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.

• The paper demonstrates that an episodic 15-year high-brightness accretion phase supplies 95% of the envelope mass triggering the recurrent nova eruption.
• It employs a high-viscosity disk model with a two order of magnitude increase in mass transfer, reproducing key pre-eruption features including convective envelope expansion.
• By rejecting thermal-viscous instability and steady nuclear burning models, the study solidifies the accretion-driven eruption paradigm for T CrB and analogous systems.

Accretion-Driven Eruption Mechanism in the Recurrent Nova T Corona Borealis

Background and Astrophysical Context

T Corona Borealis (T~CrB) is a notable symbiotic recurrent nova system, comprising a mass-transferring M4 III red giant and a massive white dwarf (WD) enveloped by an accretion disk. Historically, two nova eruptions have been documented (1866 and 1946), with recurrence intervals of approximately 80 years. These eruptions occur concomitant with 15-year-long high-brightness states, during which the system’s luminosity markedly increases, correlating with enhanced mass accretion rates onto the WD. Observationally, multi-wavelength datasets reveal a remarkably repeatable temporal alignment between the high-brightness phases and nova eruptions that strongly suggests a causal relationship.

Mass Transfer Outburst Model Formulation

The paper rigorously models the high-brightness state as a singular, prolonged accretion event driven by an increased mass transfer rate from the red giant, invoking a high-viscosity disk ($\alpha=3$). The Mass Transfer Outburst (MTO) scenario posits that the donor star’s mass transfer rate amplifies by roughly two orders of magnitude, from $$\dot{M}_\mathrm{quies} = 2 \times 10^{-9} \, M_\odot\,\mathrm{yr}^{-1}$$ to $$\dot{M}_\mathrm{out} = 1.9 \times 10^{-7} \, M_\odot\,\mathrm{yr}^{-1}$$ for $\approx$15 years. The modeling constrains these parameters by demanding that the total accreted envelope mass over a recurrence cycle matches the classical nova ignition threshold calculated for the observed recurrence interval.

A self-consistent binary configuration derived from these constraints yields $M_1 = 1.29\,M_\odot$ for the WD, $M_2 = 0.7\,M_\odot$ for the donor, and a system inclination $i = 57.3^\circ$. The model demonstrates quantitatively that the high-brightness phase is responsible for 95% of the envelope mass required to trigger a nova eruption, with the remaining 5% accumulated during lower-rate quiescent intervals. Without this episodic accretion enhancement, the recurrence timescale would extend to $\sim$5500 years, underscoring the critical nature of these MTO events in dictating nova periodicity.

Rejection of Thermal-Viscous Disk Instability and Steady Nuclear Burning Scenarios

The paper rigorously excludes both the thermal-viscous disk instability (DI) and steady nuclear burning hypotheses as mechanisms for the long-lived high-brightness state. DI models predict outburst durations significantly shorter ($\sim$10 years) than the observed state ($\sim$15 years) and cannot explain post-eruption sustained brightness since the disk mass would be disrupted and require extensive refill times inconsistent with observations. Similarly, the steady burning scenario is invalidated by the discrepant predicted accretion luminosity, which would exceed observed values by factors of 20–30, and by the sharply delineated boundary between burning regimes. In T~CrB, nova eruptions occur during high-brightness phases, which is incompatible with steady burning suppression of nova outbursts.

Pre-Eruption Dip and Convective Envelope Expansion

A distinctive feature preceding the nova eruption is the pre-eruption dip—a 1–2 year decline in brightness, most prominent in the blue bands. Spectroscopic and photometric evidence indicates this phase results from a dramatic expansion of the inner disk radius, not from reduced mass transfer. The paper models this expansion as a consequence of vigorous convective energy transport within the accreted envelope in the ‘convection phase’ that antecedents the thermonuclear runaway (TNR). The expansion rate derived is $$\dot{M}_\mathrm{quies} = 2 \times 10^{-9} \, M_\odot\,\mathrm{yr}^{-1}$$0, reproducing the observed luminosity drop across the $$\dot{M}_\mathrm{quies} = 2 \times 10^{-9} \, M_\odot\,\mathrm{yr}^{-1}$$1 bands. The pre-eruption dip aligns temporally with theoretical estimates for the convection phase, illustrating that envelope expansion and increased inner disk radius suppress accretion disk luminosity, even as mass transfer persists at high rates.

Implications and Future Directions

The findings delineate a causally linked sequence for recurrent nova eruptions in symbiotic binaries, demonstrating that enhanced mass accretion episodes fundamentally drive period shortening and eruption timing. The results affirm that TNR initiation is predicated on the previously accumulated envelope mass during the high-brightness state; thus, nova eruptions are an accretion-driven phenomenon rather than spontaneous nuclear threshold events. These insights advance the understanding of mass transfer physics, disk viscosity regimes, and envelope evolution in interacting binaries.

On the practical side, the quantitative predictions regarding mass transfer rates, envelope masses, and system parameters provide a robust groundwork for precise observational campaigns targeting both photometric and spectroscopic signatures in T~CrB and analogous systems. The rejection of alternative models is grounded in strong numerical and observational constraints. Theoretical developments in accretion disk dynamics and boundary layer physics in the convective pre-TNR phase may yield further predictive models. Broader exploration of envelope expansion rates and their observational correlates could establish new diagnostics for impending nova eruptions in cataclysmic variables and symbiotic systems.

Conclusion

The paper provides an authoritative account of the eruption mechanism in T~CrB, demonstrating through rigorous modeling that prolonged, enhanced mass accretion events onto a high-viscosity disk are indispensable for achieving the observed 80-year nova recurrence. The high-brightness state cannot be explained by thermal-viscous instabilities or steady nuclear burning. Furthermore, the pre-eruption dip is interpreted as convective envelope expansion rather than reduced mass transfer. The accretion-driven paradigm successfully connects the sequence of events observed in T~CrB, resolving longstanding interpretive ambiguities and furnishing predictive quantitative frameworks for future recurrent nova studies (2604.02708).