Letter to the Editor

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

¹ Indian Institute of Astrophysics, Koramangala II Block, Bangalore 560 034, India
² 
University of Calicut, Department of Physics, Calicut, Kerala, India e-mail: [purni;smitha]@iiap.res.in

Received: 15 June 2009
Accepted: 3 July 2009

Abstract

Context. RR Lyrae stars (RRLS) belong to population II and are generally used as a tracer of the host galaxy halo.

Aims. 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.

Methods. 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.

Results. 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 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.

Conclusions. 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.