Abstract
Blue Supergiants (BSGs) are the brightest stars in the universe at visual light with absolute magnitudes up to M V =−10 mag. They are ideal stellar objects for the determination of extragalactic distances, in particular, because the perennial uncertainties troubling most of the other stellar distance indicators, interstellar extinction and metallicity, do not affect them. The quantitative spectral analysis of low resolution spectra of individual BSGs provides accurate stellar parameters and chemical composition, which are then used to determine accurate reddening and extinction from photometry for each individual object. Accurate distances can be determined from stellar gravities and effective temperatures using the “Flux Weighted Gravity–Luminosity Relationship (FGLR)”.Most recent results of the quantitative spectral analysis of BSGs in galaxies within and beyond the Local Group based on medium and low resolution spectra obtained with the ESO VLT and the Keck telescopes on Mauna Kea are presented and distances obtained with the FGLR-method are discussed together with the effects of patchy extinction and abundance gradients in galaxies. BSG metallicities and metallicity gradients are compared with results from strong-line H ii region studies and the consequences for the empirical calibration of the metallicity dependence of the Cepheid period–luminosity relationship are pointed out. The perspectives of future work are discussed, the use of the giant ground-based telescopes of the next generation such as the TMT on Mauna Kea and the E-ELT and the tremendous value of the GAIA mission to allow for the ultimate calibration of the FGLR using galactic BSGs.
Highlights
After the detection of the accelerated expansion of the universe the physical explanation of dark energy has become the major challenge of astronomy and physics
One way to constrain the physics behind dark energy is to measure the equation-of-state parameter w = p/(ρc2)
In order to address these concerns, an independent and complementary method is desirable, which can overcome the problems of interstellar extinction and variations of chemical composition. We introduce such a method, the flux weighted gravity–luminosity relationship (FGLR), which is based on the quantitative spectroscopy of blue supergiant stars (BSGs)
Summary
After the detection of the accelerated expansion of the universe the physical explanation of dark energy has become the major challenge of astronomy and physics. HST detections of Cepheids out to 30 Mpc in galaxies with SNIa, which were detected recently and have accurate and well understood light cuves, are used to consistently calibrate SNIa as far reaching standard candles using HST H-band photometry of the host galaxy Cepheids In this way, Riess et al (2011) have been able to determine H0 with unprecedented accuracy. There are two long-standing issues with the use of Cepheids as stellar distance indicators, the perennial problem of interstellar extinction and the uncertainty about a potential metallicity dependence of the period-luminosity relationship While both problems can be mitigated to some extend by near IR observations and a strictly differential approach, as applied by Riess et al (2011), at a few percent accuracy level of H0 they continue to be a major concern. We will discuss most recent applications of the BSG-method and future work
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