Blue Supergiants as a Tool for Extragalactic Distances - Theoretical Concepts

Because of their enormous intrinsic brightness blue supergiants are ideal stellar objects to be studied spectroscopically as individuals in galaxies far beyond the Local Group. Quantitative spectroscopy by means of efficient multi-object spectrographs attached to 8m-class telescopes and modern NLTE model atmosphere techniques allow us to determine not only intrinsic stellar parameters such as effective temperature, surface gravity, chemical composition and absolute magnitude but also very accurately interstellar reddening and extinction. This is a significant advantage compared to classical distance indicators like Cepheids and RR Lyrae. We describe the spectroscopic diagnostics of blue supergiants and introduce two concepts to determine absolute magnitudes. The first one (Wind Momentum -- Luminosity Relationship) uses the correlation between observed stellar wind momentum and luminosity, whereas the second one (Flux-weighted Gravity -- Luminosity Relationship) relies only on the determination of effective temperature and surface gravity to yield an accurate estimate of absolute magnitude. We discuss the potential of these two methods.Comment: 25 pages, 16 figures; Invited review at the International Workshop on "Stellar Candles for the Extragalactic Distance Scale", held in Concepcion, Chile, December 9-11, 2002, to be published in: "Stellar Candles", Lecture Notes in Physics, Springer-Verla

Non-LTE line-formation for CNO

Accurate atomic data have become available in the recent past due to the demands of astrophysics and fusion research. We report on the impact of such data on non-LTE line-formation calculations for CNO in early-type stars. Considerable improvement is achieved by the derivation of consistent results from practically all available spectroscopic indicators, regardless of ionization stage or spin system, and the uncertainties in the analyses are drastically reduced. Moreover, systematic trends are revealed, e.g. an increase of the NI abundances from previous studies of BA-type supergiants by a factor of two is indicated. The present work promises stringent observational constraints on chemical mixing in the course of massive star evolution. First results on BA-type supergiants in the Galaxy and the Magellanic Clouds are discussed.Comment: 6 pages, 2 figures; to be published in "CNO in the Universe", ASP Conf. Series, eds. C. Charbonnel, D. Schaerer & G. Meyne

The growth history of local M33-mass bulgeless spiral galaxies

NGC7793, NGC300, M33 and NGC2403 are four nearby undisturbed and bulgeless low-mass spiral galaxies with similar morphology and stellar mass. They are ideal laboratories to study disc formation scenarios and stellar mass growth histories. We construct a simple chemical evolution model by assuming that discs grow gradually with continuous metal-free gas infall and metal-enriched gas outflow. By means of the classical $\chi^{2}$ methodology, applied to the model predictions, the best combination of free parameters capable of reproducing the corresponding present-day observations is determined, i.e. the radial dependence of the infall timescale $\tau=0.1r/{R_{\rm d}}+3.4\,{\rm Gyr}$ ($R_{\rm d}$ is the disc scale-length) and the gas outflow efficiency $b_{\rm out}=0.2$. The model results are in excellent agreement with the general predictions of the inside-out growth scenario for the evolution of spiral galaxies. About 80\% of the stellar mass of NGC\,7793 is assembled within the last 8\,Gyr and 40\% within the last 4\,Gyr. By comparing the best-fitting model results of the three other galaxies we obtain similar results, 72\% (NGC300), 66\% (NGC2403) and 79\% (M33) stellar mass were assembled within the past $\sim\rm 8\,Gyr$ (i.e. $z\,=\,1$). These four disc galaxies simultaneously increase their sizes and stellar masses as time goes by and they grow in size at $\sim\,0.30$ times the rate at which they grow in mass. The scale-lengths of these four discs are now 20\% -- 25\% larger than at $z\,=\,1$. Our best-fitting model predicted the stellar mass-metallicity relation and the metallicity gradients, constrained by the observed metallicities from HII-regions emission line analysis, agree well with the observations measured from individual massive red and blue supergiant stars and population synthesis of SDSS galaxies.Comment: 12 pages, 10 figures, accepted for publication in the A&

The influence of galaxy surface brightness on the mass-metallicity relation

We study the effect of surface brightness on the mass-metallicity relation using nearby galaxies whose gas content and metallicity profiles are available. Previous studies using fiber spectra indicated that lower surface brightness galaxies have systematically lower metallicity for their stellar mass, but the results were uncertain because of aperture effect. With stellar masses and surface brightnesses measured at WISE W1 and W2 bands, we re-investigate the surface brightness dependence with spatially-resolved metallicity profiles and find the similar result. We further demonstrate that the systematical difference cannot be explained by the gas content of galaxies. For two galaxies with similar stellar and gas masses, the one with lower surface brightness tends to have lower metallicity. Using chemical evolution models, we investigate the inflow and outflow properties of galaxies of different masses and surface brightnesses. We find that, on average, high mass galaxies have lower inflow and outflow rates relative to star formation rate. On the other hand, lower surface brightness galaxies experience stronger inflow than higher surface brightness galaxies of similar mass. The surface brightness effect is more significant for low mass galaxies. We discuss implications on the different inflow properties between low and high surface brightness galaxies, including star formation efficiency, environment and mass assembly history

The metallicity and distance of Leo A from blue supergiants

We have obtained high-quality spectra of blue supergiant candidates in the dwarf irregular galaxy Leo A with the Low Resolution Imaging Spectrometer at the Keck I telescope. From the quantitative analysis of seven B8-A0 stars we derive a mean metallicity [Z] = -1.35 +/- 0.08, in excellent agreement with the gas-phase chemical abundance. From the stellar parameters and the flux-weighted-luminosity relation (FGLR) we derive a spectroscopic distance modulus m-M = 24.77 +/- 0.11 mag, significantly larger (~0.4 mag) than the value indicated by RR Lyrae and other stellar indicators. We explain the bulk of this discrepancy with blue loop stellar evolution at very low metallicity and show that the combination of metallicity effects and blue loop evolution amounts, in the case of Leo A, to a ~0.35 mag offset of the FGLR to fainter bolometric luminosities. We identify one outlier of low bolometric magnitude as a post-AGB star. Its metallicity is consistent with that of the young population, confirming the slow chemical enrichment of Leo A.Comment: Accepted for publication by The Astrophysical Journal. 15 pages, 12 figures. Updated figure (Fig. 9

The metallicity and distance of NGC 2403 from blue supergiants

We present the first quantitative spectral analysis of blue supergiant stars in the nearby galaxy NGC 2403. Out of a sample of 47 targets observed with the LRIS spectrograph at the Keck I telescope we have extracted 16 B- and A-type supergiants for which we have data of sufficient quality to carry out a comparison with model spectra of evolved massive stars and infer the stellar parameters. The radial metallicity gradient of NGC 2403 that we derive has a slope of -0.14 (+/- 0.05) dex/r_e, and is in accordance with the analysis of H II region oxygen abundances. We present evidence that the stellar metallicities that we obtain in extragalactic systems in general agree with the nebular abundances based on the analysis of the auroral lines, over more than one order of magnitude in metallicity. Adopting the known relation between stellar parameters and intrinsic luminosity we find a distance modulus m-M = 27.38 +/- 0.08 mag. While this can be brought into agreement with Cepheid-based determinations, it is 0.14 mag short of the value measured from the tip of the red giant branch. We update the mass-metallicity relation secured from chemical abundance studies of stars in resolved star-forming galaxies.Comment: 18 pages, 11 figure. Accepted for publication in The Astrophysical Journa