Synthetic photometry for carbon rich giants, I. Hydrostatic dust-free models

TL;DR

This paper presents a comprehensive grid of hydrostatic, dust-free models for carbon-rich giant stars, providing synthetic spectra and photometry that match observations well down to 2800K, aiding stellar evolution studies.

Contribution

It introduces a large database of 746 hydrostatic COMARCS atmospheres for carbon stars, covering wide parameter ranges, and offers synthetic spectra and photometry for various filters.

Findings

Good agreement with observed colours down to 2800K

Synthetic colours are too blue below 2800K due to neglecting dust and pulsation effects

Provides publicly available tables for stellar population synthesis

Abstract

We study the spectroscopic and photometric properties of carbon stars. In the first paper of this series we focus on objects that 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. We have computed a grid of 746 spherically symmetric COMARCS atmospheres covering effective temperatures between 2400 and 4000K, surface gravities from log(g) = 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 including the results are electronically available. 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 2800K the agreement between predictions and observations of carbon stars is good. 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.