RR Lyrae stars in the inner LMC: Where did they form?

TL;DR

This study analyzes the distribution of RR Lyrae stars in the inner LMC to determine their origin, revealing they mainly trace the disk and inner halo rather than the extended halo, and suggesting a major star formation event 10-12 Gyrs ago.

Contribution

It provides new insights into the spatial distribution and origin of RR Lyrae stars in the inner LMC, linking their formation to the galaxy's disk and early star formation history.

Findings

RR Lyrae stars mimic the bar and disk inclination of the LMC.

Two populations of RR Lyrae stars with different scale heights are identified.

Majority of RR Lyrae stars formed in the LMC's disk 10-12 Gyrs ago.

Abstract

RR Lyrae stars (RRLS) belong to population II and are generally used as a tracer of the host galaxy halo. The surface as well as vertical distribution of RRLS in the inner Large Magellanic Cloud (LMC) are studied to understand whether these stars are actually formed in the halo. RRLS identified by the OGLE III survey are used to estimate their number density distribution. The scale-height of their distribution is estimated using extinction corrected average magnitudes of ab type stars. The density distribution mimics the bar, confirming results in the literature. The distribution of their scale height indicates that there may be two populations, one with smaller scale-height, very similar to the red clump stars and the other, much larger. The distribution of the reddening-corrected magnitude along the minor axis shows variation, suggesting an inclination. The inclination is estimated to be i = 31.3 (3.5) degrees, very similar to the inclination of the disk. Thus, the RRLS in the inner LMC mimic the bar and inclination of the disk, suggesting that a major fraction of RRLS is formed in the disk of the LMC. The results indicate that the RRLS in the inner LMC trace the disk and probably the inner halo. They do not trace the extended metal-poor halo of the LMC. We suggest that a major star formation event happened in the LMC at 10-12 Gyrs ago, resulting in the formation of most of the inner RRLS, as well as probably the globular clusters, inner halo and the disk of the LMC.