Carbon Stars

Wallerstein, George; Knapp, G. R.

doi:10.1146/annurev.astro.36.1.369

Cited by 215 publications

(223 citation statements)

References 172 publications

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“…It is not known why these detached shells are not observed around oxygen-rich AGB stars, and W Hya is unique in that sense. The zero-age-mainsequence mass of W Hya was determined to be 1.5 ± 0.2 M (Khouri et al 2014b), which places this object around the limit for becoming a carbon-rich star (Wallerstein & Knapp 1998). However, models for the formation of detached shells require the interaction of a faster wind with a previously slower wind (e.g.…”

Section: Recent Mass-loss History Of W Hyamentioning

confidence: 99%

Dusty wind of W Hydrae

Khouri

Waters

Koter

et al. 2015

A&A

107

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Context. Low-and intermediate-mass stars go through a period of intense mass-loss at the end of their lives, during the asymptotic giant branch (AGB) phase. While on the AGB a significant part, or even most, of their initial mass is expelled in a stellar wind. This process controls the final stages of the evolution of these stars and contributes to the chemical evolution of galaxies. However, the wind-driving mechanism of AGB stars is not yet well understood, especially so for oxygen-rich sources. Characterizing both the present-day mass-loss rate and wind structure and the evolution of the mass-loss rate of such stars is paramount to advancing our understanding of this processes. Aims. We study the dusty wind of the oxygen-rich AGB star W Hya to understand its composition and structure and shed light on the mass-loss mechanism. Methods. We modelled the dust envelope of W Hya using an advanced radiative transfer code. We analysed our dust model in the light of a previously calculated gas-phase wind model and compared it with measurements available in the literature, such as infrared spectra, infrared images, and optical scattered light fractions. Results. We find that the dust spectrum of W Hya can partly be explained by a gravitationally bound dust shell that probably is responsible for most of the amorphous Al 2 O 3 emission. The composition of the large (∼0.3 μm) grains needed to explain the scattered light cannot be constrained, but probably is dominated by silicates. Silicate emission in the thermal infrared was found to originate from beyond 40 AU from the star. In our model, the silicates need to have substantial near-infrared opacities to be visible at such large distances. The increase in near-infrared opacity of the dust at these distances roughly coincides with a sudden increase in expansion velocity as deduced from the gas-phase CO lines. The dust envelope of W Hya probably contains an important amount of calcium but we were not able to obtain a dust model that reproduces the observed emission while respecting the limit set by the gas mass-loss rate. Finally, the recent mass loss of W Hya is confirmed to be highly variable and we identify a strong peak in the mass-loss rate that occurred about 3500 years ago and lasted for a few hundred years.

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Section: Recent Mass-loss History Of W Hyamentioning

confidence: 99%

Dusty wind of W Hydrae

Khouri

Waters

Koter

et al. 2015

A&A

107

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“…Wallerstein & Knapp (1998) raised doubts about the identification of SiC 2 as the molecule responsible for the Merrill-Sanford (M-S) absorption bands observed near 490 and 497.7 nm. The SiC 2 molecule is usually mentioned to explain those bands which are either strong or absent.…”

Section: The M-s Band Intensities and The Effective Temperaturesmentioning

confidence: 99%

The effective temperatures of carbon-rich stars

Bergeat

Knapik

Rutily

2001

A&A

152

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Abstract. We evaluate effective temperatures of 390 carbon-rich stars. The interstellar extinction on their lines of sights was determined and circumstellar contributions derived. The intrinsic (dereddened) spectral energy distributions (SEDs) are classified into 14 photometric groups (HCi, CVj and SCV with i = 0,5 and j = 1,7). The new scale of effective temperatures proposed here is calibrated on the 54 angular diameters (measured on 52 stars) available at present from lunar occultations and interferometry. The brightness distribution on stellar discs and its influence on diameter evaluations are discussed. The effective temperatures directly deduced from those diameters correlate with the classification into photometric groups, despite the large error bars on diameters. The main parameter of our photometric classification is thus effective temperature. Our photometric k 1/2 coefficients are shown to be angular diameters on a relative scale for a given photometric group, (more precisely for a given effective temperature). The angular diameters are consistent with the photometric data previously shown to be consistent with the true parallaxes from HIPPARCOS observations (Knapik, et al. 1998, Sect. 6). Provisional effective temperatures, as constrained by a successful comparison of dereddened SEDs from observations to model atmosphere predictions, are in good agreement with the values directly calculated from the observed angular diameters and with those deduced from five selected intrinsic color indices. These three approaches were used to calibrate a reference angular diameter Φ0 and the associated coefficient CT eff . The effective temperature proposed for each star is the arithmetic mean of two estimates, one ("bolometric") from a reference integrated flux F0, the other ("spectral") from calibrated color indices which are representative of SED shapes. Effective temperatures for about 390 carbon stars are provided on this new homogeneous scale, together with values for some stars classified with oxygen-type SEDs with a total of 438 SEDs (410 stars) studied. Apparent bolometric magnitudes are given. Objects with strong infrared excesses and optically thick circumstellar dust shells are discussed separately. The new effective temperature scale is shown to be compatible and (statistically) consistent with the sample of direct values from the observed angular diameters. The effective temperatures are confirmed to be higher than the mean color temperatures (from 140 to 440 K). They are in good agreement with the published estimates from the infrared flux method for T eff ≥ 3170 K, while an increasing discrepancy is observed toward lower temperatures. As an illustration of the efficiency of the photometric classification and effective temperature scale, the C/O ratios and the Merrill-Sanford (M-S) band intensities are investigated. It is shown that the maximum value, mean value and dispersion of C/O increase along the photometric CV-sequence, i.e. with decreasing effective temperature. The M-S bands of SiC2 are sho...

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“…In fact, CH stars have a range of luminosities, from M V ∼ −2 at the top of the red giant branch like the CH stars in ω Centauri, down to M V ∼ +4 on the subgiant branch (Wallerstein & Knapp 1998). A high luminosity will be assumed here.…”

Section: New Carbon Stars From the Fbsmentioning

confidence: 99%

“…For example, they may be very cool and dusty giants evolving on the asymptotic giant branch (AGB) where the excess of carbon present in their atmospheres results from nucleosynthesis in the stellar interior. They can also be binary stars, often warmer giants or even dwarfs having gained carbon-rich material by accretion from a more evolved companion (for a review of C stars, see Wallerstein & Knapp 1998).…”

Section: Introductionmentioning

confidence: 99%

The properties of carbon stars found in the Byurakan and the Hamburg/ESO surveys

Mauron¹,

Gigoyan

Kendall

2006

A&A

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Aims. We analyse two samples of high latitude carbon stars found in two objective-prism surveys: the First Byurakan survey (FBS) and the Hamburg/ESO survey (HES). We determine the composition of these samples in cool AGB stars, warmer giants and dwarf carbon stars. Methods. We list 44 new carbon stars found in the FBS and use near-infrared photometric catalogues (DENIS, 2MASS) to investigate the observational and statistical properties of the surveys. The SuperCOSMOS and USNO-B1.0 catalogues are used to study proper motions and to identify dwarfs. Results. The colour-magnitude and colour-colour diagrams, either with visual magnitudes or with infrared data, show that the HES selects faint and relatively warm carbon stars, whereas FBS is less deep, but gives cooler objects. By comparing DENIS and 2MASS photometry, we find that HES objects with J − K ≤ 1 are mostly non-variable giants while clear variability is often seen for J − K ≥ 1, indicating AGB stars. By using SuperCOSMOS proper motions, 9 HES objects are found to have a proper motion µ greater than 30 mas yr −1 (3σ detection). Five of these objects are detected at the ≥5σ level. These 9 HES objects are dwarf carbon stars, including two that could belong to the halo population. The 9 carbon dwarfs are remarkably bright and deserve further study.

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Carbon Stars

Cited by 215 publications

References 172 publications

Dusty wind of W Hydrae

Dusty wind of W Hydrae

The effective temperatures of carbon-rich stars

The properties of carbon stars found in the Byurakan and the Hamburg/ESO surveys

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