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Title: Experimental measurements of the 15O(alpha,gamma)19Ne reaction rate and the stability of thermonuclear burning on accreting neutron stars

Abstract

Neutron stars in close binary star systems often accrete matter from their companion stars. Thermonuclear ignition of the accreted material in the atmosphere of the neutron star leads to a thermonuclear explosion which is observed as an X-ray burst occurring periodically between hours and days depending on the accretion rate. The ignition conditions are characterized by a sensitive interplay between the accretion rate of the fuel supply and its depletion rate by nuclear burning in the hot CNO cycle and the rp-process. For accretion rates close to stable burning the burst ignition therefore depends critically on the hot CNO breakout reaction {sup 15}O({alpha}, {gamma}){sup 19}Ne that regulates the flow between the hot CNO cycle and the rapid proton capture process. Until recently, the {sup 15}O({alpha}, {gamma}){sup 19}Ne reaction rate was not known experimentally and the theoretical estimates carried significant uncertainties. In this paper we perform a parameter study of the uncertainty of this reaction rate and determine the astrophysical consequences of the first measurement of this reaction rate. Our results corroborate earlier predictions and show that theoretically burning remains unstable up to accretion rates near the Eddington limit, in contrast to astronomical observations.

Authors:
; ; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
919621
Report Number(s):
UCRL-JRNL-230876
Journal ID: ISSN 0004-637X; ASJOAB; TRN: US0806459
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal, vol. 665, n/a, August 10, 2007, pp. 637; Journal Volume: 665
Country of Publication:
United States
Language:
English
Subject:
99 GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE; 73 NUCLEAR PHYSICS AND RADIATION PHYSICS; BINARY STARS; CNO CYCLE; IGNITION; NEUTRON STARS; PROTONS; STABILITY; STARS; THERMONUCLEAR EXPLOSIONS; THERMONUCLEAR IGNITION

Citation Formats

Fisker, J, Tan, W, Goerres, J, Wiescher, M, and Cooper, R. Experimental measurements of the 15O(alpha,gamma)19Ne reaction rate and the stability of thermonuclear burning on accreting neutron stars. United States: N. p., 2007. Web.
Fisker, J, Tan, W, Goerres, J, Wiescher, M, & Cooper, R. Experimental measurements of the 15O(alpha,gamma)19Ne reaction rate and the stability of thermonuclear burning on accreting neutron stars. United States.
Fisker, J, Tan, W, Goerres, J, Wiescher, M, and Cooper, R. Tue . "Experimental measurements of the 15O(alpha,gamma)19Ne reaction rate and the stability of thermonuclear burning on accreting neutron stars". United States. doi:. https://www.osti.gov/servlets/purl/919621.
@article{osti_919621,
title = {Experimental measurements of the 15O(alpha,gamma)19Ne reaction rate and the stability of thermonuclear burning on accreting neutron stars},
author = {Fisker, J and Tan, W and Goerres, J and Wiescher, M and Cooper, R},
abstractNote = {Neutron stars in close binary star systems often accrete matter from their companion stars. Thermonuclear ignition of the accreted material in the atmosphere of the neutron star leads to a thermonuclear explosion which is observed as an X-ray burst occurring periodically between hours and days depending on the accretion rate. The ignition conditions are characterized by a sensitive interplay between the accretion rate of the fuel supply and its depletion rate by nuclear burning in the hot CNO cycle and the rp-process. For accretion rates close to stable burning the burst ignition therefore depends critically on the hot CNO breakout reaction {sup 15}O({alpha}, {gamma}){sup 19}Ne that regulates the flow between the hot CNO cycle and the rapid proton capture process. Until recently, the {sup 15}O({alpha}, {gamma}){sup 19}Ne reaction rate was not known experimentally and the theoretical estimates carried significant uncertainties. In this paper we perform a parameter study of the uncertainty of this reaction rate and determine the astrophysical consequences of the first measurement of this reaction rate. Our results corroborate earlier predictions and show that theoretically burning remains unstable up to accretion rates near the Eddington limit, in contrast to astronomical observations.},
doi = {},
journal = {Astrophysical Journal, vol. 665, n/a, August 10, 2007, pp. 637},
number = ,
volume = 665,
place = {United States},
year = {Tue May 08 00:00:00 EDT 2007},
month = {Tue May 08 00:00:00 EDT 2007}
}
  • The stability of thermonuclear burning of hydrogen and helium accreted onto neutron stars is strongly dependent on the mass accretion rate. The burning behavior is observed to change from Type I X-ray bursts to stable burning, with oscillatory burning occurring at the transition. Simulations predict the transition at a 10 times higher mass accretion rate than observed. Using numerical models we investigate how the transition depends on the hydrogen, helium, and CNO mass fractions of the accreted material, as well as on the nuclear reaction rates of 3α and the hot-CNO breakout reactions {sup 15}O(α, γ){sup 19}Ne and {sup 18}Ne(α,more » p){sup 21}Na. For a lower hydrogen content the transition is at higher accretion rates. Furthermore, most experimentally allowed reaction rate variations change the transition accretion rate by at most 10%. A factor 10 decrease of the {sup 15}O(α, γ){sup 19}Ne rate, however, produces an increase of the transition accretion rate of 35%. None of our models reproduce the transition at the observed rate, and depending on the true {sup 15}O(α, γ){sup 19}Ne reaction rate, the actual discrepancy may be substantially larger. We find that the width of the interval of accretion rates with marginally stable burning depends strongly on both composition and reaction rates. Furthermore, close to the stability transition, our models predict that X-ray bursts have extended tails where freshly accreted fuel prolongs nuclear burning.« less
  • We present some preliminary results of calculations we have made suggesting that runaway thermonuclear burning of interstellar gas accreted onto magnetic neutron stars can account for the observed size-frequency distribution of gamma ray bursts.
  • We present some preliminary results of calculations we have made suggesting that runaway thermonuclear burning of interstellar gas accreted onto magnetic neutron stars can account for the observed size-frequency distribution of gamma ray bursts.
  • The 15O({alpha},{gamma})19Ne reaction is one of the most important breakout reactions for the hot CNO cycles. However, the relevant states in 19Ne at excitation energies of 4-5 MeV have not been well studied. The lifetimes of these states are not known and are only constrained by experimental upper/lower limits. In particular, the accurate knowledge of the {gamma}- and {alpha}- decay widths of the 4.03 MeV state of 19Ne is important, since the resonance strength of this level dominates the reaction rate for the astrophysically relevant temperatures T9 < 0.6. In this work, we employed an improved DSAM approach to obtainmore » lifetime values of this and other states via 17O(3He,n - {gamma})19Ne. For the 4.03 MeV state, the measured excitation energy is 4034.5{+-}0.8 keV and the mean lifetime, measured here for the first time, is 13{sub -6}{sup +9} fs at the confidence level of 1{sigma} and 13{sub -9}{sup +16} fs at the confidence level of 2{sigma}. This result is in excellent agreement with the 9 fs prediction by Langanke, Wiescher, Fowler, and Goerres.« less
  • A recent discussion of the {sup 18}F(p,{alpha}) reaction [N. de Sereville et al., Phys. Rev. C 79, 015801 (2009)] questions our previous analysis of interference effects in this reaction. We address these questions and demonstrate that our previous analysis is indeed appropriate.