Multi-Wavelength Observations of the AM CVn Systems

Abstract

AM CVn, GP Com, CR Boo, and V803 Cen, the so-called AM CVn systems*, are H-depleted objects which display periodicities on the order of minutes. They are very blue and present other peculiarities as well. A close binary scenario, where a very low mass He (i.e., DB type (McCook & Sion, 1984) white dwarf (WD) would be donating matter onto another He WD, has been proposed to explain their observational characteristics (Faulkner et al., 1972; Nather et al., 1981; Wood et al., 1987; O’Donoghe et al., 1987). Therefore, they are understood as a sub-group (among the nova-like type class (Ritter 1990; la Dous 1990) of the cataclysmic variables (CVs).However, despite many years of research, the cataclysmic nature of these systems has not been established beyond doubt so far (Patterson et al., 1992; Ulla 1993a, b). Thus, the validity of the proposed model has been evaluated in the present work, and their nature has been investigated in detail. For that, a comprehensive multi-wavelength study was performed for the first time ever. This “X-rays-to-radio” study of the systems makes use of old and new data, from archives (i.e., EXOSAT, HEASARC, EOLS/Einstein, IUE/ULDA and La Palma Data Archive), space- and ground-based observations. Also, a comprehensive review was made — with the aid of the SIMBAD data base — of all relevant publications where properties of the AM CVn systems have been discussed in the past.Some of the conclusions obtained are as follows: i) Including that its UV continuum increases towards red wavelengths, while the opposite happens for the other three (Ulla, 1993a), GP Com is the most different object among the AM CVn systems. However, it seems clear that it is a CV. So far, it is the only clear soft (~0.05–2 keV) and hard (~l-8 keV) X-ray emitter in the group. ii) As remarked by other authors, CR Boo and V803 Cen are the two looking most alike (apparently also like CP Eri), displaying high/low brightness transitions. These are accompanied by absorption/emission transitions of their spectral lines. iii) However, they miss an orbital period. iv) The behaviour of their UV continuum is such that when they are close to high state (i.e., V ~ 13) the slope rises towards blue wavelengths, and towards red wavelengths when close to low state (i.e., V ~ 17). This, together with the emission/absorption transitions, resembles what happens for dwarf novae (la Dous 1993). Then, CR Boo and V803 Cen have been in fact misclassified as nova-likes. v) AM CVn itself, missing as well a clear determination for an orbital period despite a most extensive photometric coverage (Solheim et al., 1984, 1991; Patterson et al., 1992), can be a cataclysmic variable, although a single-star hypothesis cannot be totally discarded. vi) It does not seem appropriate to group these four objects together, even less so under the name of AM CVn systems, among the nova-like CVs.Key wordsAM CVn systemscataclysmic variablesmultiwavelength study