Browsing by Author "Walker, A"

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    Binary microlensing events from the MACHO Project
    (2000) Alcock, C; Allsman, RA; Alves, D; Axelrod, TS; Baines, D; Becker, AC; Bennett, DP; Bourke, A; Brakel, A; Cook, KH; Crook, B; Crouch, A; Dan, J; Drake, AJ; Fragile, PC; Freeman, KC; Gal-Yam, A; Geha, M; Gray, J; Griest, K; Gurtierrez, A; Heller, A; Howard, J; Johnson, BR; Kaspi, S; Keane, M; Kovo, O; Leach, C; Leach, T; Leibowitz, EM; Lehner, MJ; Lipkin, Y; Maoz, D; Marshall, L; McDowell, D; McKeown, S; Mendelson, H; Messenger, B; Minniti, D; Nelson, C; Peterson, BA; Popowski, P; Pozza, E; Purcell, P; Pratt, MR; Quinn, J; Quinn, PJ; Rhie, SH; Rodgers, AW; Salmon, A; Shemmer, O; Stetson, P; Stubbs, CW; Sutherland, W; Thomson, S; Tomaney, A; Vandehei, T; Walker, A; Ward, K; Wyper, G
    We present the light curves of 21 gravitational microlensing events from the first six years of the MACHO Project gravitational microlensing survey that are likely examples of lensing by binary systems. These events were manually selected from a total sample of similar to 350 candidate microlensing events that were either detected by the MACHO Alert System or discovered through retrospective analyses of the MACHO database. At least 14 of these 21 events exhibit strong (caustic) features, and four of the events are well fit with lensing by large mass ratio (brown dwarf or planetary) systems, although these fits are not necessarily unique. The total binary event rate is roughly consistent with predictions based upon our knowledge of the properties of binary stars, but a precise comparison cannot be made without a determination of our binary lens event detection efficiency. Toward the Galactic bulge, we find a ratio of caustic crossing to noncaustic crossing binary lensing events of 12:4, excluding one event for which we present two fits. This suggests significant incompleteness in our ability to detect and characterize noncaustic crossing binary lensing. The distribution of mass ratios, N(q), for these binary lenses appears relatively flat. We are also able to reliably measure source-face crossing times in four of the bulge caustic crossing events, and recover from them a distribution of lens proper motions, masses, and distances consistent with a population of Galactic bulge lenses at a distance of 7 +/- 1 kpc. This analysis yields two systems with companions of similar to 0.05 M..
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    The Araucaria Project
    (2004) Gieren, W; Pietrzynski, G; Walker, A; Bresolin, F; Minniti, D; Kudritzki, RP; Udalski, A; Soszynski, I; Fouqué, P; Storm, J; Bono, G
    In a previous paper, we reported on the discovery of more than a hundred new Cepheid variables in the Sculptor Group spiral galaxy NGC 300 from wide-field images taken in the B and V photometric bands at ESO/La Silla. In this paper, we present additional VI data, derive improved periods and mean magnitudes for the variables, and construct period-luminosity relations in the V, I, and the reddening-independent V - I Wesenheit bands using 58 Cepheid variables with periods between 11 and 90 days. We obtain tightly defined relations, and by fitting the slopes determined for the LMC Cepheids by the OGLE II Project we obtain reddening-corrected distances to the galaxy in all bands, which show a slight offset to each other in the sense that the Wesenheit relation yields the smallest distance, whereas the I- and V-band distances are larger by 0.094 and 0.155 mag, respectively. We adopt as our best value the distance derived from the reddening-free Wesenheit magnitudes, which is 26.43 +/- 0.04( random) +/- 0.05( systematic) mag. The distance moduli from both the V and I bands agree perfectly with the Wesenheit value if one assumes an additional reddening of E( B - V) = 0.05 mag intrinsic to NGC 300, in addition to the Galactic foreground reddening toward NGC 300 of 0.025 mag. Such a modest intrinsic reddening is supported by recent Hubble Space Telescope (HST) images of NGC 300, which show that this galaxy is relatively dustfree but also reveal that there must be some dust absorption in NGC 300. We argue that our current distance result for NGC 300 is the most accurate that has so far been obtained using Cepheid variables and that it is largely free from systematic effects due to metallicity, blending, and sample selection. It agrees very well with the recent distance determination from the tip of the red giant branch method obtained from HST data by Butler and coworkers, and it is consistent with the Cepheid distance to NGC 300 that was derived by Freedman and coworkers from CCD photometry of a smaller sample of stars.