Evolution of Young Neutron Star Envelopes

Abstract

We extend our initial study of diffusive nuclear burning (DNB) for neutron stars (NSs) with Hydrogen atmospheres and an underlying layer of proton capturing nuclei. Our initial study showed that DNB can alter the photospheric abundance of Hydrogen on surprisingly short timescales ($10^{2-4}\yrs$). Significant composition evolution impacts the radiated thermal spectrum from the NS as well as its overall cooling rate. In this paper, we consider the case when the rate limiting step for the H consumption is diffusion to the burning layer, rather than the local nuclear timescale. This is relevant for NSs with surface temperatures in excess of $10^6 {\rm K}$, such as young ($<10^5$ yr) radio pulsars and accreting NSs in quiescence. When downward diffusion is the limiting rate in DNB, the rate of H consumption is suppressed by 1-2 orders of magnitude compared to a DNB estimate that assumes diffusive equilibrium. In order to apply our ongoing study to young neutron stars, we also include the important effects of strong magnetic fields ($B \sim 10^{12} {\rm G}$). In this initial study of magnetic modifications to DNB, we find that the H burning time is lengthened by 2-3 orders of magnitude for a $10^{12} {\rm G}$ field. However, even for NSs with dipole field strengths of $10^{12}$ G, we find that all of the H can be burned before the pulsar reaches an age of $\sim 10^5 \ {\rm yr}$, thus potentially revealing the underlying proton-capturing elements. Finally, we conclude by providing an overview of what can be learned about fallback and pulsar winds from measuring the surface composition of a young NS.