Orbital Period Change in Outburst: T Pyx Goes Rogue

ATel #4743; J. Patterson, A. Oksanen, and B. Monard, on behalf of the Center for Backyard Astrophysics (CBA) collaboration:
on 17 Jan 2013; 23:18 UT
Credential Certification: Joseph Patterson (jop@astro.columbia.edu)

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We summarize the results of our 1996-2013 study of the light curve of the recurrent nova T Pyx, based on ~3000 hours of coverage with small (0.3 m) telescopes. The star's light curve at quiescence (V=15, Mv=+1) is essentially that reported in our first paper (Patterson et al. 1998, PASP 110, 380): a broad and shallow (0.1 mag) dip occurring strictly on schedule with a mean period of 0.07622 d, but with period increasing smoothly on a timescale of 3x10⁵ yrs. The strict ephemeris and full agreement with the radial velocities (Uthas et al. 2010, MNRAS 409, 237) leave essentially no doubt that this is the orbital period. For conservative mass transfer, this implies a mass transfer rate near 10^-7 Mo/yr. A similar number is deduced for the accretion rate onto the white dwarf, assuming that the quiescent light is powered by accretion.

The 2011 outburst - discovered on 14 April, which we take to be day 0 - resembled all previous outbursts. The star rose quickly to V=8, then slowly to V=6.5, then declined very slowly, reaching V=15 after 600 days. We studied the post-eruption light curves carefully for periodic signals. By day 170 of the outburst (V=11.2), there is an obvious detection of the orbital signal, with an amplitude of 0.005 mag. The orbital signal continued through the present (day 640, V=15.0), with an amplitude smoothly growing to 0.08 mag. It is probable, though not certain, that the orbital signal was present as early as day 65 (V=8, likely amplitude 0.003 mag). The period was constant over the full 575-day time-series: 0.0762336(2) d, indicating a sudden period increase of 0.0054(7)%. Interpreted simply as the result of sudden mass ejection from the white dwarf, it implies an ejected mass of ~3x10^-5 Mo. Alternate interpretations, with greater angular momentum attributed to the ejecta, require an even greater mass. Such a high ejected mass from the "prototype" recurrent nova - apparently exceeding the accreted mass - does not augur well for the production of Type Ia supernovae. And high ejecta mass and accretion rate may make some mischief in the theory of recurrent novae (Yaron et al. 2005, ApJ 623, 398; Nomoto et al. 2007, ApJ 663, 1269).