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Nuclear Physics and Low-Metallicity Stellar Abundances: Victories and Struggles
Chris Sneden, University of Texas
speaking on behalf of many friends and colleagues in the stellar abundance & nucleosynthesis game
Two main areas of interest to me Neutron-capture elements
Z > 30 (not all are due to neutron-capture?) concentration on the r-process “complete” abundance patterns now available?
departures from scaled-solar r-process possible shortcuts to r-process enrichment predicted abundance patterns lagging current situation: observation ahead of theory
Fe-group elements Z = 21-30 lots of excellent supernova yields avaliable some observed departures from solar abundance mix but observations might not be trustworthy
steps underway to attack these worries current situation: theory ahead of observation
A prime goal (and potential trap):understanding the solar chemical composition
Sneden et al. 2008
• s-process: β-decays occur between successive n-captures• r-process: rapid, short-lived neutron blast overwhelms β-decay rates• r- or s-process element: solar-system dominance by r- or s- production
Rolfs & Rodney (1988)
The basic neutron-capture paths
A detailed look at the r- and s-process paths
Sneden et al. 2008
“s-process” element
“r-process” element
metal-poor n-capture-rich stars are common
HST UV spectra yield exotic elements in brighter low-metallicity stars
Roed
erer
et a
l. 20
12
first detections of some
elements, first believable
abundances of other elements
Roed
erer
et a
l. 20
12
see also Siqueira Mello Jr. et al. 2013
the result is a “complete” abundance setSi
quei
ra M
ello
Jr. e
t al.
2013
blue line: solar system scaled
r-process
log(X/H)+12 = log ε
But we just keep trying to fit to the solar system abundance distribution
Kratz et al. 2007
hopefully, theoretical models are now catching upSi
quei
ra M
ello
Jr. e
t al.
2013
n-capture compositions of well-studied r-rich stars: Così fan tutte??
Sned
en e
t al.
2008
confusions remain about heavy versus
light n-capture abundances was (unfortunately)
named LEPP
LEPP = lighter element primary process (Travaglio et al. 2004)
[A/B] = log(NA/NB)star – log(NA/NB)Sun
This paper suggests that
there is no known low metallicity star without neutron-capture elements
Roederer 2013
upper limits in this figure are maybe just due to
spectroscopic detection problems?
on average the points to the lower left are lowest Fe metallicity stars
increasing evidence for non-solar r-processes
Roederer et al. 2010
this is a phenomenon extending to lots of stars
Roed
erer
et a
l. 20
10
But getting detailed neutron-capture abundances requires synthetic spectrum hand (very boring) computational effort
maybe this is just r-process truncation at work
Roed
erer
et a
l. 20
10
full?
truncated?
perhaps there is an easier way:
just Sr, Ba, Eu, Yb
being done with Jesse Palmerio, John Cown, Dick Boyd, Ian Roederer
Why? Sr, Ba, Eu, Yb lines are simply strong
Sr/Ba: assessment of LEPP issues
Ba/Eu: assessment of r- or s- dominance
Ba/Yb: assessment of r-process truncation
being done with Jesse Palmerio, John Cown, Dick Boyd, Ian Roederer
let’s turn to Fe-peak elements
McW
illia
m 1
997
the “first stars” effort refined the
quantitative answers but the
qualitative trends stay the same
Cayrel et al. 2004
theoretical models can generate these elements
Kobayashi et al. 2006 Koba
yash
i et a
l. 20
06
and do so in ways that can be compared to observational observed trends
Kobayashi et al. 2006 Koba
yash
i et a
l. 20
06
there are good predictions for “zero-Z” models
Heger & Woosley 2010
for some elements the theory/observation match seems happy
Kobayashi et al. 2006
but for others, watch out!
Kobayashi et al. 2006
same theory, different observed species of the same element
A typical metal-poor giant Fe-group abundance set
there are very few lines for many species
and we often are stuck observing the wrong species
Fe-peak abundances in metal-poor stars: can you believe ANY analysis from the past?
the outcome for Bergemann et al.?
Are observers really saying that the Co/Fe ratio is 10x solar at lowest metallicities?
A new initiative to on Fe-group abundances
Kobayashi et al. 2006
this work concentrates on increasing accuracy of Fe-group elementsthe big point: must have better transition probabilitiesgroups at Wisconsin, London, Belgium lead the wayHST data at low metallicity end explores more species
dotted line: no Fe in synthesissolid line: best fit dashed lines: ±0.5 dex from best fit
red line: perfect agreementother lines: deviations
why it is worth exploring the UV spectral region
a quick report for today
the big point: Ti I & Ti II give same answer; scatter is very low; Ti is really overabundant(Wood, Lawler, Guzman, Sneden, Cowan 2013)
Ti obs/theory clashes are real,
and must now be addressed
Heger & Woosley 2010
Kobayashi et al. 2006
more work to be done!
theorists: please publish the numbers in neutron-capture predictions; continue exploring alternative ways to
produce the Z=31-50 range
observers: please produce Fe-group abundances that are useful for the theorists; especially support
improvements in lab atomic and molecular physics
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Roederer et al. 2012