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Preprint 05/10

NLTE model atmospheres for the hottest white dwarfs: Spectral analysis of the compact component in nova V4743 Sgr

T. Rauch (1), M. Orio (2, 3), R. Gonzales-Riestra (4), T. Nelson (5, 6), M. Still (7, 8), K. Werner (8), J. Wilms (9)

(1) Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Tübingen, Germany
(2) Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, Padova, Italy
(3) Department of Astronomy, University of Wisconsin-Madison, Madison, USA
(4) XMM-Newton Science Operations Centre, European Space Astronomy Centre (ESAC), Villanueva de la Canada, Madrid, Spain
(5) NASA Goddard Space Flight Center, Greenbelt, USA
(6) University of Maryland, Baltimore, USA
(7) NASA Ames Research Center, Moffett Field, USA
(8) University College London, Mullard Space Science Laboratory, Dorking, United Kingdom
(9) Dr. Remeis-Sternwarte, Astronomical Institute of the University Erlangen-Nuremberg, Bamberg, Germany

ApJ 717 (2010), 363-371

Abstract. Half a year after its outburst in September 2002, nova V4743 Sgr evolved into the brightest supersoft X-ray source in the sky with a flux maximum around 30A. We calculated grids of synthetic energy distributions (SEDs) based on NLTE model atmospheres for the analysis of the hottest white dwarfs and present the result of fits to Chandra and XMM-Newton grating X-ray spectra of V4743 Sgr of outstanding quality, exhibiting prominent resonance lines of C V, C VI, N VI, N VII, and O VII in absorption. The nova reached its highest effective temperature (T_eff = 740 ± 70kK) around April 2003 and remained at that temperature at least until September 2003. We conclude that the white dwarf is massive, about 1.1 - 1.2 M_sun. The nuclear-burning phase lasted for 2 to 2.5 years after the outburst, probably the average duration for a classical nova. The photosphere of V4743 Sgr was strongly carbon deficient (about times solar) and enriched in nitrogen and oxygen (> 5 times solar). Especially the very low C/N ratio indicates that the material at the white dwarf's surface underwent thermonuclear burning. Thus, this nova retained some of the accreted material and did not eject all of it in outburst. From March to September 2003, the nitrogen abundance is strongly decreasing, probably new material is already been accreted at this stage.
 
Key words: stars: abundances -- stars: AGB and post-AGB -- stars: atmospheres -- stars: individual: V4743 Sgr -- novae, cataclysmic variables -- white dwarfs

Preprint (1,22 Mb PDF file including figures)

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