Vientos Estelares

A&A 503, 913–928 (2009) DOI: 10.1051/0004-6361/200911703
c ESO 2009

Astronomy & Astrophysics

Synthetic photometry for carbon rich giants
I. Hydrostatic dust-free models
B. Aringer1,2,3 , L. Girardi2 , W. Nowotny1 , P. Marigo3 , and M. T. Lederer1
1 2 3

Department of Astronomy, University of Vienna, Türkenschanzstraße 17, 1180 Wien, Austria e-mail: aringer@astro.univie.ac.at OsservatorioAstronomico di Padova – INAF, Vicolo dell’Osservatorio 5, 35122 Padova, Italy e-mail: leo.girardi@oapd.inaf.it Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 2, 35122 Padova, Italy e-mail: paola.marigo@unipd.it

Received 22 January 2009 / Accepted 5 April 2009
ABSTRACT

Context. Carbon rich objects represent an important phase during the late stages of evolution of low andintermediate mass stars. They contribute significantly to the chemical enrichment and to the infrared light of galaxies. A proper description of their atmospheres is crucial for the determination of fundamental parameters such as effective temperature or mass loss rate. Aims. We study the spectroscopic and photometric properties of carbon stars. In the first paper of this series we focus on objectsthat can be described by hydrostatic models neglecting dynamical phenomena like pulsation and mass loss. As a consequence, the reddening due to circumstellar dust is not included. Our results are collected in a database, which can be used in conjunction with stellar evolution and population synthesis calculations involving the AGB. Methods. We have computed a grid of 746 spherically symmetric COMARCSatmospheres covering effective temperatures between 2400 and 4000 K, surface gravities from log(g [cm/s2 ]) = 0.0 to −1.0, metallicities ranging from the solar value down to one tenth of it and C/O ratios in the interval between 1.05 and 5.0. Subsequently, we used these models to create synthetic low resolution spectra and photometric data for a large number of filter systems. The tables includingthe results are electronically available. First tests of the application on stellar evolution calculations are shown. Results. We have selected some of the most commonly used colours in order to discuss their behaviour as a function of the stellar parameters. A comparison with measured data shows that down to 2800 K the agreement between predictions and observations of carbon stars is good and ourresults may be used to determine quantities like the effective temperature. Below this limit the synthetic colours are much too blue. The obvious reason for these problems is the neglect of circumstellar reddening and structural changes due to pulsation and mass loss. Conclusions. The warmer carbon stars with weak pulsation can be successfully described by our hydrostatic models. In order toinclude also the cooler objects with intense variations, at least a proper treatment of the reddening caused by the dusty envelopes is needed. This will be the topic of the second paper of this series.
Key words. stars: late-type – stars: AGB and post-AGB – stars: atmospheres – infrared: stars – stars: carbon –

Hertzsprung-Russell (HR) and C-M diagrams

1. Introduction
During the evolution of low tointermediate mass stars along the TP-AGB (thermal pulsing asymptotic giant Branch), material processed by the nuclear reactions in the He burning shell may be dredged up to the surface, changing the chemical abundances in the atmosphere. Especially, the amount of carbon will be increased significantly by this mechanism. If the particle density exceeds that of oxygen (the ratio of the particledensities C/O > 1), the spectral appearance of the object becomes completely different. Instead of O-bearing molecules like TiO, VO, SiO, OH and H2 O the opacities in the cool outer layers and the observed energy distributions are dominated by C2 , CN, C3 , HCN and C2 H2 (e.g. Querci et al. 1974; Jørgensen et al. 2000; Loidl et al. 2001). This characterizes a classical carbon star.
Table of the...