Refereed Papers in the Last Two Years
see also Papers Older Than Two Years
"Magnetic Massive Stars as Progenitors of 'Heavy' Stellar-Mass Black Holes," Petit, Keshthelyi, MacInnis, Cohen, Townsend, Wade, Thomas, Owocki, Puls, & ud-Doula, MNRAS, 466, 1052 (Apr. 2017).
The first LIGO discovery was a merging pair of black holes. These black holes were notably for being heavy by the standards collapsed massive stars (> 30 Msun). It was immediately assumed that the progenitors of these black holes were massive stars with significantly sub-solar metallicity – since line-driven stellar winds deplete high-mass stars of much of their original mass. And these winds are driven by metal lines and so in a low-metallicity environment, winds are weaker. However, in this paper we show that there is a channel for massive black hole formation that does not require a low metallicity environment. This involves large scale magnetic fields, which are found in roughly ten percent of massive stars. A strong, dipole field reduces the mass-loss rate by trapping the wind in a magnetosphere, from where it falls back onto the star. We present a suite of models, including one that shows a magnetized, solar metallicity 80 Msun star that retains fully 20 Msun of additional mass compared to an unmagnetized star with the same properties.
"An 'Analytic Dynamical Magnetosphere' Formalism for X-ray and Optical Emission from Slowly Rotating Magnetic Massive Stars," Owocki, ud-Doula, Sundqvist, Petit, Cohen, & Townsend, MNRAS, 462, 3830 (Nov. 2016).
We present an analytic model for the magnetospheres of slowly rotating magnetic massive stars. Without centrifugal support, magnetospheric plasma falls back onto the star periodically. Our analytic model has three plasma components along each field line: cold pre-shock wind, hot post-shock plasma, and cooled infalling plasma. The model computes the physical variables along each field line, which enables the calculation of diagnostics, like Halpha and X-ray emission. We benchmark the ADM properties against time-averaged MHD simulations.
"KELT-17b: A Hot-Jupiter Transiting an A-star in a Misaligned Orbit Detected with Doppler Tomography," Zhou, et al., AJ, 152, 136 (Nov. 2016).
The Peter van de Kamp Observatory at Swarthmore contributed to the discovery of this exoplanet (via the transit method). The host star is both bright (V = 9.2) and hot (7450 K). It is only the fourth A star known to be an exoplanet host, and one of the most rapid rotators among exoplanet host stars (44 km/s). The planet is a hot Jupiter in a 3 day orbit. The orbit is misaligned with the star's rotation – we present Doppler tomography showing this.
"X-ray, UV and optical analysis of supergiants: epsilon Ori," Puebla, Hillier, Zsargó, Cohen, & Leutenegger, MNRAS, 456, 2907 (Mar. 2016).
We use the XCMFGEN code to perform multi-wavelength modeling of the early B supergiant, epsilon Ori (middle star in Orion's belt). XCMFGEN computes statistical equilibrium (NLTE level populations) in a massive star wind, including the effects of X-ray photoionization. The goals are two-fold – to characterize the wind properties and to test the consistency of the derived parameters in different wavelength bands. In general, we do achieve good consistency for a model that has a clumping-corrected mass-loss rate of Mdot/sqrt(f_cl) = 1.6 X 10-6 Msun/yr, which is consistent with my earlier determination from modeling the X-ray profile shapes (Cohen et al. 2014). Other notable results include a spatially stratified X-ray temperature distribution, slight CNO processing in the surface/wind abundances, and a high value of the wind velocity parameter, beta ~ 2.
"The MiMeS Survey of Magnetism in Massive Stars: Introduction and Overview," Wade et al., MNRAS, 456, 2 (Feb. 2016).
This paper provides context, background information, and observational data and statistics for the Magnetism in Massive Stars (MiMeS) project. This was a large project carried out between 2008 and 2013 using spectropolarimetric measurements made at several large observatories to study the incidence and characteristics of surface magnetic fields in sample of hundreds of OB stars. The sensitivity was typically 100 G for surface dipole fields. Stable fields, consistent with a fossil origin, were detected in 5 to 10 percent of OB stars across most categories (notably, though, not Be stars, for which the incidence is zero).
"X-ray emission from the giant magnetosphere of the magnetic O-type star NGC 1624-2," Petit, Cohen, Wade, Nazé, Owocki, Sundqvist, ud-Doula, Fullerton, Leutenegger, & Gagné, MNRAS, 453, 3288 (Nov. 2015).
NGC 1624-2 is the O star with the largest magnetosphere (in excess of 10 stellar radii). New Chandra ACIS observations made at two extreme rotational phases show variability consistent with increased soft-X-ray absorption within the magnetosphere at the phase corresponding to a more edge-on view. This is consistent with the expectations of models that show a primary X-ray source location roughly one stellar radius above the surface and larger column densities through the equatorial portion of the magnetosphere.
"Detection of 610-MHz Radio Emission from Hot Magnetic Stars," Chandra, et al., MNRAS, 452, 1245 (Sep. 2015).
We report on a small survey of magnetic O and B stars using low-frequency radio observations. Radio emission is expected from the magnetospheres of these stars, and it is expected to be rotationally modulated. We do detect rotationally modulated radio emission, although at the lowest frequency band – 610 MHz – the O stars are not detected while the B stars are. This is consistent with free-free wind absorption in the O stars, which have denser winds, extending beyond their magnetospheres. We detect strong and rapid variability in one star: HD 133880, which is a very rapidly rotating late B star.
"Ground-Based Transit Observations of the HAT-P-18, HAT-P-19, HAT-P-27/WASP40 and WASP-21 Systems," Seeliger et al., MNRAS, 451, 4060 (Aug. 2015).
Transit timing variations (TTVs) in known transiting exoplanets may be expected in the presence of otherwise unseen planetary companions. In this study, carried out by the YETI collaboration, we use data from multiple observatories to search for TTVs in four systems where they are expected to be relatively large, and possibly within the reach of ground-based observatories. While we were not able to detect TTVs in any of these systems, we place upper limits on any that might be present, and also refine the determinations of the stellar, orbital, and exoplanet properties.
"Confirming HD 23478 as a New Magnetic B Star Hosting an H-alpha-Bright Centrifugal Magnetosphere," Sikora et al., MNRAS, 451, 1928 (Aug. 2015).
A B3 V star, HD 23478 had been identified, based on its IR properties, as possibly hosting a centrifugal magnetosphere. Here we report measurements of a strong magnetic field and associated H-alpha emission, consistent with the expectations for a centrifugally supported magnetosphere. We use high-quality ESPaDOnS spectra to determine the stellar properties and measure the H-alpha variability as well as to constrain the magnetic field properties. Our 23 spectropolarimetric observations are consistent with a dipole field with a polar strength of at least 9.5 kG and a field that is closely aligned with the rotation axis.
Selected Meeting Proceedings
Meeting and Talk Presentations are also available
"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).
|Return to David's home page|
|David Cohen: cohen -at- astro -dot- swarthmore -dot- edu
Last modified: March 28, 2017