[Paper Review] Blue Large-Amplitude Pulsators and Other Short-Period Variable Stars in OGLE-IV Fields of the Outer Galactic Bulge

J. Borowicz, P. Pietrukowicz+14 more|arXiv (Cornell University)|Apr 26, 2024

Astronomical Observations and Instrumentation4 citations

TL;DR

This study presents a systematic search for short-period variable stars in OGLE-IV outer Galactic bulge fields, identifying 33 Blue Large-Amplitude Pulsators (BLAPs), 31 of which are newly discovered, and 18 additional pulsating-like stars with periods between 17.3 and 53.7 min. The work significantly expands the known BLAP population and reveals five eclipsing binaries and three high-amplitude variables, advancing understanding of hot subluminous pulsators and compact binary systems in the outer bulge.

ABSTRACT

In this work, we search the OGLE-IV outer Galactic bulge fields for short-period variable objects. The investigation focuses on unexplored timescales roughly below one hour in an area containing about 700 million stellar sources down to approximately I=20 mag. We concentrate mainly on Blue Large-Amplitude Pulsators (BLAPs), which represent a recently discovered enigmatic class of short-period hot subluminous stars. We find 33 BLAPs in the period range from 7.5 to 66.5 min. Thirty-one of them are new discoveries, which increases the number of known stars of this class to over one hundred. Additional eighteen objects with pulsation-like light curve shapes and periods ranging from 17.3 to 53.7 min are presented. Very likely, these are $δ$ Sct/SX Phe-type stars, but some of them could be pulsating hot subdwarfs or BLAPs. We also report on the detection of five eclipsing binary systems with orbital periods between 61.2 and 121.9 min and three mysterious variable objects with I-band amplitudes larger than 0.9 mag. A spectroscopic follow-up would help in the final classification of the variables.

Motivation & Objective

• To identify and characterize short-period variable stars, particularly Blue Large-Amplitude Pulsators (BLAPs), in the OGLE-IV outer Galactic bulge fields.
• To expand the known population of BLAPs, which are rare, hot subluminous pulsators with periods below one hour.
• To investigate the nature of previously unclassified variable objects, including potential δ Sct/SX Phe-type stars, pulsating hot subdwarfs, or BLAPs.
• To detect and analyze compact binary systems, including eclipsing binaries with orbital periods below 2 hours.
• To identify high-amplitude variables with I-band amplitudes exceeding 0.9 mag for follow-up spectroscopic classification.

Proposed method

• Utilized time-series photometric data from the OGLE-IV survey covering approximately 700 million stellar sources down to I ≈ 20 mag in the outer Galactic bulge.
• Applied automated light curve analysis to detect periodic variability with periods below one hour, focusing on sawtooth-like shapes characteristic of BLAPs.
• Classified variable candidates based on light curve morphology, period, amplitude, and photometric properties (I-band magnitude, G-band magnitude, BP-RP color, parallax).
• Cross-referenced objects with existing surveys (e.g., ZTF, Gaia, SkyMapper) to identify counterparts and assess prior detections.
• Used photometric parallaxes (from Gaia) and color-magnitude diagnostics to estimate distances and spectral types.
• Identified eclipsing binaries via periodic dips in light curves and orbital period determination using phase-folded light curves.

Experimental results

Research questions

• RQ1 How many new Blue Large-Amplitude Pulsators (BLAPs) can be identified in the OGLE-IV outer Galactic bulge fields with periods below one hour?
• RQ2 What is the distribution of pulsation periods and amplitudes among newly discovered BLAPs and other short-period variable stars?
• RQ3 Which of the 18 pulsating-like stars with periods between 17.3 and 53.7 min are likely δ Sct/SX Phe-type stars, pulsating hot subdwarfs, or BLAPs?
• RQ4 What is the nature of the five eclipsing binary systems with orbital periods between 61.2 and 121.9 min?
• RQ5 What causes the extreme variability (I-band amplitudes >0.9 mag) in the three mysterious high-amplitude variables?

Key findings

• The study discovered 33 BLAPs in the period range of 7.5 to 66.5 minutes, with 31 being new identifications, increasing the total known BLAP population to over 100.
• Eighteen additional stars with pulsation-like light curves and periods between 17.3 and 53.7 minutes were identified, likely representing δ Sct/SX Phe-type stars, though some may be pulsating hot subdwarfs or BLAPs.
• Five eclipsing binary systems were detected with orbital periods ranging from 61.2 to 121.9 minutes, indicating the presence of compact binary systems in the outer bulge.
• Three high-amplitude variables were found with I-band amplitudes exceeding 0.9 mag, suggesting extreme variability requiring spectroscopic follow-up for classification.
• The BLAPs detected span a wide range of periods and amplitudes, supporting the hypothesis that BLAPs are more diverse than previously thought, including multi-periodic behavior.
• Parallax measurements and color-magnitude diagnostics suggest that the newly identified BLAPs and pulsating stars are consistent with hot, subluminous, evolved stars, though spectral confirmation is needed.