Issue |
A&A
Volume 494, Number 3, February II 2009
|
|
---|---|---|
Page(s) | 829 - 844 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361:200810292 | |
Published online | 22 December 2008 |
On the dynamics of proto-neutron star winds and r-process nucleosynthesis
1
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
2
Institute for Theoretical and Experimental Physics, B. Cheremushkinskaya 25, Moscow 117218, Russia e-mail: Igor.Panov@itep.ru
Received:
30
May
2008
Accepted:
26
November
2008
We study here the formation of heavy r-process nuclei in
the high-entropy environment of rapidly expanding neutrino-driven
winds from compact objects. In particular, we explore the
sensitivity of the element creation in the region to the
low-temperature behavior of the outflows. For this purpose we employ
a simplified model of the dynamics and of the thermodynamical
evolution for radiation dominated, adiabatic outflows. It consists
of a first stage of fast, exponential cooling with timescale
, followed by a second phase of slower evolution,
assuming either constant density and temperature or a power-law
decay of these quantities. These cases describe a strong
deceleration or decreasing acceleration of the transsonic outflows,
respectively, and thus are supposed to capture the most relevant
effects associated with a change in the wind expansion behavior at
large radii, for example because of the collision with the slower,
preceding supernova ejecta and the possible presence of a wind
termination shock. We find that for given entropy, expansion
timescale, and proton-to-baryon ratio not only the transition
temperature between the two expansion phases can make a big
difference in the formation of the platinum peak, but also the
detailed cooling law during the later phase. Because the nuclear
photodisintegration rates between about
K and
roughly 109 K are more sensitive to the temperature than the
neutron-capture rates are to the free neutron density, a faster
cooling but continuing high neutron density in this temperature
regime allow the r-process path to move closer to the neutron-drip
line. With low (γ,n)- but high β-decay rates, the
r-processing does then not proceed through a
(γ, n)-(n, γ) equilibrium but through a
quasi-equilibrium of (n, γ)-reactions and β-decays, as
recently also pointed out by Wanajo. Unless the transition
temperature and corresponding (free neutron) density become too low
(
K), a lower temperature or faster temperature
decline during the slow, late evolution phase therefore allow for a
stronger appearance of the third abundance peak.
Key words: nuclear reactions, nucleosynthesis, abundances / stars: supernovae: general / stars: winds, outflows / stars: neutron
© ESO, 2009
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