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Register a new userRubidium abundances of galactic disk stars
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Yoichi Takeda
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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.
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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.
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.
A Rb deficiency by a factor two with respect to the Sun has been found in M dwarfs of solar metallicity. This deficiency is difficult to understand from both the observational and nucleosynthesis point of views. To test the reliability of this Rb deficiency, we study the Rb and Zr abundances in a sample of KM-type giant stars in a similar metallicity range extracted from the AMBRE Project. We derive Rb and Zr abundances in 54 giant stars with metallicity close to solar by spectral synthesis in LTE and NLTE. The impact of the Zeeman broadening in the RbI line is also studied. The LTE analysis results in a Rb deficiency in giant stars smaller than that obtained in M dwarfs, but the NLTE [Rb/Fe] ratios are very close to solar in the full metallicity range. This contrasts with the figure found in M dwarfs. We investigate the effect of gravitational settling and magnetic activity as possible causes of the Rb deficiency found in M dwarfs. While, the former phenomenon has a negligible impact on the surface Rb abundance, the existence of an average magnetic field with intensity typical of that observed in M dwarfs may result in systematic Rb abundance underestimations if the Zeeman broadening is not considered in the spectral synthesis. The new [Rb,Zr/Fe] vs. [Fe/H] relationships can be explained when the Rb production by rotating massive stars and low-and-intermediate mass stars are considered, without the need of any deviation from the standard s-process nucleosynthesis in AGB stars as previously suggested.
Luminous Galactic OH/IR stars have been identified as massive (>4-5 M_s) AGB stars experiencing HBB and Li production. Their Rb abundances and [Rb/Zr] ratios derived from hydrostatic model atmospheres, are significantly higher than predictions from AGB nucleosynthesis models, posing a problem to our understanding of AGB evolution and nucleosynthesis. We report new Rb and Zr abundances in the full sample of massive Galactic AGB stars, previously studied with hydrostatic models, by using more realistic extended model atmospheres. We use a modified version of the spectral synthesis code Turbospectrum and consider the presence of a circumstellar envelope and radial wind. The Rb and Zr abundances are determined from the 7800 A Rb I resonant line and the 6474 A ZrO bandhead, respectively, and we explore the sensitivity of the derived abundances to variations of the stellar (Teff) and wind (M_loss, beta and vexp) parameters in the extended models. The Rb and Zr abundances derived from the best spectral fits are compared with the most recent AGB nucleosynthesis theoretical models. The new Rb abundances are much lower (even 1-2 dex) than those derived with the hydrostatic models, while the Zr abundances are similar. The Rb I line profile and Rb abundance are very sensitive to the M_loss rate but much less sensitive to variations of the wind velocity-law and the vexp(OH). We confirm the earlier preliminary results based on a smaller sample of massive O-rich AGB stars, that the use of extended atmosphere models can solve the discrepancy between the AGB nucleosynthesis theoretical models and the observations of Galactic massive AGB stars. The Rb abundances, however, are still strongly dependent of the M_loss, which is unknown in these AGB stars. Accurate M_loss rates in these massive Galactic AGB stars are needed in order to break the models degeneracy and get reliable Rb abundances in these stars.
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