Nuclear Astrophysics

Nuclear Astrophysics

Physics and Astronomy Department at the University of Washington

Elemental Abundances (lecture by Wallerstein)

6.1 Unevolved stars

Fundamental paper:

Aller and Chamberlain 1951
Found 2 stars metal-poor by factors of factors 100

Two important reasons to study unevolved stars:

1 - Need to model such stars in globular clusters, and in nearby galaxies of various types.
2 - Can provide a great deal of information on the nucleosynthesis and mass loss in the Galaxy prior to the formation of other stars of interest.

Examples:
Sun
10 M stars (type B)
metal-poor stars (subdwarfs)

Hot, young, massive (10 - 15 M) stars (see table)
He possibly enhanced
CNO
Mg, Al, Si a little low
hard to understand

Metal-poor stars
McWilliam et al. 1995, API 109, 2736.

Relative Abundance Distributions

Metal Poor Stars
- 2.0

tabular9
Data are from McWilliam et al. AJ 109, 2757.

tabular13

Gummersbach et al.

Astron and Ap. 338, 881, 1998.

Important observation log N(S) since S tends to avoid grains.

Note: CNO abundances are hard to understand. Could it be grain chemistry? It is not an effect of CNO cycling.

Composition of stars

August Comte 1810
Frauenhofer 1812
Bunsen (flame tests) and Kirehoff: atomic line lists

Astronomers get into the act:
Secchi: Observed Type III TiO bands and Type IV C-bands:
first sign of different composition for different stars

Quantitative work:
1928 Henry Norris Russel: proposed an abundance scale where arbitrarily N = 12
we know now He 11
O 9
N 8
Eu 1

Isotopes from meteorites allow very accurate determinations <1%

Are all stars the same, e.g., hot young stars vs. the sun?

e.g. Hot young stars vs. sun?

Unsold (1939): In a pioneering effort, he argued for abundances around solar values. He was convinced this was universal and seemed to believe all important synthesis occurred in the Big Bang, as in the Alpher, Bethe, Gamow paper of the late 40s.

6.2 Evolved, low-mass stars

References: RMP 1997 69, 995.
Sections by Champagne and by G.W.

The reactions of importance are (see figure)

1. O (p,F(O
followed by O(p,N etc. CNO bicycling

2. Ne Na Ne and NeNa cycle

3. Mg Al Mg and MgAl cycle

Reference RMP Review Oct. 1997 esp. section by Champagne

In M13 a globular with [Fe/H -1.5, a large spread in [OI/Fe] was found: the spread is from + 0.4 -0.5. Similar relationships in other cluster but to a varying degree.

This spreading NOT PRESENT IN FIELD STARS:
Why only globulars?

Most recent and detailed effort:
Cavallo et al., ApJ 492 (575) 1998

Background (for physicists) on composition of stars in globulars

have 10
round and compact
found in halo and galactic center
halo: typical of Baade's pop II
no galactic rotation
low metals
very old 10yrs
galactic center or disc population:
small rotation
medium metals
almost as old

Quantitative results

1958 HWG M92 [Fe/H] = -2.3
M13 [Fe/H] = - 1.3
From 1 - 3 night exposure with 200 - inch

Not much progress until new detectors become available until 1970's

Slowly, J. G. Cohen, R. Peters, GW and Leep and Oke:
Found spreads in [O/Fe], [Na/Fe], [Al/Fe]

But poor signal/noise made these results doubtful (except perhaps to the authors, of course).

BaII stars (type K pec)
(They do not show Tc)

Cameron 1955: Fundamental paper
suggesting heavy element
production by n-capture
source C( O. He later suggested Ne ( Mg
C needs T = 150
Ne needs T = 300

Predicting s-process abundances

For equilibrium

Total neutron exposure

neutron relative velocity

density n's and targets

High makes heavy s-process elements
low makes low s-process elements
by looking at ratio of hs/s you can determine . Stellar models can tell you time interval, , and so N can be estimated.