Abstract
A new classification system for carbon-rich stars is presented based on an analysis of 51 asymptotic giant branch carbon stars through the most relevant classifying indices available. The extension incorporated, which also represents the major advantage of this new system, is the combination of the usual optical indices that describe the photospheres of the objects, with new infrared ones, which allow an interpretation of the circumstellar environment of the carbon-rich stars. This new system is presented with the usual spectral subclasses and C2-, j-, MS-, and temperature indices, and also with the new SiC- (SiC/C.A. abundance estimation) and τ- (opacity) indices. The values for the infrared indices were carried out through a Monte Carlo simulation of the radiative transfer in the circumstellar envelopes of the stars. The full set of indices, when applied to our sample, resulted in a more efficient system of classification, since an examination in a wide spectral range allows us to obtain a complete scenario for carbon stars.
Highlights
The asymptotic giant branch stars with a ratio C/O > 1 have their optical spectra ruled by bands of carbon compounds, which obscure many atomic features
This New Scheme of Classification of C-Rich AGB Stars can not be applied to any carbon star
It is not a good application for AGB carbon stars that have a very thick envelope, e.g. extreme AGB stars can not be treated this way as their optical spectra are highly obscured
Summary
The asymptotic giant branch stars with a ratio C/O > 1 have their optical spectra ruled by bands of carbon compounds, which obscure many atomic features. The new classification system proposed by Keenan, the MK Revised System, re-established the spectral subclasses for the carbon rich objects and the temperature indices based on infrared intensities This Revised MK System listed four abundance indices from Yamashita (1972): the intensity of the C2 band, the isotopic carbon ratio, the SiC2 band and the CH band strength. The main difference between this scheme and the others available is that, instead of using a single region of the spectrum, we suggest a classification system based on a large wavelength range, from the blue-optical to the mid-infrared This notation draws a more complete scenario of each star.
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