Stationary neutron star envelopes at high accretion rates
arxiv(2024)
摘要
In this work we model stationary neutron star envelopes at high accretion
rates and describe our new code for such studies. As a first step we put
special emphasis on the rp-process which results in the synthesis of heavy
elements and study in detail how this synthesis depends on the mass accretion
rate and the chemical composition of the accreted matter. We show that at very
low accretion rate, Ṁ∼ 0.01 Ṁ_Edd, mostly low mass
(A≤ 24) elements are synthesized with a few heavier ones below the
^40Ca bottleneck. However, once Ṁ is above ∼
> 0.1 Ṁ_Edd this bottleneck is surpassed and nuclei in the
iron peak region (A∼ 56) are abundantly produced. At higher mass accretion
rates progressively heavier nuclei are generated, reaching A ∼ 70 at
Ṁ_Edd and A ∼ 90 at 5 Ṁ_Edd. We find
that when the rp-process is efficient, the nucleosynthesis it generates is
independent of the accreted abundance of CNO elements as these are directly and
copiously generated once the 3α-reaction is operating. We also explore
the efficiency of the rp-process under variations of the relative abundances of
H and He. Simultaneously, we put special emphasis on the density profiles of
the energy generation rate particularly at high density beyond the hydrogen
exhaustion point. Our results are of importance for the study of neutron stars
in systems in which X-ray bursts are absent but are also of relevance for other
systems in describing the low density region, mostly below 10^6 g cm,
inbetween bursts.
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