A 9-Hr CV With One Outburst in 4 Years of Kepler Data

Kavli Affiliate: Saul Rappaport

| First 5 Authors: Zhifei Yu, John Thorstensen, Saul Rappaport, Andrew Mann, Thomas Jacobs

| Summary:

During a visual search through the Kepler main-field lightcurves, we have
discovered a cataclysmic variable (CV) that experienced only a single 4-day
long outburst over four years, rising to three times the quiescent flux. During
the four years of non-outburst data the Kepler photometry of KIC 5608384
exhibits ellipsoidal light variations (`ELV’) with a $sim$12% amplitude and
period of 8.7 hours. Follow-up ground-based spectral observations have yielded
a high-quality radial velocity curve and the associated mass function.
Additionally, H$alpha$ emission lines were present in the spectra even though
these were taken while the source was presumably in quiescence. These emission
lines are at least partially eclipsed by the companion K star. We utilize the
available constraints of the mass function, the ELV amplitude, Roche-lobe
filling condition, and inferred radius of the K star to derive the system
masses and orbital inclination angle: $M_{rm wd} simeq 0.46 pm 0.02 ,
M_odot$, $M_{rm K} simeq 0.41 pm 0.03 , M_odot$, and $i gtrsim
70^circ$. The value of $M_{rm wd}$ is the lowest reported for any accreting
WD in a cataclysmic variable. We have also run binary evolution models using
MESA to infer the most likely parameters of the pre-cataclysmic binary. Using
the mass-transfer rates from the model evolution tracks we conclude that
although the rates are close to the critical value for accretion disk
stability, we expect KIC 5608384 to exhibit dwarf nova outbursts. We also
conclude that the accreting white dwarf most likely descended from a hot
subdwarf and, most notably, that this binary is one of the first bona fide
examples of a progenitor of AM CVn binaries to have evolved through the CV
channel.

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