B Stars with and without emission lines, parts 1 and 2

The spectra for B stars for which emission lines occur not on the main sequence, but only among the supergiants, and those B stars for which the presence of emission in H ahlpa is considered to be a significant factor in delineating atmospheric structure are examined. The development of models that are compatible with all known facts about a star and with the laws of physics is also discussed

High Resolution Chandra Spectroscopy of Gamma Cassiopeia (B0.5IVe)

gamma Cas has long been famous for its unique hard X-ray characteristics. We report herein on a 53 ks Chandra HETGS observation of this target. An inspection of our spectrum shows that it is quite atypical for a massive star, with abnormally weak Fe XXV, XXVI lines, Ly-alpha lines of H-like species from Fe XVII, XXIII, XXIV, S XVI, Si XIV, Mg XII, Ne X, O VII, VIII, and N VII. Also, line ratios of the rif-triplet of for a few He-like ions XVII are consistent with the dominance of collisional atomic processes. Yet, the presence of Fe and Si fluorescence K features indicates that photoionization also occurs in nearby cold gas. The line profiles indicate a mean velocity at rest and a broadening of 500 km/s. A global fitting analysis of the line and continuum spectrum finds that there are 3-4 plasma emission components. The dominant hot (12 keV) component and has a Fe abundance of 0.22 solar. Some fraction of this component (10-30%) is heavily absorbed. The other 2-3 components, with temperatures 0.1, 0.4, 3 keV, are "warm," have a nearly solar composition, a lower column absorption, and are responsible for most other emission lines. The strength of the fluorescence features and the dual-column absorption model for the hot plasma component suggest the presence near the hot sites of a cold gas structure with a column density of 10^23 cm^-2. Since this value is consistent with theoretical estimates of the vertical disk column of this star, these attributes suggest that the X-rays originate near the star or disk. It is possible that the Fe anomaly in the hot component is related to the First Ionization Potential effect found in coronal structures around active cool stars. This would be yet another indication that the X-rays -rays are produced in the immediate vicinity of the Be star.Comment: 32 pages, 4 figures (Fig. 3 colorized.) To be published in 01/10/04 Astrophysical Journal, Main Journal; included figures and updated formattin

The Remarkable Be Star HD110432

HD110432 has gained considerable attention because it is a hard, variable X-ray source similar to gamma Cas. From time-serial echelle data obtained over two weeks during 2005 January and February, we find several remarkable characteristics in the star's optical spectrum. The line profiles show rapid variations on some nights which can be most likely be attributed to irregularly occurring and short-lived migrating subfeatures. Such features have only been observed to date in gamma Cas and AB Dor, two stars for which it is believed magnetic fields force circumstellar clouds to corotate over the stellar surface. The star's optical spectrum also exhibits a number of mainly FeII and HeI emission features with profiles typical of an optically thin disk viewed edge-on. Using spectral synthesis techniques, we find that its temperature is 9800K +/-300K, that its projected area is a remarkably large 100 stellar areas, and its emitting volume resides at a distance of 1 AU from the star. We also find that the star's absorption profiles extend to +/-1000 km/s, a fact which we cannot explain. Otherwise, HD110432 and gamma Cas share similarly peculiar X-ray and optical characteristics such as high X-ray temperature, erratic X-ray variability on timescales of a few hours, optical emission lines, and submigrating features in optical line profiles. Because of these similarities, we suggest that this star is a new member of a select class of "gamma Cas analogs."Comment: 31 pages, 9 figures, accepted by ApJ (3/20/06

Ultraviolet Spectrophotometry of Variable Early-Type Be and B stars Derived from High-Resolution IUE Data

High-dispersion IUE data encode significant information about aggregate line absorptions that cannot be conveniently extracted from individual spectra. We apply a new technique in which fluxes from each echelle order of a short wavelength IUE spectrum are binned together to construct low-resolution spectra of a rapidly varying B or Be star. The ratio of binned spectra obtained bright- star and faint-star phases contains information about the mechanism responsible for a star's variability, such as from pulsations or occultations of the star by ejected matter. We model the variations caused by these mechanism by means of model atmosphere and absorbing-slab codes. Line absorptions strength changes are sensitive to conditions in circumstellar clouds with T = 8,000--13,000K. To demonstrate proofs of concept, we construct spectral ratios for circumstellar structures associated with flux variability in various Be stars: (1) Vela X1 has bow-shock wind trailing its neutron star companion and shows signatures of gas at 13,000K or 26,000K medium in different sectors, (2) 88 Her undergoes episodic outbursts as its UV flux fades, followed a year later by a dimming in visible wavelengths, a result of a gray opacity that dominates as the shell expands and cools, and (3) zeta Tau and 60 Cyg exhibit periodic spectrum and flux changes, which match model absorptions for occulting clouds. Also, ratioed UV spectra of strongly pulsating stars show unique spectrophotometric signatures which can be simulated with models. An analysis of ratioed spectra obtained for a typical sample of 18 classical Be stars known to have rapid periodic flux variations indicates that 13 of them have ratioed spectra which are relatively featureless or have signatures of pulsation. Ratioed spectra of 3 others in the sample are consistent with the presence of co-rotating clouds.Comment: Latex 49 dbl-spaced pages plus 9 figures. Accepted by ApJ. Files available at ftp://nobel.stsci.edu/pub/uv

X-ray and Optical Variations in the Classical Be Star gamma Cas

gamma Cas (B0.5e) is known to be a unique X-ray source because ot its moderate L_x, hard X-ray spectrum, and light curve punctuated by ubiquitous flares and slow undulations. Its X-ray peculiarities have led to a controversy concerning their origin: either from wind infall onto a putative degenerate companion, as for typical Be/X-ray binaries, or from the Be star per se. Recent progress has been made to address this: (1) the discovery that gamma Cas is an eccentric binary system (P = 203.59 d) with unknown secondary type, (2) the accumulation of RXTE data at 9 epochs in 1996-2000, and (3) the collation of robotic telescope B, V-band photometric observations over 4 seasons. The latter show a 3%, cyclical flux variation with cycle lengths 55-93 days. We find that X-ray fluxes at all 9 epochs show random variations with orbital phase. This contradicts the binary accretion model, which predicts a substantial modulation. However,these fluxes correlate well with the cyclical optical variations. Also, the 6 flux measurements in 2000 closely track the interpolated optical variations between the 2000 and 2001 observing seasons. Since the optical variations represent a far greater energy than that emitted as X-rays, the optical variability cannot arise from X-ray reprocessing. However, the strong correlation between the two suggests that they are driven by a common mechanism. We propose that this mechanism is a cyclical magnetic dynamo excited by a Balbus-Hawley instability located within the inner part of the circumstellar disk. In our model, variations in the field strength directly produce the changes in the magnetically related X-ray activity. Turbulence associated with the dynamo results in changes to the density distribution within the disk and creates the observed optical variations.Comment: 30 dbl-spaced pages, Latex, plus 11 figures. Accepted by Ap

The Far Ultraviolet Spectroscopic Explorer Survey of OVI Absorption in the Disk of the Milky Way

To probe the distribution and physical characteristics of interstellar gas at temperatures T ~ 3e5 K in the disk of the Milky Way, we have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to observe absorption lines of OVI toward 148 early-type stars situated at distances 1 kpc. After subtracting off a mild excess of OVI arising from the Local Bubble, combining our new results with earlier surveys of OVI, and eliminating stars that show conspicuous localized X-ray emission, we find an average OVI mid-plane density n_0 = 1.3e-8 cm^-3. The density decreases away from the plane of the Galaxy in a way that is consistent with an exponential scale height of 3.2 kpc at negative latitudes or 4.6 kpc at positive latitudes. Average volume densities of OVI along different sight lines exhibit a dispersion of about 0.26 dex, irrespective of the distances to the target stars. This indicates that OVI does not arise in randomly situated clouds of a fixed size and density, but instead is distributed in regions that have a very broad range of column densities, with the more strongly absorbing clouds having a lower space density. Line widths and centroid velocities are much larger than those expected from differential Galactic rotation, but they are nevertheless correlated with distance and N(OVI), which reinforces our picture of a diverse population of hot plasma regions that are ubiquitous over the entire Galactic disk. The velocity extremes of the OVI profiles show a loose correlation with those of very strong lines of less ionized species, supporting a picture of a turbulent, multiphase medium churned by shock-heated gas from multiple supernova explosions.Comment: Accepted for publication in ApJS. Preprint with full resolution images and all 148 spectra available at http://www.astro.princeton.edu/~dvb/o

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The spectrum of the Be star o Pu