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Register a new userStudy of extremely reddened AGB stars in the Galactic bulge
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Added by
Francisco Manuel Jim\\'enez-Esteban
Publication date
2015
fields
Physics
and research's language is
English
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Context. Extremely reddened AGB stars lose mass at high rates of >10^-5 Msun/yr. This is the very last stage of AGB evolution, in which stars in the mass range 2.0--4.0 Msun (for solar metallicity) should have been converted to C stars already. The extremely reddened AGB stars in the Galactic bulge are however predominantly O-rich, implying that they might be either low-mass stars or stars at the upper end of the AGB mass range. Aims. To determine the mass range of the most reddened AGB stars in the Galactic bulge. Methods. Using Virtual Observatory tools, we constructed spectral energy distributions of a sample of 37 evolved stars in the Galactic bulge with extremely red IRAS colours. We fitted DUSTY models to the observational data to infer the bolometric fluxes. Applying individual corrections for interstellar extinction and adopting a common distance, we determined luminosities and mass-loss rates, and inferred the progenitor mass range from comparisons with AGB evolutionary models. Results. The observed spectral energy distributions are consistent with a classification as reddened AGB stars, except for two stars, which are proto-planetary nebula candidates. For the AGB stars, we found luminosities in the range 3000--30,000 Lsun and mass-loss rates 10^-5--3x10^-4 Msun/yr. The corresponding mass range is 1.1--6.0 Msun assuming solar metallicity. Conclusions. Contrary to the predictions of the evolutionary models, the luminosity distribution is continuous, with many O-rich AGB stars in the mass range in which they should have been converted into C stars already. We suspect that bulge AGB stars have higher than solar metallicity and therefore may avoid the conversion to C-rich. The presence of low-mass stars in the sample shows that their termination of the AGB evolution also occurs during a final phase of very high mass-loss rate, leading to optically thick circumstellar shells.
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We obtained high-resolution near-IR spectra of 45 AGB stars located in the Galactic bulge. The aim of the project is to determine key elemental abundances in these stars to help constrain the formation history of the bulge. A further aim is to link the photospheric abundances to the dust species found in the winds of the stars. Here we present a progress report of the analysis of the spectra.
We present a library of 139 near-IR spectra of cool asymptotic giant branch stars that will be useful for comparison with theoretical model atmosphere calculations and for modeling the integrated emission from intermediate-age stellar populations. The source list was selected from the `extremely red region of the INT Photometric H Alpha Survey (IPHAS) colour-colour plane that is overwhelmingly dominated by very late-type stars. The spectral library also includes a large fraction of S-type and carbon stars. We present a number of spectral classification sequences highlighting the various molecular features identified and discuss a number of rare features with uncertain identifications in the literature. With its focus on particularly cool photospheres this catalogue serves as a companion to recent spectroscopic atlases of MK standards in the near-IR. Finally the relationship between IPHAS (r-i) and (r-H Alpha) colours and spectroscopically determined properties is discussed and a strong correlation between (r-H Alpha) colour and the C/O abundance index for S-type and carbon stars is noted. This relation has the potential to separate O-rich, S-type and carbon stars in the Galaxy based on their photometry alone.
An analysis of high-resolution near-infrared spectra of a sample of 45 asymptotic giant branch (AGB) stars towards the Galactic bulge is presented. The sample consists of two subsamples, a larger one in the inner and intermediate bulge, and a smaller one in the outer bulge. The data are analysed with the help of hydrostatic model atmospheres and spectral synthesis. We derive the radial velocity of all stars, and the atmospheric chemical mix ([Fe/H], C/O, $^{12}$C/$^{13}$C, Al, Si, Ti, and Y) where possible. Our ability to model the spectra is mainly limited by the (in)completeness of atomic and molecular line lists, at least for temperatures down to $T_{rm eff}approx3100$ K. We find that the subsample in the inner and intermediate bulge is quite homogeneous, with a slightly sub-solar mean metallicity and only few stars with super-solar metallicity, in agreement with previous studies of non-variable M-type giants in the bulge. All sample stars are oxygen-rich, C/O$<$1.0. The C/O and carbon isotopic ratios suggest that third dredge-up (3DUP) is absent among the sample stars, except for two stars in the outer bulge that are known to contain technetium. These stars are also more metal-poor than the stars in the intermediate or inner bulge. Current stellar masses are determined from linear pulsation models. The masses, metallicities and 3DUP behaviour are compared to AGB evolutionary models. We conclude that these models are partly in conflict with our observations. Furthermore, we conclude that the stars in the inner and intermediate bulge belong to a more metal-rich population that follows bar-like kinematics, whereas the stars in the outer bulge belong to the metal-poor, spheroidal bulge population.
An analysis of the fluorine abundance in Galactic AGB carbon stars (24 N-type, 5 SC-type and 5 J-type) is presented. This study uses the state- of-the-art carbon rich atmosphere models and improved atomic and molecular line lists in the 2.3 {mu}m region. F abundances significantly lower are obtained in comparison to previous study in the literature. The main reason of this difference is due to molecular blends. In the case of carbon stars of SC-type, differences in the model atmospheres are also relevant. The new F enhancements are now in agreement with the most recent theoretical nucleosynthesis models in low- mass AGB stars, solving the long standing problem of F in Galactic AGB stars. Nevertheless, some SC-type carbon stars still show larger F abundances than predicted by stellar models. The possibility that these stars are of larger mass is briefly discussed.
To study the nature of Bulge AGB stars and in particular their circumstellar dust, we have analysed mid-infrared spectra obtained with the ISOCAM CVF spectrometer in three Bulge fields. The ISOCAM 5-16.5 micron CVF spectra were obtained as part of the ISOGAL infrared survey of the inner Galaxy. A classification of the shape of the 10 micron dust feature was made for each case. The spectra of the individual sources were modelled using a radiative transfer model. Different combinations of amorphous silicates and aluminium-oxide dust were used in the modelling. Spectra were obtained for 29 sources of which 26 are likely to be Bulge AGB stars. Our modelling shows that the stars suffer mass loss rates in the range of 10^{-8} - 5 x 10^{-7} Msun / yr, which is at the low end of the mass-loss rates experienced on the Thermally Pulsing AGB. The luminosities range from 1,700 to 7,700 Lsun as expected for a population of AGB stars with Minit of 1.5 - 2Msun. In agreement with the condensation sequence scenario, we find that the dust content is dominated by Al_2O_3 grains in this sample of low mass-loss rate stars.
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