aspects of the astrophysics and nuclear physics of r -process nucleosynthesis

Aspects of the Astrophysics and Nuclear Physics

of r-Process Nucleosynthesis

Rebecca SurmanUnion 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 masses fission probabilities, distribution of fragments

beta decay rates neutron capture rates (?)


halo star observations

Cowan et al (2006)



Main r process

Cowan et al (2006)



Weak r process Main

r process

Cowan et al (2006)

core collapse supernovae ?


the SN neutrino-driven wind

Important parameters

outflow timescale

entropy

electron fraction€

ρ ~ e−t / 3τ ; τ ~ 0.01 s to τ ~ 1 s

€

s ~ 100 to s ~ 400

€

Ye =1

1+ n / p; Ye ~ 0.3 to Ye ~ 0.5

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

PNS

shock

€

p + ν e ↔ n + e+

n + ν e ↔ p + e−

€

Tv e> Tve


the main r process

How is a consistent pattern achieved?

Ye = 0.27

Ye = 0.26

Ye = 0.25


low Ye main r process

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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


low Ye main r process


are needed to see this picture. Fission Cycling



fission cycling and the neutrino luminosities

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

(1051 erg/s)

(1

051 e

rg/s

)

€

p + ν e ↔ n + e+

n + ν e ↔ p + e−


fission cycling: comparison with halo star data





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.


black hole - neutron star merger

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

QuickTime™ and aYUV420 codec decompressor


animation credit: NASA/SkyWorks Digital


PNS – AD-BH comparison

accretion disk

PNS BH

jet (?)

shock

outflow


PNS – AD-BH nuclear physics

accretion disk

PNS BH

jet

shock

outflow

neutrino scattering and emission

nucleosynthesis

nucleosynthesis jet (?)

nuclear physics of disk

nuclear physics of core


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


neutrino temperatures

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

€

Tv e> Tve

nv e> nve


Adiabatic flow with velocity as a function of radial distance:

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

u = v∞ 1−Ro

R

⎛

⎝ ⎜

⎞

⎠ ⎟β

our nucleosynthesis calculation

Outflow parameterization



sample nucleosynthetic outcomes


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

This is a direct consequence of the neutrino physics


sample nucleosynthetic outcomes

Möller et al (2003)

Möller et al (1997)

Möller et al (2003) + exp


Example: the importance of beta decay rates

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)


mass model - neutron capture rate comparison

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

Neutron capture rate variation

Mass model variation


nonequilibrium effects of individual capture rates

130 peakrare earth region + 195 peak€

λγ ∝T 3 / 2 exp −Sn

kT

⎛

⎝ ⎜

⎞

⎠ ⎟ σv

Z ,A

€

σv 131 Cd×10

€

σv 131 Sn×100



influential neutron capture rates

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



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


aspects of the astrophysics and nuclear physics of r -process nucleosynthesis

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