2 Data overview

Figure 2: Density maps of the LMC: DCMC (upper left), 2MASS (upper right), GSC2.2 (bottom left) and UCAC1 (bottom right) sources. The pixel size is $\sim$ $2.8^{\prime } \times 2.7^{\prime }$. Each map has been normalized to unity, but note that the highest pixel value reaches 208 for the DCMC, 186 for 2MASS, 656 for the GSC2.2 and 18 for the UCAC1.

The present work is based on public data from 2MASS, as described in Sect. 1, and on DCMC data obtained from a local copy of the catalogue that includes the missing strips of the first release (the second release is currently under process). The GSC2.2 and the UCAC1 catalogues are now also publicly available. For each catalogue, a density map is produced (see Fig. 2). The higher concentration of sources in the upper left part of each plot is in the direction of the Galactic center.

Inhomogeneities in the magnitude limit of some DENIS strips are visible on the DCMC map. White areas in the 2MASS map denote missing data due to observations not yet released. The circular gap on the upper right corner corresponds to the semi-regular pulsating star HD 29712, which is also the second brightest star in the sky in $K_{\rm s}$ (after Betelgeuse) with a magnitude lower than -4.

The GSC2.2 catalogue provides, in the direction of the LMC, ( $3^{{\rm h}}59^{{\rm m}}30^{{\rm s}}$ < RA < $7^{{\rm h}}04^{{\rm m}}30^{{\rm s}}$; $-78^{\circ}00\hbox{$^\prime$ }00\hbox{$^{\prime\prime}$ }$ < Dec < $-61^{\circ}00\hbox{$^\prime$ }00\hbox{$^{\prime\prime}$ }$), the $F_{\rm gsc2}$, $J_{\rm gsc2}$, and $V_{\rm gsc2}$ photographic bands for 6 032 541 entries. The $V_{\rm gsc2}$ magnitudes are from short exposure V plates used to deal with dense regions of the sky. The photographic magnitudes given in the GSC2.2 are in the natural systems of the photographic plates (emulsion/filter: IIIaF+OG590, IIIaJ+GG395, IIaD+W12). The photometric calibration is based on a Chebyshev polynomial fit to sequence stars from the Second Guide Star Photometric Catalogue (Bucciarelli et al. 2001) and the Tycho Catalog for the bright end. However the three magnitudes are not always present together. The $J_{\rm gsc2}$ band is missing for the innermost parts of the LMC, but is replaced by the $V_{\rm gsc2}$ band. Thus, when using the $V_{\rm gsc2}$ band in the following colour-magnitude or colour-colour diagrams of this paper, one should keep in mind that we are dealing with the central parts of the LMC only. The unusual patterns on the GSC2.2 density map match the HTM (Hierarchical Triangular Mesh) partitioning of the data, which is a recursive spatial indexing scheme dividing the unit sphere into spherical triangles. This is probably due to difficulties in producing the final catalogue for the most crowded regions.

The UCAC1 catalogue (Zacharias et al. 2000) contains, in the direction of the LMC, 267,103 entries ( $3^{{\rm h}}59^{{\rm m}}30^{{\rm s}}$ < RA < $7^{{\rm h}}04^{{\rm m}}28^{{\rm s}}$; $-78^{\circ}00\hbox{$^\prime$ }00\hbox{$^{\prime\prime}$ }$ < Dec < $-61^{\circ}00\hbox{$^\prime$ }00\hbox{$^{\prime\prime}$ }$). This is the preliminary version of an astrometric catalogue, which aims at increasing the number of optical sources with high positional accuracy. Proper motions are available, combining the UCAC1 with the USNO-A2.0 (Monet et al. 1998) positions for faint stars, and with older catalogues for bright stars. One magnitude, intermediate between Johnson V and R, is provided (579-642 nm). White regions on the UCAC1 density map denote missing digitalization frames in the center of the bar, due to difficulties in extracting sources in overcrowded regions of the sky.

The catalogues used to build the MC2 present the following differences: the observational strategy that influences the homogeneity of the final data, the passbands, the characterized stellar populations and the number of sources. These factors have a strong impact on the results of our cross-matching.