Balmer decrements as a new diagnostic for period-bounce Cataclysmic Variable stars

Abstract

Cataclysmic variable stars (CVs) evolve toward shorter orbital periods ($P_{\rm orb}$) until they reach a minimum $P_{\rm orb}$ near $P_{\rm orb}\sim80$ min. Beyond this point, the donor star becomes out of thermal equilibrium or increasingly degenerate, causing the system to "bounce back" to longer $P_{\rm orb}$ values. Such highly evolved systems are known as period-bouncers. Although 40-80\% of all CVs are expected to have reached this stage, period-bouncers come up for only 3-25\% of the observed CV population. This is likely a consequence of their intrinsic faintness associated with lower mass-transfer rates. Establishing new diagnostics to unveil this missing population is therefore crucial. Two samples of non-magnetic CVs with public SDSS optical spectra were constructed: one of short-period pre-bounce CVs and another of period-bounce CVs. For systems showing Balmer absorption from the white dwarf (WD), hydrogen-dominated atmosphere models were fitted and subtracted to correct for the WD component. H$\alpha$, H$\beta$, and H$\gamma$ fluxes were measured. We then investigated statistical relations between the Balmer decrements, the H$\beta$ line luminosity, and $P_{\rm orb}$, and compared the measured Balmer decrements with theoretical predictions from accretion disc models. Short-period pre-bounce CVs show flat Balmer decrements, that is Balmer line ratios close to unity. In contrast, systems near and beyond the period minimum exhibit progressively steeper decrements (H$\alpha$/H$\beta$>1 and H$\gamma$/H$\beta$<1). This behaviour is attributed to their lower mass accretion rates, as inferred from the H$\beta$ line luminosity. We fitted a linear logistic regression model to the diagram of H$\gamma$/H$\beta$ versus H$\alpha$/H$\beta$. We establish that this diagnostic diagram effectively separates period-bouncers from pre-bounce CVs.