Hydrodynamic simulations of the recurrent nova T Coronae Borealis: Nucleosynthesis predictions

Technical Summary: Hydrodynamic Simulations of the Recurrent Nova T Coronae Borealis: Nucleosynthesis Predictions

Problem and Context
Recurrent novae (RNe) are binary systems characterized by thermonuclear runaways (TNRs) occurring on the surface of white dwarfs (WDs) with remarkably short recurrence times, typically between 1 and 100 years. These short intervals necessitate very high mass-accretion rates, WD masses approaching the Chandrasekhar limit, and high initial WD luminosities. T Coronae Borealis (T CrB) is a prominent Galactic RN, having erupted in 1866 and 1946, with a predicted recurrence period of approximately 80 years. Given the imminence of its next outburst, there is a critical need to constrain the physical parameters of the system—specifically the WD mass, initial luminosity, accretion rate, and metallicity—to understand the conditions powering such eruptions. Furthermore, RNe are potential progenitors of Type Ia supernovae, making the characterization of their nucleosynthesis and mass accumulation vital.

Methodology
The authors employed the hydrodynamic, Lagrangian, finite-difference code SHIVA to simulate the explosion of T CrB. The simulations utilized a general equation of state including degenerate electron gas, multicomponent ion plasma, and radiation, with OPAL Rosseland mean opacities. Nuclear energy generation was calculated via a reaction network of 120 species (from $^1$H to $^{48}$Ti) linked by 630 processes, incorporating updated rates from the STARLIB database.

The study presents 11 new hydrodynamic models designed to reproduce an 80-year recurrence period ($\tau_{rec}$) under varying conditions:

• WD Masses ($M_{WD}$): 1.30, 1.35, and 1.38 $M_{\odot}$.
• Initial WD Luminosities ($L_{WD}$): 0.01, 0.1, and 1.0 $L_{\odot}$.
• Accreted Metallicity ($Z_{acc}$): Solar ($1.0 Z_{\odot}$), sub-solar ($0.1 Z_{\odot}$), and super-solar ($10 Z_{\odot}$).
• Mixing: 8 additional models were computed for 1.35 and 1.38 $M_{\odot}$ WDs to evaluate the impact of mixing (10% and 25%) between the accreted envelope and the underlying WD layers (Carbon-Oxygen and Oxygen-Neon compositions).
• Lower Mass WDs: 3 models were computed for a 1.20 $M_{\odot}$ WD to account for RNe with lower mass estimates.

For each configuration, the mass-accretion rate ($\dot{M}_{acc}$) was tuned to trigger an explosion after exactly 80 years of accretion.

Key Results

• Accretion Rates and Physical Conditions: To achieve an 80-year recurrence with solar-composition material, mass-accretion rates of $\dot{M}_{acc} \sim 10^{-8} - 10^{-7} M_{\odot} \text{yr}^{-1}$ are required for WDs with masses $1.30 - 1.38 M_{\odot}$ and luminosities $0.01 - 1.0 L_{\odot}$. Lower initial WD luminosities, lower WD masses, or reduced metallicity in the accreted material necessitate higher accretion rates to reach the critical ignition pressure within the 80-year window.
• Explosion Dynamics: Models with lower $L_{WD}$ or lower $Z_{acc}$ result in higher accumulated envelope masses and higher ignition pressures. Consequently, these models produce more violent outbursts characterized by higher peak temperatures ($T_{max}$), larger ejected masses ($\Delta M_{eje}$), and greater kinetic energies. For instance, a 1.38 $M_{\odot}$ WD with $L_{WD}=0.01 L_{\odot}$ and solar metallicity reaches $T_{max} \approx 2.92 \times 10^8$ K, whereas a 1.30 $M_{\odot}$ WD with $L_{WD}=1.0 L_{\odot}$ reaches only $2.01 \times 10^8$ K.
• Nucleosynthesis and Elemental Abundances:
  • Isotopic Ratios: The ejecta are dominated by CNO-cycle isotopes ($^{12,13}$C, $^{14,15}$N). Significant variations in isotopic ratios (e.g., $^{12}$C/$^{13}$C, $^{16}$O/$^{17}$O, $^{28}$Si/$^{29}$Si) are observed across different models.
  • WD Mass Dependence: Models with different WD masses but identical initial luminosities exhibit significant differences in the abundances of Ne, Na, Mg, Al, Si, P, S, Ar, K, Ca, and Sc. Specifically, 1.38 $M_{\odot}$ models show significant enrichment in S, Cl, K, Ca, and Sc relative to solar abundances, whereas 1.20 $M_{\odot}$ and 1.30 $M_{\odot}$ models generally show solar or slightly subsolar abundances for these elements (with the exception of model 130C). Silicon is enhanced in the 1.20 and 1.30 $M_{\odot}$ models but depleted or near-solar in the 1.38 $M_{\odot}$ models.
  • Metallicity Effects: In 1.38 $M_{\odot}$ models, varying metallicity drastically alters composition. Low metallicity ($0.1 Z_{\odot}$) leads to significant $^{40}$Ca production, while high metallicity ($10 Z_{\odot}$) shifts dominance toward N, C, Ne, and Si.
• Mixing Scenarios: Models incorporating mixing between the envelope and the WD substrate (CO or ONe) show shortened accretion phases and reduced total ejected masses compared to non-mixing models. Crucially, mixing significantly enhances the production of gamma-ray emitters ($^{7}$Be, $^{18}$F, $^{22}$Na, $^{26}$Al). While non-mixing models predict negligible gamma-ray emission, mixing models could produce detectable signatures, potentially distinguishing between CO and ONe WDs.
• Gamma-Ray Emission: For the standard non-mixing models, the authors predict that gamma-ray lines (511 keV and 1275 keV) and the associated continuum will be orders of magnitude below detection limits for current instruments due to low yields of radioactive isotopes.

Significance and Claims
The paper asserts that the compositional differences in the ejecta, particularly the elemental abundances of species such as Ne, Na, Mg, Al, Si, P, S, Ar, K, Ca, and Sc, serve as a potential diagnostic tool. These variations can constrain the parameter space and discriminate between the various T CrB models proposed in the study. Specifically, the authors claim that early spectroscopic measurements of the forthcoming T CrB outburst could determine the WD mass and the degree of mixing, thereby clarifying the physical conditions governing the system's eruptions. The study concludes that while gamma-ray detection is unlikely without significant mixing, the elemental abundance patterns offer a robust method for identifying the nature of the underlying WD and the mixing processes at play.