The period-shift effect in Oosterhoff type II globular clusters with blue horizontal-branch morphologies

Abstract

view Abstract Citations (5) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Period-Shift Effect in Oosterhoff Type II Globular Clusters With Blue Horizontal Branch Morphologies Catelan, Marcio Abstract It is shown, on the basis of data available in the literature, that the mean periods of RR Lyrae variables in Oosterhoff type II (OoII) galactic globular clusters are not significantly correlated with the HB morphology, as represented by the parameter (B - R)/(B + V + R), for objects with blue horizontal-branch (HB) morphology types (B - R)/(B + V + R) greater than 0.65. This result is compared with detailed synthetic HB model predictions based upon the evolutionary tracks of Lee & Demarque (1990). The predicted period shifts are found to be strongly correlated with (B - R)/(B + V + R), as a consequence of the effect of evolution away from the zero-age HB becoming progressively more important as the latter quantity approaches +1.0. The slopes of the predicted and observed (log P) - (B - R)/(B + V + R) relations are found to differ at confidence levels greater than or approximately equal to 88%. We argue that 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 sigmaM, for the adopted sample of 8 OoII clusters with statistically significant numbers of RR Lyrae variables (NRR greater than or equal to 11 and/or Nab greater than or equal to 7). We show that reconciliation between the models and the observations cannot be achieved without dramatic changes in the instability 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. (1991) tracks already lead to a significantly better, though not fully satisfactory, agreement between the models and the observations -- or other parameters vary among our sample of clusters besides age and/or mass loss on the red-giant branch, in which case a strong impact would be expected upon the interpretation of the second-parameter and Oosterhoff dichotomy problems. Thus, our analysis clearly represents the confirmation of the period-shift effect for OoII clusters, in a manner which is completely insensitive to uncertainties in the reddening and temperature determinations, and may also constitute evidence against its interpretation on a purely evolutionary basis. Publication: The Astronomical Journal Pub Date: June 1994 DOI: 10.1086/117018 Bibcode: 1994AJ....107.2077C Keywords: Blue Stars; Galactic Clusters; Galactic Evolution; Globular Clusters; Variable Stars; Metallicity; Morphology; Astrophysics; STARS: HORIZONTAL-BRANCH; GLOBULAR CLUSTERS: GENERAL full text sources ADS | data products SIMBAD (9)