[Paper Review] Additional Galactic Cepheids from the OGLE Survey

I. Soszyński, A. Udalski+12 more|arXiv (Cornell University)|Jul 14, 2020

Stellar, planetary, and galactic studies17 citations

TL;DR

This paper presents a comprehensive search for Cepheid variable stars in the OGLE Galaxy Variability Survey fields, identifying 742 new Galactic Cepheids—70% of which are newly discovered—along with reclassification of several previously misclassified variables. The study significantly expands the OGLE Collection of Galactic Cepheids, including the first known Galactic double-mode Cepheid pulsating in the second and third overtone, long-period double-mode classical Cepheids, and new candidates in globular clusters.

ABSTRACT

We report on the results of a comprehensive search for Cepheid variable stars in the fields photometricallymonitored by the OGLE Galaxy Variability Survey. We identify 742 Cepheids not included in the first release of the OGLE Collection of Galactic Cepheids and reclassify several dozen previously published variables. The upgraded collection comprises 1974 classical, 1625 type II, and 119 anomalous Cepheids located in the area of about 3000 square degrees covering the disk and bulge of the Milky Way. We present the most interesting objects in our sample: an isolated group of long-period doublemode classical Cepheids pulsating in the fundamental mode and first overtone, the first known Galactic double-mode Cepheid with the second and third overtone excited, double-mode type II Cepheids, candidates for single-mode first-overtone BL Her stars, and Cepheids showing simultaneous eclipsing variability. We also discuss type II and anomalous Cepheids that are potential members of globular clusters.

Motivation & Objective

• Expand the OGLE Collection of Galactic Cepheids by identifying additional classical, type II, and anomalous Cepheids in the Milky Way disk and bulge.
• Improve the accuracy and completeness of the OGLE variable star catalog by reclassifying previously misclassified variables.
• Identify rare pulsation modes, such as double-mode pulsation in second and third overtones, and eclipsing binary systems with pulsating components.
• Investigate the potential membership of Cepheids in globular clusters through spatial coincidence with tidal radii.
• Provide a final, comprehensive release of the OGLE Collection of Galactic Cepheids, integrating data from 2013–2020.

Proposed method

• Conducted a periodicity search on I-band light curves from the OGLE Galaxy Variability Survey (GVS), using the Fnpeaks algorithm to identify candidate periods.
• Selected stars with periods <50 days and high signal-to-noise ratios for further analysis based on amplitude and periodicity.
• Fitted selected candidates with template light curves derived from well-sampled OGLE Cepheids and eclipsing variables to confirm variability type.
• Used photometric data from 2013 to 2020, covering ~3000 square degrees in the Galactic disk and bulge, with I-band data points ranging from 100 to 200 per light curve.
• Cross-matched Cepheid candidates with globular cluster catalogs (Harris 2010) and variable star databases (Clement et al. 2001) to assess potential cluster membership within tidal radii.
• Re-evaluated classifications of previously published variables using light curve morphology, Fourier parameters, and brightness comparisons with known RR Lyrae stars.

Experimental results

Research questions

• RQ1 What is the distribution and nature of additional Cepheid variables in the OGLE GVS fields beyond the initial OGLE Collection?
• RQ2 Which Cepheid variables exhibit rare pulsation modes, such as double-mode pulsation in the second and third overtone?
• RQ3 How many Cepheids in the sample are likely members of globular clusters, based on spatial coincidence with tidal radii?
• RQ4 Which previously misclassified variables can be reclassified based on improved light curve analysis and template fitting?
• RQ5 What are the implications of newly discovered eclipsing binary systems with pulsating components for binary evolution and Cepheid population studies?

Key findings

• The study identifies 742 new Galactic Cepheids, with 70% being newly discovered, significantly expanding the OGLE Collection of Galactic Cepheids.
• The first known Galactic double-mode classical Cepheid pulsating in the second and third overtone is identified, marking a rare pulsation mode in the Milky Way.
• A long-period double-mode classical Cepheid pulsating in the fundamental mode and first overtone is discovered, providing insights into non-linear pulsation modes.
• The sample includes 60 type II Cepheids potentially associated with 25 globular clusters, including the first known Cepheids in NGC 6355, NGC 6540, NGC 6749, Terzan 9, and Djorg 2.
• Three anomalous Cepheids are found within tidal radii of globular clusters, including two new discoveries; one previously misclassified as an RR Lyrae star is reclassified as an anomalous Cepheid.
• The variable star V12 in NGC 6333 (M9) is reclassified from anomalous Cepheid to type II Cepheid based on light curve shape, Fourier parameters, and brightness relative to RR Lyrae stars.