A large HST program: effective temperatures of cataclysmic variable white dwarfs

We present HST/COS spectroscopy for 40 cataclysmic variables (CVs), observed as part of a 122 orbit HST program. In 31 CVs, the white dwarf (WD) is recognisable through its broad Ly$\alpha$ absorption profile and we measure the WD effective temperatures $T_{\mathrm{eff}}$) by fitting the HST data of these 31 systems assuming $\log g = 8.35$, which corresponds to the average mass for CV WDs ($0.8\,\mathrm{M}_\odot$). Our results nearly double the number of CV WDs with an accurate temperature measurement. We find that the $T_{\mathrm{eff}}$ of CV WDs above the period gap are, on average, higher and exhibit much more scatter compared to those below the gap. This behaviour broadly agrees well with theoretical predictions, reflecting different angular momentum loss mechanisms driving CV evolution above and below the period gap. Although our results show the expected decrease in $T_{\mathrm{eff}}$ below the period gap, they are not clustered in the predicted narrow track but present a relatively large spread near the period minimum. These findings are in contrast with the results from population synthesis and may point to some shortcomings in the CV evolutionary models. % and/or in other forms of angular momentum loss which could intervene at short orbital period. Reaching the minimum period, CVs are expected to evolve back towards longer periods with mean accretion rates $\dot{M} \lesssim 10^{-11} \, \mathrm{M}_\odot \, \mathrm{yr}^{-1}$, corresponding to $T_\mathrm{eff} \lesssim 10\,000$\,K. We do not identify these systems in our survey, suggesting that none of them are strong ``period bouncer'' candidates, and this major component of the predicted CV population still remains elusive to observations.