Stars as Laboratories for Fundamental Physics - MPP Theory Group

Anomalous Stellar Energy Losses Bounded by Observations 77
These authors have introduced a fourth observable, the so-called
mass to light ratio A of RR Lyrae stars. This observable has also been
used by Castellani and Degl’Innocenti (1993) to constrain a possible
core-mass excess. It amounts to a determination of the core mass of an
RR Lyrae star on the basis of its luminosity and pulsation period.
Going through similar steps as in the previous sections, Catelan,
de Freitas Pacheco, and Horvath (1995) then found the results
Y env − 0.9 δM c = 0.207 ± 0.039
from ∆M tip
HB,
Y env − 0.3 δM c = 0.218 ± 0.018 from R,
Y env + 2.1 δM c = 0.250 ± 0.049 from M RR ,
Y env + 1.5 δM c = 0.238 ± 0.008 from A. (2.38)
Bands of allowed values for Y env and δM c are shown in Fig. 2.23 which
is analogous to Fig. 2.22.
From this analysis one infers a best-fit value for the envelope helium
abundance which is somewhat low. The primordial abundance
probably is not lower than 22%. The only possibility to reduce the
envelope abundance from this level is by gravitational settling which
is counteracted by convective dredge-up, and perhaps by other effects
that might eject helium into the envelope from the core. Therefore,
Fig. 2.23. Allowed values for the envelope helium abundance of evolved
globular-cluster stars and of an anomalous core-mass excess at helium ignition
according to the analysis of Catelan, de Freitas Pacheco, and Horvath
(1995). For comparison see Fig. 2.22.