[en] We have obtained infrared (1-2.5 μm) spectroscopy for 42 halo and disk dwarfs with spectral types M1-M6.5. These data are compared to synthetic spectra generated by the latest model atmospheres of Allard and Hauschildt. Photospheric parameters metallicity, effective temperature, and radius are determined for the sample.We find good agreement between observation and theory except for known problems due to incomplete molecular data for metal hydrides and H2O. The metal-poor M subdwarfs are well matched by the models, as oxide opacity sources are less important in this case. The derived effective temperatures for the sample range from 3600 to 2600 K; at these temperatures grain formation and extinction are not significant in the photosphere. The derived metallicities range from solar to 1/10 solar. The radii and effective temperatures derived agree well with recent models of low-mass stars. The spectra are available in electronic form upon request. (c) 2000 The American Astronomical Society

[en] The MACHO collaboration has been carrying out difference image analysis (DIA) since 1996 with the aim of increasing the sensitivity to the detection of gravitational microlensing. This is a preliminary report on the application of DIA to galactic bulge images in one field. We show how the DIA technique significantly increases the number of detected lensing events, by removing the positional dependence of traditional photometry schemes and lowering the microlensing event detection threshold. This technique, unlike PSF photometry, gives the unblended colors and positions of the microlensing source stars. We present a set of criteria for selecting microlensing events from objects discovered with this technique. The 16 pixel and classical microlensing events discovered with the DIA technique are presented. (c) (c) 1999. The American Astronomical Society

[en] We present a catalog containing the measurements of 2262 sources, including 334 extended sources, 1915 point sources, and 13 known QSOs, in five SDSS passbands. Of these objects, over 1600 are measured in 15 fields covering 0.5 deg2, with a limiting magnitude of r^*<19.5, similar to the photometric limit of the SDSS spectroscopic survey. Color plots of the data show that stars, galaxies, and quasars are fairly well separated by color alone. The stellar locus populates a ribbon-like subset of color-color-color space. It is shown that stars, galaxies, and QSOs tend toward the same fundamental plane in three-dimensional color space. The stars are compared with synthetic photometry from Kurucz models; the agreement is consistent with the errors in the data. The stellar locus moves in color space by about a tenth of a magnitude from r^*=14 to r^*=19.5. The shift is consistent with a shift in the metallicity from about [Me/H] = -1 to [Me/H] = -2. We compare this with previously measured metallicity gradients as a function of distance from the galactic plane. (c) (c) 1999. The American Astronomical Society