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Register a new userGalactic chemical evolution of Lithium: interplay between stellar sources
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In this paper we study the evolution of 7Li in the Galaxy considering the contributions of various stellar sources: type II supernovae, novae, red giant stars, and asymptotic giant branch (AGB) stars. We present new results for the production of 7Li in AGB stars via the hot bottom burning process, based on stellar evolutionary models by Frost (1997). In the light of recent observations of dense circumstellar shells around evolved stars in the Galaxy and in the Magellanic Clouds, we also consider the impact of a very high mass-loss rate episode (superwind) before the evolution off the AGB phase on the 7Li enrichment in the interstellar medium. We compare the Galactic evolution of 7Li obtained with these new 7Li yields (complemented with a critical re-analysis of the role of supernovae, novae and giant stars) with a selected compilation of spectroscopic observations including halo and disk field stars as well as young stellar clusters. We conclude that even allowing for the large uncertainties in the theoretical calculation of mass-loss rates at the end of the AGB phase, the superwind phase has a significant effect on the 7Li enrichment of the Galaxy.
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We present a survey of lithium abundances in 185 main- sequence field stars with Teff between 5600 and 6600 K and [Fe/H] from -1.4 to +0.2 based on high-resolution spectra of 130 stars and a reanalysis of data from Lambert et al. (1991). The survey takes advantage of improved ways of determining effective temperature, metallicity, mass and age, offering an opportunity to investigate the behaviour of Li as a function of these parameters. An interesting result is the presence of a large gap in the Li-Teff plane, which distinguishes `Hyades-like, Li-dip stars from other stars. These Li-dip stars have a well-defined mass, which decreases with metallicity. Stars above the gap, when divided into four metallicity groups, may show a correlation between Li abundance and stellar mass, but with a large dispersion that cannot be explained by observational errors or differences in metallicity and age, which ranges from 1.5 to 15 Gyr. This suggests that Li depletion occurs early in stellar life and that other parameters, e.g. initial rotation velocity and/or the rate of angular momentum loss, affect the degree of depletion. A comparison of the distribution of stars in the Li-[Fe/H] plane with evolutionary models of Romano et al. (1999) suggests that novae are a major source for the Li production in the Galactic disk.
Lithium abundances are presented for 91 dwarf and subgiant stars in the Galactic bulge. The analysis is based on line synthesis of the 7Li line at 6707 {AA} in high-resolution spectra obtained during gravitational microlensing events, when the brightnesses of the targets were highly magnified. Our main finding is that the bulge stars at sub-solar metallicities, and that are older than about eight billion years, does not show any sign of Li production, that is, the Li trend with metallicity is flat (or even slightly declining). This indicates that no lithium was produced during the first few billion years in the history of the bulge. This finding is essentially identical to what is seen for the (old) thick disk stars in the Solar neighbourhood, and adds another piece of evidence for a tight connection between the metal-poor bulge and the Galactic thick disk. For the bulge stars younger than about eight billion years, the sample contains a group of stars at very high metallicities at [Fe/H]~+0.4 that have lithium abundances in the range A(Li)=2.6-2.8. In the Solar neighbourhood the lithium abundances have been found to peak at a A(Li)~3.3 at [Fe/H]~ +0.1 and then decrease by 0.4-0.5 dex when reaching [Fe/H]~+0.4. The few bulge stars that we have at these metallicities, seem to support this declining A(Li) trend. This could indeed support the recent claim that the low A(Li) abundances at the highest metallicities seen in the Solar neighbourhood could be due to stars from the inner disk, or the bulge region, that have migrated to the Solar neighbourhood.
We used a one-zone chemical evolution model to address the question of how many masses and metallicities are required in grids of massive stellar models in order to ensure reliable galactic chemical evolution predictions. We used a set of yields that includes seven masses between 13 and 30 Msun, 15 metallicities between 0 and 0.03 in mass fraction, and two different remnant mass prescriptions. We ran several simulations where we sampled subsets of stellar models to explore the impact of different grid resolutions. Stellar yields from low- and intermediate-mass stars and from Type Ia supernovae have been included in our simulations, but with a fixed grid resolution. We compared our results with the stellar abundances observed in the Milky Way for O, Na, Mg, Si, Ca, Ti, and Mn. Our results suggest that the range of metallicity considered is more important than the number of metallicities within that range, which only affects our numerical predictions by about 0.1 dex. We found that our predictions at [Fe/H] < -2 are very sensitive to the metallicity range and the mass sampling used for the lowest metallicity included in the set of yields. Variations between results can be as high as 0.8 dex, for any remnant mass prescription. At higher [Fe/H], we found that the required number of masses depends on the element of interest and on the remnant mass prescription. With a monotonic remnant mass prescription where every model explodes as a core-collapse supernova, the mass resolution induces variations of 0.2 dex on average. But with a remnant mass prescription that includes islands of non-explodability, the mass resolution can cause variations of about 0.2 to 0.7 dex depending on the choice of metallicity range. With such a prescription, explosive or non-explosive models can be missed if not enough masses are selected, resulting in over- or under-estimations of the mass ejected by massive stars.
Aims: We aim at measuring mass-loss rates and the luminosities of a statistically large sample of Galactic bulge stars at several galactocentric radii. The sensitivity of previous infrared surveys of the bulge has been rather limited, thus fundamental questions for late stellar evolution, such as the stage at which substantial mass-loss begins on the red giant branch and its dependence on fundamental stellar properties, remain unanswered. We aim at providing evidence and answers to these questions. Methods: To this end, we observed seven 15 times 15 arcmin^2 fields in the nuclear bulge and its vicinity with unprecedented sensitivity using the IRAC and MIPS imaging instruments on-board the Spitzer Space Telescope. In each of the fields, tens of thousands of point sources were detected. Results: In the first paper based on this data set, we present the observations, data reduction, the final catalogue of sources, and a detailed comparison to previous mid-IR surveys of the Galactic bulge, as well as to theoretical isochrones. We find in general good agreement with other surveys and the isochrones, supporting the high quality of our catalogue.
The early science results from the new generation of high-resolution stellar spectroscopic surveys, such as GALAH and the Gaia-ESO survey, will represent major milestones in the quest to chemically tag the Galaxy. Yet this technique to reconstruct dispersed coeval stellar groups has remained largely untested until recently. We build on previous work that developed an empirical chemical tagging probability function, which describes the likelihood that two field stars are conatal, that is, they were formed in the same cluster environment. In this work we perform the first ever blind chemical tagging experiment, i.e., tagging stars with no known or otherwise discernable associations, on a sample of 714 disc field stars with a number of high quality high resolution homogeneous metal abundance measurements. We present evidence that chemical tagging of field stars does identify coeval groups of stars, yet these groups may not represent distinct formation sites, e.g. as in dissolved open clusters, as previously thought. Our results point to several important conclusions, among them that group finding will be limited strictly to chemical abundance space, e.g. stellar ages, kinematics, colors, temperature and surface gravity do not enhance the detectability of groups. We also demonstrate that in addition to its role in probing the chemical enrichment and kinematic history of the Galactic disc, chemical tagging represents a powerful new stellar age determination technique.
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