Post-Supernova Accretion of Light Elements onto a New-Born Neutron Star and NS 1987A

TL;DR

The paper investigates whether late-time accretion of light elements (${${}^{4}$He, ${}^{12}$C, ${}^{16}$O}$)$ onto a newborn, hot neutron star can create a thick light-element surface layer without triggering explosive nuclear burning. By modeling stationary Fe envelopes with MESA and tracking accretion at near-Eddington rates, the authors map conditions under which stable burning preserves light elements versus when explosive burning converts most accreted material to iron-peak nuclei. They find three distinct ignition regimes: He-driven explosions within hours (very shallow depths), C-driven explosions after years, and O-driven explosions after centuries, with mixed compositions showing two characteristic timescales depending on the secondary element fraction. The results offer a plausible mechanism for light-element atmospheres on young NSs, align with the observed luminosity of NS 1987A, and highlight thresholds in composition and accretion that govern surface composition, while noting observational and nuclear-physics uncertainties that warrant further study.

Abstract

We model early accretion of light elements, He, C, and O, onto a new-born neutron star using the public stellar evolution code MESA, simulating what may happen during the first few years of its life. We find that, under the appropriate conditions, significant amounts of these elements can be accreted up to densities of 10^9 g/cc without triggering a nuclear explosion that would convert them into heavy elements. These results help to understand observations that favor light elements in the atmospheres of young cooling neutron stars, as the one found in the supernova remnant Cassiopeia A, and also add support to the recent indications for the presence of a neutron star, NS 1987A, in the remnant of SN 1987A.

Figures (4)

• Figure 1: Summary of our results: column density $y$ (and density $\rho$) reached by the accreted matter before an explosion occurs, as a function of its chemical composition for our three families of models with mixtures of $^{16}$O/$^{12}$C, $^{16}$O/$^{4}$He, and $^{12}$C/$^{4}$He. Red lines correspond to $\dot{M} = 10^{-8} \, M_\odot$ yr$^{-1}$ and blue lines to $\dot{M} = 2.6 \times 10^{-8} \, M_\odot$ yr$^{-1} \simeq \dot{M}_\mathrm{Edd}$. We apply three base luminosities: $L_b = 10^{35}$ erg s$^{-1}$, continuous lines, $\frac{1}{2} \times 10^{35}$ erg s$^{-1}$, dashed lines, and $2 \times 10^{35}$ erg s$^{-1}$, dash-dotted lines. Order of magnitude explosion times, $t_\mathrm{expl}$, are indicated: at a few centuries explosions are triggered by unstable $^{16}$O burning, at a few years by unstable $^{12}$C burning and at a few hours by $^{4}$He. The gray backgrounds indicate the range of values accessible in up to three years of accretion at the Eddington rate $\dot{m}_\mathrm{Edd} \sim 10^5$ g cm$^{-2}$ s$^{-1}$, the maximum possible heavy accretion time in the case of SN 1987A. Accretion that stops before reaching the displayed explosion depth will result in a thick light element envelope, while if it reaches the explosion depth most of the accreted matter will be converted into heavy elements.
• Figure 2: Example of a model where both $^{4}$He and $^{12}$C are exhausted before they reach their critical explosion densities and an explosion is triggered by unstable $^{16}$O burning. The left panel shows the temperature profile after $\sim 311$ years of accretion of 10% $^{4}$He immersed in 90% $^{16}$O at a rate $\dot{M} = 10^{-8} \, M_\odot$ yr$^{-1}$. Typical of all our simulations, this model exemplifies the high temperatures present in these envelopes, above $7\times 10^8$ K at $\rho \sim 10^{11}$ g cm$^{-3}$, as a result of the high flux coming from the hot young neutron star interior. In the right panel one can appreciate the depletion and then exhaustion of $^{4}$He at column depths between $10^8$ to $10^{10}$ g cm$^{-2}$ resulting in the appearance of $^{12}$C. Notice also the appearance of $^{20}$Ne, and $^{24}$Mg, as well as traces of heavier elements from succesive $(\alpha, \gamma)$ captures on $^{16}$O, $^{20}$Ne, $^{24}$Mg, $\dots$. Initiation of burning of $^{12}$C through $^{12}$C --$^{12}$C fusion is seen at $y \sim 10^{12}$ g cm$^{-2}$, resulting in the production of $^{20}$Ne and $^{24}$Mg, and traces of $^{28}$Si and $^{32}$S, rapidly leading to its depletion. Deeper, at $y \sim 3 \times 10^{13}$ g cm$^{-2}$, $^{16}$O burning from $^{16}$O --$^{16}$O (and some $^{12}$C --$^{16}$O) fusions is initiated and becomes unstable, triggering an explosion seen as a temperature peak in the left panel.
• Figure 3: Model accreting 30% $^{12}$C and 70% $^{4}$He at a rate $\dot{M} = 10^{-8} \, M_\odot$ yr$^{-1}$. The upper panel is taken at a time previous to the explosion and the lower panel illustrates the explosion itself. In each panel the left frames present the $T-\rho$ profile of the envelope, at the indicated time, with coloring depicting the hydrodynamic state of the various layers. The right frames present the composition of matter at the same times. An animated version of this figure will be available with the on-line version of the paper (MNRAS) or from the arXiv abstract page either as an "Ancillary File" or by downloading the "Tex Source" package.
• Figure 4: Model accreting 80% $^{16}$O and 20% $^{4}$He at a rate $\dot{M} = 10^{-8} \, M_\odot$ yr$^{-1}$. The panels and their frames are analogous to the ones of Figure \ref{['fig3']}. An animated version of this figure will be available with the on-line version of the paper (MNRAS) or from the arXiv abstract page either as an "Ancillary File" or by downloading the "Tex Source" package.