Carbon stars in the X-shooter Spectral Library

II. Comparison with models^⋆

¹ Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France
e-mail: anais.gonneau@astro.unistra.fr
² Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV, Groningen, The Netherlands
³ University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Wien, Austria
⁴ Dipartimento di Fisica e Astronomia Galileo Galilei, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁵ CRAL-Observatoire de Lyon, Université de Lyon, Lyon I, CNRS, UMR5574, France
⁶ New York University Abu Dhabi, Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates

Received: 19 September 2016
Accepted: 20 January 2017

Abstract

In a previous paper, we assembled a collection of medium-resolution spectra of 35 carbon stars, covering optical and near-infrared wavelengths from 400 to 2400 nm. The sample includes stars from the Milky Way and the Magellanic Clouds, with a variety of (J−K_s) colors and pulsation properties. In the present paper, we compare these observations to a new set of high-resolution synthetic spectra, based on hydrostatic model atmospheres. We find that the broad-band colors and the molecular-band strengths measured by spectrophotometric indices match those of the models when (J−K_s) is bluer than about 1.6, while the redder stars require either additional reddening or dust emission or both. Using a grid of models to fit the full observed spectra, we estimate the most likely atmospheric parameters T_eff, log (g), [Fe/H] and C/O. These parameters derived independently in the optical and near-infrared are generally consistent when (J−K_s) < 1.6. The temperatures found based on either wavelength range are typically within ±100 K of each other, and log (g) and [Fe/H] are consistent with the values expected for this sample. The reddest stars ((J−K_s) > 1.6) are divided into two families, characterized by the presence or absence of an absorption feature at 1.53 μm, generally associated with HCN and C₂H₂. Stars from the first family begin to be more affected by circumstellar extinction. The parameters found using optical or near-infrared wavelengths are still compatible with each other, but the error bars become larger. In stars showing the 1.53 μm feature, which are all large-amplitude variables, the effects of pulsation are strong and the spectra are poorly matched with hydrostatic models. For these, atmospheric parameters could not be derived reliably, and dynamical models are needed for proper interpretation.