The pre-cataclysmic system REJ0720-318

REJ0720-318 is one member of a small group of DA+dM binaries discovered as a result of the ROSAT all-sky survey, as outlined in our previous group report. This object is also one of only three white dwarfs classified as spectral type DAO, through the detection of HeII in the optical spectrum, to be detected in the EUV. All three are short period pre-cataclysmic systems which must have passed through a common-envelope phase while the white dwarf progenitor was a red giant. Hence, the presence of He in the photosphere might be a result of this or could also arise by accretion of material from the wind of the M dwarf companion. REJ0720 is the brightest object in the EUV and has the lowest column, making it a key target for spectroscopy with EUVE.

The EUV spectrum of REJ0720 (shown below) reveals a unique object. Although the total interstellar column density is low, the fraction of neutral He is far larger than in any other object observed, yielding a saturated absorption edge at 504 Angstroms. In addition, the spectrum shows both an interstellar HeII edge and photospheric HeII Lyman series absorption lines. The shape and flux level of the continuum together with the HeII lines can only be matched by a stratified H+He atmosphere with an H layer of 10 -14 solar masses. This is the only star demonstrated to have a stratified structure, although it has long been predicted theoretically. If all the supposed interstellar He really does reside in the interstellar medium the implied He and H ionization fractions are far in excess of those observed along the lines of sight to other, isolated, hot white dwarfs. Hence it is most likely that the high HeI column is in the form of a circumbinary shell that was ejected during the common envelope phase.

Spectrum of RE0720-318: a DAO+dM close binary system, with a period estimated at 1.2 days. The presence of sharp HeI and HeII edges in the spectrum provides evidence for the possible existence of circumbinary gas in the system.