Stars as Laboratories for Fundamental Physics - MPP Theory Group

Anomalous Stellar Energy Losses Bounded by Observations 75
(2.28), and (2.33) one finds
Y env − 1.0 δM c = 0.239 ± 0.024
from ∆M tip
HB,
Y env − 0.3 δM c = 0.237 ± 0.016 from R,
Y env + 2.1 δM c = 0.247 ± 0.043 from M RR . (2.35)
Bands of allowed values for Y env and δM c are shown in Fig. 2.22.
From Fig. 2.22 one reads off that approximately
Y env = 0.24 ± 0.02 and |δM c | < 0.025 M ⊙ . (2.36)
Therefore, the core mass at the helium flash is tightly constrained to
lie within 5% of its standard value.
This result allows one to constrain the operation of a novel energyloss
mechanism in the core of a red giant, i.e. at densities of around
10 6 g cm −3 and at a temperature of about 10 8 K. In some cases, however,
an energy-loss mechanism is suppressed by degeneracy effects in
a RG core, while it may be fairly efficient in the helium-burning core
of an HB star (ρ ≈ 10 4 g cm −3 , T ≈ 10 8 K) which is essentially nondegenerate.
In this case δM c ≈ 0 while the energy loss from the HB core
Fig. 2.22. Allowed values for the envelope helium abundance of evolved
globular-cluster stars and of an anomalous core-mass excess at helium ignition.
The limits were derived from the observed brightness difference ∆M tip
HB
between the HB and the RGB tip, from the “R-method” (number counts
on the HB vs. RGB), and from the brightness determination of nearby field
RR Lyrae stars (M RR ) by statistical parallaxes and the Baade-Wesselink
method as well as the brightness of RR Lyrae stars in the LMC.