6. The nature of the secondary
The properties of the secondary star are constrained by the distance derived above and the quiescent photometric magnitudes and color. A blue main-sequence star dominating the observed flux is clearly ruled out because this would imply a huge distance of kpc. An orbital period of hr corresponds either to a main-sequence M5V donor of with absolute magnitudes and (Baraffe et al. 1998), or to a much fainter brown dwarf donor of with and (Allard et al. 1996).
The large outburst amplitude and blue color suggest that the quiescent light, even in the R- and I-bands, is mostly from the white dwarf. If the secondary is a quasi-main-sequence star, however, it may also contribute some flux, in particular in the I-band. The () upper limit on the intrinsic color, , gives a lower limit on the white dwarf temperature: with an M5V secondary (using Baraffe et al. 1998) we find K ( K) for a () white dwarf. With , this in turn gives a lower limit on the distance of pc (almost independent of the white dwarf mass). Since this is clearly inconsistent with the much lower distance estimated above from the outburst magnitude and the quiescent brightness-recurrence time correlation, this strongly suggests that the secondary star in V592 Her is not a normal hydrogen burning star, but a brown dwarf.
If the secondary is indeed a brown dwarf, the R flux must be almost completely from the white dwarf. Scaling the pure-hydrogen white dwarf models of Bergeron et al. (1995) to the Nauenberg (1972) mass-radius relation (for roughly given by cm with the white dwarf mass in units of ), and taking , and , we derive a white dwarf temperature of ( K and a nominal distance pc (400-1000 pc for typical masses between 1.0 and 0.4 ). Given the uncertainties, this is quite consistent with the distance derived in the previous section.
The presence of small-scale variability suggests that some quiescent accretion was occuring a year before the 1998 outburst. At such low mass-accretion rates, however, the color of the disk contribution is likely to be very red (Paschen continuum emission), minimizing the disk contribution in R. If the disk flux does contribute in I, the inferred distance and white dwarf temperature are both somewhat higher, but our conclusions about the nature of the secondary are unchanged.
The known temperatures of white dwarfs in non-magnetic CVs range from 12 000 to 50 000 K (Gänsicke 1999). Certainly, cooler white dwarfs are more difficult to measure and the known temperatures are biased towards higher values because of selection effects. However, the estimated temperature of K for the white dwarf in V592 Her is on the low side, and is likely the result of a low long-term mean mass-accretion rate.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999