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 ($α=3$) 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.

Figures (3)

• Figure 1: $B$-band light curve of T CrB during the 1938-1955 high-brightness state and the associated 1946 nova eruption (black crosses). Red dots with error bars show the same data after median filtering with a running box of width 113 d. The recent AAVSO data from 2004 until April 2024 are shown in blue, after being processed with the same median filter and shifted in time by -78 yr to match the 1946 data. From sbl25.
• Figure 2: Diagram of accretion rate versus WD mass for T CrB. The upper black dashed line represents the limit above which steady nuclear burning occurs. The lower limit in $\dot{M}$Lunaet2018 is indicated by the black dashed-dotted line. The black solid and dotted lines mark the relationship $\dot{M} (M_1)$ from the envelope mass that produces a nova eruption, for an assumed nova eruption recurrence interval of $T_R = 80 \pm 2$ yr and for a time spent in the high-accretion state of $\Delta T_h = 14 \pm 1$ yr. The red solid lines indicate the relationship $\dot{M} (M_1)$ adopting $M_2 = 0.6$, 0.7 and 0.8 $M_\odot$ in the T CrB primary mass function of Fekelet2000. Open and filled blue diamonds mark the solutions for models I and II, respectively. From sbl25.
• Figure 3: Simulation results with model I (green solid curve) and model II (black solid curve) parameters. The points with error bars are the same as in Fig. \ref{['fig:dados']} for the $B$-band and analogous for the other bands. The green and black dashed curves are the contribution from the irradiated RG companion to models I and II, respectively. From sbl25.