[Paper Review] The OGLE Collection of Variable Stars. Classical, Type II, and Anomalous Cepheids toward the Galactic Center

I. Soszyński, A. Udalski+12 more|arXiv (Cornell University)|Dec 4, 2017

Stellar, planetary, and galactic studiesPhysics and Astronomy31 citations

TL;DR

This paper presents the largest collection of classical, type II, and anomalous Cepheids in the Galactic bulge, based on OGLE-IV VI-band photometry from 2010 to 2017. It identifies 87 classical Cepheids, 924 type II Cepheids (including BL Her, W Vir, and RV Tau stars), and 20 anomalous Cepheids—first detected in the Galactic bulge—along with 828 new RR Lyrae stars, significantly enhancing the OGLE Collection of Variable Stars for studying pulsation physics and Galactic structure.

ABSTRACT

We present a collection of classical, type II, and anomalous Cepheids detected in the OGLE fields toward the Galactic center. The sample contains 87 classical Cepheids pulsating in one, two or three radial modes, 924 type II Cepheids divided into BL Her, W Vir, peculiar W Vir, and RV Tau stars, and 20 anomalous Cepheids - first such objects found in the Galactic bulge. Additionally, we upgrade the OGLE Collection of RR Lyr stars in the Galactic bulge by adding 828 newly identified variables. For all Cepheids and RR Lyr stars, we publish time-series VI photometry obtained during the OGLE-IV project, from 2010 through 2017. We discuss basic properties of our classical pulsators: their spatial distribution, light curve morphology, period-luminosity relations, and position in the Petersen diagram. We present the most interesting individual objects in our collection: a type II Cepheid with additional eclipsing modulation, W Vir stars with the period doubling effect and the RVb phenomenon, a mode-switching RR Lyr star, and a triple-mode anomalous RRd star.

Motivation & Objective

• . The study aims to expand the OGLE Collection of Variable Stars in the Galactic bulge with newly detected classical, type II, and anomalous Cepheids.
• It seeks to improve the understanding of pulsation properties and evolutionary status of Cepheid and RR Lyrae stars in the central Milky Way.
• The research aims to provide long-term, high-cadence time-series photometry for all detected variables to enable detailed analysis of non-stationary phenomena such as mode switching and period changes.
• It updates and extends the OGLE catalog of RR Lyrae stars by identifying 828 new variables, enhancing the sample for metallicity and kinematic studies.
• The work supports testing of stellar pulsation and evolution models by providing a statistically robust, multi-moded sample of pulsating stars in a dense, highly obscured region.

Proposed method

• . The study uses VI-band photometry from 121 OGLE-IV fields toward the Galactic center, collected between 2010 and 2017 with the 1.3-m Warsaw telescope.
• A period search was conducted using the Fourier technique (FNPEAKS code) over a frequency range of 0–24 cycles per day, with a 0.00005 cycle/day step.
• Light curves with periods between 0.2 and 100 days and amplitudes >0.05 mag were fitted with a Fourier cosine series to extract low-order coefficients (R21, φ21, R31, φ31).
• Classification of pulsators was based on visual inspection of light curve morphology and Fourier parameters, with additional support from color and period ratio analysis.
• The OGLE-IV data were cross-matched with OGLE-II and OGLE-III data to extend light curve baselines, with caution taken for photometric offsets due to crowding.
• Anomalous RRd stars were identified using the Petersen diagram, with objects outside the standard RRd sequence classified as anomalous based on P1O/PF ratios and Blazhko modulation.

Experimental results

Research questions

• RQ1 . What is the spatial distribution and pulsation behavior of classical Cepheids in the Galactic bulge, and how do they relate to the period-luminosity relation?
• RQ2 How do the properties of type II Cepheids—such as BL Her, W Vir, and RV Tau stars—differ in the Galactic bulge, and what do they reveal about the old stellar population?
• RQ3 What is the nature and significance of anomalous Cepheids in the Galactic bulge, and how do they compare to those in the Magellanic Clouds?
• RQ4 What non-stationary behaviors, such as mode switching or Blazhko modulation, are observed in RR Lyrae stars in this region?
• RQ5 How do the light curve properties of triple-mode and double-mode RR Lyrae stars in the bulge compare to those in other stellar systems?

Key findings

• . The OGLE Collection of Variable Stars now includes 87 classical Cepheids, 924 type II Cepheids (including 113 W Vir stars and 10 RV Tau stars), and 20 anomalous Cepheids—first detected in the Galactic bulge.
• The study identifies 828 new RR Lyrae stars, significantly expanding the existing catalog and enabling improved studies of the old stellar population.
• A mode-switching RR Lyrae star, OGLE-BLG-RRLYR-17342, transitioned from single-mode (RRab) to double-mode (RRd) pulsation between 2010 and 2017, with the first-overtone mode appearing in 2015.
• Two triple-mode RR Lyrae stars were confirmed: OGLE-BLG-RRLYR-24137 and OGLE-BLG-RRLYR-38791, with the latter showing likely pulsation in fundamental, first-overtone, and second-overtone modes.
• The Petersen diagram reveals that 24 double-mode RR Lyr stars were reclassified as anomalous RRd stars due to their location outside the standard RRd sequence, with most showing Blazhko modulation.
• The long-baseline light curves (up to 20 years when combining OGLE-II, -III, and -IV data) enable detailed studies of non-stationary phenomena such as period changes, mode switching, and light-time effects in binary systems.