An estimate of the structural parameters of the Large Magellanic Cloud using red clump stars

¹ Indian Institute of Astrophysics, Koramangala II Block, 560034 Bangalore, India e-mail: [smitha;purni]@iiap.res.in
² Department of Physics, Calicut University, Calicut, Kerala, India

Received: 4 February 2010
Accepted: 12 June 2010

Abstract

Aims. We used the red clump stars from the VI photometric data of the Optical Gravitational Lensing Experiment (OGLE III) survey and from the Magellanic Cloud Photometric Survey (MCPS) 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 disk region is divided into sub-regions. The dereddened peak I magnitude of the red clump stars in each sub-region is used to obtain the relative distances and hence the z coordinate. The RA and Dec of each sub-region is converted into x and y cartesian coordinates. A weighted least-square plane-fitting method is applied to these x, y, z data to estimate the structural parameters of the LMC disk.

Results. We find an inclination of i =230 ± 08 and PA_lon, ϕ = 1637 ± 15 for the LMC disk using the OGLE III data and an inclination of i = 374 ± 23 and PA_lon ϕ = 1412 ± 37 for the LMC disk using the MCPS data. Extra-planar features, which are in front as well as behind the fitted plane, are seen in both the data sets.

Conclusions. Our estimates of the inclination and position angle of the line of nodes are comparable with some of the previous estimates. The effect of choice of center, reddening, and area covered on the estimated parameters are discussed. Regions in the northwest, southwest and southeast of the LMC disk are warped with respect to the fitted plane. We also identify a symmetric but off-centered warp in the inner LMC. We identify that the structure of the LMC disk inside the 3 degree radius is different from the outside disk in a way that the inner LMC has relatively less inclination and relatively large PA_lon. The 3D plot of the LMC disk suggests an off-centered increase in the inclination for the northeastern regions, which might be due to tidal effects. We suggest that the variation in the planar parameters estimated by various authors as well as in this study is caused by the difference in coverage and the complicated inner structure of the LMC disk. In the inner LMC, the stellar and the HI disk are found to have similar properties.