Kavli Affiliate: Erin Kara
| First 5 Authors: Kevin B. Burdge, Kareem El-Badry, Thomas R. Marsh, Saul Rappaport, Warren R. Brown
| Summary:
In over a thousand known cataclysmic variables (CVs), where a white dwarf is
accreting from a hydrogen-rich star, only a dozen have orbital periods below 75
minutes. One way to achieve these short periods requires the donor star to have
undergone substantial nuclear evolution prior to interacting with the white
dwarf, and it is expected that these objects will transition to helium
accretion. These transitional CVs have been proposed as progenitors of helium
CVs. However, no known transitional CV is expected to reach an orbital period
short enough to account for most of the helium CV population, leaving the role
of this evolutionary pathway unclear. Here we report observations of ZTF
J1813+4251, a 51-minute orbital period, fully eclipsing binary system
consisting of a star with a temperature comparable to that of the Sun but a
density 100 times greater due to its helium-rich composition, accreting onto a
white dwarf. Phase-resolved spectra, multi-band light curves and the broadband
spectral energy distribution allow us to obtain precise and robust constraints
on the masses, radii and temperatures of both components. Evolutionary modeling
shows that ZTF J1813+4251 is destined to become a helium CV binary, reaching an
orbital period under 20 minutes, rendering ZTF J1813+4251 a previously missing
link between helium CV binaries and hydrogen-rich CVs.
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