Carbon stars in the X-Shooter Spectral Library

A. Gonneau1*, A. Lançon2, S. C. Trager1, R. F. Peletier1, M. Lyubenova1, A. Arentsen3,B. Aringer4,5, Y.-P. Chen6, O. S. Choudhury3, M. Dries1, J. Falcón-Barroso7,8, P. Prugniel9, S. Meneses-Goytia10, W. Nowotny4, P. Sánchez-Blázquez11, A. Vazdekis7,8 and M. Koleva
1Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV, Groningen, The Netherlands
2Observatoire Astronomique de Strasbourg, CNRS, UMR 7550, F-67000 Strasbourg, France
3Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
4University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Wien, Austria
5Dipartimento di Fisica e Astronomia Galileo Galilei, Università di Padova, I-35122 Padova, Italy
6New York University Abu Dhabi, Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates
7Instituto de Astrofísica de Canarias, Vía Láctea s/n, La Laguna, Tenerife, Spain
8Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
9CRAL-Observatoire de Lyon, Université de Lyon, Lyon I, CNRS, UMR5574, France
10Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth, PO1 3FX, UK
11Universidad Autónoma de Madrid, Departamento de Física Teórica, E-28049 Cantoblanco, Madrid, Spain

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We present a new collection of spectra of 35 carbon stars, obtained as part of the X-Shooter Spectral Library project. The sample contains stars with a broad range of (J - K) colours and pulsation properties, located in the Milky Way and the Magellanic Clouds. Two families of spectra emerge when (J - K) is larger than 1.6, characterised by the presence or absence of the absorption feature at 1.53 µm, generally associated with HCN and C2H2. We compare the observations with a new set of highresolution synthetic spectra based on hydrostatic model atmospheres. The full spectrum fit of the observations to the grid of models allows us to identify the most likely atmospheric parameters. Those parameters, derived independently in the optical and near-infrared, are generally very consistent for (J - K) < 1.6. For the reddest stars showing the 1.53 µm feature, which are all large amplitude variables, the e ects of pulsation are strong and the spectra are poorly matched with hydrostatic models. Dynamical models are needed to interpret those observations.

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Keywords : X-Shooter – carbon stars – atmospheres – spectrum fitting