Calcium Excess in Novae: Beyond Nuclear Physics Uncertainties

TL;DR

This work tackles the Ca overabundance problem in classical novae by testing whether nuclear reaction-rate uncertainties can explain the observations. Using a multi-zone Nova Framework setup (MESA for evolution and NuGrid MPPNP for nucleosynthesis) and a comprehensive Monte Carlo analysis of charged-particle reaction rates, the authors identify key reactions that influence Ca yields but find that even large rate variations cannot reconcile models with observed Ca (and Ar) enhancements. The hottest nova model shows the strongest Ca production among the scenarios tested, yet remains short of the observed abundances, implying that astrophysical factors or observational biases are more likely drivers than nuclear physics uncertainties. The study highlights the need to explore non-solar initial compositions, alternative mixing scenarios, and broader, multi-wavelength observations, while confirming that targeted nuclear data in the Ca–K–Ar region remain valuable for constraining nucleosynthesis in novae.

Abstract

We examine Ca abundances in classical novae from spectroscopic observations spanning 65 years and investigate whether they are systematically high compared to those predicted by nova models. For the first time, we perform Monte Carlo simulations assessing the impact of nuclear reaction rate uncertainties on abundances predicted by multi-zone nova models. While the Ca abundances in the models are sensitive to variations of rates of the reactions 37Ar(p,gamma)38K and 38K(p,gamma)39Ca, the nuclear physics uncertainties of these reactions cannot account for the discrepancy between the observed and predicted Ca abundances in novae. Furthermore, the overabundance of Ca has important implications for measuring 7Be in nova ejecta, as Ca lines are used to estimate 7Be abundances. If the Ca abundance is incorrectly determined, it could lead to inaccurate 7Be abundance estimates. Possible alternative explanations for the observed Ca overabundance are discussed.

Figures (5)

• Figure 1: Comparison of elemental mass fractions from our multi-zone nova models (MESA and MPPNP simulations in the Nova Framework, solid-color lines) with those of the Barcelona Group (black dashed and dotted lines). Models selected from the Barcelona group have parameters closest to ours for both CO and ONe novae jose1998. The gray dashed and dotted lines represent more recent models from jose_2020.
• Figure 2: Comparison of observed abundances in CO novae V705 Cas arkhipova2000, GQ Mus morisset1996, V977 Sco andrea1994, V443 Sct andrea1994, and DQ Her Pottasch1959 with predicted abundances from Model 1 and Model 2. Observational data are shown with blue and green symbols, while model predictions are represented by pink and purple circles and diamonds connected by a solid line.
• Figure 3: Comparison of observed abundances in novae of uncertain type (CO or ONe) with predicted abundances from multi-zone nova models. Observational data for V1224 Oph andrea1994, V603 Aql Pottasch1959, RR Pic Pottasch1959, and GK Per Pottasch1959 are shown with blue and green symbols. Model predictions are represented by purple, pink, yellow, orange, and red symbols connected by solid lines.
• Figure 4: Distributions of the abundances for the selected elements relative to their default value in the multi-zone MC simulation for nova Model 5. The size and color of the circles represent the number of MC runs with that abundance.
• Figure 5: Comparison between the elemental mass fractions of the pre-mixed accreted envelope (blue line) and the post-explosion mass-averaged envelope composition from our multi-zone nova models (red line). The dashed line shows the solar composition and the dotted line shows the fraction of H in the initial composition compared to the H at the end of the simulation. The top panel shows this for Model 1 and the bottom panel shows this for Model 5. For both of the models shown the pre-mixed material is assumed to be 50% solar material and 50% WD material.