No abstract

A new grid of 65 faint near-infrared standard stars is presented. They are spread around the sky, lie between 10th and 12th mag at K, and are measured to precisions better than 0.005 mag in the J, H, K and Ks bands; the latter is a medium-band modi ed K. A secondary list of red stars suitable for determining color transformations between photometric systems is also presented.

A b s t r a c t . We describe an ongoing survey to search for dark m a t t e r via lensing events of stars in the Galactic Bulge. T h e principal properties of the survey are described, a n d some preliminary results are shown for newly-discovered variables. We discuss some of the projects related to the s t u d y of the Galactic Bulge t h a t can b e addressed using these data, a n d describe the f u t u r e plans for the survey over the coming few years. K e y w o r d s : methods: observational -surveys -stars: imaging 311 //. Dejonghe and H.

Near-infrared J, H, and K s photometric measurements of 92 Cepheids in the Large Magellanic Cloud are presented. The stars are spread over the face of the Cloud, their periods range from 3 to 100 days, and their light curves are sampled at an average of 22 phase points per star. The intensity-weighted mean magnitudes and colors define period-luminosity-color ( PL or PLC) relations whose uncertainties due to differential metal abundance and reddening/extinction effects are minimal. The dispersions in the infrared PL, PLC, and extinction-free periodWesenheit relations are extremely small, amounting to less than 0.10 mag (or 5% in distance). The orientation of the disk plane of the sample (inclination angle and line of nodes) agrees well with the 2001 results of van der Marel & Cioni. The PL and PLC fits are the best-determined such relationships yet found for any sample of Cepheids and establish a calibration that can be used to precisely anchor ground-and space-based near-infrared Cepheid data to external galaxies, as well as back to Cepheid calibrators in the Galaxy. As an example, we use the 1998 Galactic Cepheid calibration of Gieren and coworkers to obtain the distance modulus to the centroid of our LMC sample. The true modulus of the LMC is thus determined to be 18:50 AE 0:05 mag. Currently, the dominant source of uncertainty in this number is the scatter in the Galactic calibrator sample. The PLC fits and dispersions and the dependence of the PLC on metal abundance are compared with theoretical versions computed from the 1999 work of Alibert and coworkers. Overall, the agreement is excellent, indicating that at near-infrared wavelengths the slope and dispersion of the PLC depend very weakly on metal abundance. The shift in the JHK PLC relations is $0.02 mag for a change in metal abundance from solar to one-half solar.

The color-magnitude diagrams of ∼ 7 × 10 5 stars obtained for 12 fields across the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump giants. We find that the distributions of the apparent magnitudes of the red clump stars are systematically fainter when moving towards lower galactic l fields. The most plausible explanation of this distinct trend is that the Galactic bulge is a bar, whose nearest end lies at positive galactic longitude. We model this Galactic bar by fitting for all fields the observed luminosity functions in the red clump region of the color-magnitude diagram. We find that almost regardless of the analytical function used to describe the 3-D stars distribution of the Galactic bar, the resulting models have the major axis inclined to the line of sight by 20 − 30 deg, with axis ratios corresponding to x 0 : y 0 : z 0 = 3.5 : 1.5 : 1. This puts a strong constraint on the possible range of the Galactic bar models. Gravitational microlensing can provide us with additional constrains on the structure of the Galactic bar.