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Title: Nuclear Reactions in the Crusts of Accreting Neutron Stars

X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density is $$\rho =2\times {10}^{12}\,{\rm{g}}\,{\mathrm{cm}}^{-3}$$ using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond $$\rho =2\times {10}^{12}\,{\rm{g}}\,{\mathrm{cm}}^{-3}$$. This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A ≥ 102 nuclei, where the inner crust remains impure with an impurity parameter of Q imp ≈ 20 owing to the N = 82 shell closure. In agreement with previous work, we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. As a result, this work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.
Authors:
 [1] ;  [2] ;  [3] ; ORCiD logo [4] ;  [5] ;  [4] ; ORCiD logo [6] ;  [7] ;  [8] ; ORCiD logo [9] ;  [10] ;  [11] ;  [12] ; ORCiD logo [13] ;  [2] ;  [14]
  1. Michigan State Univ., East Lansing, MI (United States); Univ. of Notre Dame, Notre Dame, IN (United States); Technological and Higher Education Institute of Hong Kong (Hong Kong)
  2. Univ. of Notre Dame, Notre Dame, IN (United States)
  3. Indian Institute of Technology Ropar, Punjab (India)
  4. Michigan State Univ., East Lansing, MI (United States); Univ. of Notre Dame, Notre Dame, IN (United States)
  5. Mississippi State Univ., Mississippi State, MS (United States)
  6. Michigan State Univ., East Lansing, MI (United States); Univ. of Notre Dame, Notre Dame, IN (United States); Indiana Univ., Bloomington, IN (United States)
  7. Instituto de Fisica da Univ. de Sao Paulo, Sao Paulo (Brazil)
  8. Coastal Carolina Univ., Conway, SC (United States)
  9. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Univ. of Tennessee, Knoxville, TN (United States)
  10. Michigan State Univ., East Lansing, MI (United States); Univ. of Notre Dame, Notre Dame, IN (United States); Univ. of Maryland, College Park, MD (United States)
  11. Univ. of Notre Dame, Notre Dame, IN (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  12. Ioffe Institute, Saint Petersburg (Russia)
  13. Univ. of Notre Dame, Notre Dame, IN (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Univ. of Tennessee, Knoxville, TN (United States)
  14. Extreme Light Infrastructure-Nuclear Physics, Ilfov (Romania)
Publication Date:
Report Number(s):
LA-UR-18-22864
Journal ID: ISSN 1538-4357
Grant/Contract Number:
AC05-00OR22725; SC0013037; AC52-06NA25396
Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 859; Journal Issue: 1; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Research Org:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Mississippi State Univ., Mississippi State, MS (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org:
USDOE Office of Science (SC), Nuclear Physics (NP) (SC-26); USDOE National Nuclear Security Administration (NNSA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; dense matter; nuclear reactions; nucleosynthesis; abundances; stars: neutron; X-rays: binaries; 73 NUCLEAR PHYSICS AND RADIATION PHYSICS; dense matter – nuclear reactions, nucleosynthesis, abundances – stars: neutron – X-rays: binaries; Atomic and Nuclear Physics; Astronomy and Astrophysics
OSTI Identifier:
1454393
Alternate Identifier(s):
OSTI ID: 1459442; OSTI ID: 1463562

Lau, Rita, Beard, Mary, Gupta, Sanjib S., Schatz, H., Afanasjev, A. V., Brown, Edward F., Deibel, A. T., Gasques, Leandro R., Hitt, George Wesley, Hix, William Raphael, Keek, Laurens, Moller, Peter, Shternin, Peter S., Steiner, Andrew W., Wiescher, Michael, and Xu, Yi. Nuclear Reactions in the Crusts of Accreting Neutron Stars. United States: N. p., Web. doi:10.3847/1538-4357/aabfe0.
Lau, Rita, Beard, Mary, Gupta, Sanjib S., Schatz, H., Afanasjev, A. V., Brown, Edward F., Deibel, A. T., Gasques, Leandro R., Hitt, George Wesley, Hix, William Raphael, Keek, Laurens, Moller, Peter, Shternin, Peter S., Steiner, Andrew W., Wiescher, Michael, & Xu, Yi. Nuclear Reactions in the Crusts of Accreting Neutron Stars. United States. doi:10.3847/1538-4357/aabfe0.
Lau, Rita, Beard, Mary, Gupta, Sanjib S., Schatz, H., Afanasjev, A. V., Brown, Edward F., Deibel, A. T., Gasques, Leandro R., Hitt, George Wesley, Hix, William Raphael, Keek, Laurens, Moller, Peter, Shternin, Peter S., Steiner, Andrew W., Wiescher, Michael, and Xu, Yi. 2018. "Nuclear Reactions in the Crusts of Accreting Neutron Stars". United States. doi:10.3847/1538-4357/aabfe0.
@article{osti_1454393,
title = {Nuclear Reactions in the Crusts of Accreting Neutron Stars},
author = {Lau, Rita and Beard, Mary and Gupta, Sanjib S. and Schatz, H. and Afanasjev, A. V. and Brown, Edward F. and Deibel, A. T. and Gasques, Leandro R. and Hitt, George Wesley and Hix, William Raphael and Keek, Laurens and Moller, Peter and Shternin, Peter S. and Steiner, Andrew W. and Wiescher, Michael and Xu, Yi},
abstractNote = {X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density is $\rho =2\times {10}^{12}\,{\rm{g}}\,{\mathrm{cm}}^{-3}$ using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond $\rho =2\times {10}^{12}\,{\rm{g}}\,{\mathrm{cm}}^{-3}$. This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A ≥ 102 nuclei, where the inner crust remains impure with an impurity parameter of Q imp ≈ 20 owing to the N = 82 shell closure. In agreement with previous work, we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. As a result, this work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.},
doi = {10.3847/1538-4357/aabfe0},
journal = {The Astrophysical Journal (Online)},
number = 1,
volume = 859,
place = {United States},
year = {2018},
month = {5}
}