Carbon stars in the X-Shooter Spectral Library^{⋆,⋆⋆,⋆⋆⋆}

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, UMR 5574, 69007 Lyon, France
⁶ New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁸ Instituto de Astrofísica de Canarias, vía Láctea s/n, La Laguna, 38205 Tenerife, Spain
⁹ Departamento de Astrofísica, Universidad de La Laguna, 38205 , La Laguna, Tenerife, Spain
¹⁰ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281, 9000 Gent, Belgium
¹¹ Universidad Autónoma de Madrid, Departamento de Física Teórica, 28049 Cantoblanco, Madrid, Spain
¹² Instituto de Astrofísica, Facultad de Fisica, Pontificia Universidad Catolica de Chile, Santiago 22, Chile

Received: 9 April 2015
Accepted: 20 December 2015

Abstract

We provide a new collection of spectra of 35 carbon stars obtained with the ESO/VLT X-Shooter instrument as part of the X-Shooter Spectral Library project. The spectra extend from 0.3 μm to 2.4 μm with a resolving power above ~8000. The sample contains stars with a broad range of (J − K) color and pulsation properties located in the Milky Way and the Magellanic Clouds. We show that the distribution of spectral properties of carbon stars at a given (J − K) color becomes bimodal (in our sample) when (J − K) is larger than about 1.5. We describe the two families of spectra that emerge, characterized by the presence or absence of the absorption feature at 1.53 μm, generally associated with HCN and C₂H₂. This feature appears essentially only in large-amplitude variables, though not in all observations. Associated spectral signatures that we interpret as the result of veiling by circumstellar matter, indicate that the 1.53 μm feature might point to episodes of dust production in carbon-rich Miras.