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Register a new userBoron Abundances in the Galactic Disk
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Katia Cunha
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When compared to lithium and beryllium, the absence of boron lines in the optical results in a relatively small data set of boron abundances measured in Galactic stars to date. In this paper we discuss boron abundances published in the literature and focus on the evolution of boron in the Galaxy as measured from pristine boron abundances in cool stars as well as early-type stars in the Galactic disk. The trend of B with Fe obtained from cool F-G dwarfs in the disk is found to have a slope of 0.87 +/- 0.08 (in a log-log plot). This slope is similar to the slope of B with Fe found for the metal poor halo stars and there seems to be a smooth connection between the halo and disk in the chemical evolution of boron. The disk trend of boron with oxygen has a steeper slope of ~1.5. This slope suggests an intermediate behavior between primary and secondary production of boron with respect to oxygen. The slope derived for oxygen is consistent with the slope obtained for Fe provided that [O/Fe] increases as [Fe/H] decreases, as observed in the disk.
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The chemical evolution of fluorine is investigated in a sample of Milky Way red giantstars that span a significant range in metallicity from [Fe/H] $sim$ -1.3 to 0.0 dex. Fluorine abundances are derived from vibration-rotation lines of HF in high-resolution infraredspectra near $lambda$ 2.335 $mu$m. The red giants are members of the thin and thick disk / halo,with two stars being likely members of the outer disk Monoceros overdensity. At lowermetallicities, with [Fe/H]<-0.4 to -0.5, the abundance of F varies as a primary element with respect to the Fe abundance, with a constant subsolar value of [F/Fe] $sim$ -0.3 to -0.4 dex. At larger metallicities, however, [F/Fe] increases rapidly with [Fe/H] anddisplays a near-secondary behavior with respect to Fe. Comparisons with various models of chemical evolution suggest that in the low-metallicity regime (dominated hereby thick disk stars), a primary evolution of $^{19}$F with Fe, with a subsolar [F/Fe] valuethat roughly matches the observed plateau can be reproduced by a model incorporatingneutrino nucleosynthesis in the aftermath of the core collapse in supernovae of type II (SN II). A primary behavior for [F/Fe] at low metallicity is also observed for a model including rapid rotating low-metallicity massive stars but this overproduces [F/Fe] atlow metallicity. The thick disk red giants in our sample span a large range of galactocentric distance (Rg $sim$ 6--13.7 kpc), yet display a $sim$constant value of [F/Fe], indicating a very flat gradient (with a slope of 0.02 $pm$ 0.03 dex/kpc) of this elemental ratio over asignificant portion of the Galaxy having|Z|>300 pc away from the Galaxy mid-plane.
Spectroscopic determinations of Rubidium abundances were conducted by applying the spectrum fitting method to the Rb I 7800 line for an extensive sample of ~500 late-type dwarfs as well as giants (including Hyades cluster stars) belonging to the galactic disk population, with an aim of establishing the behaviour of [Rb/Fe] ratio for disk stars in the metallicity range of -0.6<[Fe/H]<+0.3. An inspection of the resulting Rb abundances for Hyades dwarfs revealed that they show a systematic Teff-dependent trend at >5500K; this means that the results for mid-G to F stars (including the Sun) are not reliable (i.e., more or less overestimated), which might be due to some imperfect treatment of surface convection in classical model atmospheres. As such, it was decided to confine only to late-G and K stars at Teff<5500K and adopt the solar-system (meteoritic) value as the reference Rb abundance. The [Rb/Fe] vs.[Fe/H] relations derived for field dwarfs and giants turned out to be consistent with each other, showing a gradual increase of [Rb/Fe] with a decrease in [Fe/H] (with d[Rb/Fe]/d[Fe/H] gradient of ~-0.4 around the solar metallicity), which is favourably compared with the theoretical prediction of chemical evolution models. Accordingly, this study could not confirm the anomalous behaviour of [Rb/Fe] ratio (tending to be subsolar but steeply increasing toward supersolar metallicity) recently reported for M dwarf stars of -0.3<[Fe/H]<+0.3.
Context. The measurement of $alpha$-elements abundances provides a powerful tool to put constraints on chemical evolution and star formation history of galaxies. The majority of studies on the $alpha$-element Sulfur (S) are focused on local stars, making S behavior in other environments an astronomical topic yet to be analyzed. Aims. The investigation of S in the Galactic bulge has only recently been considered for the first time. This work aims to improve our knowledge on S behavior in this component of the Milky Way. Methods. We present S abundances of 74 dwarf and sub-giant stars in the Galactic bulge, 21 and 30 F and G thick and thin disk stars. We performed local thermodynamic equilibrium analysis and applied corrections for non-LTE on high resolution and high signal-to-noise UVES spectra. S abundances were derived from multiplets 1, 6 and 8 in the metallicity range $-2<$[Fe/H]$<$0.6, by spectrosynthesis or line equivalent widths. Results. We confirm that S behaves like an $alpha$-element within the Galactic bulge. In the [S/Fe] versus [Fe/H] diagram, S presents a plateau at low metallicity followed by a decreasing of [S/Fe] with the increasing of [Fe/H], until reaching [S/Fe]$sim0$ at super-solar metallicity. We found that the Galactic bulge is S-rich with respect to both the thick and thin disks at $-1<$[Fe/H]$<0.3$, supporting a more rapid formation and chemical evolution of the Galactic bulge than the disk.
An extensive study on the potassium abundances of late-type stars was carried out by applying the non-LTE spectrum-fitting analysis to the K I resonance line at 7698.96A to a large sample of 160 FGK dwarfs and 328 late-G /early-K giants (including 89 giants in the Kepler field with seismologically known ages) belonging to the disk population (-1 < [Fe/H] < 0.5), which may provide important observational constraint on the nucleosynthesis history of K in the galactic disk. Special attention was paid to clarifying the observed behaviors of [K/Fe] in terms of [Fe/H] along with stellar age, and to checking whether giants and dwarfs yield consistent results with each other. The following results were obtained. (1) A slightly increasing tendency of [K/Fe] with a decrease in [Fe/H] (d[K/Fe]/d[Fe/H] ~ -0.1 to -0.15; a shallower slope than reported by previous studies) was confirmed for FGK dwarfs, though thick-disk stars tend to show larger [K/Fe] deviating from this gradient. (2) Almost similar characteristics was observed also for apparently bright field giants locating in the solar neighborhood (such as like dwarfs). (3) However, the [K/Fe] vs. [Fe/H] relation for more distant {it Kepler} giants shows larger scatter and is systematically higher (by <~0.1dex) than that of dwarfs, implying that chemical evolution of K is rather diversified depending on the position in the Galaxy. (4) Regarding the age-dependence, a marginal trend of increasing [K/Fe] with age is recognized for dwarfs, while any systematic tendency is not observed for Kepler giants. These consequences may suggest that evolution of [K/Fe] with time in the galactic disk does exist but proceeded more gradually than previously thought, and its condition is appreciably location-dependent.
The aim of this study is to analyse and determine elemental abundances for a large sample of distant B stars in the outer Galactic disk in order to constrain the chemical distribution of the Galactic disk and models of chemical evolution of the Galaxy. Here, we present preliminary results on a few stars along with the adopted methodology based on securing simultaneous O and Si ionization equilibria with consistent NLTE model atmospheres.
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