[Paper Review] The OGLE Collection of Variable Stars. Anomalous Cepheids in the Magellanic Clouds

I. Soszyński, A. Udalski+11 more|arXiv (Cornell University)|Aug 4, 2015

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper presents a catalog of 250 anomalous Cepheids (ACs) in the Large and Small Magellanic Clouds (LMC and SMC), including the first confirmed ACs in the SMC and four fundamental-mode ACs in the Milky Way foreground. Using Fourier light curve decomposition, the authors demonstrate that $φ_{21}$ and $φ_{31}$ coefficients effectively distinguish ACs from classical Cepheids, particularly in the LMC and Galactic field, while light curve similarities in the SMC complicate classification and may explain non-linear period-luminosity relations in short-period Cepheids.

ABSTRACT

We present a collection of 250 anomalous Cepheids (ACs) discovered in the OGLE-IV fields toward the Large (LMC) and Small Magellanic Cloud (SMC). The LMC sample is an extension of the OGLE-III Catalog of ACs published in 2008, while the SMC sample contains the first known bona fide ACs in this galaxy. The total sample is composed of 141 ACs in the LMC and 109 ACs in the SMC. All these stars pulsate in single modes: fundamental (174 objects) or first overtone (76 objects). Additionally, we report the discovery of four ACs located in the foreground of the Magellanic Clouds. These are the first fundamental-mode ACs known in the Galactic field. We demonstrate that the coefficients phi_21 and phi_31 determined by the Fourier light curve decomposition are useful discriminators between classical Cepheids and ACs, at least in the LMC and in the field of the Milky Way. In the SMC, the light curve shapes and mean magnitudes of short-period classical Cepheids make them similar to ACs, which is a source of difficulties in the discrimination of both classes of pulsators. The presence of unidentified ACs in the catalogs of classical Cepheids may be partly responsible for the observed non-linearity of the period-luminosity relation observed for short-period Cepheids in the SMC. We compare spatial distributions of ACs, classical Cepheids and RR Lyr stars. We show that the distribution of ACs resembles that of old stars (RR Lyr variables), although in the LMC there are visible structures typical for young population (classical Cepheids): the bar and spiral arms. This may suggest that ACs are a mixture of relatively young stars and mergers of very old stars.

Motivation & Objective

• To identify and catalog anomalous Cepheids (ACs) in the OGLE-IV survey fields toward the Large and Small Magellanic Clouds.
• To resolve the ambiguity in distinguishing ACs from classical Cepheids in the SMC, where light curves and mean magnitudes are similar due to low metallicity.
• To investigate the origin of ACs by analyzing their spatial distribution in relation to classical Cepheids and RR Lyrae stars.
• To identify and characterize ACs in the Galactic halo, particularly fundamental-mode ACs, using photometric data from the OGLE-IV survey.

Proposed method

• Time-series I and V-band photometry was collected using the 1.3-m Warsaw Telescope at Las Campanas Observatory from 2010 to 2015.
• Data reduction was performed using the Difference Image Analysis technique to enhance detection sensitivity for variable stars.
• Fourier decomposition of light curves was applied to extract $φ_{21}$ and $φ_{31}$ coefficients as discriminants between ACs and classical Cepheids.
• Period-luminosity (PL) relations were derived for ACs in the LMC and compared with those in the SMC to assess differences in pulsation behavior.
• Spatial distributions of ACs, classical Cepheids, and RR Lyrae stars were analyzed to infer evolutionary origins.
• Stellar distances were estimated using the $ \log{P}$ – $W_{I}$ relation calibrated on LMC ACs, assuming a LMC distance of 49.97 kpc.

Experimental results

Research questions

• RQ1 What is the distribution and nature of anomalous Cepheids in the Small Magellanic Cloud, and how do they differ from classical Cepheids in this environment?
• RQ2 Can Fourier light curve parameters $φ_{21}$ and $φ_{31}$ reliably distinguish anomalous Cepheids from classical Cepheids in the LMC and in the Galactic field?
• RQ3 Why is the period-luminosity relation for short-period Cepheids in the SMC observed to be non-linear, and could misclassified ACs contribute to this effect?
• RQ4 What does the spatial distribution of ACs in the LMC reveal about their formation mechanism—intermediate-age metal-poor stars or evolved binary mergers?
• RQ5 Are anomalous Cepheids in the Galactic halo more numerous than previously thought, and can they be reliably identified using photometric light curve morphology?

Key findings

• The OGLE-IV survey discovered 141 anomalous Cepheids in the Large Magellanic Cloud and 109 in the Small Magellanic Cloud, including the first confirmed bona fide ACs in the SMC.
• Four fundamental-mode anomalous Cepheids were identified in the Galactic foreground of the Magellanic Clouds, marking the first such discovery in the Milky Way field.
• The Fourier coefficients $φ_{21}$ and $φ_{31}$ are effective discriminators between classical Cepheids and anomalous Cepheids in the LMC and in the Galactic field.
• In the SMC, the light curve shapes and mean magnitudes of short-period classical Cepheids are similar to those of ACs, complicating classification and potentially contributing to the observed non-linearity in the period-luminosity relation.
• The spatial distribution of ACs in the LMC shows traces of the bar and spiral arms, suggesting a young population, while the distribution in the SMC and at large angular distances from the LMC center resembles that of old stars (RR Lyrae), indicating a mixed origin.
• The presence of ACs in the Galactic halo, confirmed by their light curve morphology and Fourier parameters, suggests they are more numerous than previously recognized, especially in the field of the Milky Way.