The Hydrogen-Burning Turnoff of RS Ophiuchi (2006)

We report a coordinated multi-band photometry of the RS Oph 2006 outburst andhighlight the emission line free y-band photometry that shows a mid-plateauphase at y ~ 10.2 mag from day 40 to day 75 after the discovery followed by asharp drop of the final decline. Such mid-plateau phases are observed in othertwo recurrent novae, U Sco and CI Aql, and are interpreted as a bright diskirradiated by the white dwarf. We have calculated theoretical light curvesbased on the optically thick wind theory and have reproduced the observed lightcurves including the mid-plateau phase and the final sharp decline. This finaldecline is identified with the end of steady hydrogen shell-burning, whichturned out the day ~80. This turnoff date is consistent with the end of asupersoft X-ray phase observed with Swift. Our model suggests a white dwarfmass of 1.35 \pm 0.01 M_\sun, which indicates that RS Oph is a progenitor ofType Ia supernovae. We strongly recommend the y-filter observation of novae todetect both the presence of a disk and the hydrogen burning turn-off.Comment: to appear in ApJL, 4 pages including 4 figure