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تسجيل مستخدم جديدBlue luminous stars in nearby galaxies - UIT005: a possible link to the luminous blue variable stage
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(abridged) A detailed study of the blue supergiant UIT005 (B2-2.5Ia+) in M33 is presented. The results of our quantitative spectral analysis indicate that the star is a very luminous, log(L/Lsun)~5.9 dex, and massive, M~50 Msun, object, showing a very high nitrogen-to-oxygen ratio in its surface (N/O~8, by mass). Based on the derived Mg and Si abundances, we argue that this high N/O ratio cannot be the result of an initial low O content due to its location on the disk of M33, known to present a steep metallicity gradient. In combination with the He abundance, the most plausible interpretation is that UIT005 is in an advanced stage of evolution, showing in its surface N enrichment and O depletion resulting from mixing with CNO processed material from the stellar interior. A comparison with the predictions of current stellar evolutionary models indicates that there are significant discrepancies, in particular with regard to the degree of chemical processing, with the models predicting a much lower degree of O depletion than observed. At the same time, the mass-loss rate derived in our analysis is an order of magnitude lower than the values considered in the evolutionary calculations. Based on a study of the surrounding stellar population and the nearby cluster NGC588, using WFPC2 photometry, we suggest that UIT005 could be in fact a runaway star from this cluster.
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Context. Luminous Blue Variables (LBVs) are thought to be in a transitory phase between O stars on the main-sequence and the Wolf-Rayet stage. Recent studies suggest that they might be formed through binary interaction. Only a few are known in binary
systems but their multiplicity fraction is uncertain. Aims. This study aims at deriving the binary fraction among the Galactic (confirmed and candidate) LBV population. We combine multi-epoch spectroscopy and long-baseline interferometry. Methods. We use cross-correlation to measure their radial velocities. We identify spectroscopic binaries through significant RV variability (larger than 35 km/s). We investigate the observational biases to establish the intrinsic binary fraction. We use CANDID to detect interferometric companions, derive their parameters and positions. Results. We derive an observed spectroscopic binary fraction of 26 %. Considering period and mass ratio ranges from Porb=1 to 1000 days, and q = 0.1-1.0, and a representative set of orbital parameter distributions, we find a bias-corrected binary fraction of 62%. From interferometry, we detect 14 companions out of 18 objects, providing a binary fraction of 78% at projected separations between 1 and 120 mas. From the derived primary diameters, and the distances of these objects, we measure for the first time the exact radii of Galactic LBVs to be between 100 and 650 Rsun, making unlikely to have short-period systems. Conclusions. This analysis shows that the binary fraction among the Galactic LBV population is large. If they form through single-star evolution, their orbit must be initially large. If they form through binary channel that implies that either massive stars in short binary systems must undergo a phase of fully non-conservative mass transfer to be able to sufficiently widen the orbit or that LBVs form through merging in initially binary or triple systems.
The Luminous Blue Variable stars exhibit behavior ranging from light curve `microvariations on timescales of tens of days, to `outbursts accompanied by mass loss of up to 10e-03 solar masses per year, occurring decades apart, to `giant eruptions such
as seen in Eta Carinae ejecting one or more solar masses and recurring on timescales of centuries. Here we review the work of the Los Alamos group since 1993 to investigate pulsations and instabilities in massive stars using linear pulsation models and non-linear hydrodynamic models. The models predict pulsational variability that may be associated with the microvariations. Using a nonlinear pulsation hydrodynamics code with a time-dependent convection treatment, we show that, in some circumstances, the Eddington limit is exceeded periodically in the pulsation driving region of the stellar envelope, accelerating the outer layers, and perhaps initiating mass loss or LBV outbursts. We discuss how pulsations and mass loss may be responsible for the location of the Humphreys-Davidson Limit in the H-R diagram. The `giant eruptions, however, must involve much deeper regions in the stellar core to cause such large amounts of mass to be ejected. We review and suggest some possible explanations, including mixing from gravity modes, secular instabilities, the epsilon mechanism, or the SASI instability as proposed for Type II supernovae. We outline future work and required stellar modeling capabilities to investigate these possibilities.
We study five Luminous Blue Variable (LBV) candidates in the Andromeda galaxy and one more (MN112) in the Milky Way. We obtain the same-epoch near-infrared (NIR) and optical spectra on the 3.5-meter telescope at the Apache Point Observatory and on th
e 6-meter telescope of the SAO RAS. The candidates show typical LBV features in their spectra: broad and strong hydrogen lines, HeI, FeII, and [FeII] lines. We estimate the temperatures, reddening, radii and luminosities of the stars using their spectral energy distributions. Bolometric luminosities of the candidates are similar to those of known LBV stars in the Andromeda galaxy. One candidate, J004341.84+411112.0, demonstrates photometric variability (about 0.27 mag in V band), which allows us to classify it as a LBV. The star J004415.04+420156.2 shows characteristics typical for B[e]-supergiants. The star J004411.36+413257.2 is classified as FeII star. We confirm that the stars J004621.08+421308.2 and J004507.65+413740.8 are warm hypergiants. We for the first time obtain NIR spectrum of the Galactic LBV candidate MN112. We use both optical and NIR spectra of MN112 for comparison with similar stars in M31 and notice identical spectra and the same temperature in the J004341.84+411112.0. This allows us to confirm that MN112 is a LBV, which should show its brightness variability in longer time span observations.
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