A STUDY OF A HIGH-RESOLUTION IUE SPECTRUM OF AM CANUM-VENATICORUM

We have obtained the first high resolution IUE spectrum1 of the helium-rich, cataclysmic variable star AM CVn. The spectrum is greatly underexposed, but we can still detect both wide and narrow line profiles. We report broad, shortward-shifted, P-Cygni-like absorption and in some cases emission lines in the far UV high ionization resonance lines of C, N, O and Si, but the profiles are considerably more complex than the collimated disk/boundary layer outflows, absorbing disk continuum light in H-rich CVs. The highest ionizations show evidence of a narrow jet or conical flow. For other, lower ionized lines, we find some evidence of a stellar origin. The broad He II (lambda1640 angstrom) absorption profile with blue shifted emission core has a remarkably similar overall structure to the He II (lambda 1640 angstrom) broad absorption trough in the IUE spectrum of the prototypical cool DO white dwarf HZ 21. In fact, on closer examination, we note that the weak absorption superimposed on the broad emission, remarked upon in Fritz et al. (1990) is also present in the AM CVn profile! This supports the possibility that AM CVn, a helium-rich nova-like cataclysmic binary, consisting of two interacting helium-rich degenerates may have an evolutionary link to a ''single'' DO star like HZ 21. The sharp absorption lines seen most convincingly in the resonance doublets of N V (lambda1238 angstrom, lambda1242 angstrom) and C IV (lambda1548 angstrom, lambda1550 angstrom ) and in He H (lambda 1640 angstrom) exhibit a precise velocity coincidence. These sharp features are almost certainly due to circumbinary matter because they are obviously unaffected by the rapid orbital motion (or rapid stellar rotation) in this short period system during the long (9.3 hour) IUE echelle exposure. To our knowledge this is the first direct evidence of circumstellar/circumbinary matter in the line of sight to the system. Our observations support an evolution through shell episodes of a close binary system which ends up with an expanding hot envelope as seen for HZ 21, and suggests one possible evolutionary channel for production of DOs (DBs).