Prediction of the Supersoft X-Ray Phase, Helium Enrichment, and Turnoff Time in the 2000 Outburst of the Recurrent Nova Ci Aquilae

Abstract

Recurrent nova CI Aquilae is still bright 300 days after the optical maximum, showing the slowest evolution among recurrent novae. We predict the turnoff time of the CI Aql 2000 outburst coming in 2001 August after a supersoft X-ray source (SSS) phase lasts 250 days. We also predict helium enrichment of ejecta, He/H ~ 0.25 by number. Observational confirmations are urgently required. Based on the optically thick wind mass-loss theory of the thermonuclear runaway model, we have also estimated the white dwarf (WD) mass to be M_WD = 1.2 ± 0.05 M_☉ by fitting our theoretical light curves with the 1917 and 2000 outbursts. The mass of the hydrogen-rich envelope on the WD is also estimated to be ΔM ~ 6 × 10^-6 M_☉ at the optical maximum, indicating an average mass accretion rate of _acc ~ 0.7 × 10^-7 M_☉ yr^-1 during the quiescent phase between the 1917 and 2000 outbursts. Using these obtained values, we have consistently reproduced the light curve in quiescence as well as the two outburst phases. Thus, we predict the turnoff time to be in 2001 August for the 2000 outburst. We strongly recommend soft X-ray observations to detect the SSS until 2001 August because the massive wind phase already ended in 2000 December and was followed by an SSS phase that very likely will last until 2001 August.