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Register a new userQuantitative Spectroscopy of 24 A supergiants in the Sculptor galaxy NGC 300: Flux weighted gravity luminosity relationship, metallicity and metallicity gradient
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R.-P. Kudritzki
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A quantitative spectral analysis of 24 A supergiants in the Sculptor Group spiral galaxy NGC 300 at a distance of 1.9 Mpc is presented. A new method is introduced to analyze low resolution (~5 AE) spectra, which yields metallicities accurate to 0.2 dex including the uncertainties arising from the errors in Teff (5%) and log g (0.2 dex). For the first time the stellar metallicity gradient based on elements such as titanium and iron in a galaxy beyond the Local Group is investigated. Solar metallicity is measured in the center and 0.3 solar in the outskirts and a logarithmic gradient of -0.08 dex/kpc. An average reddening of E(B-V)~0.12 mag is obtained, however with a large variation from 0.07 to 0.24 mag. We also determine stellar radii, luminosities and masses and discuss the evolutionary status. Finally, the observed relationship between absolute bolometric magnitudes M_{bol} and flux weighted gravities g_{F} = g/Teff^4 is investigated. At high temperatures the strengths of the Balmer lines depends solely on the flux-weighted gravity, which allows a precise direct determination of log g_{F} with an accuracy of 0.05 to 0.1 dex. We find a tight relationship between M_{bol} and log g_{F} in agreement with stellar evolution theory. Combining these new results with previous work on Local Group galaxies we obtain a new flux weighted gravity luminosity relationship (FGLR), which is very well defined and appears to be an excellent alternative tool to determine distances to galaxies.
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The quantitative spectral analysis of low resolution Keck LRIS spectra of blue supergiants in the disk of the giant spiral galaxy M81 is used to determine stellar effective temperatures, gravities, metallicities, luminosites, interstellar reddening and a new distance using the Flux-weighted Gravity--Luminosity Relationship (FGLR). Substantial reddening and extinction is found with E(B-V) ranging between 0.13 to 0.38 mag and an average value of 0.26 mag. The distance modulus obtained after individual reddening corrections is 27.7+/-0.1 mag. The result is discussed with regard to recently measured TRGB and Cepheid distances. The metallicities (based on elements such as iron, titanium, magnesium) are supersolar (~0.2 dex) in the inner disk (R<=5kpc) and slightly subsolar (~ -0.05 dex) in the outer disk (R>10 kpc) with a shallow metallicity gradient of 0.034 dex/kpc. The comparison with published oxygen abundances of planetary nebulae and metallicities determined through fits of HST color-magnitude diagrams indicates a late metal enrichment and a flattening of the abundance gradient over the last 5 Gyrs. This might be the result of gas infall from metal rich satellite galaxies. Combining these M81 metallicities with published blue supergiant abundance studies in the Local Group and the Sculptor Group a galaxy mass metallicity-relationship based solely on stellar spectroscopic studies is presented and compared with recent studies of SDSS star forming galaxies.
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High quality spectra of 90 blue supergiant stars in the Large Magellanic Cloud are analyzed with respect to effective temperature, gravity, metallicity, reddening, extinction and extinction law. An average metallicity, based on Fe and Mg abundances, relative to the Sun of [Z] = -0.35 +/- 0.09 dex is obtained. The reddening distribution peaks at E(B-V) = 0.08 mag, but significantly larger values are also encountered. A wide distribution of the ratio of extinction to reddening is found ranging from Rv = 2 to 6. The results are used to investigate the blue supergiant relationship between flux-weighted gravity, and absolute bolometric magnitude. The existence of a tight relationship, the FGLR, is confirmed. However, in contrast to previous work the observations reveal that the FGLR is divided into two parts with a different slope. For flux-weighted gravities larger than 1.30 dex the slope is similar as found in previous work, but the relationship becomes significantly steeper for smaller values of the flux-weighted gravity. A new calibration of the FGLR for extragalactic distance determinations is provided.
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We present abundances for seven stars in the (extremely) low-metallicity tail of the Sculptor dwarf spheroidal galaxy, from spectra taken with X-shooter on the ESO VLT. Targets were selected from the Ca II triplet (CaT) survey of the Dwarf Abundances and Radial Velocities Team (DART) using the latest calibration. Of the seven extremely metal-poor candidates, five stars are confirmed to be extremely metal-poor (i.e., [Fe/H]<-3 dex), with [Fe/H]=-3.47 +/- 0.07 for our most metal-poor star. All are around or below [Fe/H]=-2.5 dex from the measurement of individual Fe lines. These values are in agreement with the CaT predictions to within error bars. None of the seven stars is found to be carbon-rich. We estimate a 2-13% possibility of this being a pure chance effect, which could indicate a lower fraction of carbon-rich extremely metal-poor stars in Sculptor compared to the Milky Way halo. The [alpha/Fe] ratios show a range from +0.5 to -0.5, a larger variation than seen in Galactic samples although typically consistent within 1-2sigma. One star seems mildly iron-enhanced. Our program stars show no deviations from the Galactic abundance trends in chromium and the heavy elements barium and strontium. Sodium abundances are, however, below the Galactic values for several stars. Overall, we conclude that the CaT lines are a successful metallicity indicator down to the extremely metal-poor regime and that the extremely metal-poor stars in the Sculptor dwarf galaxy are chemically more similar to their Milky Way halo equivalents than the more metal-rich population of stars.
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