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19 94MNRAS.210. .4 99B Mon. Not. R. Astron. Soc. 270,499 Mon. Not. R. Astron. Soc. 270,499-515 ( 1994) 99B .4 A ROSAT survey of hot DA white dwarfs in non-interacting binary systems 94MNRAS.210. 19 M. A. Barstow,lllrf^: J. B. Holberg,2* T. A. Fleming,3 4 M. C. Marsh,1 *t D. Koester5 and D. Wonnacott6f 1 Department of Physics and Astronomy, University of Leicester, University Road, Leicester LEI 7RH 2 Lunar and Planetary Laboratory, University of Arizona, Gould-Simpson Building, Tucson, AZ 85721, USA 3 Steward Observatory, University of Arizona, Tucson, AZ 87521, USA A Max-Planck-Institut für Extraterrestriche Physik, Giessenbachstrasse, D-8046 Garching, Germany 5 Institut fir Theoretische Physik und Sternwarte, Olshausenstrasse, Physikzentrum, Kiel, Germany 6Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Holmbury St Mary, Dorking, Surrey RH56NT Accepted 1994 April 25. Received 1994 April 20; in original form 1994 January 5 ABSTRACT A number of new non-interacting binary systems comprising a white dwarf plus a normal stellar companion have been discovered by the ROSAT X-ray and EUV sky surveys. We discuss the identification of nine of these objects, determine the temperatures and gravities of the white dwarfs, and compare their atmospheric compositions with the population of isolated white dwarfs. As a result, it may be poss- ible to estimate how many hot white dwarfs in total reside in binary systems. Coronal emission is detected in several systems, and its nature, in the light of the likely age of the binaries, is discussed. These binary systems are important representatives of several possible evolutionary paths, including common-envelope and mass-transfer phases. Their further study will be of great importance in understanding stellar evolu- tion in general, and binaries in particular. Key words: stars: atmospheres - binaries: general - white dwarfs - ultraviolet: stars - X-rays: stars. of the Sirius binary system (distance 2.64 pc) the two stars 1 INTRODUCTION could not be resolved, and the visible light from Sirius B, the The majority of the known white dwarfs are isolated stars. white dwarf component, would then be completely hidden. This is perhaps not surprising, as they have mostly been Several unresolved binaries with hot white dwarf com- identified through optical blue star colour and proper motion panions have been discovered serendipitously. The white surveys (e.g. Green, Schmidt & Liebert 1986). Thus there dwarf in the well-studied system V471 Tauri was revealed as exists a well-known inherent bias against detection of white a result of its eclipse by the K dwarf primary (Nelson & dwarfs in unresolved binary systems. For example, the spec- Young 1970). The spectral signatures of others have been trum of any companion star of type K or earlier will com- found in the IUE spectra of a number of stars - ÇCap pletely dominate that of the white dwarf, rendering it (Böhm-Vitense 1980), 56 Peg (Schindler et al. 1982; Stencel, undetectable at optical wavelengths. Even if the companion Neff & McClure 1984), 4 o1 Ori (Johnson & Ake 1986) and is an M star, the composite spectrum is likely to redden the HD 27483 (Böhm-Vitense 1993). However, a systematic colours, thereby preventing the white dwarf appearing as a search of the then-existing IUE archive by Shipman & Geczi blue object. Indeed, were it not for the proximity to the Earth ( 1989) revealed no further white dwarf companions. The ROSAT X-ray and EUV sky surveys (Triimper 1992 and Pounds et al. 1993, respectively) have given us an oppor- * Guest Observer with the International Ultraviolet Explorer {IUE) tunity to search for hot white dwarfs in unresolved binaries satellite. through detection of EUV and X-ray emission from the fGuest Observer with the Voyager Ultraviolet Spectrometers (UVS). hidden white dwarf component. A number of such dis- ÍGuest Observer at the South African Astronomical Observatory coveries with companions covering a large range of spectral (SAAO). types, from A to M, have already been presented in several © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 99B 500 M A. Barstow et al .4 papers (e.g. Fleming et al. 1991; Cooke et al. 1992; Barstow generally greater than even the most active late-type stars. et al. 1993c; Hodgkin et al. 1993; Landsman, Simon & However, the distance of an object must be known for this to Bergeron 1993; Tweedy et al. 1993; Wonnacott, Kellett & be a useful discriminator. An additional means of confirming Stickland 1993). Here we concentrate in particular on those the presence of a white dwarf source is to study the pulse- binary systems with companions in the range A to K, exclud- height spectrum of the ROSAT position-sensitive propor- 94MNRAS.210. ing the DA+M pre-CV systems which we will consider tional counter (PSPC) data. For white dwarfs, all the photons 19 separately. The ROSAT discoveries have enlarged the total lie below the 0.28-keV carbon K edge of the counter win- sample of hot white dwarfs with unresolved A to K com- dow. In contrast, all other objects generally have spectra panions by nine systems. As relatively few such systems were extending to higher energies. already known, this is a highly significant increase. Interest- When a likely white dwarf system has been found in the ingly, the Hyades triple system HD 27483, observed by survey data, further observations are needed to confirm its Böhm-Vitense (1993), is an EUV source (Pounds et al. nature. For white dwarfs with companions later than K, their 1993), but it is far from clear whether or not the emission is binary nature is evident in composite optical spectra, show- from the white dwarf, which may be too cool (T= 23 000K) ing the H Balmer absorption series typical of DA white to be detected at that distance, or the F6V companions. dwarfs and TiO bands from the M star companion. For com- Using IUE and Voyager observations, we have carried out panions of earlier spectral type, the white dwarf component a systematic survey of these objects, determining their effec- cannot be discerned in the optical spectrum. However, it is tive temperatures and gravities from the Lyman line profiles possible to discriminate between the two stars using far-UV and UV continuum. Knowing the temperatures and gravities, spectra. Several of the white dwarfs in this sample have the ROSAT X-ray and EUV fluxes may be used to deter- already been identified in this way using IUE observations mine whether the white dwarf atmospheres are pure hydro- (see references in Tables 1 and 2). gen or contain any additional sources of opacity. Although Deep exposures on bright A stars, performed during the we do not, in general, know the mass ratio for these systems, calibration phase of the ROSAT mission to quantify any UV the companions to the white dwarfs are all K or earlier. leak in the filters, demonstrate that, in general, they are not Therefore, in most cases, the white dwarf is probably the less EUV/X-ray sources. Therefore any EUV/X-ray source massive component. For the purposes of differentiation, we apparently associated with an A star is of immediate interest. will refer to the white dwarf as the secondary, and to its com- While it is clear that any A star ‘source’ must have some kind panion star as the primary. of companion which is responsible for the EUV/X-ray emis- sion, stars of spectral type F-K can be intrinsic sources in 2 SELECTION AND IDENTIFICATION OF their own right, and finding those with white dwarf com- WHITE DWARFS IN UNRESOLVED BINARIES panions is more difficult. Initially, we selected a sample of these on the grounds of white dwarf-like EUV colours and Approximately 120 isolated, hot white dwarfs have been luminosities. After eliminating field white dwarfs which are detected in either the ROSAT X-ray or EUV all-sky surveys, chance associations with catalogued late-type stars, and and more than 50 per cent of these are previously uncata- those objects where the colours are biased by EUV flares in logued as degenerate stars (Fleming et al. 1993). Comparison the S2 filter, seven stars with spectral type in the range F to K of the EUV and X-ray properties of these stars with normal appear to have white dwarf companions, including the sys- stars and other sources shows that they have very distinctive tem HD 33959C, published earlier (Hodgkin et al. 1993). soft spectra, when the interstellar H i column is relatively low. Table 1 lists the new white dwarf binaries discovered so In particular, the ratio of the Wide Field Camera (WFC) far in the ROSAT sky survey, indicating the spectral type of survey S2 and SI filter count rates typically exceeds a factor the companion, their visual magnitudes and references to 3. Secondly, the EUV luminosities of these white dwarfs are earlier publications. On the basis of this information, an Table 1. Physical parameters of the companion stars. RE No. Cat. Name Spectral type V magnitude d est. (pc) References RE0044+093 BD+08o102 G 10.0 66-132 RE0228-611 HD 15638 F3-F6V 8.8 110-174 1 RE0459-101 HR1608 K0IV 5.38 38-54 1,2 RE0515+324 HD33959C F4V 7.95 89 1.3 RE1111-224 ß Ort A1V/A2III 4.48 19-87 1.4 RE1925—566 G2-G8 10.6 100-153 RE2126+192 HR8210 (IK Peg) A8m 6.07 43-54 1,2,5 RE2300-070 HD217411 G5 9.8 87 RE2353-702 HD223816 F5IV/G0 9.9 115-330 Refs. (1) Barstow et al. (1993c); (2) Landsman, Simon & Bergeron (1993); (3) Hodgkin et al.
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