Papers

"On the Importance of the Interclump Medium for Superionization: O VI Formation in the Wind of ζ Pup," Zsargo, Hillier, Bouret, Lanz, Leutenegger, & Cohen, Ap.J.L., in press (Aug. 2008).

It's increasingly clear that hot-star winds are highly clumped, with the majority of the mass assumed to reside in the over-dense clumps, while the tenuous interclump medium fills a majority of the volume. The interclump medium is often taken to be completely empty. However, in this paper we show that a tenuous - but not void - interclump medium is the site of the the superion O VI, which produces the strong UV wind features at 1032, 1038 Angstroms. The XCMFGEN calculations we present here show that X-ray photoionization plays a key role in producing O VI, but in the dense clumps, recombination reduces the amount of O VI to negligible levels. The lower density in the interclump medium leads to a much lower recombination rate, enabling the survival of O VI. The O VI profiles we synthesize with our two-component model reproduce the data well. Models either without X-rays or without any material in the interclump medium fail to. Our calculations also show that C IV is produced within the clumps, while the situation with N V is intermediate between O VI and C IV.

"High-Resolution Chandra X-ray Imaging and Spectroscopy of the Sigma Orionis Cluster," Skinner, Sokal, Cohen, Gagne, Owocki, & Townsend, Ap.J., in press (Apr. 2008).

The sigma Ori system contains five stars, including an O9.5 V + B0.5 V close binary (AB) and a magnetically strong B2 V star (sigma Ori E). This paper analyzes the X-ray emission properties of several dozen stars in the ~3 Myr old cluster surrounding this system as well as presenting detailed analysis of the broad-band and grating spectra of the AB and E components of sigma Ori itself. Unlike other X-ray observations of sigma Ori E, this one does not show any flaring, although the observed quiescent level of X-ray emission is several times higher than that seen in the XMM observation. Our data shows a hint of sinusoidal X-ray variability modulated on the rotation period. The grating spectrum of the O star is characteristically soft and modestly strong. The emission lines are resolved, but quite narrow (HWHM < 300 km/s, which is only about 1/4 of the UV wind terminal velocity). The small f/i ratio of Mg XI implies a location for the x-ray emitting plasma less than one stellar radius above the photosphere. The narrow lines make it difficult to understand the X-rays in the context of line-driven instability wind shocks (the same can be said for beta Cru and delta Ori). The magnetically channeled wind shock scenario would be able to explain the narrow lines, but no magnetic field has been detected on sigma Ori A, and the MCWS scenario would likely lead to higher X-ray temperatures than observed. Finally, the softness of the spectrum rules out contamination by an unseen low-mass companion and the f/i constraints rule out wind-wind binary X-ray production.

"Chandra Spectroscopy of the Hot Star β Crucis and the Discovery of a Pre-Main-Sequence Companion," Cohen, Kuhn ('07), Gagne, Jensen, & Miller, MNRAS, 386, 1855 (June 2008).

Beta Crucis, or Mimosa, is one of the bright stars in the Southern Cross and due both to its high luminosity and its proximity, it is one of the very brightest stars in the sky. At 16 solar masses and spectral type B0.5 III, it is not quite as massive - nor as luminous or hot - as the O stars with their dense, radiation-driven stellar winds. The wind of beta Cru is several orders of magnitude weaker, but still seems to generate a significant X-ray luminosity. In this paper, we use Chandra spectroscopy to test the wind-shock paradigm for X-ray emission in early B stars. We find a very large X-ray filling factor is necessary to explain the overall level of X-ray emission, and that the X-ray emitting plasma is distributed in the wind (from f/i ratios of Ne IX and Mg XI), though the rather narrow X-ray line widths (HWHM ~ 150 km/s) make it quite hard to understand how it is produced by the standard line-driven instability mechanism. We detect modulation of the hard X-rays on the known (beta Cephei) optical pulsation period. We also have discovered an apparent binary companion at a separation of 4 arc seconds. This putative companion has never been seen in optical images, likely because of the extreme brightness contrast with the B star primary. Its X-ray properties (hard, variable, thermal spectrum) are consistent with the companion being a low-mass pre-main-sequence star, which would make the beta Cru system a Lindroos binary.

The above-linked annotated Palomar image came from a posting on the Psychohistorian blog. You can also check out our press release, article in the New York Times, (tangentially) related sound clip from The Incredibles, and the drink menu from the aptly named B Star Bar. Of course, the press interest was due not to the science, but rather to the presence of beta Crucis on the flag of Australia (the design of which is being reconsidered, though not because of our work) and at least four other national flags, plus many other flags. Note that the image is flipped on the flag of Brazil which has an old-fashioned "outside" view of the celestial sphere.

"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.

"X-ray Emission from O Stars," Cohen, IAU 250: Massive Stars as Cosmic Engines, Kauai, Hawaii (2007). To appear in IAU Symposium No. 250, eds. Bresolin, Crowther, & Puls (2008). This is a color version, with one extra figure (and with the transcribed questions and answers that followed the talk). There is also a black-and-white version, identical to the six-page article that will appear in the bound conference proceedings.

"X-rays from Magnetically Channeled Winds of OB Stars," Cohen, IAU 250: Massive Stars as Cosmic Engines, Kauai, Hawaii (2007). To appear in IAU Symposium No. 250, eds. Bresolin, Crowther, & Puls (2008).

"Quantitative Analysis of Resolved X-ray Emission Line Profiles of O Stars," Cohen, Leutenegger, & Townsend, International Workshop on Clumping in Hot-Star Winds, University of Potsdam, Germany (2007). To appear electronically: University of Potsdam (Universitats-Verlag).

"Resonance Scattering in the X-ray Emission Line Profiles of ζ Puppis," Leutenegger, Cohen, Kahn, Owocki, & Paerels, International Workshop on Clumping in Hot-Star Winds, University of Potsdam, Germany (2007). To appear electronically: University of Potsdam (Universitats-Verlag).

"Flow Dynamics and Plasma Heating of Spheromaks in SSX," Brown, Cothran, Cohen, Horwitz ('07), & Chaplin ('07), Innovative Confinement Concepts Workshop, University of Maryland, (2007). To appear in Journal of Fusion Energy.

"Diagnostics of Disks Around Hot Stars," Cohen, Hanson, Townsend, Bjorkman, & Gagne, The Nature and Evolution of Disks Around Hot Stars, Eastern Tennessee State University, Johnson City, TN (2004). Appears in ASP Conf. Ser., 337, p. 59 (2005).

"Analysis of Doppler-Broadened X-ray Emission Line Profiles from Hot Stars," Cohen, Kramer, & Owocki, MSSL Workshop on High-Resolution X-ray Spectroscopy, Dorking, Surrey, UK, (2002). Available via cd, and on-line.

"X-rays from Hot Stars and Young Stellar Objects," Gagne, Cohen, Owocki, & ud-Doula, X-rays in Sharp Focus: Chandra Science Symposium, St. Paul, MN (2001). Appears in ASP Conf. Ser., 262, p. 31 (2002).

"EUV/X-ray Emission and the Thermal and Ionization Structure of B Star Winds," Cohen, Thermal and Ionization Aspects of Flows from Hot Stars: Observation and Theory, Tartu, Estonia, (1999). Appears in ASP Conf. Ser., 204, p. 65 (2000).

"X-ray Emission from Isolated Be Stars," Cohen, The Be Phenomenon in Early-Type Stars, IAU colloquium 175, Alicante, Spain, (1999). Appears in ASP Conf. Ser., 214, p. 156 (2000).

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