Browsing by Author "Dalal, N"

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    Macho project limits on black hole dark matter in the 1-30 M⊙ range
    (2001) Alcock, C; Allsman, RA; Alves, DR; Axelrod, TS; Becker, AC; Bennett, DP; Cook, KH; Dalal, N; Drake, AJ; Freeman, KC; Geha, M; Griest, K; Lehner, MJ; Marshall, SL; Minniti, D; Nelson, CA; Peterson, BA; Popowski, P; Pratt, MR; Quinn, PJ; Stubbs, CW; Sutherland, W; Tomaney, AB; Vandehei, T; Welch, DL
    We report on a search for long-duration microlensing events toward the Large Magellanic Cloud. We find none and therefore put limits on the contribution of high-mass objects to the Galactic dark matter. At a 95% confidence level, we exclude objects in the mass range of 0.3-30.0 M-circle dot from contributing more than 4 x 10(11) M-circle dot to the Galactic halo. Combined with earlier results, this means that objects with masses under 30 M-circle dot cannot make up the entire dark matter halo if the halo is of typical size. For a typical dark halo, objects with masses under 10 M-circle dot contribute less than 40% of the dark matter.
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    The macho project Hubble Space Telescope follow-up
    (2001) Alcock, C; Allsman, RA; Alves, DR; Axelrod, TS; Becker, AC; Bennet, DP; Cook, KH; Dalal, N; Drake, AJ; Freeman, KC; Geha, M; Griest, K; Lehner, MJ; Marshall, SL; Minniti, D; Nelson, CA; Peterson, BA; Popowski, P; Pratt, MR; Quinn, PJ; Stubbs, CW; Sutherland, W; Tomaney, AB; Vandehei, T
    We attempt to determine whether the MACHO microlensing source stars are drawn from the average population of the LMC or from a population behind the LMC by examining the Hubble Space Telescope (HST) color-magnitude diagram (CMD) of microlensing source stars. We present WFPC2 HST photometry of eight MACHO microlensing source stars and the surrounding fields in the LMC. The microlensing source stars are identified by deriving accurate centroids in the ground-based MACHO images using difference image analysis (DIA) and then transforming the DIA coordinates to the HST frame. We consider in detail a model for the background population of source stars based on that presented by Zhao, Graff, & Guhathakurta. In this model, the source stars have an additional reddening of [E(B - V)] = 0.13 mag and a slightly larger distance modulus, [Delta mu] similar to 0.3 mag, than the average LMC population. We also investigate a series of source star models, varying the relative fraction of source stars drawn from the average and background populations and the displacement of the background population from the LMC. Because of the small number of analyzed events, the distribution of probabilities of different models is rather flat. A shallow maximum occurs at a fraction s(LMC) similar to 0.8 of the source stars in the LMC. This is consistent with the interpretation that a significant fraction of observed microlensing events are due to lenses in the Milky Way halo, but does not definitively exclude other models.