Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........71c&link_type=abstract
PhD Dissertation, Calgary Univ. Alberta Canada
Astronomy and Astrophysics
Astronomy
8
Stellar Evolution, Carbon Stars, Infrared Astronomy Satellite, Color, Infrared Stars, M Stars, Main Sequence Stars, Spectroscopy, Stellar Spectra
Scientific paper
This thesis is concerned with the evolution of carbon stars. A carbon star is a late-type star in which the abundance of carbon in the photosphere is greater than that of oxygen. Data from the Infrared Astronomical Satellite (IRAS) survey were used to show that carbon stars which are identified from optical surveys and those identified from the SiC dust features in the IRAS Low Resolution Spectrometer (LRS) spectra have different IRAS colors. The former ( "visual carbon stars" ) are visually bright and have large excesses at 60 mu m, while the latter group ( "infrared carbon stars" ) have blackbody energy distributions. All catalogued visual carbon stars with IRAS 12 mu m fluxes greater than 5 Jy were searched for LRS spectra in the IRAS LRS data base. 152 new LRS spectra with reasonable good signal-to-noise ratio along with 575 sources with previously released LRS spectra have been classified in the classification scheme of Volk and Cohen (1989a). The LRS spectra of all these sources were examined, and 15 were found to show the 10 mu m silicate emission feature. Eight of these are new discoveries. This group of "silicate carbon stars" may represent transition objects between oxygen-rich and carbon stars on the asymptotic giant branch (AGB). A radiative transfer model of optically thick detached shells for these silicate carbon stars is presented which produces excellent fits to the observed energy distribution of silicate carbon stars. J stars (l3C-rich carbon stars) have been suggested to be transition objects between M-type stars and C-type stars. An optical spectroscopic study of these silicate carbon stars was performed. Four sources have been confirmed to be J stars. Two more are provisionally identified as J stars. A preliminary spectral analysis has also been carried out. Model calculations are presented on the evolution from the visual carbon stars to infrared carbon stars, and on the evolution of infrared carbon stars. A new empirical opacity function for the SiC grain is derived from the LRS spectra of infrared carbon stars. A two-shell system model (oxygen-rich detached shell and newly-forming SiC dust shell), the Interrupted Mass Loss Model, has been developed. The energy distribution of approximately 110 transition objects with developing SiC dust shells are fitted with the Interrupted Mass Loss Model. Furthermore, the model tracks successfully explain the 'C' shaped distribution of these objects in the IRAS 12 mu m/25 mu m/60 mu m colour-colour diagram. The energy distributions of approximately 150 infrared carbon stars are also matched with a radiative transfer SiC dust shell model. The evolution of infrared carbon stars (from the SiC shell model) can be understood by a continuous increase in mass loss rate on the AGB. A evolutionary scenario of AGB stars is suggested. There is a branching of M-type and C-type stars on the AGB with each branch evolving independently to the planetary nebula stage. The initial mass of the star in the main sequence may be the factor that determines which branch the star will follow.
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