Evolution of Young Neutron Star Envelopes

We extend our initial study of diffusive nuclear burning (DNB) for neutronstars (NSs) with Hydrogen atmospheres and an underlying layer of protoncapturing nuclei. Our initial study showed that DNB can alter the photosphericabundance of Hydrogen on surprisingly short timescales ($10^{2-4}\yrs$).Significant composition evolution impacts the radiated thermal spectrum fromthe NS as well as its overall cooling rate. In this paper, we consider the casewhen the rate limiting step for the H consumption is diffusion to the burninglayer, rather than the local nuclear timescale. This is relevant for NSs withsurface 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 thelimiting rate in DNB, the rate of H consumption is suppressed by 1-2 orders ofmagnitude compared to a DNB estimate that assumes diffusive equilibrium. Inorder to apply our ongoing study to young neutron stars, we also include theimportant effects of strong magnetic fields ($B \sim 10^{12} {\rm G}$). In thisinitial study of magnetic modifications to DNB, we find that the H burning timeis 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 thatall 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 aboutfallback and pulsar winds from measuring the surface composition of a young NS.Comment: 10 pages, 8 figures, to appear in Ap