Hubble Space Telescope Far-UV Spectroscopy of the Short Orbital Period Recurrent Nova CI Aql: Implications for White Dwarf Mass Evolution

An Hubble Space Telescope Cosmic Object Spectrograph Far UV spectrum (1170 to 1800) was obtained for the short orbital period recurrent novae (T Pyxidis subclass), CI Aquilae. CI Aql is the only classical Cataclysmic variable (CV) known to have two eclipses of a sensible depth per orbit cycle and also to have pre- and post-outburst light curves that are steady enough to allow estimates of mass and orbital period changes. Our far-ultraviolet (FUV) spectral analysis with model accretion disks and non-LTE high-gravity photospheres, together with the Gaia parallax, reveal that CI Aql's FUV light is dominated by an optically thick accretion disk with an accretion rate of the order of 4 × 10 ^-8 M yr ^-1 . Its database of light curves, radial velocity curves, and eclipse timings is among the best for any CV. Its orbit period (P), dP/dt, and reference time are rederived via a simultaneous analysis of the three data types, giving a dimensionless post-outburst dP/dt of (-2.49 ± 0.95) × 10 ^-10 . The lack of information on the loss of orbital to rotational angular momentum leads to some uncertainty in the translation of dP/dt to the white dwarf (WD) mass change rate, dM ₁ /dt, but within the modest range of to +7.8 × 10 ^-8 . The estimated WD mass change through outburst for CI Aql, based on simple differencing of its pre- and post-outburst orbit period, is unchanged from the previously published . At the WD's estimated mass increase rate, it will terminate as a Type Ia supernova within 10 million years.