Stars as Laboratories for Fundamental Physics - MPP Theory Group

Anomalous Stellar Energy Losses Bounded by Observations 69
d) Brightness of RR Lyrae Stars
Theoretically, details of the evolution of stars on the HB are difficult to
account for. Notably, they may move in and out of the RR Lyrae instability
strip so that the stars found there cannot be trivially associated
with a specific age after the beginning of helium burning. Therefore,
it is easiest to use the absolute bolometric brightness of zero-age HB
stars with a mass chosen such that they fall into the RR Lyrae strip
(Buzzoni et al. 1983; Raffelt 1990b)
M RR = 0.66 − 3.5 Y 23 + 0.16 Z 13 − ∆ RR − 7.3 δM c , (2.20)
where ∆ RR is an unknown amount of deviation between real RR Lyrae
stars and the zero-age HB models of Sweigart and Gross (1976) that
served to derive this relation.
It is expected that ∆ RR is a positive number of order 0.1 mag, i.e.
on average RR Lyrae stars are thought to be somewhat brighter than
zero-age HB star models. This conclusion is supported by Sandage’s
(1990a) investigation of the vertical height of the HB by means of the
pulsational properties of RR Lyrae stars. Sandage found an intrinsic
width between 0.2 mag for the most metal-poor and about 0.4 mag for
the most metal-rich clusters, i.e. an average deviation between 0.1 and
0.2 mag between zero-age and average HB stars.
Lee, Demarque, and Zinn (1990) have constructed synthetic HBs
for a range of metallicities and helium content on the basis of new
evolutionary sequences. For a MS helium content of 0.20, which in their
calculation amounts to Y env = 0.22, they found (see also Lee 1990)
M RR = 0.70 + 0.22 Z 13 . (2.21)
Comparing this with Eq. (2.20) at Y 23 = −0.01 and δM c = 0 one
finds ∆ RR ≈ 0. This is not in contradiction with the brightening of
RR Lyrae stars relative to zero age, it only means that there is a slight
offset relative to the analytic representation Eq. (2.20) derived from the
Sweigart and Gross (1976) calculations. ∆ RR shall always refer to the
brightness difference of real RR Lyrae stars relative to Eq. (2.20), it
does not refer to an offset relative to real zero-age HB stars.
e) Brightness Difference between HB and RGB Tip
The main observable to constrain a deviation from the standard core
mass at the helium flash is the brightness difference between the HB