[Paper Review] Over 78 000 RR Lyrae Stars in the Galactic Bulge and Disk from the OGLE Survey

I. Soszyński, A. Udalski+12 more|CaltechAUTHORS (California Institute of Technology)|Dec 31, 2019

Stellar, planetary, and galactic studiesPhysics and Astronomy1 references37 citations

TL;DR

The paper upgrades the OGLE Collection of RR Lyrae stars to 78,350 objects in the Galactic bulge and disk, detailing classifications (RRab, RRc, RRd, anomalous RRd), completeness, cross-matching, and multimode pulsators.

ABSTRACT

We present an upgrade of the OGLE Collection of RR Lyrae stars in the Galactic bulge and disk. The size of our sample has been doubled and reached 78 350 RR Lyr variables, of which 56 508 are fundamental-mode pulsators (RRab stars), 21 321 pulsate solely in the first-overtone (RRc stars), 458 are classical double-mode pulsators (RRd stars), and 63 are anomalous RRd variables (including five triple-mode pulsators). For all the newly identified RR Lyr stars, we publish time-series photometry obtained during the OGLE Galaxy Variability Survey. We present the spatial distribution of RR Lyr stars on the sky, provide a list of globular clusters hosting RR Lyr variables, and discuss the Petersen diagram for multimode pulsators. We find new RRd stars belonging to a compact group in the Petersen diagram (with period ratios P_1O/P_F = 0.74 and fundamental-mode periods P_F = 0.44 d) and we show that their spatial distribution is roughly spherically symmetrical around the Milky Way center.

Motivation & Objective

• Quantify the Galactic RR Lyrae population in the bulge and disk using the OGLE Galaxy Variability Survey (GVS).
• Produce a highly complete and pure catalog of RR Lyrae stars across a wide footprint.
• Characterize pulsation modes (RRab, RRc, RRd, anomalous RRd) and multimode behavior.
• Analyze spatial distribution, cross-match with external catalogs, and assess completeness and contamination.

Proposed method

• Two-stage RR Lyrae selection in GVS I-band light curves using the Fnpeaks period search and Discrete Fourier Transform.
• Light-curve template fitting and visual inspection to classify pulsation types.
• Fourier decomposition of light curves to distinguish RRab, RRc, and RRd candidates.
• Cross-match with major external RR Lyrae catalogs (VSX, Gaia DR2, Pan-STARRS, etc.) to assess completeness and recover overlooked objects.
• Tatry code (Schwarzenberg-Czerny 1996) used to refine pulsation periods.
• Finding charts of 60″×60″ for each star and publication of time-series photometry.

Experimental results

Research questions

• RQ1 What is the completeness and contamination level of the OGLE GVS RR Lyrae collection across bulge and disk fields?
• RQ2 How does the RR Lyrae population in the Galactic bulge and disk distribute spatially and in terms of pulsation modes?
• RQ3 What is the incidence of multimode RR Lyrae stars (RRd and anomalous RRd) in this large Galactic sample, and how do their properties relate to metallicity and Galactic structure?
• RQ4 How do the newly identified RR Lyrae stars compare with existing catalogs, and what fraction represents new detections?
• RQ5 What can the Petersen diagram of double-mode RR Lyrae stars reveal about stellar evolution and possible accretion remnants?

Key findings

• The OGLE collection now contains 78,350 Galactic RR Lyrae stars: 56,508 RRab, 21,321 RRc, and 521 RRd (including 63 anomalous RRd).
• About 40% of the newly identified RR Lyrae stars are not in existing catalogs, indicating substantial new detections.
• Completeness is approximately 94% for the whole sample, with 96% for RRab and 91% for RRc; completeness declines for fainter RRc stars (I>17 mag) and especially I>18 mag.
• A significant increase in multimode RR Lyrae stars is reported, with RRd counts rising from 148 to 458; anomalous RRd stars increase from 31 to 63, including three triple-mode RR Lyr stars.
• A compact group of RRd stars with P_F ≈ 0.44 d and P_1O/P_F ≈ 0.74 is confirmed (about 50 objects), largely bulge-centered and roughly spherically distributed around the Galactic center, challenging a previous stellar stream interpretation.
• The study provides detailed spatial distributions, identifies globular clusters hosting RR Lyrae (27 clusters noted), and discusses the Petersen diagram as a probe of mass/metallicity in double-mode pulsators.