M. Catelan, THE PERIOD-SHIFT EFFECT IN OOSTERHOFF TYPE-II GLOBULAR-CLUSTERS WITH BLUE HORIZONTAL-BRANCH MORPHOLOGIES, The Astronomical journal, 107(6), 1994, pp. 2077-2092
It is shown, on the basis of data available in the literature, that th
e mean periods of RR Lyrae variables in Oosterhoff type II (OoII) gala
ctic globular clusters are not significantly correlated with the HB mo
rphology, as represented by the parameter (B - R)/(B + V + R), for obj
ects with blue horizontal-branch (HB) morphology types [(B - R)/(B + V
+ R)>0.65]. This result is compared with detailed synthetic HB model
predictions based upon the evolutionary tracks of Lee & Demarque [ApJS
, 73, 709 (1990)]. The predicted period shifts are found to be strongl
y correlated with (B - R)/(B + V + R), as a consequence of the effect
of evolution away from the zero-age HB becoming progressively more imp
ortant as the latter quantity approaches + 1.0. The slopes of the pred
icted and observed [log P]-(B - R)/(B + V + R) relations are found to
differ at confidence levels greater than or similar to 88%. We argue t
hat these results are likely not a spurious consequence of statistical
fluctuations affecting the clusters with the bluest HB types, nor of
our choices of metallicities Z, HB morphology parameters (B - R)/(B V + R), and mass dispersions on the HB sigma(M), for the adopted sampl
e of 8 OoII clusters with statistically significant numbers of RR Lyra
e variables (N(RR) greater-than-or-equal-to II and/or N(ab) greater-th
an-or-equal-to 7). We show that reconciliation between the models and
the observations cannot be achieved without dramatic changes in the in
stability strip topology and/or period-mean density relation, and thus
suggest that either the adopted evolutionary tracks are inadequate-we
find evidence that the Castellani et al. [ApJS, 76, 911 (1991)] track
s already lead to a significantly better, though not fully satisfactor
y, agreement between the models and the observation-or other parameter
s vary among our sample of clusters besides age and/or mass loss on th
e red-giant branch, in which case a strong impact would be expected up
on the interpretation of the second-parameter and Oosterhoff dichotomy
problems. Thus, our analysis clearly represents the confirmation of t
he period-shift effect for OoII clusters, in a manner which is complet
ely insensitive to uncertainties in the reddening and temperature dete
rminations, and may also constitute evidence against its interpretatio
n on a purely evolutionary basis.