Browsing by Author "Drake, AJ"

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    A search for stellar obscuration events due to dark clouds
    (2003) Drake, AJ; Cook, KH
    The recent detections of a large population of faint submillimeter sources, an excess halo gamma-ray background, and the extreme scattering events observed for extragalactic radio sources have been explained as being due to baryonic dark matter in the form of small, dark gas clouds. In this paper, we present the results of a search for the transient stellar obscurations such clouds are expected to cause. We examine the MACHO project light curves of 48 x 10(6) stars toward the Galactic bulge, Large Magellanic Cloud, and Small Magellanic Cloud for the presence of dark cloud extinction events. We find no evidence for a population of dark gas clouds with A(V) > 0.2 in the masses range from similar to10(-4) to 2 x 10(-2) M-., in either the Galactic disk or halo. However, it is possible that such dark cloud populations could exist if they are clustered in regions away from the observed lines of sight.
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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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    Chromospherically active stars in the Galactic bulge
    (2006) Drake, AJ
    We present the results from the discovery and study of similar to 3000 chromospherically active giant and subgiant stars toward the Galactic bulge. We find that these stars are predominantly RS CVn binaries with rotation periods between 10 and 100 days. We discover that the average rotational period of these stars decreases with their distance from the Galactic plane. We find that the primary stars in the RS CVn systems are predominantly first-ascent giants. Our research also suggests that if these stars have spot cycles like the Sun, then the cycle period must be longer than 10 yr on average. We confirm that the amplitude of the spot-induced modulations observed in the light curves of these objects is generally larger at minimum light than at maximum. Furthermore, we confirm that the amplitudes of the modulations due to stellar spots generally increase as the observed change in average brightness increases. We find no evidence for a relationship between a chromospherically active star's brightness and its rotational period. However, the average period does increase with color for stars with periods <= 30 days.
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    Difference image analysis of galactic microlensing. II. Microlensing events
    (1999) Alcock, C; Allsman, RA; Alves, D; Axelrod, TS; Becker, AC; Bennett, DP; Cook, KH; Drake, AJ; Freeman, KC; Griest, K; Lehner, MJ; Marshall, SL; Minniti, D; Peterson, BA; Pratt, MR; Quinn, PJ; Stubbs, CW; Sutherland, W; Tomaney, A; Vandehei, T; Welch, DL
    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.
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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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    On the selection of photometric planetary transits
    (2003) Drake, AJ
    We present a new method for differentiating between planetary transits and eclipsing binaries on the basis of the presence of ellipsoidal light variations. These variations can be used to detect stellar secondaries with masses of similar to0.2 M-circle dot orbiting Sun-like stars at a photometric accuracy level that has already been achieved in transit surveys. By removing candidates exhibiting this effect, it is possible to greatly reduce the number of objects requiring spectroscopic follow-up with large telescopes. Unlike the usual candidate selection methods, which are based primarily on the estimated radius of the orbiting object, this technique is not biased against bona. de planets and brown dwarfs with large radii because the amplitude of the effect depends on the transiting object's mass and orbital distance. In many binary systems where a candidate planetary transit is actually due to the partial eclipse of two normal stars, the presence of flux variations due to the gravity darkening effect will show the true nature of these systems.
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    Photometric transits from the MACHO project database
    (2004) Drake, AJ; Cook, KH
    We have searched the MACHO project photometry database for light curves consistent with the transits of low-luminosity objects. By analyzing the 180,000 brightest stars in 14 0.5 deg(2) fields toward the Galactic bulge, we have found a few hundred low-amplitude eclipsing binary systems. We have used the shapes and colors of these objects to separate the transits of objects with relatively small radii from the grazing eclipses of stars with larger radii. We find nine objects with short-period eclipses consistent with either giant extrasolar planets or late-type dwarf stars.
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    Resolving the nature of the Large Magellanic Cloud microlensing event MACHO-LMC-7
    (2004) Drake, AJ; Cook, KH; Keller, SC
    We present the results from an analysis of Hubble Space Telescope high-resolution camera data for the Large Magellanic Cloud microlensing event MACHO-LMC-5. By determining the parallax and proper motion of this object, we find that the lens is an M dwarf star at a distance of 578(-53)(+65) pc with a proper motion of 21.39 +/- 0.04 mas yr(-1). On the basis of the kinematics and location of this star, it is more likely to be part of the Galactic thick disk than the thin disk population. We confirm that the microlensing event LMC-5 is a jerk-parallax event.
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    Searching for periodicities in the MACHO light curve of LMC X-2
    (2000) Alcock, C; Allsman, RA; Alves, DR; Axelrod, TS; Becker, AC; Bennett, DP; Charles, PA; Cook, KH; Drake, AJ; Freeman, KC; Geha, M; Griest, K; Groot, P; Lehner, MJ; Marshall, SL; McGowan, KE; Minniti, D; Nelson, CA; Peterson, BA; Popowski, P; Pratt, MR; Quinn, PJ; Sutherland, W; Tomaney, AB; Vandehei, T; van Paradijs, J
    Using the exceptional long-term monitoring capabilities of the MACHO project, we present here the optical history of LMC X-2 for a continuous 6-yr period. These data were used to investigate the previously claimed periodicities for this source of 8.15 h and 12.54 d: we find upper limits of 0.10 mag and 0.09 mag, respectively.
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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.