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- Allende Prieto, C., et al.
(författare)
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Deep SDSS optical spectroscopy of distant halo stars I. Atmospheric parameters and stellar metallicity distribution
- 2014
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 568
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We analyze a sample of tens of thousands of spectra of halo turnoff 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 flux calibrators for the Baryonic Oscillations Spectroscopic Survey (ROSS). Methods. Our analysis is based on an automated method that determines the set of parameters of a model atmosphere that reproduces each obserxrci spectrum best. We used an optimization algorithm and evaluate model fluxes by means of inter-polation in a precomputed grid, In our analysis, we account for the spectrograph's varying resolution as a function of fiber 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, confirming the systematics identified in the comparison with the SSPP. We find that our rnetallicity estimates are, significantly more precise than the SSPP results. We also find excellent agreement with several independent analyses. We show that the SDSS color criteria for selecting F-type halo turnoff stars as flux calibrators efficiently excludes stars with high metallieities, but does not significantly distort the shape of the metallicity distribution at low rnetallicity. We obtain a halo metathcity 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.
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| 3. |
- Bensby, Thomas, et al.
(författare)
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OGLE-2009-BLG-076S: THE MOST METAL-POOR DWARF STAR IN THE GALACTIC BULGE
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X. ; 699:2, s. 174-177
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Tidskriftsartikel (refereegranskat)abstract
- Measurements based on a large number of red giant stars suggest a broad metallicity distribution function (MDF) for the Galactic bulge, centered on [Fe/H] approximate to -0.1. However, recently, a new opportunity emerged to utilize temporary flux amplification (by factors of similar to 100 or more) of faint dwarf stars in the Bulge which are gravitationally lensed, making them observable with high-resolution spectrographs during a short observational window. Surprisingly, of the first six stars measured, five have [Fe/H] > +0.30, suggesting a highly skewed MDF, inconsistent with observations of giant stars. Here we present a detailed elemental abundance analysis of OGLE-2009-BLG-076S, based on a high-resolution spectrum obtained with the UVES spectrograph at the ESO Very Large Telescope. Our results indicate it is the most metal-poor dwarf star in the Bulge yet observed, with [Fe/H] = -0.76. Our results argue against a strong selection effect disfavoring metal-poor microlensed stars. It is possible that small number statistics is responsible for the giant/dwarf Bulge MDF discrepancy. Should this discrepancy survive when larger numbers of Bulge dwarf stars (soon to be available) are analyzed, it may require modification of our understanding of either Bulge formation models, or the behavior of metal-rich giant stars.
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| 4. |
- Berger, E., et al.
(författare)
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An Intermediate Luminosity Transient in NGC 300 : The Eruption of a Dust-Enshrouded Massive Star
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 699, s. 1850-1865
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Tidskriftsartikel (refereegranskat)abstract
- We present multi-epoch high-resolution optical spectroscopy, UV/radio/X-ray imaging, and archival Hubble and Spitzer observations of an intermediate luminosity optical transient recently discovered in the nearby galaxy NGC 300. We find that the transient (NGC 300 OT2008-1) has a peak absolute magnitude of M bol ≈ -11.8 mag, intermediate between novae and supernovae, and similar to the recent events M85 OT2006-1 and SN 2008S. Our high-resolution spectra, the first for this event, are dominated by intermediate velocity (~200-1000 km s-1) hydrogen Balmer lines and Ca II emission and absorption lines that point to a complex circumstellar environment, reminiscent of the yellow hypergiant IRC+10420. In particular, we detect asymmetric Ca II H&K absorption with a broad red wing extending to ~103 km s-1, indicative of gas inflow at high velocity (possibly the wind of a massive binary companion). The low luminosity, intermediate velocities, and overall similarity to a known eruptive star indicate that the event did not result in a complete disruption of the progenitor. We identify the progenitor in archival Spitzer observations, with deep upper limits from Hubble data. The spectral energy distribution points to a dust-enshrouded star with a luminosity of about 6 × 104 L sun, indicative of a ~10-20 M sun progenitor (or binary system). This conclusion is in good agreement with our interpretation of the outburst and circumstellar properties. The lack of significant extinction in the transient spectrum indicates that the dust surrounding the progenitor was cleared by the outburst. We thus predict that the progenitor should be eventually visible with Hubble if the transient event marks an evolutionary transition to a dust-free state, or with Spitzer if the event marks a cyclical process of dust formation.
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| 7. |
- Simmerer, Jennifer, et al.
(författare)
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A study of the metal-rich, thick disk globular cluster NGC5927, A stellar inventory
- 2013
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 556
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Tidskriftsartikel (refereegranskat)abstract
- We present a list of 72 radial velocity member stars in the metal-rich globular cluster NGC5927. The radial velocities are based on multiepoch, multifibre spectra. We identify 46 RGB/HB stars and 26 turn-off stars that are radial velocity members in the cluster. This cluster is situated quite close to the disk and hence fore-and/or background contamination, especially in the outskirts of the cluster, can be quite severe. Fortunately, the cluster has a radial velocity (we determine it to v(r) = -104.03 +/- 5.03 km s(-1)) that sets it clearly apart from the bulk velocities of the surrounding, background, and foreground stellar populations. Hence, our identification of members is clean and we can quantify a 50% contamination when stars in the outer part of the cluster are selected solely based on position in the color-magnitude diagram as opposed to selections based on radial velocities.
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