Accretion Rates of 42 Nova-Like Stars with IUE and Gaia Data

We analyzed more than 700 ultraviolet spectra of 45 nova-like stars (NLs) observed with the International Ultraviolet Explorer (IUE) satellite, obtaining reliable data for 42 of them. Combining these with the distances from the Gaia Early Data Release 3 (EDR3) and with results from the literature, for each object we determined the reddening EB - V, the disk spectral energy distribution (SED), the reference (i.e., inclination-corrected) absolute magnitude and disk luminosity (M-V(ref), L-disk(ref)), and the mass accretion rate ((M) over dot), all with propagated errors. The de-reddened UV continuum of NLs in a high state is well approximated by a power-law distribution with index alpha in the range -2.4 <= alpha <= -0.2. The agreement between the power-law extrapolation to the V band and the observed V magnitude is outstanding and implies that for NLs in a high state, the disk continuum dominates not only in the UV but also in the optical, with other possible contributions (white dwarf, M dwarf, and hot spot) being minor. We note that the accretion rate correlates with the period, power-law index, and M-V(ref), making them convenient proxies for (M) over dot). The strongest correlation (p(H0) < 10(-6)) is log (M) over dot) = -0.57 +/- 0.06 M-V(ref)-5.98 +/- 0.29. Nine of the 42 NLs fall within the period gap but all have (M) over dot) very similar to that of the objects above the gap, contrary to theory expectations but in agreement with other observational work, and indicating that - at least for NLs - the theoretical assumptions of the standard model of the evolution of CVs need substantial revision. Medians and weighted means of log (M) over dot) (approximate to -8.5) are very similar among NL classes, and also to those of old novae, dispelling the prejudice that stars belonging to the SW Sex class of NLs have "exceptionally high" (M) over dot) compared to other NLs (and old novae). In fact, it is one of the most interesting results of this study that NLs and old novae are indistinguishable in terms of (M) over dot) and its correlation with M-V(ref). Two NLs (V1315 Aql and BZ Cam) have shells around them, a likely fingerprint of a past nova eruption, but the suggested association with "guest stars" of ancient Chinese chronicles is questionable.