Stars as Laboratories for Fundamental Physics - MPP Theory Group

356 Chapter 10
Table 10.3. Extrapolation of S 17 to E = 0 for measurements of 7 Be p → 8 B γ
according to Johnson et al. (1992).
Experiment S 17 (0) [eV b]
Kavanagh et al. (1960) 15 ± 6
Parker (1966, 1968) 27 ± 4
Kavanagh et al. (1969) 25.2 ± 2.4
Vaughn et al. (1970) 19.4 ± 2.8
Wiezorek et al. (1977) 41.5 ± 9.3
Filippone et al. (1983) 20.2 ± 2.3
S 17 down to the astrophysically interesting regime. The most recent
comprehensive reanalysis was performed by Johnson et al. (1992) who
found the values listed in Tab. 10.3. They tend to be smaller by around
10% relative to previous extrapolations which did not take into account
that for laboratory energies there is a contribution from d-waves in the
entrance channel. Johnson et al. used two different interaction models
for the extrapolation which yielded identical results within 2%. This
does not necessarily imply that the theoretical extrapolation is known
with this precision; for example, Riisager and Jensen (1993) suggest
much smaller values for S 17 (0).
As stressed by Johnson et al. (1992) it is problematic to combine the
results of Tab. 10.3 to a “world average” because they show systematic
discrepancies. In fact, the data of the two “low-energy” experiments
(Kavanagh et al. 1969; Filippone et al. 1983) agree very well with each
other over a wide range of energies with a systematic offset by a factor
1.34. A similar offset exists between the “high-energy” data of Parker
(1966, 1968) and Vaughn et al. (1970) with a factor 1.42 (Gai 1995).
Because it is unknown which of the experiments is right (if any)
Johnson et al. (1992) combined the data according to a prescription
adopted by the Particle Data Group (1994) for such cases. It amounts
to the usual weighted average, but increasing the error by a certain factor
derived from the statistical significance of the discrepancies. Johnson
et al. then arrive at a world average
S 17 (0) = (22.4 ± 2.1) eV b. (10.8)
Bahcall and Pinsonneault (1992, 1995) used this value, i.e. they used
a 1σ uncertainty of 9%. Turck-Chièze and Lopes (1993) used (22.4 ±
1.3 stat ± 3.0 syst ) eV b, i.e. they adopted an uncertainty of ±15%.