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Register a new userLMC Blue Supergiant Stars and the Calibration of the Flux-weighted Gravity--Luminosity Relationship
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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 calculate models of stellar evolution for very massive stars and include the effects of modified gravity to investigate the influence on the physical properties of blue supergiant stars and their use as extragalactic distance indicators. With shielding and fifth force parameters in a similar range as in previous studies of Cepheid and tip of the red giant branch (TRGB) stars we find clear effects on stellar luminosity and flux-weighted gravity. The relationship between flux weighted gravity, g_F = g/Teff^4, and bolometric magnitude M_bol (FGLR), which has been used successfully for accurate distance determinations, is systematically affected. While the stellar evolution FGLRs show a systematic offset from the observed relation, we can use the differential shifts between models with Newtonian and modified gravity to estimate the influence on FGLR distance determinations. Modified gravity leads to a distance increase of 0.05 to 0.15 magnitudes in distance modulus. These change are comparable to the ones found for Cepheid stars. We compare observed FGLR and TRGB distances of nine galaxies to constrain the free parameters of modified gravity. Not accounting for systematic differences between TRGB and FGLR distances shielding parameters of 5*10^-7 and 10^-6 and fifth force parameters of 1/3 and 1 can be ruled out with about 90% confidence. Allowing for potential systematic offsets between TRGB and FGLR distances no determination is possible for a shielding parameter of 10^-6. For 5*10$^-7 a fifth force parameter of 1 can be ruled out to 92% but 1/3 is unlikely only to 60%.
The flux-weighted gravity-luminosity relation (FWGLR) is investigated for a sample of 477 classical Cepheids (CCs), including stars that have been classified in the literature as such but are probably not. The luminosities are taken from the literature, based on the fitting of the spectral energy distributions (SEDs) assuming a certain distance and reddening. The flux-weighted gravity (FWG) is taken from gravity and effective temperature determinations in the literature based on high-resolution spectroscopy. There is a very good agreement between the theoretically predicted and observed FWG versus pulsation period relation that could serve in estimating the FWG (and $log g$) in spectroscopic studies with a precision of 0.1~dex. As was known in the literature, the theoretically predicted FWGLR relation for CCs is very tight and is not very sensitive to metallicity (at least for LMC and solar values), rotation rate, and crossing of the instability strip. The observed relation has a slightly different slope and shows more scatter (0.54~dex). This is due both to uncertainties in the distances and to the pulsation phase averaged FWG values. Data from future Gaia data releases should reduce these errors, and then the FWGLR could serve as a powerful tool in Cepheid studies.
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.
The characteristics of light variation of RSGs in SMC are analyzed based on the nearly 8-10 year long data collected by the ASAS and MACHO projects. The identified 126 RSGs are classified into five categories accordingly: 20 with poor photometry, 55 with no reliable period, 6 with semi-regular variation, 15 with Long Secondary Period (LSP) and distinguishable short period and 30 with only LSP. For the semi-regular variables and the LSP variables with distinguishable short period, the Ks band period-luminosity (P-L) relation is analyzed and compared with that of the Galaxy, LMC and M33. It is found that the RSGs in these galaxies obey similar P-L relation except the Galaxy. In addition, the P-L relations in the infrared bands, namely the 2MASS JHKs, Spitzer/IRAC and Spitzer/MIPS 24 {mu}m bands, are derived with high reliability. The best P-L relation occurs in the Spitzer/IRAC [3.6] and [4.5] bands. Based on the comparison with the theoretical calculation of the P-L relation, the mode of pulsation of RSGs in SMC is suggested to be the first overtone radial mode.
We examine the recent star formation associated with four supergiant shells (SGSs) in the Large Magellanic Cloud (LMC): LMC 1, 4, 5, and 6, which have been shown to have simple expanding-shell structures. H II regions and OB associations are used to infer star formation in the last few Myr, while massive young stellar objects (YSOs) reveal the current ongoing star formation. Distributions of ionized, H I, and molecular components of the interstellar gas are compared with the sites of recent and current star formation to determine whether triggering has taken place. We find that a great majority of the current star formation has occurred in gravitationally unstable regions, and that evidence of triggered star formation is prevalent at both large and local scales.
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