The accretion-driven eruption of the recurrent nova T Corona Borealis

Abstract

T Corona Borealis (T CrB) is a symbiotic recurrent nova with an $\simeq 80$ yr recurrence interval, the eruptions of which occur on top of a $\simeq 15$ yr long high-brightness state. We show that the high-brightness state is best explained as the response of a high-viscosity ($\alpha=3$) accretion disk to a unique event in which the mass transfer rate from the donor star increases by a factor $\simeq 100$, from $\dot{M}\mathrm{(quies)}= 2 \times 10^{-9} M_\odot$ yr$^{-1}$ up to $\dot{M}\mathrm{(out)}= 1.9 \times 10^{-7} M_\odot$ yr$^{-1}$; it can not be a thermal-viscous disk instability outburst neither a steady nuclear burning event. The constraint that the matter accreted onto the white dwarf in between eruptions equals the envelope mass $M_{ig}$ needed to trigger nova eruptions at the observed recurrence interval requires a white dwarf mass of $M_1= 1.29 M_\odot$, a donor star mass of $M_2= 0.7 M_\odot$, and an inclination of $i= 57.3^o$. As the high-brightness state responds for 95% of $M_{ig}$, the nova eruptions of T CrB are induced by accretion events. Without the 15 yr long enhanced mass transfer events, its nova recurrence interval would be significantly longer, $\simeq 5500$ yr. T CrB exhibits a conspicuous decrease in brightness during the 1-2 yr prior to the nova event. We argue that this pre-eruption dip occurs during the convection phase that precedes the nova eruption and is best explained by the slow, accelerated expansion of the accreted envelope (and inner disk radius) at an average velocity of $v_\mathrm{exp}= 0.02$ km s$^{-1}$ over a 2 yr timescale, likely as a consequence of excess heat being increasingly deposited at the accreted layer by thermonuclear reactions before the nova eruption stage.