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The (Double) White Dwarf Binary SDSS 1257+5428

Kulkarni, S. R. and van Kerkwijk, M. H. (2010) The (Double) White Dwarf Binary SDSS 1257+5428. Astrophysical Journal, 719 (2). pp. 1123-1131. ISSN 0004-637X.

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SDSS 1257+5428 is a white dwarf in a close orbit with a companion that has been suggested to be a neutron star. If so, it hosts the closest known neutron star, and its existence implies a great abundance of similar systems and a rate of white dwarf neutron-star mergers similar to that of the type Ia supernova rate. Here, we present high signal-to-noise spectra of SDSS 1257+5428, which confirm an independent finding that the system is in fact composed of two white dwarfs, one relatively cool and with low mass and the other hotter and more massive. With this, the demographics and merger rate are no longer puzzling (various factors combine to lower the latter by more than 2 orders of magnitude). We show that the spectra are fit well with a combination of two hydrogen model atmospheres, as long as the lines of the higher-gravity component are broadened significantly relative to what is expected from just pressure broadening. Interpreting this additional broadening as due to rotation, the inferred spin period is short, about 1 minute. Similarly rapid rotation is only seen in accreting white dwarfs that are magnetic; empirically, it appears that in non-magnetized white dwarfs, accreted angular momentum is lost by nova explosions before it can be transferred to the white dwarf. This suggests that the massive white dwarf in SDSS 1257+5428 is magnetic as well, with B ≃ 10^5 G. Alternatively, the broadening seen in the spectral lines could be due to a stronger magnetic field, of ~10^6 G. The two models can be distinguished by further observations.

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Kulkarni, S. R.0000-0001-5390-8563
Additional Information:© 2010 American Astronomical Society. Received 2010 March 10; accepted 2010 June 9; published 2010 July 27. We thank E. S. Phinney, G. Nelemans, C. Badenes, and L. Bildsten for discussions. LRIS has been upgraded since its commissioning and as a result is perhaps now the most efficient single object optical spectrometer. We thank the teams that made these improvements possible (J. McCarthy and C. Steidel, leaders for LRIS-Blue upgrade; C. Rokosi, leader for the LRIS-Red upgrade; and J. Miller and D. Phillips, leaders for the ADC sub-system). We are grateful to the staff of the WM Keck Observatory for their excellent service and to the librarians who maintain the ADS and Simbad databases.
Subject Keywords:binaries: close; stars: individual (SDSS J125733.63+542850.5); white dwarfs
Issue or Number:2
Classification Code:PACS: 97.20.Rp; 97.60.Jd; 97.10.Gz; 95.30.Ky; 97.10.Ld; 97.10.Kc
Record Number:CaltechAUTHORS:20100903-105234075
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Official Citation:S. R. Kulkarni and M. H. van Kerkwijk 2010 ApJ 719 1123 doi: 10.1088/0004-637X/719/2/1123
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:19780
Deposited By: Jason Perez
Deposited On:08 Sep 2010 14:40
Last Modified:03 Oct 2019 02:01

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