The low wind expansion velocity of metal‐poor carbon stars in the Halo and the Sagittarius stream★ (original) (raw)
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Probing the mass loss history of carbon stars using CO line and dust continuum emission
Astronomy & Astrophysics, 2002
An extensive modelling of CO line emission from the circumstellar envelopes around a number of carbon stars is performed. By combining radio observations and infrared observations obtained by ISO the circumstellar envelope characteristics are probed over a large radial range. In the radiative transfer analysis the observational data are consistently reproduced assuming a spherically symmetric and smooth wind expanding at a constant velocity. The combined data set gives better determined envelope parameters, and puts constraints on the mass loss history of these carbon stars. The importance of dust in the excitation of CO is addressed using a radiative transfer analysis of the observed continuum emission, and it is found to have only minor effects on the derived line intensities. The analysis of the dust emission also puts further constraints on the mass loss rate history. The stars presented here are not likely to have experienced any drastic long-term mass loss rate modulations, at least less than a factor of about 5, over the past thousands of years. Only three, out of nine, carbon stars were observed long enough by ISO to allow a detection of CO far-infrared rotational lines.
2010
Context. Outflows from asymptotic giant branch (AGB) and red supergiant (RSG) stars inject dust into the interstellar medium. The total rate of dust return provides an important constraint to galactic chemical evolution models. However, this requires detailed radiative transfer (RT) modeling of individual stars, which becomes impractical for large data sets. An alternative approach is to select the best-fit spectral energy distribution (SED) from a grid of dust shell models, allowing for a faster determination of the luminosities and mass-loss rates for entire samples. Aims. We have developed the Grid of RSG and AGB ModelS (GRAMS) to measure the mass-loss return from evolved stars. The models span the range of stellar, dust shell and grain properties relevant to evolved stars. The GRAMS model database will be made available to the scientific community. In this paper we present the carbon-rich AGB model grid and compare our results with photometry and spectra of Large Magellanic Cloud (LMC) carbon stars from the SAGE (Surveying the Agents of Galaxy Evolution) and SAGE-Spec programs. Methods. We generate models for spherically symmetric dust shells using the 2Dust code, with hydrostatic models for the central stars. The model photospheres have effective temperatures between 2600 and 4000 K and luminosities from ∼2000 L ⊙ to ∼40000 L ⊙ . Assuming a constant expansion velocity, we explore five values of the inner radius R in of the dust shell (1.5, 3, 4.5, 7 and 12 R star ). We fix the outer radius at 1000 R in . Based on the results from our previous study, we use amorphous carbon dust mixed with 10% silicon carbide by mass. The grain size distribution follows a power-law and an exponential falloff at large sizes. The models span twenty-six values of 11.3 µm optical depth, ranging from 0.001 to 4. For each model, 2Dust calculates the output SED from 0.2 to 200 µm. Results. Over 12 000 models have dust temperatures below 1800 K. For these, we derive synthetic photometry in optical, nearinfrared and mid-infrared filters for comparison with available data. We find good agreement with magnitudes and colors observed for LMC carbon-rich and extreme AGB star candidates from the SAGE survey, as well as spectroscopically confirmed carbon stars from the SAGE-Spec study. Our models reproduce the IRAC colors of most of the extreme AGB star candidates, consistent with the expectation that a majority of these enshrouded stars have carbon-rich dust. Finally, we fit the SEDs of some well-studied carbon stars and compare the resulting luminosities and mass-loss rates with those from previous studies.
Abundances and kinematics of carbon-enhanced metal-poor stars in the Galactic halo
Astronomy & Astrophysics, 2019
Carbon-enhanced metal-poor (CEMP) stars span a wide range of stellar populations, from bona fide second-generation stars to later-forming stars that provide excellent probes of binary mass transfer and stellar evolution. Here we analyse 11 metal-poor stars (8 of which are new to the literature), and demonstrate that 10 are CEMP stars. Based on high signal-to-noise ratio (S/N) X-shooter spectra, we derive abundances of 20 elements (C, N, O, Na, Mg, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Sr, Y, Ba, La, Ce, Pr, Nd, and Eu). From the high-S/N spectra, we were able to trace the chemical contribution of the rare earth elements (REE) from various possible production sites, finding a preference for metal-poor low-mass asymptotic giant branch (AGB) stars of 1.5 M⊙ in CEMP-s stars, while CEMP-r/s stars may indicate a more massive AGB contribution (2–5 M⊙). A contribution from the r-process – possibly from neutron star–neutron star mergers (NSM) – is also detectable in the REE stellar abundances, especia...
Carbon-Enhanced Metal-Poor Stars in the Inner and Outer Halo Components of the Milky Way
The Astrophysical Journal, 2012
The carbon-enhanced metal-poor (CEMP) stars in the inner-and outer-halo components of the Milky Way are explored, based on accurate determinations of the carbon-to-iron abundance ratios and kinematic quantities for over 30000 calibration stars from the Sloan Digital Sky Survey (SDSS) and its first extension, (SDSS-II). The carbon abundance estimates are based on fits to the CH G-band at 4305 Å; the kinematic and orbital parameters have been evaluated for a subsample of the calibration stars (with distances d < 10 kpc from the Sun), employing previously developed methods. We find that 1%, 6%, and 18% of stars in the metallicity intervals −1.5 < [Fe/H] < −0.5, −2.5 < [Fe/H] < −1.5, and [Fe/H] < −2.5, respectively, exhibit carbon-to-iron abundance ratios ([C/Fe], or "carbonicity") in excess of [C/Fe]= +0.7, our adopted definition for CEMP stars. We examine the carbonicity distribution function (CarDF) as a function of distance from the Galactic plane, |Z|, and demonstrate that its nature changes dramatically with increasing distance. For |Z| < 5 kpc, relatively few CEMP stars are identified (over all metallicities). For distances |Z| > 5 kpc, the CarDF exhibits a strong tail towards high values, up to [C/Fe] > +3.0. For low metallicity stars located above |Z| = 5 kpc we confirm a significant increase in the fraction of CEMP stars with declining metallicity, as indicated by previous work. In the low metallicity regime, for both [Fe/H] < −1.4 and [Fe/H] < −1.8, we find a clear increase in the CEMP star fraction with |Z|, approaching ∼15% in the most distant bins. We have sought to understand whether these behaviors are metallicity driven, due to differences in the mean [Fe/H] of the inner-and outer-halo components, or population driven, due to differences in the fractions of CEMP stars associated with each population at a given metallicity. To accomplish this, we have obtained a statistical separation of the inner-and outerhalo components in velocity space using the Extreme Deconvolution technique, which provides membership probabilities for each star. At low metallicity (−2.5 < [Fe/H] < −2.0), where the dominant component is the outer halo ([Fe/H] peak,out = −2.2), there is evidence for a significant contrast in the CEMP star fraction between the inner-and outer-halo components, such that F CEMPout ∼ 2×F CEMPin . This may continue to even lower metallicities, but the sample is too small to be certain. These results are considered in the context of the formation of the halo system of the Milky Way, and the likely progenitors of the inner-and outer-halo populations.
Carbon enrichment of the evolved stars in the Sagittarius dwarf spheroidal
Monthly Notices of the Royal Astronomical Society, 2012
We present spectra of 1142 colour-selected stars in the direction of the Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy, of which 1058 were taken with VLT/FLAMES multi-object spectrograph and 84 were taken with the SAAO Radcliffe 1.9-m telescope grating spectrograph. Spectroscopic membership is confirmed (at >99 per cent confidence) for 592 stars on the basis of their radial velocity, and spectral types are given. Very slow rotation is marginally detected around the galaxy's major axis. We identify five S stars and 23 carbon stars, of which all but four carbon stars are newly determined and all but one (PQ Sgr) are likely Sgr dSph members. We examine the onset of carbon richness in this metal-poor galaxy in the context of stellar models. We compare the stellar death rate (one star per 1000-1700 yr) with the known planetary nebula dynamical ages and find that the bulk population produce the observed (carbon-rich) planetary nebulae. We compute average lifetimes of S and carbon stars as 60-250 and 130-500 kyr, compared to a total thermal-pulsing asymptotic giant branch lifetime of 530-1330 kyr. We conclude by discussing the return of carbon-rich material to the interstellar medium.