Stars as Laboratories for Fundamental Physics - MPP Theory Group

Anomalous Stellar Energy Losses Bounded by Observations 63
Fig. 2.17. Brightness difference between main-sequence turnoff (TO) and
horizontal branch (HB) in 19 galactic globular clusters according to Buonanno,
Corsi, and Fusi Pecci (1989). The shaded band indicates the 1σ statistical
error of the best fit Eq. (2.13).
distribution about the mean. This result illustrates the level of precision
that presently can be achieved at determining ∆VHB TO .
In principle, the color of the TO also specifies the location on the
MS and thus the TO mass and cluster age. In practice, the color of
a star is theoretically less well determined than its luminosity because
it depends on the treatment of the photosphere and on the surface
area and thus on the radius which is partly fixed by the treatment
of convection. Still, the TO color is a useful measure for the relative
ages between clusters of identical metallicities. Notably, the distanceindependent
color difference ∆(B − V ) between the TO and the base
of the RGB (Fig. 2.16) has been used to establish an age difference of
about (3±1) Gyr between the clusters NGC 288 and 362 (VandenBerg,
Bolte, and Stetson 1990; Sarajedini and Demarque 1990).
In order to constrain the operation of a novel energy-loss mechanism
the brightness of the RGB tip is particularly useful because particle
emission (neutrinos, axions) from a red-giant core delays helium ignition.
This delay allows the stars to develop a more massive core and
thus to turn brighter before they become HB stars. Again, the inferred
luminosity of the brightest star on the RGB depends on the distance
whence the most useful observable is the distance-independent brightness
difference between the RGB tip and the HB. However, because
the color of RR Lyrae stars and red giants is very different one must