Kim Griest

Microlensing optical depth toward the galactic bulge using clump giants from the MACHO survey

Using 7 years of MACHO survey data, we present a new determination of the optical depth to microlensing towards the Galactic bulge. We select the sample of 62 microlensing events (60 unique) on clump giant sources and perform a detailed efficiency analysis. We use only the clump giant sources because these are bright bulge stars and are not as strongly affected by blending as other events. Using a subsample of 42 clump events concentrated in an area of 4.5 deg{sup 2} with 739000 clump giant stars, we find {tau} = 2.17{sub -0.38}{sup +0.47} x 10{sup -6} at (l,b) = (1{sup o}.50, -2{sup o}.68), somewhat smaller than found in most previous MACHO studies, but in excellent agreement with recent theoretical predictions. We also present the optical depth in each of the 19 fields in which we detected events, and find limits on optical depth for fields with no events. The errors in optical depth in individual fields are dominated by Poisson noise. We measure optical depth gradients of (1.06 {+-} 0.71) x 10{sup -6}deg{sup -1} and (0.29 {+-} 0.43) x 10{sup -6}deg{sup -1} in the galactic latitude b and longitude l directions, respectively. Finally, we discuss the possibility of anomalous durationmorexa0» distribution of events in the field 104 centered on (l,b) = (3{sup o}.11, -3{sup o}.01) as well as investigate spatial clustering of events in all fields.«xa0less

MACHO observations of LMC red giants: Mira and semi-regular pulsators, and contact and semi-detached binaries

The MACHO data base has been used to examine light curves of all red giant stars brighter than Mbol rv -2 in a 0.5 0 x0.5 0 area of the LMC bar. Periods, often multiple, have been searched for in all stars found to be variable. Five distinct period-luminosity sequences have been found on the low mass (M ~ 2.25M0 ) giant branch. Comparison of observed periods, luminosities and period ratios with theoretical models identifies Miras unambiguously as radial fundamental mode pulsators, while semi-regular variables can be pulsating in the 1st, 2nd or 3rd overtone, or even the fundamental. All these variables lie on just 3 of the 5 distinct sequences, and they all appear to be on the AGB. The fourth sequence contains red giants on the first giant branch (FGB) or at the red end of the core-helium burning loops of intermediate mass stars (M ~ 2.25M0 ) . The light curves of these stars strongly suggest that they are contact binaries, and they make up rvO.5% of stars within 1 mag. of the FGB tip. Stars on the fifth sequence show semiregular, eclipse-like light curves. The light curves and periods of these stars suggest that they are in semi-detached binaries, transfering mass to an invisible companion via a stellar wind or Roche lobe overflow. They make up rv25% of AGB stars. If the existence of these red giant contact and semi-detached binaries is confirmed, then extant theories of binary star evolution will require substantial modification. IMount Stromlo and Siding Spring Observatories, Australian National University. 2Center for Particle Astrophysics, University of California, Berkeley. 3Supercomputing Facility, Australian National University. 4Department of Physics, University of California, Davis. 5Lawrence Livermore National Laboratory. 6Departments of Astronomy and Physics, University of Washington. 7Department of Physics, University of Notre Dame. 8Department of Physics, University of California, San Diego. 9Department of Physics, University of Sheffield. lODepartamento de Astronomia, P. Universidad Cat6lica, Santiago. 11 Center for Space Research, MIT. 12European Southern Observatory. 13Department of Physics, University of Oxford. 14Department of Physics and Astronomy, McMaster University.