Chemical and structural analysis of the Large Magellanic Cloud using the fundamental mode RR Lyrae stars

Abstract

We present a careful and detailed light curve analysis of publicly available $I$-band data on fundamental mode RR Lyrae (RRab) stars of the Large Magellanic Cloud (LMC) obtained by the Optical Gravitational Lensing Experiment (OGLE) phase-III project. Using the Fourier parameters of $13,095$ RRab stars, metallicities and absolute magnitudes of individual stars are obtained. The representation of stars on the $P-{\varphi }_{31}^V$ plane shows the existence of three significant metallicity groups with mean metallicities as $-1.20\pm 0.12$ dex, $-1.57\pm 0.10$ dex and $-1.89\pm 0.09$ dex. The corresponding absolute magnitudes of these three groups are obtained as $0.70\pm 0.08$ mag, $0.59\pm 0.06$ mag and $0.49\pm 0.08$ mag, respectively. Distribution of these three groups as a function of vertical $|z|$ distance indicates that the formation of the LMC disk predates the formation of the inner halo. Issue of the existence of a metallicity gradient as a function of galactocentric distances has also been addressed. Approximating the structure of the LMC disk as a triaxial ellipsoid, the inclination angle ($i$) relative to the plane of the sky and the position angle of the line of nodes (${\theta }_{lon}$) were estimated as $24^{\circ }.20$ and $176^{\circ }.01$, respectively. The axes ratios and the eccentricity were also determined using the principal axes transformation method.

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