Luminous blue variable

Abstract: Some of the most luminous stars have sporadic, violent mass-loss events whose cuases are not understood. These evolved hot stars are called Luminous Blue Variables (LBV's), and their instability may shape the appearance of the upper HR diagram. LBV eruptions are interestingly reminiscent of geysers, or even volcanos. They have received considerable observational attention since 1980, but theoretical work to explain the instability has been scarce. In a typical LBV eruption, the star's photosphere expands and the apparent temperature decreases to near 8000 K. During these normal eruptions the bolometric luminosity remains constant, as typified by S Dor, AG Car and R127. A few LBV's, specifically eta Car, P Cyg, V12 in NGC 2403 and SN1961V, have giant eruptions in which the total luminosity actually increases by more than one or two magnitudes. The star may expel as much as a solar mass or more with a total luminous output rivalling a supernova. The classical LBV's have luminosities greater than MBol approaching -9.6 mag, suggesting initial mass greater than 50 Solar Mass. These stars have very lkikely not been red supergiants as there are no evolved cool stars of comparable luminosity. Their instaibility may prevent their evolution to the red supergiant region. There is also a group of less luminous LBV's (MBol approaching -8 to -9 mag) with low temperatuers, smaller amplitudes and lower mass loss rates. These stars have probably been red supergiants and have shed a lot of mass prior to their current unstable state. Although the physical cuase of the LBV instability is not yet understood, the most likely mechanism involve radiation pressure (the opacity-modified Eddington limit) or dynamical instabilities in the outer layers as the star evolves off the main sequence. In this review, we summarize the physical characteristics and behavior of LBV's, and discuss their brief but critical role in massive star evolution, and possible mechanisms for their remarkable instability.

Abstract: (abridged) We report our discovery and observations of the peculiar Type IIn supernova SN2006gy in NGC1260, revealing that it reached a peak magnitude of -22, making it the most luminous supernova ever recorded. It is not yet clear what powers the total radiated energy of 1e51 erg, but we argue that any mechanism -- thermal emission, circumstellar interaction, or 56Ni decay -- requires a very massive progenitor star. The circumstellar interaction hypothesis would require truly exceptional conditions around the star probably experienced an LBV eruption like the 19th century eruption of eta Carinae. Alternatively, radioactive decay of 56Ni may be a less objectionable hypothesis. That power source would imply a large Ni mass of 22 Msun, requiring that SN2006gy was a pair-instability supernova where the star's core was obliterated. SN2006gy is the first supernova for which we have good reason to suspect a pair-instability explosion. Based on a number of lines of evidence, we rule out the hypothesis that SN 2006gy was a ``Type IIa'' event. Instead, we propose that the progenitor may have been a very massive evolved object like eta Carinae that, contrary to expectations, failed to completely shed its massive hydrogen envelope before it died. Our interpretation of SN2006gy implies that the most massive stars can explode earlier than expected, during the LBV phase, preventing them from ever becoming Wolf-Rayet stars. SN2006gy also suggests that the most massive stars can create brilliant supernovae instead of dying ignominious deaths through direct collapse to a black hole.

Abstract: We present UBVRI photometry obtained from Mosaic images of M31 and M33 using the Kitt Peak National Observatory 4 m telescope. We describe our data reduction and automated photometry techniques in some detail, as we will shortly perform a similar analysis of other Local Group galaxies. The present study covered 2.2 deg2 along the major axis of M31 and 0.8 deg2 on M33, chosen so as to include all of the regions currently active in forming massive stars. We calibrated our data using photometry from the Lowell 1.1 m telescope, and this external method resulted in millimagnitude differences in the photometry of overlapping fields, providing some assurance that our photometry is reliable. The final catalog contains 371,781 and 146,622 stars in M31 and M33, respectively, where every star has a counterpart in (at least) the B, V, and R passbands. Our survey goes deep enough to achieve 1%-2% photometry at 21 mag (corresponding to stars more massive than 20 M⊙) and achieves <10% errors at U ~ B ~ V ~ R ~ I ~ 23 mag. Although our typical seeing was only modest (08-14, with median 10) by some standards, we find excellent correspondence between our catalog sources and those we see in our Hubble Space Telescope ACS data for OB48, a crowded region in M31. We compare our final photometry with that of others and find good agreement with the CCD catalog of M31 stars by Magnier et al., although our study covers twice the area and goes about 2 mag deeper. There is also excellent agreement with the CCD "DIRECT" surveys of M31 and M33. The photographic studies of others fare less well, particularly at the faint end in V, where accurate background subtraction is needed for good photometry. We provide cross-references to the stars confirmed as members by spectroscopy and compare the locations of these to the complete set in color-magnitude diagrams. While follow-up spectroscopy is needed for many projects, we demonstrate the success of our photometry in being able to distinguish M31/M33 members from foreground Galactic stars. Finally, we present the results of a single night of spectroscopy on the WIYN 3.5 m telescope, examining the brightest likely members of M31. The spectra identify 34 newly confirmed members, including B-A supergiants, the earliest O star known in M31, and two new luminous blue variable candidates whose spectra are similar to that of P Cygni.

Abstract: We suggest that the mass lost during the evolution of very massive stars may be dominated by optically thick, continuum-driven outbursts or explosions, instead of by steady line-driven winds. In order for a massive star to become a Wolf-Rayet star, it must shed its hydrogen envelope, but new estimates of the effects of clumping in winds from O-type stars indicate that line driving is vastly insufficient. We discuss massive stars above roughly 40-50 M☉, which do not become red supergiants and for which the best alternative is mass loss during brief eruptions of luminous blue variables (LBVs). Our clearest example of this phenomenon is the 19th century outburst of η Carinae, when the star shed 12-20 M☉ or more in less than a decade. Other examples are circumstellar nebulae of LBVs and LBV candidates, extragalactic η Car analogs (the so-called supernova impostors), and massive shells around supernovae and gamma-ray bursters. We do not yet fully understand what triggers LBV outbursts or what supplies their energy, but they occur nonetheless, and they present a fundamental mystery in stellar astrophysics. Since line opacity from metals becomes too saturated, the extreme mass loss probably arises from a continuum-driven wind or a hydrodynamic explosion, both of which are insensitive to metallicity. As such, eruptive mass loss could have played a pivotal role in the evolution and ultimate fate of massive metal-poor stars in the early universe. If they occur in these Population III stars, such eruptions would also profoundly affect the chemical yield and types of remnants from early supernovae and hypernovae thought to be the origin of long gamma-ray bursts.