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Aspects of the Astrophysics and Nuclear Physics of r -Process Nucleosynthesis

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Aspects of the Astrophysics and Nuclear Physics of r-Process Nucleosynthesis

Rebecca Surman

Union College

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology

July 2008

r-process nucleosynthesis

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 2/25

r-process nucleosynthesis - current challenges

Astrophysics

The astrophysical site(s) not conclusively known; possibilities include:

• core collapse supernovae e.g., Meyer et al (1992), Woosley et al (1994), Takahashi et al (1994)

• neutron star mergers e.g., Meyer (1989), Frieburghaus et al (1999), Rosswog et al (2001)

• shocked surface layers of O-Ne-Mg cores e.g., Wanajo et al (2003), Ning et al (2007)

• gamma-ray bursts e.g., Surman et al (2005)

Nuclear Physics

Nuclear properties for ~3000 nuclei far from stability

nuclear massesfission probabilities, distribution of fragments

beta decay ratesneutron capture rates (?)

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 3/25

halo star observations

Cowan et al (2006)

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 4/25

halo star observations

Main r process

Cowan et al (2006)

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 5/25

halo star observations

Weak r process

Main r process

- core collapse supernovae ?

Cowan et al (2006)

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 6/25

the SN neutrino-driven wind

Important parameters

outflow timescale

entropy

electron fraction

shock

PNS

p, n 4He + n seed nuclei + n r process

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 7/25

the main r process

How is a consistent pattern achieved?

Ye = 0.25

Ye = 0.26

Ye = 0.27

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 8/25

low Ye main r process

Beun, McLaughlin, Surman, & Hix, PRC 77, 035804 (2008)

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 9/25

low Ye main r process

Fission Cycling

Beun, McLaughlin, Surman, & Hix, PRC 77, 035804 (2008)

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 9/25

fission cycling and the neutrino luminosities

(1051 erg/s)

(1051 erg/s)

Surman, Beun, McLaughlin, Kane, & Hix, J Phys G 35, 014059 (2008)

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 10/25

fission cycling: comparison with halo star data

Beun, McLaughlin, Surman, & Hix, PRC 77, 035804 (2008)

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 11/25

fission cycling and the main r process

In the SN neutrino-driven wind, the electron neutrino flux determines whether a successful r process is possible

The electron neutrino flux can be reduced by:

fast outflow

active-sterile neutrino oscillations

other new physics

If a sufficient reduction in the electron neutrino flux occurs, fission cycling may insure a stable abundance distribution consistent with the pattern in metal-poor halo stars

Accurate fission probabilities and fragment distributions are required to correctly predict the details of the final abundance distribution for a fission cycling main r process.

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 12/25

Orbit of a black hole - neutron star binary decays by gravitational wave emission

Tidal disruption of the neutron star produces a rapidly accreting disk around the black hole (AD-BH)

possible engine for a short gamma-ray burst

black hole - neutron star merger

animation credit: NASA/SkyWorks Digital

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 13/25

PNS – AD-BH comparison

jet (?)

shock

outflow

PNS

BH

accretion disk

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 14/25

PNS – AD-BH nuclear physics

nucleosynthesis

jet

jet (?)

shock

nucleosynthesis

outflow

PNS

BH

accretion disk

neutrino scattering and emission

nuclear physics of disk

nuclear physics of core

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 15/25

3D black hole - neutron star merger model

1.6 M neutron star + 2.5 M black hole with a = 0.6

Evolved until remains of neutron star form an accretion disk

Model by M. Ruffert and H.-Th. Janka

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 16/25

neutrino temperatures

Surman, McLaughlin, Ruffert, Janka, and Hix, arXiv:0803.1785

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 17/25

our nucleosynthesis calculation

Outflow parameterization

Adiabatic flow with velocity as a function of radial distance:

with v~ 104 km/s, 0.2 < < 1.4, 10 < s/k < 50

Surman, McLaughlin, Ruffert, Janka, and Hix, arXiv:0803.1785

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 18/25

sample nucleosynthetic outcomes

All trajectories from the inner disk region make r-process nuclei

This is a direct consequence of the neutrino physics

Surman, McLaughlin, Ruffert, Janka, and Hix, arXiv:0803.1785

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 19/25

sample nucleosynthetic outcomes

Example: the importance of beta decay rates

Möller et al (2003)

Möller et al (1997)

Möller et al (2003) + exp

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 20/25

neutron capture rates and the r process

- Do they make any difference?
- can influence time until onset of freezeout
e.g., Goriely (1997,8), Farouqi et al, Rauscher (2005)

- can shape local details of the abundance distribution
e.g., Surman et al (1998), Surman & Engel (2001)

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 21/25

mass model - neutron capture rate comparison

Neutron capture rate variation

Mass model variation

Surman, Beun, McLaughlin, and Hix, arXiv:0806.3753

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 22/25

nonequilibrium effects of individual capture rates

130 peak

rare earth region + 195 peak

Surman, Beun, McLaughlin, and Hix, arXiv:0806.3753

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 23/25

influential neutron capture rates

Surman, Beun, McLaughlin, and Hix, arXiv:0806.3753

Capture rates that affect a 5-40% change in the global r-process abundance pattern for increases to the rate by a factor of:

10

50

100-1000

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 24/25

summary

We still don’t know where the r process takes place

evidence increasingly points to core collapse supernovae for the site of the main r process (fission cycling would help)

list of potential sites should include hot outflows from black hole-neutron star mergers, particularly for the weak r process

Everybody knows we need nuclear masses and beta decay rates

individual neutron capture rates are also important

fission probabilities and fragment distributions may be crucial

R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 25/25