Effective temperatures of cataclysmic-variable white dwarfs as a probe of their evolution

Abstract

We present HST spectroscopy for 45 cataclysmic variables (CVs), observed with\nHST/COS and HST/STIS. For 36 CVs, the white dwarf is recognisable through its\nbroad Ly$\\alpha$ absorption profile and we measure the white dwarf effective\ntemperatures ($T_{\\mathrm{eff}}$) by fitting the HST data assuming\n$\\log\\,g=8.35$, which corresponds to the average mass for CV white dwarfs\n($\\simeq\\,0.8\\,\\mathrm{M}_\\odot$). Our results nearly double the number of CV\nwhite dwarfs with an accurate temperature measurement. We find that CVs above\nthe period gap have, on average, higher temperatures ($\\langle T_{\\mathrm{eff}}\n\\rangle \\simeq 23\\,000\\,$K) and exhibit much more scatter compared to those\nbelow the gap ($\\langle T_{\\mathrm{eff}} \\rangle \\simeq 15\\,000\\,$K). While\nthis behaviour broadly agrees with theoretical predictions, some discrepancies\nare present: (i) all our new measurements above the gap are characterised by\nlower temperatures ($T_{\\mathrm{eff}} \\simeq 16\\,000 - 26\\,000\\,$K) than\npredicted by the present day CV population models ($T_{\\mathrm{eff}} \\simeq\n38\\,000 - 43\\,000\\,$K); (ii) our results below the gap are not clustered in the\npredicted narrow track and exhibit in particular a relatively large spread near\nthe period minimum, which may point to some shortcomings in the CV evolutionary\nmodels. Finally, in the standard model of CV evolution, reaching the minimum\nperiod, CVs are expected to evolve back towards longer periods with mean\naccretion rates $\\dot{M}\\lesssim 2 \\times\n10^{-11}\\,\\mathrm{M}_\\odot\\,\\mathrm{yr}^{-1}$, corresponding to\n$T_\\mathrm{eff}\\lesssim 11\\,500\\,$K. We do not unambiguously identify any such\nsystem in our survey, suggesting that this major component of the predicted CV\npopulation still remains elusive to observations.\n