evolution of the gamma-ray horizon (the effective VHE photosphere) (Fig. 6).

Vol. 606
In section 7. Stellar structure and evolution

The brightness of the red giant branch tip. Theoretical framework, a set of reference models, and predicted observables

by A. Serenelli, A. Weiss, S. Cassisi, M. Salaris, and A. Pietrinferni, A&A 606, A33


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>>> The brightness of the tip of the red giant branch is a useful reference quantity for several fields of astrophysics, and thus a theoretical model that accurately predicts its properties is important. This paper provides a solid theoretical model, valid for a reference set of standard physical assumptions, that is mostly independent of numerical details. The paper examines the dependence of the brightness of the tip of the red giant branch on physical assumptions and numerical details for a wide range of metallicities and masses based on two different stellar evolution codes. The authors present a reference set of models based on the most up to date physical inputs in infrared colours suitable for interpreting observations of resolved stellar populations. Also included are analytic fits to facilitate their use. This paper demonstrates that careful stellar modelling allows an accurate prediction of the luminosity of the red giant branch tip. Differences from empirically determined brightnesses may result either from inaccurate colour transformations or from deficits in the constitutional physics. The best-tested theoretical reference values are presented.

Vol. 606
In section 8. Stellar atmospheres

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. VI. First chromosphere model of a late-type giant

by S. Wedemeyer, A. Kucinskas, J. Klevas, and H.-G. Ludwig A&A 606, A26


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This paper presents the first exploratory 3D hydrodynamical model atmosphere of the chromosphere of a cool red giant star to study the dynamical and thermodynamic properties, as well as the influence on the observable properties of such a star. Three-dimensional radiation hydrodynamics simulations were carried out with the CO^5BOLD model atmosphere code for a star with atmospheric parameters that are similar to those of the K-type red giant star Aldebaran. The authors compute the emergent continuum intensity maps at different wavelengths, spectral line profiles of Ca II K, the Ca II infrared triplet line at 854.2 nm, and H alpha, as well as the spectral energy distribution of the emergent radiative flux. The initial model is found to quickly develop a dynamical chromosphere that is characterised by propagating and interacting shock waves. The peak temperatures in the chromospheric shock fronts reach values up to 5000K although the shock fronts remain quite narrow. As for the Sun,the gas temperature distribution in the upper layers is composed of a cool component due to adiabatic cooling in the expanding post-shock regions and a hot component due to the shock waves. These simulations show that the atmospheres of red giant stars are dynamic. This means that many observable properties cannot be reproduced with a one-dimensional static model but require advanced 3D hydrodynamical modelling. Furthermore, including a chromosphere in the models can produce a significant contribution to the emergent UV flux.