Papers

"Discovery of a Magnetic Field in the O9 Sub-Giant Star HD 57682 by the MiMeS Collaboration," Grunhut, et al. MNRAS, in press (Oct. 2009).

Using the ESPaDOnS spectropolarimeter on the CFHT, we have detected a magnetic field in HD 57682, making it only the fourth O star with a measured magnetic field. The significance of the field detection is very high, but there are large uncertainties on some of the field parameters. Overall, the data are consistent with a 1700 G dipole field (Bayesian probabilities for the field parameters are shown in the right-hand figure, below) and very slow rotation (consistent with the 31 day upper limit inferred from the vsini and the star's radius). We also present time-variable Hα emission, consistent with rotationally modulated magnetospheric emission, and archival IUE spectra from which we derive a low mass-loss rate of only 10^-9 solar masses per year, implying a large magnetic confinement parameter (η_* ~ 10⁴). ROSAT measurements indicate a large X-ray filling factor, which would be consistent with MCWS X-rays, but also soft emission, which implies that the MCWS-related shocks are not very strong.
 

"No Transition Disk? Infrared Excess, PAH, H₂, and X-rays from the Weak-Lined T Tauri Star DoAr 21," Jensen, Cohen, & Gagne Ap.J., 730, 252 (Sep. 2009).

DoAr 21 is an exceptionally X-ray bright low-mass pre-main-sequence star in the rho Oph star formation region. It is relatively high mass (~2 Msun) and young (< 1 Myr) and there is mixed information in the literature about the presence of an accretion disk around it. We present here not only a new, high-resolution Chandra X-ray spectrum, but also new optical spectroscopy and infrared imaging. We incorporate this new, multi-wavelength spectral data with existing information from the literature to establish that while there is, in fact, circumstellar material within several 100 AU of the star, it appears not to be in the form of an accretion disk but rather is more akin to a photodissociation region that is strongly irradiated by the high energy emission of DoAr21. This highlights both the very young ages at which some stars are found without disks, and the extreme radiation environment around even late-type pre-main-sequence stars.
 

"Spectroscopic Measurements of Temperature and Plasma Impurity Concentration During Magnetic Reconnection at the Swarthmore Spheromak Experiment," Chaplin ('07), Brown, Cohen, Gray, & Cothran, Phys. Plasmas, 16, 042505 (Apr. 2009).

My colleague Michael Brown studies magnetic reconnection in the Swarthmore Spheromak Experiment, in which two spheromaks merge and undergo magnetic reconnection and relaxation. He and his group study the dynamical evolution of the magnetic field (using in situ magnetic probes) and the production of non-thermal particles during reconnection [pdf], among other topics. In this paper, we use UV and soft X-ray spectroscopy to study the time-dependent temperature during the reconnection process in the experiment. We also use the UV spectra to constrain the relative amounts of C, N, and O in the primarily hydrogen plasma. We accomplish both of these goals via detailed non-LTE statistical equilibrium and spectral modeling, for which we use the PrismSpect and ATBASE codes. The results we report on in this paper are augmented by spectrally resolved UV line profile measurements, from which we derive time-dependent ion temperatures in the reconnection region of the plasma. We derive from the data peak temperatures of about 20 eV, consistent with a simple model of reconnection energetics. We also find that the carbon emission is consistent with about one out of every hundred atoms being carbon, whereas the nitrogen and oxygen line intensities imply a much lower fraction of these impurities. We detect a burst of soft X-rays during reconnection, but their spectral evolution is not consistent with a single temperature, equilibrium spectral model.

This work grew out of Vernon Chaplin's senior thesis work at Swarthmore, for which he was a finalist for the Apker Award, in 2007. Vernon is currently in the Physics Ph.D. program at Caltech.

"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., 685, L149 (Oct. 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., 683, 796 (Aug. 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.

"X-ray Spectroscopy of the Radiation-Driven Winds of Massive Stars: Line Profile and Line Ratio Diagnostics," Cohen, 16th International Conference on Atomic Processes in Plasmas, Monterey, CA (2009). To appear in AIP-published proceedings.

"Modelling, Design and Diagnostics for a Photoionised Plasma Experiment," Hall, Durmaz, Mancini, Bailey, Rochau, Rosenberg, Cohen, Golovkin, MacFarlane, Sherril, Abdallah, Heeter, Foord, Glenzer, & Scott, High Energy Density Laboratory Astrophysics, St. Louis (2008). To appear in Astrophysics and Space Science, vol. 307.

"X-ray Emission from O Stars," Cohen, IAU 250: Massive Stars as Cosmic Engines, Kauai, Hawaii (2007). Appears in IAU Symposium No. 250, eds. Bresolin, Crowther, & Puls, Cambridge University Press, p. 17 (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). Appears in IAU Symposium No. 250, eds. Bresolin, Crowther, & Puls, Cambridge University Press, p. 577 (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). Appears 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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