Stars as Laboratories for Fundamental Physics - MPP Theory Group

Anomalous Stellar Energy Losses Bounded by Observations 79
into question in recent years (e.g. Wheeler, Sneden, and Truran 1989).
It is thought that in these systems the “α elements” (mostly oxygen)
are enhanced relative to iron. RG sequences calculated by Raffelt and
Weiss (1992) with somewhat extreme α enhancements indicated that
the red-giant core mass and luminosity at the helium flash are only
moderately changed. Most of the change that did occur was due to
the reduction of Fe at constant Z because of the α enhancement. Put
another way, if Z 13 = [Fe/H] + 1.3 is used as the defining equation for
the above “reduced metallicity” the effect of α enhancements appear
to be rather minimal.
Catelan, de Freitas Pacheco, and Horvath (1995) have taken the
point of view that for enhanced α elements one should rescale the
metallicity according to a recipe given by Chieffi, Straniero, and Salaris
(1991). Put another way, the metallicity parameter of Eq. (2.16) that
was used in the previous sections should be redefined as
Z 13 ≡ [Fe/H] + 1.3 + log(0.579 f + 0.421). (2.39)
Here, f is the enhancement factor of the abundance of α elements
relative to the solar value. For f = 3 one finds that Z 13 must be
offset by +0.33. Catelan, de Freitas Pacheco, and Horvath (1995) went
through their analysis with this modification, causing the allowed bands
of Fig. 2.23 to be slightly shifted relative to each other. However, the
overall change is small, well within the stated upper limit on δM c .
In summary, no effect has been discussed in the literature that would
cause the predicted core mass at helium ignition to deviate from its
standard value beyond the adopted limit of ±0.025 M ⊙ . Therefore, any
new energy-loss mechanism that would cause a significantly larger coremass
excess would have to be compensated by a hitherto unidentified
other novel effect.
2.5 Particle Bounds from Globular-Cluster Stars
2.5.1 Helium-Burning Lifetime
A particularly simple argument to constrain the properties of novel
particles arises from the observed duration of helium burning of lowmass
stars, i.e. form the lifetime of stars on the horizontal branch (HB).
In Sect. 2.4.4 it was argued that the number ratio R of stars on the HB
vs. RGB in globular clusters agreed with standard predictions to within
10%. Therefore, the helium-burning lifetime t He agrees with standard
predictions to within this limit. A less significant confirmation arises