[Paper Review] The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. XIII. Long-Period Variables in the Small Magellanic Cloud

I. Soszyński, A. Udalski+8 more|arXiv (Cornell University)|Sep 6, 2011

Astronomical Observations and Instrumentation22 citations

TL;DR

This paper presents the OGLE-III Catalog of Long-Period Variables (LPVs) in the Small Magellanic Cloud, comprising 19,384 stars including 352 Miras, 2,222 semiregular variables (SRVs), and 16,810 OSARGs. Using 13 years of VI-band photometry from the 1.3-m Warsaw telescope, the study classifies stars by pulsation type and spectral class, revealing a higher fraction of carbon-rich stars in the SMC than in the LMC due to lower metallicity, and identifies extreme objects such as a Mira with a 1,860-day period—the longest known pulsation period in any environment.

ABSTRACT

The thirteenth part of the OGLE-III Catalog of Variable Stars (OIII-CVS) contains 19384 long-period variables (LPVs) detected in the Small Magellanic Cloud. The sample is composed of 352 Mira stars, 2222 semiregular variables (SRVs) and 16810 OGLE Small Amplitude Red Giants (OSARGs). Sources are divided into oxygen-rich and carbon-rich stars. The catalog includes time-series VI photometry obtained between 1997 and 2009. Methods used to select and classify variable stars are described. We show some statistical properties of the sample, and compare it with LPVs in the Large Magellanic Cloud. Additionally, we present objects of particular interest, e.g. a SRV with outbursts, and a Mira star with the longest known pulsation period P=1860 days.

Motivation & Objective

• To compile the largest published catalog of long-period variables (LPVs) in the Small Magellanic Cloud (SMC) using photometric data from the OGLE-III survey.
• To classify LPVs into Mira stars, semiregular variables (SRVs), and OGLE Small Amplitude Red Giants (OSARGs), distinguishing between oxygen-rich and carbon-rich stars.
• To analyze the period-luminosity (PL) relations and statistical properties of LPVs in the SMC and compare them with those in the Large Magellanic Cloud (LMC).
• To identify rare or exceptional variable stars, such as those with outbursts or extremely long pulsation periods, to probe pulsation mechanisms and mass loss processes.
• To investigate the influence of metallicity on the evolution of asymptotic giant branch (AGB) stars, particularly the earlier onset of carbon enrichment in low-metallicity environments like the SMC.

Proposed method

• Time-series VI-band photometry was collected from 1997 to 2009 using the 1.3-m Warsaw telescope at Las Campanas Observatory, with observations spanning 14 square degrees across 41 fields.
• The Difference Image Analysis (DIA) technique was applied to the photometric data to enhance detection sensitivity and improve light curve extraction for faint and variable stars.
• Variable stars were classified using period-luminosity (PL) relations and light curve morphology, with stars grouped into OSARGs, SRVs, and Miras based on pulsation characteristics and amplitude.
• Reddening-free magnitude $W_{JK}$ was computed using J and K-band photometry to minimize extinction effects and improve PL relation analysis.
• The sample was divided into oxygen-rich and carbon-rich stars based on spectral classification and color-color diagrams, with emphasis on metallicity effects on stellar evolution.
• Statistical analysis of period distributions and luminosity functions was performed, and unusual light curves were examined for signs of outbursts, variable amplitudes, or complex pulsation modes.

Experimental results

Research questions

• RQ1 What is the distribution and classification of long-period variables in the Small Magellanic Cloud, and how does it compare to the Large Magellanic Cloud?
• RQ2 How does the lower metallicity of the SMC affect the relative abundance of oxygen-rich versus carbon-rich LPVs?
• RQ3 What are the period-luminosity relations for OSARGs, SRVs, and Miras in the SMC, and do they differ from those in the LMC?
• RQ4 What causes the long-period variations observed in sequence D stars, and why do their amplitudes and mean luminosities correlate in some cases?
• RQ5 What can extreme objects like the 1,860-day Mira or a SRV with outbursts reveal about pulsation mechanisms and mass loss in evolved stars?

Key findings

• The catalog contains 19,384 long-period variables in the SMC, including 352 Mira stars, 2,222 semiregular variables (SRVs), and 16,810 OGLE Small Amplitude Red Giants (OSARGs).
• A Mira star, OGLE-SMC-LPV-08137, exhibits a pulsation period of 1,860 days—the longest known in any environment.
• Carbon-rich stars dominate among brighter LPVs in the SMC, with a higher fraction of C-rich stars than in the LMC, consistent with the lower metallicity of the SMC.
• The distribution of SRVs in the PL diagram reveals a possible additional PL ridge between sequences C and C′, suggesting possible distinct pulsation modes or stellar populations.
• A SRV (OGLE-SMC-LPV-00861) shows two distinct outbursts with amplitudes of 0.38 and 0.65 mag, a rare phenomenon in red giants possibly linked to planetary engulfment.
• Sequence D stars, such as OGLE-SMC-LPV-14045, show correlated variations in amplitude and mean luminosity, suggesting obscuration by a variable circumstellar dust cloud during enhanced mass loss episodes.