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星期六, 十月 18, 2008

Metzger 2008 Neutron-Rich Freeze-Out in compat mergers

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Title:
Neutron-Rich Freeze-Out in Viscously Spreading Accretion Disks Formed from Compact Object Mergers
Authors:
Metzger, B. D.; Piro, A. L.; Quataert, E.
Publication:
eprint arXiv:0810.2535
Publication Date:
10/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
12 pages, 9 figures, 1 table; submitted to MNRAS
Bibliographic Code:
2008arXiv0810.2535M

Abstract

Accretion disks with masses ~0.001-0.1 Msun form during the merger of neutron star (NS)-NS and black hole-NS binaries. Initially, such hyper-accreting disks cool efficiently by neutrino emission and their composition is driven neutron-rich by pair captures under degenerate conditions. However, as the disk viscously spreads and its temperature drops, cooling becomes inefficient and the disk becomes advective. Analytic arguments and numerical simulations suggest that once this occurs, powerful winds likely drive away most of the disk's remaining mass. We calculate the thermal evolution and nuclear composition of viscously spreading accretion disks formed from compact object mergers using one-dimensional height-integrated simulations. We show that freeze-out from weak equilibrium necessarily accompanies the disk's late-time transition to an advective state. As a result, hyper-accreting disks generically freeze out neutron-rich (with electron fraction Ye ~ 0.2-0.4), and their late-time outflows robustly synthesize rare neutron-rich isotopes. Using the measured abundances of these isotopes in our solar system, we constrain the compact object merger rate in the Milky Way to be <> 30(M_d,0/0.1 Msun)^(1/2) degrees. This is consistent with other evidence that short duration GRB outflows are less collimated than those produced in long duration GRBs.
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