Carbon stars in the X-shooter Spectral Library

TL;DR

This paper presents a new spectral library of 35 carbon stars observed with the X-shooter instrument, revealing bimodal spectral properties related to dust production and variability, with implications for understanding stellar evolution.

Contribution

The study provides a comprehensive spectral library of carbon stars across a broad wavelength range, highlighting bimodal spectral features linked to pulsation and dust formation, which was not previously documented.

Findings

Spectral properties of carbon stars are bimodal at (J-K) > 1.5.

The 1.53μm absorption feature is associated with large-amplitude variables.

Spectral signatures suggest episodic dust production in carbon-rich Miras.

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$\mu$m to 2.4$\mu$m with a resolving power above $\sim$ 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$\mu$m, generally associated with HCN and C$_2$H$_2$. 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$\mu$m feature might point to episodes of dust production in carbon-rich Miras.