On the Oosterhoff dichotomy in the Galactic bulge: I. spatial distribution

Abstract

We present a study of the Oosterhoff (Oo) dichotomy in the Galactic bulge using 8 141 fundamental mode RR Lyrae stars. We used public photometric data from the Optical Gravitational Lensing Experiment (OGLE) and the Vista Variables in the Vía Láctea survey (VVV). We carefully selected fundamental mode stars without modulation and without association with any globular cluster located toward the Galactic bulge. Subsequently, we identified and separated the Oosterhoff groups I and II on the basis of their period-amplitude distribution and using a relation fitted to the Oosterhoff I locus. Both Oosterhoff groups were then compared to observations of two bulge globular clusters and with models of stellar pulsation and evolution. We found that some of the variables classified as Oo II belong to a third Oo group. The Oosterhoff II variables are more metal-poor on average, more massive, and cooler than their Oosterhoff I counterparts. The analysis of their spatial distribution shows a systematic difference between foreground, central and background regions in the occurrence of the Oosterhoff II group. The difference between the Oo I and II groups is also seen in their distance distributions with respect to the Galactic bar, but neither group is associated with the bar.

Blazhko effect in the Galactic bulge fundamental mode RR Lyrae stars – II. Modulation shapes, amplitudes, and periods

“…15 outlines the boundary between Oosterhoff type I (hereafter OoI) and Oosterhoff type II (hereafter OoII) variables. This line is based on third-degree polynomial relation from table 2 in Prudil et al (2019) and a shift in the pulsation period by 0.045 day from Miceli et al (2008). We note, that the boundary between OoI and II globular clusters shifts with metallicity and the transformation of V amplitudes into I band can affect the association of individual modulated stars in the Oosterhoff groups.…”

“…The fact that majority of modulated stars is located in the OoI group also tells a little bit about their physical properties in comparison with OoII stars. For example, the difference in masses between both Oosterhoff groups has been suggested in the past (Catelan 1992;Cacciari & Bruzzi 1993;Sandage 2006;Prudil et al 2019), where the masses of OoII stars should be equal or larger in comparison with the OoI group. More importantly, the OoI variables lie close to the blue-edge of the instability strip and on are on average intrinsically hotter by several hundred Kelvins (Prudil et al 2019).…”

“…For example, the difference in masses between both Oosterhoff groups has been suggested in the past (Catelan 1992;Cacciari & Bruzzi 1993;Sandage 2006;Prudil et al 2019), where the masses of OoII stars should be equal or larger in comparison with the OoI group. More importantly, the OoI variables lie close to the blue-edge of the instability strip and on are on average intrinsically hotter by several hundred Kelvins (Prudil et al 2019). This would mean that modulated stars are on average hotter and possibly less massive than non-modulated stars, under the assumption that they are located on a zero-age horizontal giant branch.…”

“…For transformation of V-band amplitudes into I-band for RRLs in globular clusters with missing I observations we used eq. 16 from Prudil et al (2019). The black line in Fig.…”

Evidence for Galactic disc RR Lyrae stars in the solar neighbourhood

“…The pericenter peak RR Lyrae stars fall into the regions typical for the fundamental mode RR Lyrae stars with short pulsation periods and asymmetric light curves. In the period-amplitude diagram they can be all clearly associated with the Oosterhoff type I group (Oosterhoff 1939) identified in the Galactic bulge (Prudil et al 2019a). Moreover, a large fraction of the pericenter-peak RR Lyrae stars occupy the regions of high-amplitude short-period (from hereon referred to as HASP, Fiorentino et al 2015) RR Lyrae stars, which are found mainly in the most metal-rich systems (cf.…”