2024A&A...681A..83G

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 E_B - V_, the disk spectral energy distribution (SED), the reference (i.e., inclination-corrected) absolute magnitude and disk luminosity ($ M_V^{{\rm ref}} $ , $ L_{\mathrm{disk}}^{\mathrm{ref}} $), 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 $ M_V^{{\rm ref}} $ , making them convenient proxies for Ṁ. The strongest correlation (p_H0 < 10^–6) is $ \ \log \dot{M} = -0.57\pm0.06 \ M_{V}^{\mathrm{ref}} - 5.98\pm0.29 $. 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 $ M_{V}^{\mathrm{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.