- Email: [email protected]
Thermonuclear runaways on neutron stars: Nucleosynthesis and hydrodynamics
ELSEVIER
Nuclear
Physics
A7 18 (2003)
602~404~ www.elsevier.com/locate/npe
Thermonuclear Hydrodynamics Jordi Jo&”
Runaways
and Fermin
on Neutron
Stars:
Nucleosynthesis
and
Morenoa
&Departament de Fisica i Enginyeria Nuclear, Universitat Institut d’Estudis Espacials de Catalunya (IEEC-UPC),
Politkcnica de Catalunya, Barcelona, Spain.
and
We report on preliminary results of thermonuclear runaways in the H/He-rich envelopes of accreting neutron stars, in connection with x-ray bursts of type I. Calculations are carried by means of a one-dimensional, implicit, hydrodynamic code. Special emphasis is focused on the physical properties of the explosion as well as on the accompanying nucleosynthesis. 1. Introduction Thermonuclear runaways (TN&) in the accreted H/He-rich envelopes of neutron stars (NS) have been extensively studied by different groups, mainly in connection with type 1 z-ray bursts. Usually, limited reaction networks are adopted, a compromise between computing time and accuracy (in terms of the energy generated during the TNR). Endpoints include Ni [6,7,10], Se [I], Kr [3], or Y [8]. Calculations by Wallace & Woosley (1984) [9] reached ‘%d, but in thecontext of a reduced 16 nuclei network. On the contrary, Schatz et al. [4,5] have carried very detailed nucleosynthetic calculations, with a complete reaction network up to the SnSbTe mass region, but using a simple parametric model. We present here some preliminary nucleosynthesis results obtained through the coupling of an extended nuclear reaction network and a hydrodynamic code. 2. The
model
Simulations are carried by means of a modified version of the code SHIVA, a onedimensional, spherically symmetric, hydrodynamic code, in Lagrangian formulation, extensively used in the modeling of classical nova outbursts [2]. For the present study, we have considered a 1.4 M. NS, with an initial luminosity LNs = 5 x lo-‘L,. At the present, stage, we have assumed an envelope ‘in place’, with an arbitrary mass ranging from lo-l2 to 10-gMo. We have adopted a simple Harrison-Wheeler equation of state (EOS) for the NS interior. As for the envelope, we have adopted an EOS that includes contributions from an electron gas (with different degrees of degeneracy), an ion plasma, and radiation. Coulomb corrections to the electronic pressure have been taken into account. We plan to use a more detailed EOS for the NS interior in future calculations. Nevertheless, our current, EOS seems to be accurate enough for the modeling of TNRs that take place in the NS envelope, as reported in this work. Energy transport is carried by radiat,ion 0375-9474/03/s
- see front
matter
doi: 10.1016/SO375-9474(03)00898-4
0 2003 Published
by Elsevier
Science
B.V
J Josk. B Moreno /Nuclear Physics A 718 (2003) 602~~604~
603~
Figure 1. Snapshots of the evolution of the main nuclear path at different stages of the TNR that takes place in a lo-“MD H/He-rich envelope on top of a 1.4 M, NS. Panels correspond to Tbs = 3 x lo8 K (upper left), 6 x lo8 K (upper right), 10” K (lower left), and 1.7 x 10’ K (i.e., Tpeak, lower right).
and convection. A time-dependent formalism for convective transport has been included whenever the characteristic convective timescale becomes larger than the integration time step. Partial mixing between adjacent convective shells is treated by means of a diffusion equation (see [2], for details). Reaction rates have been obtained through the Network Generator tool (Netgen) of the Institut d’Astronomie et d’Astrophysique (ULB, Brussels). The network includes a neutron sink to mimic the effect of slow neutron captures not considered in the calculations. Electron on heavy elements (i.e., from “‘Pd to “‘Bi), screening is also taken into account. 3. Hydrodynamics
and nuclear
reaction
networks
One of the main difficulties in the modeling of these explosions regards large nuclear reaction network to a hydrodynamic code. In a first step, a limited nuclear reaction network, with 100 isotopes -from ‘H to 40Caat the reactions, to the hydrodynamic code. When the temperatame reaches Tbs = lo8 K, we switch to a more ext,ended network, including
how to couple a we have coupled and 250 nuclear envelope’s base 200 isotopes -‘H
604~
1 Josk. l? Motzno/Nuclear
Physics A718 (2003) 602c-604~
to 6”Ga- and nearly 600 nuclear react,ions. With this network we follow, the course of the TNR through the peak of the explosion up to the time when Tbs decreases below 2 x 107 K. Then, we use the temperature and density profiles from these hydrodynamic calculations to obtain a detailed nucleosynthesis (i.e., post-processing) using a complete nuclear reaction network wit,h 500 nuclei (up to “‘Pd), linked through an updated network that includes more than 3000 nrlclear reactions (i.e., P-decays, p-, n-, and cu-captures, plus the corresponding reverse reactions, including photodisintegrations). This procedure is applied to each individual mass-shell adopted for the hydrodynamic calculations (typically, 50-100 shells). 3.1. Nucleosynthesis: preliminary results The main nuclear path follows a series of p- and a-capture reactions. Snapshots of the evolution of the main nuclear path at different, stages of the TNR. in a lo-“Ma H/He-rich envelope on top of a 1.4M, NS, are shown in Fig. 1. The early evolution of the TNR is driven by the 3a reaction plus a series of (p,y) reactsions. A few P-decays (i.e., 150, 13N, 17F 1 show also large reaction fluxes at this stage. As the temperature rises, the role of the photodisintegration reactions, (y,p) and (+~,a), increases dramatically. In this particular model, the nuclear activity extends essentially up t,o 61Ni , which reaches an abundance of 2.8 x 10p5, by mass. These preliminary results will be improved soon with the inclusion of the full accretion phase and extended to a wider range of models, in an attempt to explore in detail the parameter space. We plan to shed some light into several critical issues, such as the predicted endpoint for nucleosynthesis in x-ray bursts (up to now, discussed only in the framework of one-zone calculations), or t,he role of convection (and other multidimensional effects) in the course of the explosion itself. We acknowledge partial support from the Spanish MCYT 04094-CO3-02/03 and AY\A2000-1785, and by the CIRIT.
through
grants
AYA2002-
REFERENCES 1. 2. 3. 4. 5.
Hanawa, T., Sugimoto, D., & Hashimoto, M.-A. (1983), PASJ 35, 491. Jo&, J., & Hernanz, M. (1998), ApJ 494, 680. Koike, O., Hashimoto, M., Arai, K., & Wanajo, S. (1999), A&A 342, 464. Schatz, H., Bildsten, L., Cumming, A., & Wiescher, M. (1999), ApJ 524, 1014. Schatz, H., Aprahamian, A., Barnard, V., Bildsten, L., Cumming, A., Ouellette, M.: Rauscher, T., Thielemann, F.-K., & Wiescher, M. (2001), Nucl. Phys. A688, 150. 6. Taam, R.E., Woosley, SE., Weaver, T.A., & Lamb, D.Q. (1993), ApJ 413, 324. 7. Taam, R.E., Woosley, SE., & Lamb, D.Q. (1996), Ap,J 459, 271. 8. Wallace, R.K., & Woosley, SE. (1981), ApJS 45, 389. 9. Wallace, R.K., & Woosley, S.E. (1984), in High EneTq:qy Tra,nsients in Astrophysics, S.E. Woosley (ed.), New York: AIP, p. 319. Transients in Astrophysics, 10. Woosley, S.E., & Wallace, R..K. (1984), in High Eneqy S.E. Woosley (ed.), New York: AIP, p. 273.
Nuclear
Physics
A7 18 (2003)
602~404~ www.elsevier.com/locate/npe
Thermonuclear Hydrodynamics Jordi Jo&”
Runaways
and Fermin
on Neutron
Stars:
Nucleosynthesis
and
Morenoa
&Departament de Fisica i Enginyeria Nuclear, Universitat Institut d’Estudis Espacials de Catalunya (IEEC-UPC),
Politkcnica de Catalunya, Barcelona, Spain.
and
We report on preliminary results of thermonuclear runaways in the H/He-rich envelopes of accreting neutron stars, in connection with x-ray bursts of type I. Calculations are carried by means of a one-dimensional, implicit, hydrodynamic code. Special emphasis is focused on the physical properties of the explosion as well as on the accompanying nucleosynthesis. 1. Introduction Thermonuclear runaways (TN&) in the accreted H/He-rich envelopes of neutron stars (NS) have been extensively studied by different groups, mainly in connection with type 1 z-ray bursts. Usually, limited reaction networks are adopted, a compromise between computing time and accuracy (in terms of the energy generated during the TNR). Endpoints include Ni [6,7,10], Se [I], Kr [3], or Y [8]. Calculations by Wallace & Woosley (1984) [9] reached ‘%d, but in thecontext of a reduced 16 nuclei network. On the contrary, Schatz et al. [4,5] have carried very detailed nucleosynthetic calculations, with a complete reaction network up to the SnSbTe mass region, but using a simple parametric model. We present here some preliminary nucleosynthesis results obtained through the coupling of an extended nuclear reaction network and a hydrodynamic code. 2. The
model
Simulations are carried by means of a modified version of the code SHIVA, a onedimensional, spherically symmetric, hydrodynamic code, in Lagrangian formulation, extensively used in the modeling of classical nova outbursts [2]. For the present study, we have considered a 1.4 M. NS, with an initial luminosity LNs = 5 x lo-‘L,. At the present, stage, we have assumed an envelope ‘in place’, with an arbitrary mass ranging from lo-l2 to 10-gMo. We have adopted a simple Harrison-Wheeler equation of state (EOS) for the NS interior. As for the envelope, we have adopted an EOS that includes contributions from an electron gas (with different degrees of degeneracy), an ion plasma, and radiation. Coulomb corrections to the electronic pressure have been taken into account. We plan to use a more detailed EOS for the NS interior in future calculations. Nevertheless, our current, EOS seems to be accurate enough for the modeling of TNRs that take place in the NS envelope, as reported in this work. Energy transport is carried by radiat,ion 0375-9474/03/s
- see front
matter
doi: 10.1016/SO375-9474(03)00898-4
0 2003 Published
by Elsevier
Science
B.V
J Josk. B Moreno /Nuclear Physics A 718 (2003) 602~~604~
603~
Figure 1. Snapshots of the evolution of the main nuclear path at different stages of the TNR that takes place in a lo-“MD H/He-rich envelope on top of a 1.4 M, NS. Panels correspond to Tbs = 3 x lo8 K (upper left), 6 x lo8 K (upper right), 10” K (lower left), and 1.7 x 10’ K (i.e., Tpeak, lower right).
and convection. A time-dependent formalism for convective transport has been included whenever the characteristic convective timescale becomes larger than the integration time step. Partial mixing between adjacent convective shells is treated by means of a diffusion equation (see [2], for details). Reaction rates have been obtained through the Network Generator tool (Netgen) of the Institut d’Astronomie et d’Astrophysique (ULB, Brussels). The network includes a neutron sink to mimic the effect of slow neutron captures not considered in the calculations. Electron on heavy elements (i.e., from “‘Pd to “‘Bi), screening is also taken into account. 3. Hydrodynamics
and nuclear
reaction
networks
One of the main difficulties in the modeling of these explosions regards large nuclear reaction network to a hydrodynamic code. In a first step, a limited nuclear reaction network, with 100 isotopes -from ‘H to 40Caat the reactions, to the hydrodynamic code. When the temperatame reaches Tbs = lo8 K, we switch to a more ext,ended network, including
how to couple a we have coupled and 250 nuclear envelope’s base 200 isotopes -‘H
604~
1 Josk. l? Motzno/Nuclear
Physics A718 (2003) 602c-604~
to 6”Ga- and nearly 600 nuclear react,ions. With this network we follow, the course of the TNR through the peak of the explosion up to the time when Tbs decreases below 2 x 107 K. Then, we use the temperature and density profiles from these hydrodynamic calculations to obtain a detailed nucleosynthesis (i.e., post-processing) using a complete nuclear reaction network wit,h 500 nuclei (up to “‘Pd), linked through an updated network that includes more than 3000 nrlclear reactions (i.e., P-decays, p-, n-, and cu-captures, plus the corresponding reverse reactions, including photodisintegrations). This procedure is applied to each individual mass-shell adopted for the hydrodynamic calculations (typically, 50-100 shells). 3.1. Nucleosynthesis: preliminary results The main nuclear path follows a series of p- and a-capture reactions. Snapshots of the evolution of the main nuclear path at different, stages of the TNR. in a lo-“Ma H/He-rich envelope on top of a 1.4M, NS, are shown in Fig. 1. The early evolution of the TNR is driven by the 3a reaction plus a series of (p,y) reactsions. A few P-decays (i.e., 150, 13N, 17F 1 show also large reaction fluxes at this stage. As the temperature rises, the role of the photodisintegration reactions, (y,p) and (+~,a), increases dramatically. In this particular model, the nuclear activity extends essentially up t,o 61Ni , which reaches an abundance of 2.8 x 10p5, by mass. These preliminary results will be improved soon with the inclusion of the full accretion phase and extended to a wider range of models, in an attempt to explore in detail the parameter space. We plan to shed some light into several critical issues, such as the predicted endpoint for nucleosynthesis in x-ray bursts (up to now, discussed only in the framework of one-zone calculations), or t,he role of convection (and other multidimensional effects) in the course of the explosion itself. We acknowledge partial support from the Spanish MCYT 04094-CO3-02/03 and AY\A2000-1785, and by the CIRIT.
through
grants
AYA2002-
REFERENCES 1. 2. 3. 4. 5.
Hanawa, T., Sugimoto, D., & Hashimoto, M.-A. (1983), PASJ 35, 491. Jo&, J., & Hernanz, M. (1998), ApJ 494, 680. Koike, O., Hashimoto, M., Arai, K., & Wanajo, S. (1999), A&A 342, 464. Schatz, H., Bildsten, L., Cumming, A., & Wiescher, M. (1999), ApJ 524, 1014. Schatz, H., Aprahamian, A., Barnard, V., Bildsten, L., Cumming, A., Ouellette, M.: Rauscher, T., Thielemann, F.-K., & Wiescher, M. (2001), Nucl. Phys. A688, 150. 6. Taam, R.E., Woosley, SE., Weaver, T.A., & Lamb, D.Q. (1993), ApJ 413, 324. 7. Taam, R.E., Woosley, SE., & Lamb, D.Q. (1996), Ap,J 459, 271. 8. Wallace, R.K., & Woosley, SE. (1981), ApJS 45, 389. 9. Wallace, R.K., & Woosley, S.E. (1984), in High EneTq:qy Tra,nsients in Astrophysics, S.E. Woosley (ed.), New York: AIP, p. 319. Transients in Astrophysics, 10. Woosley, S.E., & Wallace, R..K. (1984), in High Eneqy S.E. Woosley (ed.), New York: AIP, p. 273.