Spectra of carbon-rich asymptotic giant branch stars between 0.5 and 2.5 μm: Theory meets observation (original) (raw)
Related papers
Astronomy & Astrophysics, 2004
We have calculated synthetic opacity sampling spectra for carbon-rich Asymptotic Giant Branch (AGB) stars based on dynamic model atmospheres which couple time-dependent dynamics and frequency-dependent radiative transfer, as presented in the third paper of this series. We include the molecules CO, CH, CN, C2, CS, HCN, C2H2 and C3 in our calculations, both when computing the atmospheric structures, and the synthetic spectra. A comparison of the synthetic spectra with various observed colours and spectra in the wavelength range between 0.5 and 25 μm,of TX Psc, WZ Cas, V460 Cyg, T Lyr and S Cep is presented. We obtain good agreement between observations gathered at different phases and synthetic spectra of one single hydrodynamical model for each star in the wavelength region between 0.5 and 5 μm. At longer wavelengths our models showing mass loss offer a first self-consistent qualitative explanation of why a strong feature around 14 μm, which is predicted by all hydrostatic models as well as dynamical models showing no mass loss, is missing in observed AGB carbon star spectra.
Dynamic model atmospheres of AGB stars II. Synthetic near infrared spectra of carbon stars
We have calculated synthetic opacity sampling spec- tra for carbon-rich Asymptotic Giant Branch (AGB) stars based on dynamic model atmospheres presented in the first paper of this series. We discuss how different model parameters influence the resulting synthetic spectra and how the spectra vary with phase. The molecules included are: CO, CH, CN, C 2 , HCN, C 2 H 2 and C 3 . We show in which atmospheric layers the dif- ferent molecules form, in an attempt to understand the qual- itatively different variation with pulsation phase exhibited by various spectral features. Almost all features are blends of tran- sitions from more than one molecule, and we therefore identify the most important transitions and molecules that contribute to the main spectral features from 0.5 to 12 μm. Furthermore, we demonstrate the effect on the individual spectral features due to the carbon depletion when dust is formed in the atmosphere.
ISO-SWS spectra of the carbon stars TX Psc, V460 Cyg, and TT Cyg
During our open and discretionary time ISO pro- grams we have observed a number of carbon-rich and oxygen- rich AGB stars. We present here a hydrostatic analysis of 3 carbon rich stars, TX Psc, V460 Cyg, and TT Cyg, which show only modest variability. We identify absorption features of the molecules C2, CN, CH, CO, CS, HCN, C2H2, and C3 in the three stars. The relative intensities of the corresponding bands put strict limits on the possible values of Teff , log g, and C/O. In particular the ratio of the intensity of the 3 μm band (due to HCN and C2 H2 ) and the 5 μm band (due to CO and C3 ) is a sensitive measure of the C/O ratio. We show that our model atmospheres and corresponding synthetic spectra are able to reproduce the observed spectra quite accurately from the visual region through the infrared out to approximately 10 μm. Beyond 10 μm the flux in the observed spectra increases compared to the computed spectra, and the photospheric 14μm band dominated by C2H2 and HCN is al- most absent in the observed spectra whereas it is very strong in the synthetic ones. This discrepancy is not an artefact in the reduction or in the SWS response function, but a physical phe- nomenon in the stars, which does not appear in oxygen-rich giants. We exclude a so-called “warm molecular envelope” as an explanation of the discrepancies, and suggest that the flux excess at long wavelengths can be interpreted as clumps of ≈ 500 K dense material which obscures about 10% of the photosphere. Weak spectral features seen around 13.7 μm can be understood as even cooler gas, rich in C2H2, above the clumps. Other possi- ble models to explain the long wavelength discrepancy between the computed and observed spectra need to be explored too.
Infrared observations of peculiar carbon stars
1997
We present a uniform and high quality set of infrared photometric (JHK) observations of the 6 peculiar carbon giant stars V Ari, UV Cam, BD+34 911, TU Gem, BD+57 2161 and BD+34 4134. All of these belong to the small group of known cool CH giants populating the Galactic halo. Comparison of the J − H and H − K colours to "normal" C stars show our stars to be "bluer" (i.e., having lower values of J − H and H − K) than the bulk of the Galactic C stars. Comparison with synthetic JHK colours reveal 5 of our 6 stars as having considerably lower metallicities and/or higher temperatures than the bulk. Using standard assumptions we derive estimates of their effective temperatures, gravities, luminosities and distances. Their derived luminosities place them close to (or below) the theoretical first He shell flash luminosity, although other observations indicate their carbon excess to be intrinsic.
The Chromospheres of Carbon Stars
Proceedings of the International Astronomical Union
Most oxygen-rich late-type giant stars show evidence for chromospheres in their visual spectra (e.g. Call Η & Κ emission features). Cool (i.e. N-type) non-Mira carbon stars, however, have never been observed to have chromospheric emission in their Call Η & Κ lines. However, faint Mgll h & k lines were detected in emission in low-dispersion IUE spectra of the brightest cool carbon stars in the early 1980s. May 1984 saw the first (and only) successful high-dispersion IUE spectrum taken of a cool carbon star, TX Psc (NO; C6,2). Armed with this high-dispersion spectrum, as well as low-dispersion IUE and ground-based spectra, Luttermoser et al. (1989) made the first detailed attempt to semiempirically model the chromosphere of a cool carbon star. This model was successful in reproducing the Mgll lines, but it was not well constrained due to the lack of other observed high-resolution chromospheric profiles for comparison. Modeling carbon star chromospheres can now be addressed more accurately with HST/GHRS high-resolution spectra. New fluoresced emission features have been discovered in the GHRS spectra of carbon stars that are not present in their oxygen-rich counterparts.
Atomic Carbon in the Envelopes of Carbon‐rich Post–Asymptotic Giant Branch Stars
The Astrophysical Journal, 2000
Atomic carbon has been detected in the envelopes of three carbon-rich evolved stars : HD 44179 (\AFGL 915, the "" Red Rectangle ÏÏ), HD 56126, and, tentatively, the carbon star V Hya. This brings to seven the number of evolved star envelopes in which C I has been detected. Upper limits were found for several other stars, including R CrB. C I was not detected in several oxygen-rich postÈasymptotic giant branch (AGB) stars (OH 231.8]4.2, for example), although it is detected in their carbon-rich analogs. Two trends are evident in the data. First, circumstellar envelopes with detectable C I are overwhelmingly carbon-rich, suggesting that much of the C I is produced by the dissociation of molecules other than CO. Second, the more evolved the envelope away from the AGB, the higher the C I/CO ratio. The oxygenrich supergiant star a Ori remains the only oxygen-rich star with a wind containing detectable C I. These data suggest an evolutionary sequence for the C I/CO ratio in cool circumstellar envelopes. This ratio is small (a few percent) while the star is on the AGB, and the C I is located in the outer envelope and produced by photodissociation. The ratio increases to about 0.5 as the star evolves away from the AGB because of the dissociation of CO and other carbon-bearing molecules by shocks caused by the fast winds which appear at the end of evolution on the AGB. Finally, the ratio becomes ?1 as the central star becomes hot enough to photodissociate CO.
Interpretation of carbon overabundances in the atmospheres of carbon stars
Astrophysics and Space Science, 1978
An attempt is made to estimate the rate of accretion of interstellar grains by red giants. It seems possible for a red giant traversing dust clouds to acquire, under realistic conditions, an amount of carbon sufficient to turn its spectrum into that of a carbon star. Coarse grains exceeding 10-3 cm in diameter are most effective in this process.