Accretion rates of 42 nova-like stars with IUE and Gaia data

Abstract

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 (MVref, Ldiskref), and the mass accretion rate (Ṁ), 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 α in the range −2.4 ≤ α ≤ −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 MVref, making them convenient proxies for Ṁ. The strongest correlation (pH0 < 10−6) is log Ṁ = −0.57 ± 0.06 MVref−5.98 ± 0.29. Nine of the 42 NLs fall within the period gap but all have Ṁ 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 Ṁ (≈ −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” Ṁ 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 Ṁ and its correlation with MVref. 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.