J/A+A/662/A63 Partially accreted crusts of neutron stars (Suleiman+, 2022)
Partially accreted crusts of neutron stars.
Suleiman L., Zdunik J.L., Haensel P., Fortin M.
<Astron. Astrophys. 662, A63 (2022)>
=2022A&A...662A..63S 2022A&A...662A..63S (SIMBAD/NED BibCode)
ADC_Keywords: Atomic physics
Keywords: stars: neutron - equation of state - dense matter - X-rays: binaries -
accretion, accretion disks
Abstract:
Neutron stars in low-mass binary systems are subject to accretion when
material originating from the companion star accumulates on the
surface. In most cases, the justified and common assumption in
studying the properties of the neutron star crust is the fully
accreted crust approximation. However, observations of some X-ray
transient sources indicate that the original crust has not been
completely replaced by accreted material, but is partly composed of
the compressed original crust.
The crust of an accreting neutron star beyond the fully accreted crust
approximation was studied; a two-part (or hybrid) crust made of the
original crust that is compressed and of the accreted material
crashing onto it was reconstructed as a function of the accretion
stage. The differences in the composition and energy sources for the
fully accreted and hybrid crusts influence the cooling and transport
properties.
A simple semi-empirical formula of a compressible liquid drop was used
to compute the equation of state and composition of the hybrid crust.
Calculations were based on the single-nucleus model, with a more
accurate treatment of the neutron drip point. We compared the nuclear
reactions triggered by compression in the original crust and in the
accreted matter part of the hybrid crust. We discuss another crust
compression astrophysical phenomenon related to spinning neutron
stars.
The compression of the originally catalyzed outer crust triggers
exothermic reactions (electron captures and pycnonuclear fusions) that
deposit heat in the crust. The heat sources are cataloged as a
function of the compression until the fully accreted crust
approximation is reached. The pressure at which neutron drip occurs is
a nonmonotonic function of the depth, leading to a temporary neutron
drip anomaly. The additional potential source of energy for partially
accreted crusts is the occurrence of a density inversion phenomenon
between some compressed layers.
The original crust of a neutron star cannot be neglected for the
initial period of accretion, when the original crust is not fully
replaced by the accreted matter. The amount of heat associated with
the compression of the original crust is on the same order of
magnitude as that from the sources acting in the accreted part of the
hybrid crust.
Description:
The tables present the reactions triggered by the compression of an
originally catalyzed outer crust calculated in the Mackie & Baym
(1977) nuclear model. Reactions are presented up to the neutron drip
point calculated in the single nucleus model for the file
ReactionOCCOCSNM.res and calculated with the continuous approach (see
Chamel et al. (2015, Phys. Rev. C, 91, 055803)) for the file
ReactionOCCOCContinuous.res.
The tables are in 24 parts for the file ReactionOCCOCSNM.res and 23
parts for ReactionOCCOCContinuous.res, each part corresponding to the
compression of one shell of the originally catalyzed crust. At the
beginning of each line, we present the pressure and density at the top
of the shell in its catalyzed state as P_ini and rho_ini, the nuclei
of the shell in its catalyzed state, and the shell number (from 1 at
the surface to 24 at inner crust boundary).
The other columns present:
- the pressure at which the reaction occurs (P) in dyn/cm^2,
- the density rho_ini in g/cm^3 right before the reaction,
- L = Δρ/ρ the relative change in density due to the
reaction in percent,
- the nucleon (A_i) and proton (Z_i) number of the parent nucleus of
the reaction,
- the nucleon (A_f), proton (Z_f) and neutron (N_f) number of the
daughter nuclei of the reaction,
- the energy per nucleon Q in keV per nucleon of the reaction,
- the compression DP in dyn/cm^2 at which the reaction is triggered,
- the relative compression dP/P in percent at which the reaction
is triggered.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
snm.dat 132 85 Reactions triggered by the compression of an
originally catalyzed outer crust
(ReactionOCCOCSNM.res)
cont.dat 132 65 Reactions triggered by the compression of an
originally catalyzed outer crust calculated with the
continuous approach (ReactionOCCOCContinuous.res)
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Byte-by-byte Description of file: snm.dat cont.dat
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Bytes Format Units Label Explanations
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1- 12 E12.7 dyn/cm2 Pini Pressure at the top of the shell in its
catalyzed state (1)
14- 25 E12.7 g/cm3 rhoini Density of the shell in its catalyzed
state (1)
27- 37 A11 --- Nuclei Original catalyzed nuclei (A,Z,N) (1)
39- 43 A5 --- --- [Shell]
45- 46 I2 --- Shell [1/24] Shell number (1)
48- 59 E12.7 dyn/cm2 P Pressure at which the reaction occurs
61- 72 E12.7 g/cm3 rho Density
74- 78 F5.2 % lambda Relative change in density due to the
reaction, Δρ/ρ
80- 82 I3 --- Ai Nucleon number of the parent nucleus of the
reaction
84- 85 I2 --- Zi Proton number of the parent nucleus of the
reaction
87- 89 I3 --- Af Nucleon number of the daughter nucleus of the
reaction
91- 92 I2 --- Zf Proton number of the daughter nucleus of the
reaction
94- 95 I2 --- Nf Neutron number of the daughter nucleus of the
reaction
97-104 F8.4 keV Q Energy per nucleon Q in keV per nucleon of the
reaction
106-117 E12.7 dyn/cm2 DP Compression at which the reaction is triggered
119-132 F14.4 % dP/P Relative compression at which the reaction is
triggered
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Note (1): Parameters corresponding to the compression of one shell of the
originally catalyzed crust.
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Acknowledgements:
Lami Suleiman, lsuleiman(at)camk.edu.pl
(End) Lami Suleiman [CAMK PAN], Patricia Vannier [CDS] 29-Mar-2022