OB stars with strong radiation-driven stellar winds and large-scale magnetic fields generate strong and hard X-ray emission via the Magnetically Channeled Wind Shock (MCWS) mechanism. In this brief paper, I describe four separate X-ray diagnostics of the MCWS mechanism in OB stars, with applications to the prototype young O star, theta-1 Ori C

Cohen, David H.

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Swarthmore College Works Physics & Astronomy Faculty Works Physics & Astronomy 12-1-2007 X-Rays From Magnetically Channeled Winds Of OB Stars, In 'Magnetic Massive Stars' David H. Cohen Swarthmore College, dcohen1@swarthmore.edu This work is brought to you for free and open access by . It has been accepted for inclusion in Physics & Astronomy Faculty Works by an authorized administrator of Works. For more information, please contact myworks@swarthmore.edu. Follow this and additional works at: https://works.swarthmore.edu/fac-physics  Part of the Astrophysics and Astronomy Commons Let us know how access to these works benefits you  Recommended Citation David H. Cohen. (2007). "X-Rays From Magnetically Channeled Winds Of OB Stars, In 'Magnetic Massive Stars'". Massive Stars As Cosmic Engines: Proceedings Of The 250th Symposium Of The International Astronomical Union. Volume 250, 581-582. DOI: 10.1017/S1743921308020966 https://works.swarthmore.edu/fac-physics/62 arXiv:0801.4780v1  [astro-ph]  30 Jan 2008Massive Stars as Cosmic EnginesProceedings IAU Symposium No. 250, 2008F. Bresolin, P.A. Crowther & J. Puls, eds.c© 2008 International Astronomical UnionDOI: 00.0000/X000000000000000XX-rays from Magnetically Channeled Windsof OB StarsDavid H. Cohen11Swarthmore College, Department of Physics and Astronomy, 500 College Ave., Swarthmore,Pennsylvania 19081OB stars with strong radiation-driven stellar winds and large-scale magnetic fieldsgenerate strong and hard X-ray emission via the Magnetically Channeled Wind Shock(MCWS) mechanism (Shore & Brown 1990; Babel & Montmerle 1997; ud-Doula &Owocki 2002). There are four separate X-ray diagnostics that confirm the MCWS sce-nario for the young, magnetized O star that illuminates the Orion Nebula, θ1 Ori C, andconstrain the physical properties of its X-ray emitting magnetosphere:1. High X-ray temperatures, determined from thermal spectral model fitting. Thedifferential emission measure of θ1 Ori C peaks at temperatures above 10 MK, whichis in contrast to the few million K peak temperatures in mature, unmagnetized O stars(Wojdowski & Schulz 2005), and which is well reproduced by MHD simulations of theMCWS mechanism (Gagne´ et al. 2005).2. Relatively narrow X-ray emission lines. The X-ray emitting plasma in the MCWSscenario is predominantly in the closed magnetic field regions and thus the plasma velocityis relatively low and the associated Doppler line broadening is modest. This is seen inthe MHD simulations and confirmed by the Chandra grating observations.3. The rotational modulation of the X-ray emission is consistent with part of themagnetosphere being eclipsed near phase 0.5, when the viewing orientation is magneticequator-on. The depth of the eclipse provides information about the location of the X-ray emitting plasma (deeper eclipses imply more plasma close to the star). The observedeclipse depth for θ1 Ori C implies that the bulk of the plasma is within a stellar radiusof the photosphere. This is a somewhat closer than the MHD simulations predict (e.g.Gagne´ et al. 2005).4. The ratio of the forbidden to intercombination line strengths in helium-like ionsalso puts a constraint on the location of the X-ray emitting plasma, via the sensitivityof these line ratios to the local UV mean intensity. The closer the hot plasma is to thephotosphere, the stronger the UV photoexcitation of electrons from the upper level ofthe forbidden line to the upper level of the intercombination line, and the smaller the f/iratio. This is demonstrated in Fig. 1 for the Mg xi complex in the co-added (over fourobservations) Chandra grating spectrum of θ1 Ori C, where we see that the very weakforbidden line requires a plasma location below r ≈ 2 R∗.ReferencesBabel, J. & Montmerle, T. 1997, ApJ (Letters), 485, L29Gagne´, M., Oksala, M., Cohen, D.H., Tonnesen, S.K., ud-Doula, A., Owocki, S.P., Townsend,R.H.D., & MacFarlane, J.J. 2005, ApJ, 628, 986Shore, S.N. & Brown, D.N. 1990, ApJ, 365, 665ud-Doula, A. & Owocki, S.P. 2002, ApJ, 576, 413Wojdowski, P. & Schulz, N.S. 2005, ApJ, 627, 9531PREPRINT2 David H. CohenFigure 1. A snapshot from a 2-D MHD simulation of θ1 Ori C, taken from Gagne´ et al. (2005),showing emission measure in grayscale, magnetic field lines as white contours, and with a thick,black contour enclosing plasma with temperature above 106 K (lower left). The three otherpanels show the Chandra spectrum in the vicinity of the helium-like Mg xi complex, with amodel of the resonance, intercombination (i), and forbidden (f) lines overplotted. The relativestrengths of the f and i lines are different in each of the three panels, as the three models werecalculated assuming a source location of 1.2 R∗, 2.1 R∗, and 4.0 R∗, respectively, starting at thetop left and moving clockwise. The arrows indicate the approximate location in each case. Theintermediate case – r = 2.1 R∗, which seems to agree with the MHD simulation – is marginallyconsistent with the data (68% confidence limit).

X-Rays From Magnetically Channeled Winds Of OB Stars, In \u27Magnetic Massive Stars\u27

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X-Rays From Magnetically Channeled Winds Of OB Stars, In \u27Magnetic Massive Stars\u27

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