Structure of the Large Magellanic Cloud from near infrared magnitudes of red clump stars

Indian Institute of Astrophysics, Koramangala II Block, 560034 Bangalore, India
e-mail: smitha@iiap.res.in; purni@iiap.res.in

Received: 2 April 2012
Accepted: 30 January 2013

Abstract

Context. The structural parameters of the disk of the Large Magellanic Cloud (LMC) are estimated.

Aims. We used the JH photometric data of red clump (RC) stars from the Magellanic Cloud Point Source Catalog (MCPSC) obtained from the InfraRed Survey Facility (IRSF) to estimate the structural parameters of the LMC disk, such as the inclination, i, and the position angle of the line of nodes (PA_lon), φ.

Methods. The observed LMC region is divided into several sub-regions, and stars in each region are cross-identified with the optically identified RC stars to obtain the near infrared magnitudes. The peak values of H magnitude and (J − H) colour of the observed RC distribution are obtained by fitting a profile to the distributions and by taking the average value of magnitude and colour of the RC stars in the bin with largest number. Then the dereddened peak H₀ magnitude of the RC stars in each sub-region is obtained from the peak values of H magnitude and (J − H) colour of the observed RC distribution. The right ascension (RA), declination (Dec), and relative distance from the centre of each sub-region are converted into x,y, and z Cartesian coordinates. A weighted least square plane fitting method is applied to this x,y,z data to estimate the structural parameters of the LMC disk.

Results. An intrinsic (J − H)₀ colour of 0.40    ±    0.03 mag in the Simultaneous three-colour InfraRed Imager for Unbiased Survey (SIRIUS) IRSF filter system is estimated for the RC stars in the LMC and a reddening map based on (J − H) colour of the RC stars is presented. When the peaks of the RC distribution were identified by averaging, an inclination of 25°.7  ±  1°.6 and a PA_lon = 141°.5  ±  4°.5 were obtained. We estimate a distance modulus, μ = 18.47  ±  0.1 mag to the LMC. Extra-planar features which are both in front and behind the fitted plane are identified. They match with the optically identified extra-planar features. The bar of the LMC is found to be part of the disk within 500 pc.

Conclusions. The estimates of the structural parameters are found to be independent of the photometric bands used for the analysis. The radial variation of the structural parameters are also studied. We find that the inner disk, within  ~3°.0, is less inclined and has a larger value of PA_lon when compared to the outer disk. Our estimates are compared with the literature values, and the possible reasons for the small discrepancies found are discussed.