Refereed Papers in the Last Two Years
"Helium-like X-ray line complexes show that the hottest plasma on the O supergiant zeta Puppis is in its wind," Cohen, Overdorff, Leutenegger, Gagne, Petit, David-Uraz, MNRAS, 513, 1609 (Jun. 2022).
Certain emission lines in X-ray spectra are affected by the strength of the local UV radiation field, which enables the so-called helium-like forbidden-to-intercombination line strength ratio to be used as a diagnostic of the distance the hot, X-ray emitting plasma in O star winds is from the star's surface. Here we apply this diagnostic to two Chandra datasets of our favorite O star, zeta Puppis. The analysis we present includes detailed treatment of the many satellite lines that are blended with the helium-like diagnostic lines, which is something that hasn't been done before. We find a location of the X-ray plasma well-out in the wind flow (above r ~ 1.5 Rstar) and we find changes in the implied properties between the two sets of observations.
"Chandra grating spectroscopy of embedded wind shock X-ray emission from O stars shows low plasma temperatures and significant wind absorption," Cohen, Parts, Doskoch, Wang, Petit, Leutenegger, Gagne, MNRAS, 503, 715 (May 2021).
We fit the Chandra spectra of six O and B stars with X-ray emission primarily due to embedded wind shocks using a multi-temperature emission model (bvapec) combined with a wind absorption model (vwindtab). We find quite similar differential emission measure distributions for all the stars — consistent with a power law (n ~ -2.3) truncated at about T = 10 million K. X-ray absorption by the dominant, cool component of the wind is significant for all the stars. For ζ Pup, 95% of the generated soft X-rays (λ > 15 A) are absorbed.
"Chandra spectral measurements of the O supergiant zeta Puppis indicate a surprising increase in the wind mass-loss rate over 18 years," Cohen, Wang, Petit, Leutenegger, Dakir, Mayhue, & David-Uraz, MNRAS, 499, 6044 (Dec. 2020).
A new (2018-19) long Chandra data set shows a surprising change in the X-ray emission line profile shapes, consistent with an increase in the wind mass-loss rate, since the previous observation, taken 18 years earlier. We show the profile shape changes via both a moment analysis and fitting of Gaussians to each individual, relatively unblended line seen in both the new and the old datasets. To interpret these changes, we fit line profile models that include Doppler broadening and wind absorption and find a mass loss rate of 2.47 X 10-6 Msun/yr. That is 40 percent higher than the value derived from the profile fitting of the older data. The X-ray line fluxes have also increased, though by less than 40 percent. The shorter wavelength lines, which are less subject to absorption, show larger flux increases compared to the more absorbed, longer wavelength lines. These results are consistent with a mass-loss rate increase that causes an increase in both wind emission and wind absorption.
"KELT-25 b and KELT-26 b: A Hot Jupiter and a Substellar Companion Transiting Young A Stars Observed by TESS," Martínez, et al., AJ, 160, 111 (Sep. 2020).
These two KELT planets both orbit A stars and have short periods. KELT 25b may be as massive as 68 Jupiter masses, while KELT 26b is a very inflated hot Jupiter on a highly misaligned orbit.
Selected Meeting Proceedings
"Chandra grating spectroscopy finds a surprising change in the wind mass-loss rate of the O supergiant zeta Puppis over 18 years," Cohen, Rak, & Wang. A talk presented at the 19th High Energy Astrophysics Division meeting of the AAS (2022).
"The X-ray View of OB Star Wind Structure and Dynamics," Cohen, Circumstellar Dynamics at High Resolution, Foz do Iguacu, Brazil (2012). Appears in ASPC, 464, 267 (2012).
"X-ray Spectroscopy of the O2 If* star HD 93129A," Cohen, Four Decades of Research on Massive Stars, Lac Taureau (2011). Appears in ASPC, 465, 147 (2013). Also, from the same meeting and conference proceedings, "An X-ray Survey of Colliding Wind Binaries," Gagné, Fehon, Savoy, Cartagena, Cohen, & Owocki; and "The Physical Basis of the Lx/Lbol Empirical Law for O-Star X-rays," Owocki, Sundqvist, Cohen, & Gayley.
"X-ray Spectral Diagnostics of Activity in Massive Stars," Cohen, Wollman, & Leutenegger, IAU 272: Active OB Stars: Structure, Evolution, and Mass Loss, Paris (2010). Appears in IAU Symposium No. 272, eds. Neiner, Wade, Meynet, & Peters, Cambridge University Press, p. 348 (2011).
"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). Appears in AIP conf. ser. 1161, p. 132 (2009).
"Modelling, Design and Diagnostics for a Photoionised Plasma Experiment," [or ADS] Hall, Durmaz, Mancini, Bailey, Rochau, Rosenberg, Cohen, Golovkin, MacFarlane, Sherril, Abdallah, Heeter, Foord, Glenzer, & Scott, High Energy Density Laboratory Astrophysics, St. Louis (2008). Appears in Astrophysics and Space Science, 322, 117 (2009).
"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 appears in the bound conference proceedings.
"X-rays from Magnetically Channeled Winds of OB Stars," [or ADS] 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, & Gagné, 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,"
Gagné, 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).