An Oosterhoff Analysis of the Galactic Bulge Field RR Lyrae stars: Implications On Their Absolute Magnitudes

We present an analysis of the period-$V$-amplitude plane for RR0 Lyrae stars (fundamental mode pulsators) with "normal" light curves in the bulge using the MACHO bulge fields. Although bulge globular clusters have RR Lyraes that divide into two reasonable distinct groups according to the average period of the RR0 Lyraes (Oosterhoff 1939), there is no evidence of a gap between Oosterhoff I and II stars in the bulge field star sample. The majority of the bulge RR0 Lyrae field star population have a difference in period compared to the Oosterhoff I cluster M3 (Delta log P) that is shifted by about 0.02 days with regard to the Milky Way Oosterhoff I population, and the sample includes stars with Delta log P > 0.06 days, a characteristic hardly seen in Milky Way globular clusters. The metal-rich RR0 Lyrae stars in the Galactic bulge sample have Delta log P values on the other side of the spectrum as those in the metal-rich globular clusters NGC 6388 and NGC 6441. We find that the $V$-amplitude for a given period is a function of Delta log P, and not of metal abundance, similar to the result found by Clement & Shelton (1999) for RR Lyrae stars in Milky Way globular clusters. A comparative study of the bulge field stars with similar metallicities but different Oosterhoff types is carried out. Bulge field RR0 Lyrae variables with Delta log P values similar to Oosterhoff II clusters are about 0.2 mag brighter than RR0 Lyrae variables with Delta log P similar to Oosterhoff I clusters. Reliance upon a single absolute magnitude-[Fe/H] relation may be inappropriate when considering populations with different Delta log P.Comment: accepted to AJ, 9 figure

Amplitude Fine-Structure in the Cepheid P-L Relation I: Amplitude Distribution Across the RR Lyrae Instability Strip Mapped Using the Accessibility Restriction Imposed by the Horizontal Branch

The largest amplitude light curves for both RR Lyrae (RRL) variables and classical Cepheids with periods less than 10 days and greater than 20 days occur at the blue edge of the respective instability strips. It is shown that the equation for the decrease in amplitude with penetration into the strip from the blue edge, and hence the amplitude fine structure within the strip, is the same for RRL and the Cepheids despite their metallicity differences. However, the manifestation of this identity is different between the two classes of variables because the sampling of the RRL strip is restricted by the discrete strip positions of the horizontal branch, a restriction that is absent for the Cepheids in stellar aggregates with a variety of ages. To show the similarity of the strip amplitude fine structure for RRL and Cepheids we make a grid of lines of constant amplitude in the HR diagram of the strip using amplitude data for classical Cepheids in the Galaxy, LMC, and SMC. The model implicit in the grid, that also contains lines of constant period, is used to predict the correlations between period, amplitude, and color for the two Oosterhoff RRL groups in globular clusters. The good agreement of the predictions with the observations using the classical Cepheid amplitude fine structure also for the RRL shows one aspect of the unity of the pulsation processes between the two classes of variables.Comment: 24 pages, 3 tables, 5 figures, submitted to The Astrophysical Journa

Horizontal Branch Morphology and Mass Loss in Globular Clusters

The connection between mass loss on the red giant branch (RGB) and horizontal branch (HB) morphology in globular clusters (GCs) has long been acknowledged but the mechanisms governing mass loss remains poorly understood from a theoretical perspective. The present study uses synthetic HB models to demonstrate for the first time that alpha-enhancement and a simple relation between mass loss and metallicity can explain the entire range of HB morphology (characterized by the HB type index) observed in old, coeval GCs. The mass loss-metallicity relation accounts naturally for the fact that the most metal poor GCs ([Fe/H] < -2) have redder HBs than is typical of GCs with -2 < [Fe/H] < -1.5 without invoking younger ages. These results may prove useful in studying the contribution of HB stars to integrated light via stellar population synthesis.Comment: 13 pages, 5 figures, to appear in ApJ Letters (figure 2 may not display correctly in some PDF viewers