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Non-LTE effects on the lead and thorium abundance determinations for cool stars
Context. Knowing accurate lead abundances of metal-poor stars providesconstraints on the Pb production mechanisms in the early Galaxy.Accurately deriving thorium abundances permits a nucleo-chronometric agedetermination of the star. Aims: We aim to improve thecalculation of the Pb i and Th ii lines in stellar atmospheres based onnon-local thermodynamic equilibrium (non-LTE) line formation, and toevaluate the influence of departures from LTE on Pb and Th abundancedeterminations for a range of stellar parameters by varying themetallicity from the solar value down to [Fe/H] = -3. Methods: Wepresent comprehensive model atoms for Pb i and Th ii and describecalculations of the Pb i energy levels and oscillator strengths. Results: The main non-LTE mechanism for Pb i is the ultravioletoverionization. We find that non-LTE leads to systematically depletedtotal absorption in the Pb i lines and accordingly, positive abundancecorrections. The departures from LTE increase with decreasingmetallicity. Using the semi-empirical model atmosphere HM74, wedetermine the lead non-LTE abundance for the Sun to be log?Pb, &sun; = 2.09, in agreement with the meteoriticlead abundance. We revised the Pb and Eu abundances of the two stronglyr-process enhanced stars CS 31082-001 and HE1523-0901 and the metal-poor stellar sample. Our new resultsprovide strong evidence of universal Pb-to-Eu relative r-process yieldsduring the course of Galactic evolution. The stars in the metallicityrange -2.3 < [Fe/H] < -1.4 have Pb/Eu abundance ratios that are,on average, 0.51 dex higher than those of strongly r-process enhancedstars. We conclude that the s-process production of lead started asearly as the time when Galactic metallicity had reached [Fe/H] = -2.3.The average Pb/Eu abundance ratio of the mildly metal-poor stars, with-1.4 ? [Fe/H] ? -0.59, is very close to the corresponding SolarSystem value, in line with the theoretical predictions that AGB starswith [Fe/H] ? -1 provided the largest contribution to the solarabundance of s-nuclei of lead. The departures from LTE for Th ii arecaused by the pumping transitions from the levels with Eexc< 1 eV. Non-LTE leads to weakened Th ii lines and positive abundancecorrections. Overall, the abundance correction does not exceed 0.2 dexwhen collisions with H i atoms are taken into account in statisticalequilibrium calculations.
| Element abundances in the stars of the MILES spectral library: the Mg/Fe ratio
We have obtained [Mg/Fe] measurements for 76.3 per cent of the stars inthe Mid-resolution Isaac Newton Telescope Library of Empirical Spectra(MILES) spectral library used for understanding stellar atmospheres andstellar populations in galaxies and star clusters. These abundanceratios were obtained through (1) a compilation of values from theliterature using abundances from high-resolution (HR) spectroscopicstudies and (2) a robust spectroscopic analysis using the MILESmid-resolution (MR) optical spectra. All the [Mg/Fe] values werecarefully calibrated to a single uniform scale, by using an extensivecontrol sample with results from HR spectra. The small averageuncertainties in the calibrated [Mg/Fe] values [respectively 0.09 and0.12 dex with methods (1) and (2)] and the good coverage of the starswith [Mg/Fe] over stellar atmospheric parameter space of the librarywill permit the building of new simple stellar populations (SSPs) withempirical ?-enhancements. These will be available for a range of[Mg/Fe], including both sub-solar and super-solar values, and forseveral metallicities and ages. These models will open up new prospectsfor testing and applications of evolutionary stellar populationsynthesis.
| Forming the first planetary systems: debris around Galactic thick disc stars
The thick disc contains stars formed within the first Gyr of Galactichistory, and little is known about their planetary systems. The SpitzerMIPS instrument was used to search 11 of the closest of these oldlow-metal stars for circumstellar debris, as a signpost that bodies atleast as large as planetesimals were formed. A total of 22 thick discstars has now been observed, after including archival data, but dust isnot found in any of the systems. The data rule out a high incidence ofdebris among star systems from early in the Galaxy's formation. However,some stars of this very old population do host giant planets, atpossibly more than the general incidence among low-metal Sun-like stars.As the Solar system contains gas giants but little cometary dust, thethick disc could host analogue systems that formed many Gyr before theSun.
| A Keck HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. II. On the Frequency of Giant Planets in the Metal-Poor Regime
We present an analysis of three years of precision radial velocity (RV)measurements of 160 metal-poor stars observed with HIRES on the Keck 1telescope. We report on variability and long-term velocity trends foreach star in our sample. We identify several long-term, low-amplitude RVvariables worthy of followup with direct imaging techniques. We placelower limits on the detectable companion mass as a function of orbitalperiod. Our survey would have detected, with a 99.5% confidence level,over 95% of all companions on low-eccentricity orbits with velocitysemiamplitude K gsim 100 m s–1, orMp sin i gsim 3.0 M J(P/yr)(1/3), fororbital periods P lsim 3 yr. None of the stars in our sampleexhibits RV variations compatible with the presence of Jovian planetswith periods shorter than the survey duration. The resulting averagefrequency of gas giants orbiting metal-poor dwarfs with–2.0lsim[Fe/H]lsim–0.6 is fp < 0.67% (at the1σ confidence level). We examine the implications of this nullresult in the context of the observed correlation between the rate ofoccurrence of giant planets and the metallicity of their main-sequencesolar-type stellar hosts. By combining our data set with the Fischer& Valenti (2005) uniform sample, we confirm that the likelihood of astar to harbor a planet more massive than Jupiter within 2 AU is asteeply rising function of the host's metallicity. However, the data forstars with –1.0lsim[Fe/H]lsim0.0 are compatible, in a statisticalsense, with a constant occurrence rate fp sime 1%. Ourresults can usefully inform theoretical studies of the process ofgiant-planet formation across two orders of magnitude in metallicity.
| Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
While the strong anticorrelation between chromospheric activity and agehas led to the common use of the Ca II H and K emission index(R'HK=LHK/Lbol) as anempirical age estimator for solar-type dwarfs, existing activity-agerelations produce implausible ages at both high and low activity levels.We have compiled R'HK data from the literature foryoung stellar clusters, richly populating for the first time the youngend of the activity-age relation. Combining the cluster activity datawith modern cluster age estimates and analyzing the color dependence ofthe chromospheric activity age index, we derive an improved activity-agecalibration for F7-K2 dwarfs (0.5 mag
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