Galactic fundamental mode RR Lyrae stars. Period - amplitude diagram, metallicities and distribution

TL;DR

This study analyzes 1455 Galactic field RR Lyrae stars using ASAS data, revealing clear Oosterhoff group distinctions, metallicity relations, and spatial distributions, with implications for understanding Galactic structure and stellar evolution.

Contribution

It provides the first large-scale analysis of Galactic field RR Lyrae stars showing Oosterhoff groups and compares photometric metallicity methods, highlighting discrepancies and spatial distribution patterns.

Findings

Oosterhoff groups are clearly manifested in the period-amplitude diagram.

Photometric metallicities from Jurcsik & Kovacs (1996) are more reliable than Sandage (2004).

No metallicity gradient with distance from the Galactic Center was observed.

Abstract

We have analysed 1455 fundamental mode RR Lyrae stars of the Galactic field, using the All Sky Automated Survey (ASAS) data. The sample covers 75% of the sky and contains objects in the close neighbourhood of the Sun, within 4 kpc distance. Unlike in the previous analysis of the close field RRab, we see a clear manifestation of the Oosterhoff groups on the period - amplitude diagram. The relation for Oosterhoff I type variables becomes strongly flattened at large V amplitudes, which was not observed for globular cluster RR Lyrae. We calculate photometric metallicities using two available methods: one of Jurcsik & Kovacs (1996) and the other of Sandage (2004). We find significant discrepancies between results from both methods. Comparison with spectroscopic metallicities undoubtedly favors the method of Jurcsik & Kovacs (1996). In addition, we notice that RRab of Oosterhoff II type might follow a different metallicity-period-phase relation than Oosterhoff I type variables. The spatial distribution of Galactic field RRab does not show any metallicity gradients with distance from the Galactic Center in either of the Oosterhoff groups. However, both the older, metal poor Oosterhoff II variables and the metal rich Oosterhoff I RRab become more concentrated to the Galactic plane with increasing metal content.