Papers

"Measurement and Analysis of Helium-Like Triplet Ratios in the X-ray Spectra of O-Type Stars," Leutenegger, Paerels, Kahn, & Cohen, Ap.J., 650, 1096 (Oct. 2006).

We explore the effects of photospheric spectral structure and a spatially distributed emitting plasma on the forbidden-to-intercombination line ratio diagnostics, as applied to O star x-ray spectra. We do this by simultaneously fitting simple wind-broadened line profiles to the resonance, forbidden, and intercombination components of the He-like triplets of, primarily, Mg XI, Si XIII, and S XV in the Chandra spectra of four O stars: ζ Pup, ζ Ori, ι Ori, and δ Ori. We show that if a spatial distribution of x-ray emitting plasma in the wind is assumed then the otherwise important effect of spectral structure can be safely ignored (since the spatial integral translates into a frequency integral, effectively smearing out absorption lines in the photospheric spectrum). We find completely consistent results between the spatial distributions implied by the f/i ratios and those implied by the fitted profile shapes. These distributions typically involve an onset radius of roughly 1.5 Rstar, in accord with the general wind-shock scenario. Earlier claims of plasma very close to the photosphere in ζ Pup and ζ Ori need not be invoked, based on the results of our new analysis. These differences with previous claims are not, however, related to either of the assumptions we investigate in this paper. Rather, they are due to a combination of newer model atmospheres and atomic physics, and, apparently, different fits to the data themselves.

"The Effects of Porosity on X-ray Emission Line Profiles From Hot-Star Winds," Owocki & Cohen, Ap.J., 648, 565 (Sep. 2006).

We explore the extent to which non-uniformities (clumping and accompanying porosity) in hot-star winds can affect the shapes of x-ray emission line profiles. We show that the key parameter is the porosity length, which is the ratio of the clump scale to the volume filling factor. It can also be thought of as the mean free path between clumps. For porosity lengths that scale linearly with radius in the wind, we find that the porosity length at the base of the wind must approach unity before significant effects are seen on the line profiles. This requirement is difficult to reconcile with the small-scale structure seen in multi-dimensional simulations of hot-star winds, and thus lends support to the idea that hot-star mass-loss rates have been overestimated.

"Wind Signatures in the X-ray Emission Line Profiles of the Late O Supergiant ζ Orionis," Cohen, Leutenegger, Grizzard (St. John's College, '06), Reed ('05), Kramer ('03), & Owocki, MNRAS, 368, 1905 (June 2006).

X-ray line profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line profiles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line profile provides information about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper we apply several analysis techniques to the emission lines in the Chandra HETGS spectrum of the late-O supergiant ζ Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blue shifts and profile asymmetry, as well as broadening in the X-ray emission lines of ζ Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for ζ Ori are very similar to those for the earlier O star, ζ Pup, which we have previously shown to be well-fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in ζ Ori, as compared to ζ Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both ζ Ori and ζ Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from ζ Ori can be understood within the framework of the standard wind-shock scenario for hot stars.

"Numerical Modeling of Hohlraum Radiation Conditions," Cohen, Landen, & MacFarlane, Phys. Plasmas, 12, 122703 (Dec. 2005).

Small gold enclosures (approx. 1 mm in scale), called hohlraums, can be heated by short, intense laser pulses to temperatures of several million degrees. The intense, thermal radiation field inside these hohlraums can be used to drive the implosion of fusion fuel capsules or to generate shock waves and/or rapidly heat material samples placed inside these enclosures, for the purpose of studying material properties or aspects of shock physics, among other things. We have undertaken an extensive modeling study of the conditions within hohlraums. In this paper, we use the VisRad view-factor code to model the radiation conditions inside laser-heated hohlraums and halfraums (half-hohlraums). Specifically, we explore the spatial and spectral variations of the radiation field within halfraums, and compare the radiation drive onto an experimental package with that measured by an absolutely calibrated x-ray detector (such as DANTE) looking into the halfraum through a laser entrance hole. We also explore the effects of beam pointing, halfraum length, and both internal and external shields on both the radiation conditions inside the halfraum and on DANTE measurements.

"Chandra HETGS Multi-phase Spectroscopy of the Young Magnetic O Star θ¹ Ori C," Gagne, Oksala (West Chester University, '04), Cohen, Tonnesen ('03), ud-Doula, Owocki, Townsend, & MacFarlane, Ap.J., 628, 986 (Aug. 2005). [b/w or color]

The young, hot star, θ¹ Ori C, is the primary source of ionizing photons in the Orion Nebula, and is also one of the only massive stars with a detected magnetic field. A picture of an oblique magnetic rotator has emerged from combined optical, UV, Zeeman splitting, and broadband x-ray measurements. In this paper, we analyze grating spectra from four separate Chandra pointings covering a range of rotational phases (and thus magnetic field orientations). We also present numerical MHD modeling of the magnetically channeled wind of this star. The Chandra data is remarkably consistent with the MHD models as well as the older datasets mentioned above. And, in fact, the different x-ray spectral diagnostics we employ are consistent with each other: The emission lines are relatively narrow, but resolved, consistent with a channeled wind flow, which the MHD simulations indicate produces the bulk of the x-ray emission within one stellar radius of the photosphere; the He-like f/i ratios are consistent with hot plasma confined within one stellar radius of the photosphere, as is the x-ray lightcurve, which is nearly completely gray, consistent with a picture of occultation of an x-ray emitting torus; the emission measure distribution derived from the spectra are consistent with the predictions of the MHD simulations as well. The MCWS model is thus very successful at explaining the high-energy emission from this magnetized hot star, and in fact, may be applicable to other young, early-type stars as well.

After the paper was accepted, we discovered a small error with the atomic model we had used in the analysis of the f/i ratios. We reanalyzed these line ratios, incorporating an updated estimate of the effective temperature and also elaborating on the predictions of our MHD models in the context of the contstraints imposed by the f/i measurements. This new information has been published in an erratum, appearing in vol. 634 of the Ap.J. (20 Nov 2005).

"Tracer Spectroscopy Diagnostics of Doped Ablators in Inertial Confinement Fusion Experiments on OMEGA," Cohen, MacFarlane, Jaanimagi, Landen, Haynes, Conners ('03), Penrose ('04), & Shupe ('05), Phys. Plasmas, 11, 2702 (May 2004). [on-line article with link to color figure or direct link to archive containing color figure and supporting material]

We report on the results of a series of experiments carried out on the OMEGA laser to study the effects of ablator dopants on radiation propagation in laser fusion fuel capsules. We used point-projection backlit tracer spectroscopy to study the localized physical conditions in the interiors of plastic samples mounted on laser-heated gold enclosures, or hohlraums. We show, using detailed modeling to better understand the physics, that the effects of adding a small amount of germanium to a plastic ablator sample is quite significant, in terms of the coupling of the x-ray radiation field to the plasma and the associated radiation wave velocity through the material.

"High-Resolution Chandra Spectroscopy of γ Cassiopeia (B0.5e)," Smith, Cohen, Gu, Robinson, Evans, & Schran, Ap.J., 600, 972 (Jan. 2004).

This Be star was already known to have unusual properties, including a very hard and bright X-ray spectrum. Our analysis of the first high-resolution X-ray spectrum of γ Cas has revealed several new things: (1) the hottest plasma (12 keV) has two components with different absorption columns, with the higher column density component plausibly associated with absorption by the Be disk, (2) there are also several lower temperature components, though their emission measure is too high to correspond to the normal X-ray emission levels of early B stars, (3) the hot component is more metal deficient than the cooler components, (4) line ratios and the absence of narrow recombination continua indicate that the X-ray emitting plasma is collisional and not dominated by photoionization, (5) the X-ray line widths (rms of roughly 500 km s^-1) are consistent with the star-disk corotation velocity, and (6) the cold iron K fluorescence feature is present in the spectrum, consistent with reprocessing in the Be disk. A scenario in which magnetic activity heats plasma near the surface of the star is consistent with these observed properties.

"X-ray Emission Line Profile Modeling of O Stars: Fitting a Spherically-Symmetric Analytic Wind-Shock Model to the Chandra Spectrum of ζ Puppis," Kramer ('03), Cohen, & Owocki, Ap.J., 592, 532 (Jul. 2003).

We have applied the X-ray line profile model of Owocki & Cohen (2001) to the analysis of the Chandra MEG spectrum of the early O supergiant ζ Puppis. We fit eight lines in the spectrum and find the following: (1) general consistency with a distributed wind shock picture, but (2) too little continuum absorption, based on what is known about the star's mass-loss rate, and (3) a spatial distribution of X-ray emitting plasma that scales roughly as the square of the wind density beyond some minimum radius that is roughly one stellar radius above the photosphere. Additionally, we surprisingly find that (4) there is no strong trend of wind absorption, or any other physical parameter, with wavelength, indicating that the wind opacity does not vary strongly with wavelength, contrary to the expectations of models.

"Chandra Spectroscopy of τ Scorpii: A Narrow Lined Spectrum from a Hot Star," Cohen, de Messieres ('04), MacFarlane, Miller, Cassinelli, Owocki, & Liedahl, Ap.J., 586, 495 (Mar. 2003).

The hot star τ Sco is quite unusual, with very strong X-rays, a very low projected rotational velocity, infalling UV absorbing plasma, and relative youth. The Chandra spectrum we report on in this paper shows the narrowest lines yet of any hot star observed with the Chandra HETGS. Although the emission lines are much narrower than would be expected based on the wind terminal velocity, they are not completely narrow, making them unlike X-ray emission lines seen in any known coronal source. Additionally, line ratio diagnostics indicate that at least some of the plasma on this star is several stellar radii above the photosphere. The existence of slow moving very hot plasma located well above the surface of τ Sco seems to be consistent with the expectations of the magnetically-confined wind-shock model, which might also naturally lead to material falling from the tops of closed magnetic loops back onto the photosphere.

"X-ray Spectral Diagnostics of Neon Photoionization Experiments on the Z-machine," Cohen, MacFarlane, Bailey, & Liedahl, Rev. Sci. Inst., 74, 1962 (Mar. 2003).

We report on X-ray diagnostics, in conjunction with computer modeling, of radiatively heated, X-ray photoionized plasmas on the Z-Machine at Sandia National Lab. Backlit absorption spectroscopy along with time-resolved emission spectroscopy may enable us to test models of X-ray photoionized plasmas that are used by the astrophysics community to analyze spectra from X-ray binaries and other accretion powered sources. Initial experimental results are shown.

"X-ray Emission Line Profile Modeling of Hot Stars," Kramer ('03), Tonnesen ('03), Cohen, Owocki, ud-Doula, & MacFarlane, Rev. Sci. Inst., 74, 1966 (Mar. 2003).

With the advent of high-resolution X-ray spectroscopy in astrophysics, line-profile analysis of hot stars has become possible. Hot stars are one of the few sources for which line profiles can be resolved at a resolution of λ/Δλ = 1000 because of the prodigious outflow velocities of their massive stellar winds. We report on both spherically-symmetric wind-emission models and equitorially enhanced wind models (related to the presence of large-scale magnetic fields in some hot stars). These models make predictions of line shapes that enable us to constrain the various physical models of X-ray production in hot stars.

"New Challenges for Wind Shock Models: The Chandra Spectrum of the Hot Star δ Orionis," Miller, Cassinelli, Waldron, MacFarlane, & Cohen, Ap.J., 577, 951 (Oct. 2002).

The late-O supergiant δ Ori has a Chandra spectrum that differs significantly from that measured in the O4 supergiant ζ Puppis. Its emission lines are narrower and show no signs of asymmetry, in contrast to those seen in ζ Puppis. This poses a serious challenge to the generally accepted wind-shock scenario, which would predict broader lines, consistent with the wind velocity measured in the UV and asymmetric lines due to wind absorption.

"Radiation Science Using Z-Pinch X-rays," Bailey, Chandler, Cohen, et al., Phys. Plasmas, 9, 2186 (May 2002).

Sandia's Z-Machine is the world's most powerful X-ray source. We describe a range of radiation physics experiments that are being carried out on the Z-Machine. Backlit inner-shell absorption spectroscopy is proving to be a very useful diagnostic tool for determining the physical conditions in different experimental packages fielded on the Z-Machine.

"X-ray Line Profiles from Parameterized Emission Within an Accelerating Stellar Wind," Owocki & Cohen, Ap.J., 559, 1108 (Oct. 2001).

We introduce a simple and flexible wind model that accounts for X-ray emission from hot plasma and continuum absorption by a cool wind component for the purpose of synthesizing emission line profiles. This spherically symmetric model has only four parameters and can be related to a host of different physical models of X-ray production on hot stars. We show that line shapes vary widely depending on the spatial distribution of the X-ray emitting plasma and the degree of continuum absorption. Emission line profiles, as observed recently with the new Chandra and XMM spectrometers, are thus shown to be a very powerful diagnostic of the physical conditions in the winds of hot stars.

"Neon Photoionization Experiments Driven by Z-Pinch Radiation," Bailey, Cohen, et al., J. Quant. Spec. Rad. Trans., 71, 151 (Oct. 2001).

This is the initial paper describing the experimental set-up for X-ray photoionization experiments at Sandia's Z-machine, in which a centimeter-scale cell containing neon gas is exposed to a strong X-ray field. We acquired time-integrated neon spectra showing hydrogen-like and helium-like absorption lines. In the future, we will obtain both absorption and emission spectra using time-resolved spectrometers. Among other applications, these experiments should provide information useful for the benchmarking of codes used to model X-ray binaries and other astrophysical plasmas.

"Chandra Detection of Doppler Shifted X-ray Line Profiles from the Wind of ζ Puppis (O4f)," Cassinelli, Miller, Waldron, MacFarlane, & Cohen, Ap.J. Lett., 554, L55 (Jun. 2001).

This paper reports on the high-spectral resolution observations of the canonical hot star wind X-ray source: ζ Puppis. The spectrum shows very broad lines, due to the massive outflowing stellar wind on this star. The lines are broad and somewhat asymmetric, in qualitative agreement with wind-shock models of X-ray production. Line ratio diagnostics are also employed to show that the hot plasma in the wind of this star is concentrated not far from the surface of the star.

"Statistical Fitting Analysis of Stark-Broadened Optically Thick Ar II Spectra Measured in Ion Beam Transport Experiments," Chung, Cohen, MacFarlane, Bailey, Wang, & Moses, J. Quant. Spec. Rad. Trans., 65, 135 (Apr. 2000).

We report on the analysis and modeling of partially optically thick argon emission line spectra taken during a light ion beam experiment at the PBFA-II facility at Sandia National Lab (see Bailey et al. 1999). We use a technique borrowed from X-ray astronomy data analysis to constrain the multi-parameter model that describes the observed optical argon spectra. We compare the derived parameter values (optical depths and densities) with the results of detailed non-LTE, time-dependent modeling of the argon excitation and ionization in the presence of thermal and non-thermal electrons.

"A Simple Scaling Analysis of X-Ray Emission and Absorption in Hot-Star Winds," Owocki & Cohen, Ap.J., 520, 833 (Aug. 1999).

After 20 years of observations of hot stars with X-ray telescopes, the most robust trend observed has been the correlation of X-ray luminosity with bolometric luminosity, the so-called L_X/L_Bol=10^-7 law. We show that this observation does not arise naturally from the wind-shock scenario, in which X-ray luminosity should scale with the wind density squared (for optically thin winds) or like the wind density (for optically thick winds). Here we show that the observed trend can be recovered if there is a weak dependence of the X-ray filling factor on radius in the wind. We also show that the overall spectral shape (in low-resolution observations) is governed much more by the absorption properties of the wind than the emission properties, and that X-ray spectra of hot stars are characterized by an optically thick soft X-ray portion and an optically thick harder X-ray portion, with the transition coming at the peak of the spectrum. These spectral trends are discussed as diagnostics. Finally, the fall-off of X-ray luminosity (i.e. deviation from the L_X/L_Bol=10^-7 law) in stars later than about B1 can be understood as a transition to winds that are completely optically thin to X-rays.

"Spectroscopic Measurements of Argon Plasma Formation by a High-Intensity Lithium Ion Beam," Bailey, Chung, Carlson, Cohen, Johnson, Lake, MacFarlane, Wang, & Welch, Phys. Rev. Lett., 82, 739 (Jan. 1999).

This is the first paper on the argon plasma formation in PBFA-II lithium ion beam experiments at Sandia National Lab. In these experiments, a litium ion beam is propagated through an argon-filled gas cell. The beam induces ionization and excitation in the argon, which we would like to understand so that the conditions in the gas cell can be optimized for beam propagation and so beam divergence can be minimized. We describe the experiment and spectral diagnostics, in the context of light ion beam transport physics.

"ROSAT and HIPPARCOS Observations of Isolated Pre-Main-Sequence Stars Near HD 98800," Jensen, Cohen, & Neuhauser, Astroph. J., 116, 414 (Jul. 1998).

Stars are usually thought to be formed in large star-forming complexes. But recently, a few pre-main-sequence (PMS) stars have been found not near any known regions of star formation. We report on ROSAT HRI observations of the field around the previously known, isolated PMS stars HD 98800 and CD -33 7795. We confirm the youth of these stars (via their high X-ray luminosities) and identify several other possible PMS stars in the field. However, only one of them, CD -36 7429, meets all the criteria for youth (several others lie above the main sequence, but appear to be giants and sub-giants). An analysis of the proper motions of the three PMS stars indicates that they probably did not form in the same cloud.

"ORFEUS-SPAS II Extreme-Ultraviolet Spectroscopy of ε CMa (B2 II)," Cohen, Hurwitz, Cassinelli, & Bowyer, Ap.J. Lett., 500, L51 (Jun. 1998).

We use the EUV spectrometer on the ORFEUS mission (flown on the Shuttle) to analyze the photospheric Lyman continuum of the B bright giant, ε CMa, which we have previously observed with EUVE (Cassinelli et al. 1995). This observation has significantly higher resolution (though lower signal to noise) which allows us to much more cleanly separate the numerous absorption lines seen in this part of the spectrum. Several of the lines (O V, Si IV) are clearly wind-broadened, consistent with a terminal velocity of 800 km/sec.

"ROSAT PSPC Observations of 27 Near-Main-Sequence B Stars," Cohen, Cassinelli, & MacFarlane, Ap.J., 487, 867 (Oct. 1997).

This paper follows up on an earlier, smaller survey of the X-ray properties of B stars (Cassinelli et al. 1994) and extends it to later spectral subtypes (down to B7). We explore the region in the HR diagram (around B2-B3) where the X-ray luminosity falls off precipitously. We detect X-ray emission from 22 of the 27 B stars in the survey, counter to the perception that B stars are not usually sources of X-rays. The X-rays from these B stars are very soft, and though relatively weak, actually exceed the levels expected from shocks embedded in their thin, radiatively-driven winds. We also find very little X-ray variability, and note modestly higher X-ray flux levels from the Be stars in the sample (though the subsample is not very large).

"ASCA X-Ray Spectroscopy of the Unusual B0 V Star τ Scorpii," Cohen, Cassinelli, & Waldron, Ap.J., 488, 376 (Oct. 1997).

The ASCA SIS spectrometers provide the highest spectral resolution of any X-ray telescopes at this time (mid-1990s). The early B star τ Sco was already known to be a source of unusually strong and hard X-rays from earlier, low-resolution observations. We quantify the temperature distribution in the X-ray emitting plasma (at least a quarter of the emission measure derives from plasma with temperatures above 25 million K). We also confirm, for the first time, that the emission is primarily thermal (with line complexes of Mg, Si, and S detected and their fluxes measured). We place strong limits on the flux from any non-thermal component. The derived X-ray properties are very difficult to explain in the context of line-force instability wind shocks, and might be associated with this star's relative youth (less than a million years on the main sequence).

"EUVE Spectroscopy of β Canis Majoris (B1 II-III) from 500 Angstrom to 700 Angstrom," Cassinelli, Cohen, et al., Ap.J., 460, 949 (Apr. 1996).

This star is one of only two hot stars with large EUV fluxes at the Earth (ε CMa is the other). This is due to its position in a "tunnel" of unusually low interstellar hydrogen column density. We report on the photospheric Lyman continuum spectrum and compare it with the predictions of model atmospheres. The primary result of this study is that the Lyman continuum flux level is several times higher than any available atmosphere model. We detect no firm evidence of wind signatures in any of the absorption lines.

"Evidence for Wind Attenuation and a Multitemperature Plasma in the Combined EUVE and ROSAT Observations of ε Canis Majoris (B2 II)," Cohen, Cooper, MacFarlane, Owocki, Wang, & Cassinelli, Ap.J., 460, 506 (Mar. 1996).

The B bright giant ε CMa is the only hot star with measurable emission lines in the EUVE spectrometers. These spectrometers had a resolution of several hundred, far in excess of any previous X-ray or EUV instruments. Thus, the data set reported on here is the first -- and for several years only -- hot star high energy spectrum with resolved emission lines. These emission lines were reported on initially by us (Cassinelli et al. 1994), but here we analyze them in conjunction with the low-resolution ROSAT PSPC soft X-ray spectrum. Because the spectra from these two instruments sample the same, multi-million degree plasma, but some of the EUV lines are subject to significant attenuation by the cool component of the wind, the combined analysis yields information about both the temperature distribution of the plasma and the degree of wind attenuation (and hence the physical location). The results show relatively conclusively that there is a spatial distribution of hot plasma within the stellar wind (as opposed to a coronal scenario, in which the wind lies above all of the X-ray emitting plasma). Furthermore, the temperature distribution we infer is consistent wit the predictions of the line-force instability wind-shock mechanism, based on numerical simulations.

"The X-Ray Halo of Nova V1974 Cygni (Nova Cygni 1992) and the Nature of Interstellar Dust," Mathis, Cohen, Finley, & Krautter, Ap.J., 449, 320 (Aug. 1995).

X-rays are preferentially forward scattered by dust grains, leading to observations of X-ray haloes around bright point sources with significant ISM columns along their lines of sight. Nova Cyg 92 is one such source, and in this paper we use the radial intensity of its X-ray halo observed with the ROSAT PSPC, combined with optical observations of hydrogen line ratios and broad-band reddening to model the physical properties of the interstellar dust along the line of sight to this object. We find that the dust grains are composite, consisting of a significant fraction of vacuum. And we also put some constraints on the dust size distribution.

"EUVE Spectroscopy of ε Canis Majoris (B2 II) from 70 Angstrom to 730 Angstrom," Cassinelli, Cohen, et al., Ap.J., 438, 932 (Jan. 1995).

We report on the first spectroscopic measurements of a hot star below the Lyman edge. This observation yielded several surprising and interesting results: (1) The photospheric continuum levels in the Lyman and Helium continua are ten to a hundred times higher than the predictions of state of the art model atmospheres. (2) The photospheric continua are heavily line-blanketed. (3) The initial and final stages of the Bowen fluorescence mechanism (He II Ly-α at 304 Angstrom and O III at 374 Angstrom) are detected for the first time in an object beyond the solar system. This emission arises in the cold component of the stellar wind. (4) Five emission lines from highly ionized iron are detected in the SW and MW spectrometers (between 170 and 335 Angstroms). Each one of these lines has a flux approaching the total flux in the entire ROSAT PSPC bandpass.

"X-ray Induced Ionization in the Winds of Near-Main-Sequence O and B Stars," MacFarlane, Cohen, & Wang, Ap.J., 437, 351 (Dec. 1994).

Using non-LTE modeling, constrained by ROSAT PSPC observations, we show that X-ray (and EUV) photoionization plays an important role in setting the ionization balance in the winds of OB stars. This effect is increasingly important as one looks toward later spectral subtypes. The reasons for this are two-fold: (1) Recombination is slower in the lower density winds of the later spectral subtypes. And (2) X-ray dominate photospheric continuum emission at lower photon energies in the cooler stars found at later spectral subtypes. For τ Sco (B0 V) and later stars, the inclusion of X-ray photoionization can change the dominant ionization state by one charge state or even more. For hotter stars, it can still have a significant effect on superionization states that are seen in the UV. UV absorption line diagnostics, including mass-loss rate diagnostics must take the effects of X-ray photoionization into account.

"X-ray Emission From Near-Main-Sequence B Stars" Cassinelli, Cohen, MacFarlane, Sanders, & Welsh, Ap.J., 421, 705 (Feb. 1994).

We report on the initial phase of a ROSAT PSPC survey of early B stars. Our results show that all the B stars in our sample (B0 to B2) are X-ray sources. Previous surveys probably did not show a high rate of X-ray production due to lack of sensitivity. The X-ray luminosities of even early B stars are significantly below those of O stars. And in fact, the well-known L_X/L_Bol=10^-7 law breaks down around B1, below which the ratio fo X-ray luminosity to bolometric luminosity is considerably lower. The spectral characteristics of these B stars are, with the exception of τ Sco, very soft.

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