The Galaxy Evolution Explorer (GALEX) satellite has obtained high time resolution ultraviolet photometry during a large flare on the M4 dwarf star GJ 3685A. Simultaneous Near-ultraviolet (NUV, 1750-2800 Å) and Far-ultraviolet (FUV, 1350-1750 Å) time-tagged photometry with time resolution better than 0.1 s shows that the overall brightness in the FUV band increased by a factor of 1000 in 200 s. Under the assumption that the NUV emission is mostly due to a stellar continuum, and that the FUV flux is shared equally between emission lines and continuum, there is evidence for two distinct flare components for this event. The first flare type is characterized by an exponential increase in flux with little or no increase in temperature. The other involves rapid increases in both temperature and flux. While the decay time for the first flare component may be several hours, the second flare event decayed over less than 1 minute, suggesting that there was little or no confinement of the heated plasma.

AbstractThe Australia Telescope and Anglo-Australian Telescope were used in May 2000 to record the radio and optical emissions from the dMe flare star Proxima Centauri. Eight bright optical flares over a two-day interval resulted in no detectable excess short-term radio emission at 1.38 and 2.50 GHz. However, a slowly declining 1.38 GHz emission over the two-day interval was nearly 100% right circular polarised and was restricted to a relatively narrow bandwidth with total intensity (I) and circular polarisation (V) varying significantly over the 104 MHz receiver bandwidth. These are the first observations to show that highly-polarised narrowband flare star emission can persist for several days. This signature is attributed to sources of coherent radio emission in the star's corona. Similarities with various solar radio emissions are discussed; however, it is not possible with the existing observations to distinguish between fundamental plasma emission and electron–cyclotron maser emission as the responsible mechanism.

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.

We report on far-ultraviolet (FUV) observations of flares on the dMOe star AU Microscopii obtained on 1998 September 6 with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. The data consist of medium-resolution echelle spectra covering the wavelength region from 1170 to 1730 Angstrom with a spectral resolution of lambda/Delta lambda = 45,800. The observations were obtained using the TIME-TAG observing mode in which the time and position of each detected photon is recorded. This allows a study of variability that is constrained only by counting statistics. During a total on-source time of 10,105 s, we observed numerous microflare bursts as well as four well-defined flare events. These flares lasted between 10 s and 3 minutes and were most easily detected in the FUV continuum and the Si Iv and C IV resonance lines. Variations in both the cooler (e.g., Ly alpha, C II, O I) and hotter (O V, N V, Fe XXI) emission lines were much less pronounced. We examined the physical characteristics of the flare events, including the time history of the wavelength-integrated fluxes in the continuum and the various emission lines. In particular, we searched for, but did not find, evidence for increased emission in the red wing of the Ly alpha line, which would indicate the presence of moderately energetic proton beams. We integrated the emission over the entire time of major flare activity to investigate the average line and continuum properties of the time-averaged spectrum, including flows and turbulence as a function of temperature. We also considered the shape of the FUV continuum, which may actually be caused by the enhancement of numerous weak emission lines. Unfortunately, none of the events were strong enough to allow a detailed examination of line profiles as a function of time.

With the Hubble Space Telescope (HST) we have observed Hyades F stars, using the Space Telescope Imaging Spectrograph (STIS), in order to get more information about the heating mechanism(s) for the chromospheres and transition layers and their dependence on rotation and age. In this paper we study the Mg II lines at 2800 Angstrom. We include earlier observations with the International Ultraviolet Explorer (IUE) satellite. The Mg II emission lines become observable for B-V >0.3. The emission line fluxes increase steeply until B-V similar to 0.40. For single stars there is a steep decrease in flux between B-V = 0.41 and B-V = 0.44, similar to the behavior of the Ca II emission line cores. For larger B-V the Mg II emission line fluxes again increase, but much more slowly than for the Ca II lines. Generally, the low point of the emission is reached between B-V = 0.43 and B-V = 0.45, i.e., similar to the Ca II emission cores. For the Hyades F stars there appears to be a difference between the emissions for single stars and those for binaries. We find that for Hyades stars with surface line fluxes larger than 10(6) ergs cm(-2) s(-1) the emission line fluxes decrease with increasing v sin i. For smaller fluxes they may increase with increasing v sin i. We have only three stars that perhaps show this. We study the flux ratios of the Mg II k and h lines at 2795.7 and 2802.5 Angstrom in order to determine where the lines fall on the AZ curve of growth. For the earliest F stars studied here the ratio is close to 2, as expected for optically thin lines. Generally, it seems that the optical depths in the line centers are less than 10. There remain problems in understanding the size of the line widths. We discuss the interpretation of the Wilson-Bappu effect. For the Hyades F stars there is a strong dependence of the line width on the effective temperature.

We present the results of magnetohydrodynamic (MHD) modeling of winds from luminous late-type stars using a 2.5-dimensional, nonlinear MHD computer code. We assume that the wind is generated within an initially hydrostatic atmosphere and is driven by torsional Alfven waves generated at the stellar surface. Two cases of atmospheric topology are considered: case I has longitudinally uniform density distribution and isotropic radial magnetic field over the stellar surface, and case II has an isotropic, radial magnetic field with a transverse density gradient, which we refer to as an "atmospheric hole." We use the same set of boundary conditions for both models. The calculations are designed to model a cool luminous star, for which we assume an initial hydrostatic pressure scale height of 0.072 R-*, an Alfven wave speed of 92 km s(-1) at the surface, and a wave period of 76 days, which roughly corresponds with the convective turnover time. For case I the calculations produce a wind with terminal velocity of similar to 22 km s(-1) and a mass loss rate comparable to the expected value of 10(-6) M-circle dot yr(-1). For case II we predict a two-component wind: a fast (25 km s(-1)) and relatively dense wind outside of the atmospheric hole and a slow (15 km s(-1)), rarefied wind inside of the hole.

On 1996 March 14-15 we conducted a campaign with the Hubble Space Telescope GHRS to observe the Si IV λλ1394, 1403 lines of the B0.5e star γ Cas at high temporal and spectral resolution. As a part of this ~22 hr campaign, the Rossi X-Ray Timing Explorer (RXTE) was also used to monitor this star's copious and variable X-ray emission. In this fourth paper of a series we present an analysis of the rapid variations of the discrete absorption components (DACs) of the Si IV doublet. The DACs attain a maximum absorption at -1280 km s-1, taper at higher velocities, and extend to -1800 km s-1. The DACs in this star's resonance lines have been shown to be correlated with a ≳6 yr cycle in the Balmer line emission V/R ratio, and in 1996 this DAC strength was near its maximum. We derive hydrogen densities of 109-1010 cm-3 in the DAC material using a curve-of-growth method and find that the plasma becomes marginally optically thick near -1280 km s-1. The "mean DAC" probably represents a broad "plateau" with a volume density intermediate between the star's midlatitude wind and equatorial disk. We also follow the blueward evolution of subfeatures in the DACs. These features appear to emanate primarily from one or two discrete azimuths on the star and accelerate much more slowly than expected for the background wind, thereby exhibiting an enhanced opacity spiral stream pattern embedded within the structure forming the DAC. In the first two papers in this series, we suggested that active X-ray centers are associated with at least two major cool clouds forced into corotation. Several correlations of flickering in the Si IV DACs are found in our data, which support the idea that changes in X-ray ionizing flux cause changes in the ionization of material at various sectors along the spiral pattern. We demonstrate that similar flickering is visible in archival IUE data from 1982 and may also be responsible for earlier reports from Copernicus of rapid changes in this star's UV and optical lines. Finally, we discovered that flickering of the DAC fluxes in the 1982 data is correlated with rotation phase and shows a modulation with a 7.5 hr cyclical cessation of X-ray flares that was observed recently by RXTE. This confirms our basic picture that lulls in X-ray activity close to the star's surface cause both a lower Si V ionization fraction and an increase in Si IV variability within the DAC structures.

We report on observations of the dM1e flare star AU Microscopii obtained with the Space Telescope Imaging Spectrograph (STIS) on 1998 September 6. A total of 10,105 s of observations were obtained with the medium-resolution E140M grating. We report here on the 9200 s of "quiescent" data when the star did not obviously flare. In this data set, we identify 142 emission lines from 28 species, including low-temperature chromospheric lines (e.g., C I, O I), transition-region lines (e.g., C II-IV, N IV, O III-V, Si II-IV), and the coronal line Fe XXI 1354 Å. There are a number of intersystem lines that are useful for measuring electron densities. We discuss line redshifts and the broad wings of transition-region lines that provide evidence of microflare heating. We derive the emission-measure distribution and compare it with the solar one. We also discuss the shape and formation mechanisms of the He II 1640 Å lines. The analysis of the flare data and of the interstellar absorption features seen in the H I, D I, C II, and O I resonance lines will be published elsewhere.

With the Hubble Space Telescope (HST) and the Goddard High Resolution Spectrograph we have observed four barium and three weak barium stars in the ultraviolet spectral region, together with two nonpeculiar giant standard stars. An additional suspected Ba star was observed with HST and the Space Telescope Imaging Spectrograph. In the H-R diagram, three of the observed Ba stars lie on the same evolutionary tracks as the Hyades giants. Using International Ultraviolet Explorer (IUE) spectra of previously studied giants together with our HST spectra, we investigate whether the chromospheric and transition layer emission-line spectra of the Ba stars are different from those of nonpeculiar giants and from those of giants with peculiar carbon and/or nitrogen abundances. Except for the Ba star HD 46407 and the suspected Ba star HD 65699, the Ba star and mild Ba star emission-line fluxes are, for a given effective temperature and for a given luminosity, lower than those for the nonpeculiar giants observed with IUE. In comparison with the HST-observed standard stars, the C IV λ1550-to-C II λ1335 line flux ratios are smaller, but not necessarily so in comparison with all IUE-observed nonpeculiar giants. However, the C IV-to-C II line flux ratios for the Ba stars decrease with increasing carbon abundances. This shows that the energy balance in the lower transition layer is influenced by the carbon abundance. The temperature gradient appears to be smaller in the C II line-emitting region. There does not seem to be a difference in chromospheric electron densities for the Ba and non-Ba stars, though this result is rather uncertain.

In this paper we discuss X-ray observations of gamma Cas obtained in 1998 November with the Rossi X-Ray Timing Explorer (RXTE). The data were obtained nearly continuously over 54 hr, which is about twice the expected rotational period. An earlier RXTE light curve obtained in 1996 March over a 27 hr period showed X-ray flux arising from short-duration shots (flares) superimposed on an undulating "basal" component that was anticorrelated with fluctuations of the UV continuum over a timescale of similar to 10 hr. The object of the present study was to (1) examine the long-term variations of the X-ray characteristics through comparisons with this earlier data and (2) to determine whether variations of the basal flux repeat during a second rotation period. A comparison of the results with the 1996 data set shows a number of similarities and differences in the X-ray behavior: (a) the mean X-ray level in 1998 was only 60% of the 1996 level, (b) the basal fluxes in 1998 vary over shorter timescales (less than 2 hr) than in 1996, (c) the shots were found to show a slightly softer (cooler) mean color than the basal component in 1998, although they were slightly hotter in 1996, (d) fluctuations in the colors of the shot and basal fluxes generally track one another in both data sets, (e) cyclical patterns of X-ray flux decrease with a period of about 7.5 hr occurred in both data sets, and (f) the frequency of shots with a given integrated energy was found to decrease exponentially with energy, although the rate of decrease in 1996 was slower than in 1998. There was only marginal evidence for a repetition during the second half of the time sequence of long-term basal flux variations seen during the first half of the observations. We suspect, however, that the large intrinsic variability of the X-ray source would have masked a true replication. We also present archival IUE data that shows the presence of UV continuum variations in 1982 with similar characteristics to those seen in 1996. This suggests that the regions responsible for the UV variability are very long lived. The data also provide the basis for a refined but still tentative rotational period of 1.12277 days. Assuming a flare paradigm and a very simple electron beam model, we examine the atmospheric heating expected for the shot events. We conclude that it is possible to explain how the measured shot temperature can be smaller than the temperature deduced for the basal X-ray emission. We also discover that if the beam model is correct then the electrons within the beam have relatively high energies (>200 keV) and are nearly monoenergetic. In three appendices we discuss arguments, first, against the idea that the X-ray emission from gamma Cas arises from mass accretion onto a hypothetical white dwarf companion or from an active late-type star and, second, in favor of its origin from near the surface of gamma Gas.

We report on time-series photometric observations of the dM4.5e flare star YZ Canis Minoris obtained in 1993 November with the High Speed Photometer on board the Hubble Space Telescope. The data consist of five 30 minute time sequences with a sampling rate of 0.01 a that were taken through the F240W filter (centered at 240 nm with an 80 nm width). At these wavelengths the stellar photospheric background is small, so relatively small flares can be detected. The observations show a stellar background of 120 counts s(-1) on which are superposed 54 flare events ranging in integrated flux from 2.0 x 10(28) to 3.0. 10(30) ergs, as well as longer term variations with an amplitude of up to 50% of the average continuum intensity and timescales ranging from several minutes to hours. A statistical analysis of this background suggests that it may be composed of unresolved microflaring activity that has an energy distribution considerably steeper than that deduced for the larger flare events. This is consistent with previous observations as well as the self-organized criticality and reconnecting current sheet flare theories. These results are compared with data from the dM8e flare star CN Leonis, which was obtained earlier with the same experimental setup. CN Leo has both a smaller stellar background and a lower hare occurrence rate than YZ CMi The fact that CN Leo also has a quiescent X-ray flux that is less than 10% of the YZ CMi emission suggests a Link between chromospheric and coronal heating.

High signal-to-noise ratio spectra of RR Tel obtained at medium resolution with the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) are used to test available atomic data for the O IV 2s(2)2p P-2-2s2p(2) P-4 multiplet (UV 0.01). The fine-structure intervals of the 2s2p(2) P-4 term given by Moore (1983) appear to need revision. The flux ratios of lines within multiplet UV (0.01), which have a common upper level, depend only on transition probabilities. The observed flux ratio of lines from the P-4(3/2) level differs from that predicted by theory, but this difference cannot be attributed to a blend with a line of S IV]. At the electron densities in the RR Tel nebula, other flux ratios give information on the relative electron excitation rates between the P-2 and P-4 fine-structure levels. Using the collision strengths calculated by Zhang, Graziani & Pradhan, the rate to the P-4(5/2) level, relative to the rates to the other J states, appears to be underestimated by similar to 10 per cent, which is within the expected uncertainty of 20 per cent. We also discuss the S IV 3s(2)3p P-2-3s3p(2) P-4 multiplet. (Less)

UV spectra of lambda Velorum taken with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope are used to probe the structure of the outer atmospheric layers and wind and to estimate the mass-loss rate from this K5 Ib-II supergiant. VLA radio observations at lambda = 3.6 cm are used to obtain an independent check on the wind velocity and mass-loss rate inferred from the UV observations. Parameters of the chromospheric structure are estimated from measurements of UV line widths, positions, and fluxes and from the UV continuum flux distribution. The ratios of optically thin C II] emission lines indicate a mean chromospheric electron density of log N-e approximate to 8.9 +/- 0.2 cm(-3). The profiles of these lines indicate a chromospheric turbulence (upsilon(0) approximate to 25-36 km s(-1)), which greatly exceeds that seen in either the photosphere or wind. The centroids of optically thin emission lines of Fe II and of the emission wings of self-reversed Fe II lines indicate that they are formed in plasma approximately at rest with respect to the photosphere of the star. This suggests that the acceleration of the wind occurs above the chromospheric regions in which these emission line photons are created. The UV continuum detected by the GHRS clearly traces the mean flux-formation temperature as it increases with height in the chromosphere from a well-defined temperature minimum of 3200 K up to about 4600 K. Emission seen in lines of C III] and Si III] provides evidence of material at higher than chromospheric temperatures in the outer atmosphere of this noncoronal star. The photon-scattering wind produces self-reversals in the strong chromospheric emission lines, which allow us to probe the velocity held of the wind. The velocities to which these self-absorptions extend increase with intrinsic line strength, and thus height in the wind, and therefore directly map the wind acceleration. The width and shape of these; self-absorptions reflect a wind turbulence of approximate to 9-21 km s(-1). We further characterize the wind by comparing the observations with synthetic profiles generated with the Lamers et al. Sobolev with Exact Integration (SEI) radiative transfer code, assuming simple models of the outer atmospheric structure. These comparisons indicate that the wind in 1994 can be described by a model with a wind acceleration parameter beta similar to 0.9, a terminal velocity of 29-33 km s(-1), and a mass-loss rate similar to 3 x 10(-9) M. yr(-1). Modeling of the 3.6 cm radio flux observed in 1997 suggests a more slowly accelerating wind (higher beta) and/or a higher mass-loss rate than inferred from the UV line profiles. These differences may be due to temporal variations in the wind or from limitations in one or both of the models. The discrepancy is currently under investigation.

The Goddard High-Resolution Spectrograph (GHRS) has observed the K5 III star a Tau in the 2330 Angstrom region on three separate occasions. These spectra show marked changes with time, with the UV continuum varying by a factor of 2, and with the emission lines changing in flux by 30% or more, with the amount of change dependent upon the opacity of the line. The variations suggests a restructuring of the atmosphere rather than simply a change in the surface area covered by chromospheric material. Surprisingly, there was no detectable change in the chromospheric turbulence on timescales of hours or years. On average, the lower part of the atmosphere was found to be fairly static, with a slight infall of 1-2 km s(-1). At higher altitudes, probed by observation of the stronger Fe rr lines as well as of the O I (UV 2) and Mg II (UV 1) resonance lines, there is evidence for the acceleration of a slow wind, similar to that seen in the M giants gamma Cru and mu Gem. This wind is much less massive than for the later type giants, however, since its effects are seen in only the most optically thick of the Fe II lines. Comparison of the alpha Tau observations with similar data for the K5 VT hybrid star gamma Dra shows remarkable similarity in the photosphere and lower chromosphere. Both stars have pronounced UV continua, identical turbulences and chromospheric densities, and very similar line fluxes and profiles for all lines formed in the lower chromosphere, including C II], Co I, Si II], and Fe II. A deep exposure near 1500 Angstrom also shows the first evidence for hot plasma in the atmosphere of alpha Tau through the detection of the C Iv (UV 1) doublet with a surface flux about 30% of that observed in gamma Dra. Most of the evidence for the stellar wind is in the Mg II (UV 1) and O I (UV 2) resonance Lines. Modeling these lines using the Sobolev with Exact Integration (SEI) radiative transfer code shows that the wind in gamma Dra accelerates faster and reaches a higher terminal velocity than does the wind in alpha Tau. However, the wind turbulent velocity in gamma Dra is only about one-third of the value seen in alpha Tau. We conclude that the observations support the suggestion by Judge & Stencel that the processes that heat the chromosphere are distinct from those that drive the stellar winds.

We review changes to the characteristics of the Goddard High Resolution Spectrograph (GHRS) which resulted from the installation of the Corrective Optics Space Telescope Axial Replacement (COSTAR) on the Hubble Space Telescope. The introduction of two new optical elements into the light path altered the spectral distribution of the light, decreasing the amount of light striking the instrument by about 30% at wavelengths greater than 1200 Å and effectively eliminated all radiation at wavelengths less than 1130 Å. However, at the longer wavelengths the improved focus offset this loss when the Large Science Aperture (LSA) was used and increased the overall throughput of the Small Science Aperture (SSA) by a factor of 2. The improved focus also enhanced the spectral resolution of LSA observations and improved the ability of the instrument to observe in crowded fields.

In 1996 March we obtained simultaneous Rossi X-Ray Timing Explorer RXTE Proportional Counter Array (PCA) and Hubble Space Telescope (HST) Goddard High-Resolution Spectrograph (GHRS) light curves for the B0.5e star gamma Cas in order to compare its X-ray and ultraviolet continuum flux behaviors. The GHRS data set consisted of a nearly continuous sequence of UV spectra covering a 21+ hr interval. Each 40 Angstrom spectrum was centered on the Si IV lambda lambda 1394-1403 lines and registered 8100 counts in each 1 s exposure. Combining spectra and integrating over greater-than 100 continuum pixels allowed us to define a UV continuum light curve binned to 1 minute with a signal-to-noise ratio of a few thousand pixel(-1). We found that the light curve exhibited variations over a time comparable to the rotation period of the star, showing two broad minima 10 hr apart, which had depths of 0.8% and 1.8%. The long-term trends in the UV are anticorrelated with the X-ray fluxes, with the X-rays exhibiting increases of similar to 10% and similar to 40% during times of UV flux minima. The stability of the long-term X-ray variations on gamma Cas is supported by phasing our March data with contemporaneous ASCA data, suggesting a possible period of 1.125 days (or a close alias). We also get agreement of dip patterns for an assumed 1.123 day period by phasing the GHRS continuum flux curve with IUE light curves in various wavelengths from 2 months earlier. We take this as an estimate of the star's rotational period. We conclude that the X-ray emission from gamma Cas probably consists of two components. The first is a slowly varying "basal" flux representing the minimum level seen during any given phase. Superimposed on this are rapid fluctuations ("shots") that have lifetimes ranging from greater-than 10 a to greater-than-or-equal-to 10 minutes. The character of these components varies from one spacecraft orbit to the next, indicating that the emissions are not produced in a truly "stationary" chaotic environment. Moreover, both the number and amplitude of the shots increase during UV minima. The shot profiles are typically symmetric and can have decay times of a few seconds or less. The shots also have a slightly harder flux distribution than the basal component, suggesting that the two emission regions are not cospatial. The time-averaged X-ray spectrum indicates a quasi temperature of similar to 10(8) K, in agreement with earlier studies.We present a picture in which magnetically generated structures on and over the star's surface are responsible for the basal and shot X-ray components. The energies and luminosities of the shots are so high that even the weakest events we measure are comparable in strength to the most luminous flares on cool active stars. Using general cooling relations for a thermal plasma, one finds that the source region for the shots probably have a size scale of less-than-or-equal-to 10(4) km and densities of similar to 10(13-14) cm(-3). From a simple flare model, we find that generally only a small fraction of the shot energy is radiated during the event itself. The remainder of the hot plasma expands to fill a confined volume, possibly a magnetic loop, connected to the original hare site. A collection of these loops may then account for the basal emission. With this model, we estimate that the individual loops have a characteristic density of similar to 10(11) cm(-3) and dimensions of greater than or equal to 0.1R(*). We note that the magnetic interpretation for the shot and basal emission poses several theoretical questions, such as how complex, dynamic fields can exist on a star that does not have a convective envelope. These results suggest that gamma Cas is a member of an arguably new group of hot stars that flare continuously in X-rays. This group may represent an extension of the hotter Bp stars to high values of rotation.

UV spectra of the "noncoronal" single K supergiant λ Vel and of the single M giant γ Cru obtained with IUE and the Hubble Space Telescope at various epochs indicate that the profiles of many lines formed in the wind exhibit striking alterations in shape with time. We parameterize the wind profiles in terms of an empirical optical depth τemp by reflecting the red wing about line center and comparing the reflected intensity with that of the blue wing. In the λ Vel wind the terminal velocity v∞ was found to be close to 40 km s-1 in 1978, 1982, and 1994, but was at least 20 km s-1 greater in 1990. The faster wind in 1990 also had a total optical depth that was a factor of 2-6 times greater than at the other epochs.

We present the results of magnetohydrodynamic (MHD) modeling of winds from luminous late-type stars using a 2.5-dimensional, nonlinear MHD computer code. We assume that the wind is generated within an initially hydrostatic atmosphere and is driven by torsional Alfven waves generated at the stellar surface. Two cases of topology are considered: case I has longitudinally uniform density distribution and isotropic radial magnetic field over the stellar surface, and case II has an isotropic, radial magnetic field with a transverse density gradient, which we refer to as an atmospheric We use the same set of boundary conditions for both models. The calculations are designed to model a cool luminous star, for which we assume an initial hydrostatic pressure scale height of 0.072 Stellar Radius, an Alfven wave speed of 92 km/s at the surface, and a wave period of 76 days, which roughly corresponds with the convective turnover time. For case I the calculations produce a wind with terminal velocity of about 22 km/s and a mass loss rate comparable to the expected value of 10(exp -6) Solar Mass/yr. For case II we predict a two-component wind: a fast (25 km/s) and relatively dense wind outside of the hole and a slow (1.5 km/s), rarefied wind inside of the hole.

Simultaneous X-ray/UV observations over a full day on 1996 March 14-15 have been made of the prototypical B0.5e star gamma Cas using the Rossi X-Ray Timing Explorer satellite and the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope. The GHRS spectra, taken in the region of the Si Iv lambda lambda 1394-1403 doublet, also permitted the construction of an extremely precise light curve from a nearby "pseudocontinuum" region. The continuum UV and X-ray light curves reveal a pair of X-ray maxima similar to 10 hr apart that coincide in time with UV continuum flux "dips" of similar to 1%. In the first paper in this series we attributed the long-term X-ray Variations to magnetic activity sites on the surface of the star that undergo rotational modulation on a similar to 1.125 days period. In the current study we find that flux and color curves generated from a 33 hr sequence of Inter national Ultraviolet Explorer (IUE) echellegrams obtained in 1996 January display dip features similar to those in the GHRS data. Comparing the timings of the continuum flux dips and the Si Iv line strength variations in both the GHRS and IUE data sets gives a slightly revised period of 1.123 days for both the UV and X-ray activities. This strengthens the argument that high-energy activity on gamma Cas is modulated by rotation of long-lived structures close to its surface. Analysis of the pseudocontinuum light curves constructed from the GHRS and IUE light curves shows at least two surprising characteristics for the flux dips: (1) the dips last only similar to 0.3 cycles, which is too brief for rotation modulation of surface features, and (2) their amplitudes increase from long to short wavelengths, which attain a maximum near 1206 Angstrom. The character of the variations of the photospheric Si Iv line profiles is unexpected in that the equivalent width fluctuations do not correlate with the slow undulations of the continuum flux. Moreover, the profile variations do not show an expected blue-to-red migration of microfeatures. We show that the continuum characteristics and absence of migration of features in the Si Iv lines can be explained by the presence of very cool, optically thin clouds that corotate with the star. Assuming a tilt of the rotational axis of +45 degrees to the observer's line of sight, our model simulations of the two major dips in the UV light curves indicate that the clouds have radii of a few tenths of a stellar radius and are attached to points on the surface at low to mid-latitudes on the near hemisphere. These findings support the conclusion of the first paper in this series that gamma Cas is a member of a small group of OB stars that have magnetospheres associated with X-ray activity.

The star HD 72089 is located behind the Vela supernova remnant and shows a complex array of high- and low-velocity interstellar absorption features arising from shocked clouds. A spectrum of this star was recorded over the wavelength range 1196.4-1397.2 Å at a resolving power of λ/Δλ = 110,000 and a signal-to-noise ratio of 32 by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We have identified seven narrow components of C I and have measured their relative populations in excited fine-structure levels. Broader features at heliocentric velocities ranging from -70 to 130 km s-1 are seen in C II, N I, O I, Si II, S II, and Ni II. In the high-velocity components, the unusually low abundances of N I and O I, relative to S II and Si II, suggest that these elements may be preferentially ionized to higher stages by radiation from hot gas immediately behind the shock fronts.