Kris Davidson

The luminous blue variables: Astrophysical geysers

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 (LBVs), 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 stars 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 LBVs, 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 LBVs 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 LBVs (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 LBVs, and discuss their brief but critical role in massive star evolution, and possible mechanisms for their remarkable instability.

Neutron-Star Accretion in a Stellar Wind: Model for a Pulsed X-Ray Source

Description of a relatively simple stellar-wind-driven accretion model that appears to satisfactorily account for many of the observed features of the pulsing X-ray sources Cen X-3 and Her X-1. A rotating magnetized neutron star orbits a more massive slightly evolved star and accretes mass from the stellar wind emanating from that star. Some of the implications of the model are discussed.

Latitude-dependent Effects in the Stellar Wind of η Carinae*

The Homunculus reflection nebula around η Carinae provides a rare opportunity to observe the spectrum of a star from more than one direction. In the case of η Car, the nebulas geometry is known well enough to infer how line profiles vary with latitude. We present Space Telescope Imaging Spectrograph (STIS) spectra of several positions in the Homunculus, showing directly that η Car has an aspherical stellar wind. P Cygni absorption in Balmer lines depends on latitude, with relatively high velocities and strong absorption near the polar axis. Stronger absorption at high latitudes is surprising, and it suggests higher mass flux toward the poles, perhaps resulting from radiative driving with equatorial gravity darkening on a rotating star. Reflected profiles of He I lines are more puzzling, offering clues to the wind geometry and ionization structure. During η Cars high-excitation state in 2000 March, the wind was fast and dense at the poles, with higher ionization at low latitudes. Older STIS data obtained since 1998 reveal that this global stellar wind geometry changes during η Cars 5.5 yr cycle and may suggest that this stars spectroscopic events are shell ejections. Whether or not a companion star triggers these outbursts remains ambiguous. The most dramatic changes in the wind occur at low latitudes, while the dense polar wind remains relatively undisturbed during an event. The apparent stability of the polar wind also supports the inferred bipolar geometry. The wind geometry and its variability have critical implications for understanding the 5.5 yr cycle and long-term variability but do not provide a clear alternative to the binary hypothesis for generating η Cars X-rays.

The Asymmetric Nebula Surrounding the Extreme Red Supergiant Vy Canis Majoris

We present HST/WFPC2 images plus ground-based infrared images and photometry of the very luminous OH/IR star VY Canis Majoris. Our WFPC2 data show a complex distribution of knots and filamentary arcs in the asymmetric reflection nebula around the obscured central star. The reflection arcs may result from multiple, asymmetric ejection episodes due to localized events on VY CMas surface. Such events probably involve magnetic fields and convection, by analogy with solar activity. Surface photometry indicates that the star may have experienced enhanced mass loss over the past 1000 yr. We also demonstrate that the apparent asymmetry of the nebula results from a combination of high extinction and backscattering by dust grains. Thermal-infrared images reveal a more symmetric distribution, elongated along a nearly east-west direction. VY CMa probably has a flattened disklike distribution of dust with a northeast-southwest polar axis and may be experiencing activity analogous to solar prominences. The presence of an axis of symmetry raises interesting questions for a star the size of Saturns orbit. Magnetic fields and surface activity may play an important role in VY CMas mass-loss history.

The Shape and Orientation of the Homunculus Nebula Based on Spectroscopic Velocities

Doppler velocities give information about the three-dimensional shape and orientation of the bipolar ejecta of η Car. We report slit spectroscopy obtained using the Space Telescope Imaging Spectrograph (HST/STIS), with spatial resolution adequate for good models. These data allow an independent estimate of the distance to this object. Equatorial velocities, not clearly recognized in earlier work, provide the first definite measure of the bipolar inclination angle relative to our line of sight. Equatorial structure is arguably the most significant new spectroscopic result reported here, and some of the equatorial gas has higher and lower velocities which probably represent later and earlier ejection events. A range of bipolar lobe shapes fit the data reasonably well; the worst ambiguities result from the nature of the object more than from instrumental limitations.

The relation between apparent temperature and mass-loss rate in hypergiant eruptions

On utilise un modele de vent opaque simplifie pour estimer la temperature de radiation emergente en fonction de la masse pour des eruptions geantes de variables bleues lumineuses

X-Ray Spectral Variation of η Carinae through the 2003 X-Ray Minimum

We report the results of an observing campaign on η Car around the 2003 X-ray minimum, mainly using the XMM-Newton observatory. These are the first spatially resolved X-ray monitoring observations of the stellar X-ray spectrum during the minimum. The hard X-ray emission, associated with the wind-wind collision (WWC) in the binary system, varied strongly in flux on timescales of days, but not significantly on timescales of hours. The X-ray flux in the 2-10 keV band seen by XMM-Newton was only 0.7% of the flux maximum seen by RXTE. The slope of the X-ray continuum above 5 keV did not vary in any observation, which suggests that the electron temperature of the hottest plasma did not vary significantly at any phase. Through the minimum, the absorption to the stellar source increased by a factor of 5-10 to NH ~ (3-4) × 1023 cm-2. These variations were qualitatively consistent with emission from the WWC plasma entering into the dense wind of the massive primary star. During the minimum, X-ray spectra also showed significant excesses in the thermal Fe XXV emission line on the red side, while they showed only a factor of 2 increase in equivalent width of the Fe fluorescence line at 6.4 keV. These features are not fully consistent with the eclipse of the X-ray plasma and may suggest an intrinsic fading of the X-ray emissivity. The drop in the WWC emission revealed the presence of an additional X-ray component that exhibited no variation on timescales of weeks to years. This component may be produced by the collision of high-speed outflows at v ~ 1000-2000 km s-1 from η Car with ambient gas within a few thousand AU from the star.

HIGH-EXCITATION EMISSION LINES NEAR ETA CARINAE, AND ITS LIKELY COMPANION STAR*

In order to study the distribution of gas and ionizing radiation around η Car and their implications for its likely companion star, we have examined high-excitation emission lines of [Ne III], [Fe III], etc., in spectra obtained with the Hubble Space Telescope/Space Telescope Imaging Spectrograph instrument during 1998-2004. Our principal results, some of them unexpected, are as follows. (1) The high-excitation fluxes varied systematically and non-trivially throughout η Cars 5.5 year spectroscopic cycle. Instead of rising to a plateau after the 1998 event, they changed continuously with a maximum in mid-cycle. (2) At one significant location a brief, strong secondary maximum occurred just before the 2003.5 spectroscopic event. (3) These emission lines are strongly concentrated at the Weigelt knots several hundred AU northwest of the star. With less certainty, [Ne III] appears to be somewhat more concentrated than [Fe III]. (4) A faster, blueshifted component of each feature appears concentrated near the star and elongated perpendicular to the systems bipolar axis. This structure may be related to the equatorial outflow and/or to dense material known to exist along our line of sight to the star. (5) Using the photoionization program Cloudy, we estimated the range of parameters for the hot secondary star that would give satisfactory high-excitation line ratios in the ejecta. T eff 39, 000 K and L ~ 4 × 105 L ☉, for example, would be satisfactory. The allowed region in parameter space is wider (and mostly less luminous) than some previous authors suggested.

On the He II Emission in η Carinae and the Origin of Its Spectroscopic Events

We describe and analyze Hubble Space Telescope (HST) observations of transient emission near 4680 8 inCar, reportedearlierbySteiner&Damineli.If,asseemsprobable,thisisHeiik4687,thenitisauniquecluetoCars5.5yr cycle. According to our analysis, several aspects of this feature support a mass-ejection model of the observed spectro- scopicevents,andnotaneclipsemodel.TheHeiiemissionappearedinearly2003,grewtoabriefmaximumduringthe 2003.5spectroscopicevent,andthenabruptlydisappeared.ItdidnotappearinanyotherHSTspectrabeforeorafter the event. The peak brightness was larger than previously reported and is difficult to explain even if one allows for an un- certaintyfactoroforder3.Thestellarwindmustprovideatemporarylarger-than-normalenergysupply,andwedescribe a special form of radiative amplification that may also be needed. These characteristics are consistent with a class of mass-ejectionorwind-disturbancescenarios,whichhaveimplicationsforthephysicalstructureandstabilityofCar. Subject headingg binaries: general — line: profiles — stars: individual (� Car) — stars: variables: other — stars: winds, outflows

Spitzer Space Telescope Infrared Imaging and Spectroscopy of the Crab Nebula

We present 3.6, 4.5, 5.8, 8.0, 24, and 70 μm images of the Crab Nebula obtained with the Spitzer Space Telescope IRAC and MIPS cameras, low- and high-resolution Spitzer IRS spectra of selected positions within the nebula, and a near-infrared ground-based image made in the light of [Fe II] 1.644 μm. The 8.0 μm image, made with a bandpass that includes [Ar II] 7.0 μm, resembles the general morphology of visible Hα and near-IR [Fe II] line emission, while the 3.6 and 4.5 μm images are dominated by continuum synchrotron emission. The 24 and 70 μm images show enhanced emission that may be due to line emission or the presence of a small amount of warm dust in the nebula on the order of less than 1% of a solar mass. The ratio of the 3.6 and 4.5 μm images reveals a spatial variation in the synchrotron power-law index ranging from approximately 0.3 to 0.8 across the nebula. Combining this information with optical and X-ray synchrotron images, we derive a broadband spectrum that reflects the superposition of the flatter spectrum of the jet and torus with the steeper spectrum of the diffuse nebula. We also see suggestions of the expected pileup of relativistic electrons just before the exponential cutoff in the X-ray. The pulsar, and the associated equatorial toroid and polar jet structures seen in Chandra and Hubble Space Telescope images (Hester et al. 2002), can be identified in all of the IRAC images. We present the IR photometry of the pulsar. The forbidden lines identified in the high-resolution IR spectra are all double due to Doppler shifts from the front and back of the expanding nebula and give an expansion velocity of ≈1264 km s-1.