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Deep SDSS optical spectroscopy of distant halo stars
摘要： Aims. We analyze a sample of tens of thousands of spectra of halo turno? stars, obtained with the optical spectrographs of the Sloan Digital Sky Survey (SDSS), to characterize the stellar halo population “in situ” out to a distance of a few tens of kpc from the Sun. In this paper we describe the derivation of atmospheric parameters. We also derive the overall stellar metallicity distribution based on F-type stars observed as ?ux calibrators for the Baryonic Oscillations Spectroscopic Survey (BOSS). Methods. Our analysis is based on an automated method that determines the set of parameters of a model atmosphere that reproduces each observed spectrum best. We used an optimization algorithm and evaluate model ?uxes by means of interpolation in a precomputed grid. In our analysis, we account for the spectrograph’s varying resolution as a function of ?ber and wavelength. Our results for early SDSS (pre-BOSS upgrade) data compare well with those from the SEGUE Stellar Parameter Pipeline (SSPP), except for stars with log g (cgs units) lower than 2.5. Results. An analysis of stars in the globular cluster M 13 reveals a dependence of the inferred metallicity on surface gravity for stars with log g < 2.5, con?rming the systematics identi?ed in the comparison with the SSPP. We ?nd that our metallicity estimates are signi?cantly more precise than the SSPP results. We also ?nd excellent agreement with several independent analyses. We show that the SDSS color criteria for selecting F-type halo turno? stars as ?ux calibrators e?ciently excludes stars with high metallicities, but does not signi?cantly distort the shape of the metallicity distribution at low metallicity. We obtain a halo metallicity distribution that is narrower and more asymmetric than in previous studies. The lowest gravity stars in our sample, at tens of kpc from the Sun, indicate a shift of the metallicity distribution to lower abundances, consistent with what is expected from a dual halo system in the Milky Way.
关键词： techniques: spectroscopic,stars: atmospheres,methods: observational,Galaxy: halo,stars: Population II,stars: fundamental parameters
更新于2025-09-23 15:19:57
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Benchmark stars for <i>Gaia</i> Fundamental properties of the Population II star HD?140283 from interferometric, spectroscopic, and photometric data
摘要： Metal-poor halo stars are important astrophysical laboratories that allow us to unravel details about many aspects of astrophysics, including the chemical conditions at the formation of our Galaxy, understanding the processes of diffusion in stellar interiors, and determining precise effective temperatures and calibration of colour-effective temperature relations. To address any of these issues the fundamental properties of the stars must first be determined. HD 140283 is the closest and brightest metal-poor Population II halo star (distance = 58 pc and V = 7.21), an ideal target that allows us to approach these questions, and one of a list of 34 benchmark stars defined for Gaia astrophysical parameter calibration. In the framework of characterizing these benchmark stars, we determined the fundamental properties of HD 140283 (radius, mass, age, and effective temperature) by obtaining new interferometric and spectroscopic measurements and combining them with photometry from the literature. The interferometric measurements were obtained using the visible interferometer VEGA on the CHARA array and we determined a 1D limb-darkened angular diameter of θ1D = 0.353 ± 0.013 milliarcsec. Using photometry from the literature we derived the bolometric flux in two ways: a zero reddening solution (AV = 0.0 mag) of Fbol of 3.890 ± 0.066 × 10?8 erg s?1 cm?2, and a maximum of AV = 0.1 mag solution of 4.220 ± 0.067 × 10?8 erg s?1 cm?2. The interferometric Teff is thus between 5534 ± 103 K and 5647 ± 105 K and its radius is R = 2.21 ± 0.08 R⊙. Spectroscopic measurements of HD 140283 were obtained using HARPS, NARVAL, and UVES and a 1D LTE analysis of Hα line wings yielded Teff spec = 5626 ± 75 K. Using fine-tuned stellar models including diffusion of elements we then determined the mass M and age t of HD 140283. Once the metallicity has been fixed, the age of the star depends on M, initial helium abundance Yi, and mixing-length parameter α, only two of which are independent. We derive simple equations to estimate one from the other two. We need to adjust α to much lower values than the solar one (～2) in order to fit the observations, and if AV = 0.0 mag then 0.5 ≤ α ≤ 1. We give an equation to estimate t from M, Yi (α), and AV. Establishing a reference α = 1.00 and adopting Yi = 0.245 we derive a mass and age of HD 140283: M = 0.780 ± 0.010 M⊙ and t = 13.7 ± 0.7 Gyr (AV = 0.0 mag), or M = 0.805 ± 0.010 M⊙ and t = 12.2 ± 0.6 Gyr (AV = 0.1 mag). Our stellar models yield an initial (interior) metal-hydrogen mass fraction of [Z/X]i = ?1.70 and log g = 3.65 ± 0.03. Theoretical advances allowing us to impose the mixing-length parameter would greatly improve the redundancy between M, Yi, and age, while from an observational point of view, accurate determinations of extinction along with asteroseismic observations would provide critical information allowing us to overcome the current limitations in our results.
关键词： stars: fundamental parameters,techniques: spectroscopic,surveys,stars: individual: HD 140283,techniques: interferometric,stars: Population II
更新于2025-09-19 17:15:36