HOT WHITE DWARFS IN DETACHED BINARIES

FROM THE ROSAT WFC ALL SKY SURVEY

Thesis submitted for the degree of Doctor of Philosophy

at the University of Leicester.

by

Matthew R. Burleigh X-ray Astronomy Group

Department of Physics and Astronomy University of Leicester

7th February 1997

UMI Number: U529765

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D eclaration

I hearby declare that no part of this thesis has been previously submitted to this or any other University as part of the requirement for a higher degree. The work described herein was conducted by the undersigned except for contributions from colleagues as acknowledged in the text.

f t \

Matthew R. Burleigh, 7th February 1997

HOT W HITE DWARFS IN DETACHED BINARIES

FROM THE ROSAT WFC ALL SKY SURVEY

M atthew R . Burleigh

A B ST R A C T

White dwarfs in unresolved pairs with normal stars (spectral type K or earlier) are invisible at optical wavelengths, due to the close proximity of the much more luminous main sequence companion. ROSAT has provided evidence for the existence of a growing sample of these hidden white dwarfs through the detection of EUV and soft X-ray emission. For companions of spectral type ~A5 or earlier, the white dwarf can be spectroscopically identified at far-ultraviolet wavelengths by IUE. Eleven such systems had previously been found in this way from ROSAT, EUVE and IUE observations. A search for fainter, less obvious examples of these binaries is presented, and five new systems have been discovered.

Three new close, pre-CV WD+dM binaries have also been found in the ROSAT WFC survey. Intriguingly, all three degenerates are rare mixed hydrogen/helium atmosphere DAO white dwarfs. The EUVE spectrum of one of these new systems, RE J0720318, is analysed in detail. In particular, it is found that, while the optical spectrum can only be reproduced with a homogeneously mixed atmosphere, the EUVE spectrum can only be matched by a layered model, implying that the underlying structure of the white dwarf is stratified. The hydrogen layer mass of 3xlO-14M is the lowest measured for any white dwarf from EUVE spectra. In addition, an unprecedented Hel/HI ratio of ~1 is detected for the absorbing column along the line of sight, implying a hydrogen ionisation fraction of >90%, if all this material resides in the local interstellar medium. It is suggested that most of the helium lies in the vicinity of the star, possibly in the form of a circumbinary disk left over from the common envelope phase. These results have important implications for our understanding of the evolutionary status of DAO white dwarfs in particular, and for post-common envelope systems in general.

A catalogue of all the detached white dwarf binaries found in the ROSAT survey is presented, with an analysis of the white dwarf mass distribution. Compared with optically selected samples, a significant excess of hot, massive objects is detected. This excess probably arises from the slower cooling rates of massive (>0.9 M) white dwarfs in comparison to normal mass (0.6 M) stars.

P ublications

Barstow, M.A., Holberg, J.B., Marsh, M.C., Tweedy, R.W., Burleigh, M.R., Fleming, T.A., Koester, D., Penny, A.J., Sansom, A.E., 1994, Mon. N ot R. ast. Soc., 271, 175 RE 1738+665, the hottest DA white dwarf detected by ROSAT

Barstow, M.A., Burleigh, M.R., Fleming, T.A., Holberg, J.B., Koester, D., Marsh, M.C., Rosen, S.R., Rutten, R.G.M., Sakai, S., Tweedy, R.W. & Wegner, G., 1995, Mon. Not. R. ast. Soc., 272,531The orbital period of the pre-cataclysmic binary RE 2013+400 and a study of the atmosphere of the DAO white dwarf primary

Burleigh, M.R. & Barstow, M.A., 1995, In: White Dwarfs, eds., Koester, D. & Werner, K., Springer-Verlag, 318Constraints on DAO white dwarf composition from the ROSAT EUV survey

Marsh, M.C., Barstow, M.A., Holberg, J.B., ODonoghue, D., Buckley, D.A., Fleming, T.A., Koester, D. & Burleigh, M.R., 1995, In: White Dwarfs, eds., Koester, D. & Werner, K., Springer- Verlag, 328An EUV selected sample of DA white dwarfs

Barstow, M.A., ODonoghue, D., Kilkenny, D., Burleigh, M.R. h Fleming, T.A., 1995, Mon. Not. R. ast. Soc., 273, 711RE0720-318: A pre-cataclysmic binary system with a DAO white dwarf primary

Barstow, M.A., Jordan, S., ODonoghue, D., Burleigh, M.R., Napiwotzki, R. & Harrop-Allin, M.K., 1995, Mon. Not. R. ast. Soc., 277, 971RE J0317 853: The hottest known highly magnetic DA white dwarf

Jeffries, R.D., Burleigh, M.R. & Robb, R.M., 1996, Astr. & Astrophys. Letts., 305, L45Is 2RE J0357+283 the most coronally active star in the Galaxy?

Burleigh, M.R., Barstow, M.A. & Dobbie, P.D., 1997, Astr. & Astrophys. Letts., 317, L27 The close DAO+dM binary RE J0720318: a stratified white dwarf with a thin H layer and a

possible circumbinary disk

Marsh, M.C., Barstow, M.A., Buckley, D.A., Burleigh, M.R., Holberg, J.B., Koester, D., Penny, A.J. &: Sansom, A.E., 1997, Mon. Not. R. ast. Soc., 286, 369An EUV selected sample of DA white dwarfs from the ROSAT all-sky survey. I: Opticallyderived stellar parameters

Marsh, M.C., Barstow, M.A., Buckley, D.A., Burleigh, M.R., Holberg, J.B., Koester, D., Penny, A.J. & Sansom, A.E., 1997, Mon. Not. R. ast. Soc., 287, 705An EUV selected sample of DA white dwarfs from the ROSAT all-sky survey. II: EUV and soft X-ray properties

Burleigh, M.R., Barstow, M.A. & Fleming, T.A., 1997, Mon. Not. R. ast. Soc., 287, 381 A search for hidden white dwarfs in the ROSAT EUV survey

Burleigh, M.R. & Barstow, M.A., 1997, In: White Dwarfs, eds., Isern, J., Hernanz, M. & Garcia- Berro, E., Kluwer, 329Hot white dwarfs in non-interacting binary systems from the ROSAT EUV survey

Burleigh, M.R., Barstow, M.A. & Holberg, J.B., 1997, Mon. Not. R. ast. Soc., in p repara tion A search for hidden white dwarfs in the ROSAT EUV survey II: Discovery of a distant DA-\-F5VG0V binary system in a direction of low density neutral hydrogen

To my parents

Thank you

A cknow ledgem ents

My introduction to the field of white dwarfs came in the summer of 1992 when, as an undergraduate student earning a few pounds to pay off my overdraft, I found myself helping my future supervisor Martin Barstow to organise a conference on the subject. That week, I spent most of the time operating a video camera to record each post-talk discussion. I didnt realise that two years later I would be attending the next white dwarf conference in Kiel, Germany, as a delegate, and that many of the astronomers I had been serving coffee to would later become my friends and colleagues. In the autumn of 1993, when I came back to Leicester to start a PhD with the X-ray astronomy group, Martin took me to one side and asked if I would be happy to work for him. I have never regretted saying yes. Martin has been as good a supervisor as I could have asked for. He has introduced me to a subject that I find fascinating, and hope to pursue for many years to come. He has always been willing to make time for me, to teach me and discuss our field with me, and, although I must have tried his patience on many occasions, he has never turned me away. From him I have learnt not only how to do good science, but how best to express and communicate results. Thank you for your support and confidence, Martin, and I hope that I shall be able to work with you on many future projects.

Through my subject I have met many bright, and brilliant scientists and characters from around the world. Special thanks to Jay Holberg for his hospitality during the week I spent with him and his family in Tucson, and for many enlightening discussions; to Rob Jeffries for always providing answers to my naive email questions; to Darragh ODonoghue, Dave Buckley and Dave Kilkenny for teaching me about optical astronomy and letting me come to Cape Town, surely one of the most beautiful cities in the world; to Stefan Jordan and Rex Saffer for drinking me under the table in Kiel and Blanes; to Richard Monier and Constance la Dous for their hospitality at IUE Vilspa.

Throughout my PhD I have never been afraid to take advantage of the PPARC funds available for travel to conferences, meetings and observing runs around the world. Thanks to all the friends I have made in Cape Town (Hi Margie!), Blanes , Sydney and Los Angeles, for taking me out, pouring beer down my throat and forcing me to eat enormous steaks.

Of course, this overly-sentimental acknowledgements section would not be complete without mentioning my colleagues at Leicester. Thank you Jon, for knowing how to go out and have fun, Tim for being a fellow real-ale lover and suffering the lows of English cricket with me, Paul for Table Mountain and that Spanish resort, Gareth for being eternally optimistic about Waless prospects, Dennis for being cool, Richard, Steve, Graham and Big Dave for endless conversations down the pub, and also to John, Keith, Ian, Jezza, Geoff, Geoff, Sarah, Helen, Simon, Ben and Matt (keep striving Marshy, were all behind you). Than