Results for "Andrew Gould"

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The Effect of Differential Limb Magnification on Abundance Analysis of Microlensed Dwarf StarsOct 19 2009Finite source effects can be important in observations of gravitational microlensing of stars. Near caustic crossings, for example, some parts of the source star will be more highly magnified than other parts. The spectrum of the star is then no longer ... More
Geosynchronous Microlens ParallaxesNov 27 2012Dec 29 2012I show that for a substantial fraction of planets detected in a space-based survey, it would be possible to measure the planet and host masses and distances, if the survey satellite were placed in geosynchronous orbit. Such an orbit would enable measurement ... More
A New Method to Calibrate the Stellar Color/Surface-Brightness RelationJul 22 2014I show that the standard microlensing technique to measure the angular radius of a star using color/surface-brightness relations can be inverted, via late-time proper motion measurements, to calibrate these relations. The method is especially useful for ... More
A Second Kelvin-Helmholtz Timescale of Post Helium-Flash EvolutionNov 29 2011I show that after the "helium flash" abruptly ends its first ascent red giant evolution, a solar-type star is powered primarily by gravitational contraction of its helium core, rather than by nuclear fusion. Because this energy is released in the core ... More
Hexadecapole Approximation in Planetary MicrolensingJan 17 2008Apr 03 2008The frequency of microlensing planet detections, particularly in difficult-to-model high-magnification events, is increasing. Their analysis can require tens of thousands of processor hours or more, primarily because of the high density and high precision ... More
Microlens Parallax Measurements with a Warm SpitzerMay 26 2007Because Spitzer is an Earth-trailing orbit, losing about 0.1 AU/yr, it is excellently located to perform microlens parallax observations toward the Magellanic Clouds (LMC/SMC) and the Galactic bulge. These yield the so-called ``projected velocity'' of ... More
Probing MACHOs Toward the Galactic BulgeMar 28 2005If the massive compact halo object (MACHO) fraction of the Galactic dark halo is f ~ 20% as suggested by some microlensing experiments, then about 1.2% of lensing events toward the Galactic bulge are due to MACHOs. For the 40% of these that lie nearby ... More
Recent Developments in Gravitational MicrolensingMar 30 2008Twenty-one years after Bohdan's seminal paper launched the field of gravitational microlensing, it has radically diversified from a method narrowly focused on finding dark matter to a very general astronomical tool. Microlensing has now detected 12 planets, ... More
Completeness of USNO-B for High Proper-Motion StarsDec 31 2002I test the completeness of USNO-B detections of high proper-motion (mu>0.18"/yr) stars and the accuracy of its measurements by comparing them to the revised NLTT (rNLTT) catalog of Salim & Gould. For 14.5<V<18.5, only 6% of such stars are missing from ... More
Measuring the Remnant Mass Function of the Galactic BulgeJun 29 1999I show that by observing microlensing events both astrometrically and photometrically, the Space Interferometry Mission (SIM) can measure the mass function of stellar remnants in the Galactic bulge including white dwarfs, neutron stars, and black holes. ... More
LMC Microlenses: Dark or Luminous?Sep 25 1997Zhao has proposed that the microlensing events observed toward the Large Magellanic Cloud (LMC) could be due to faint stars in a dwarf galaxy or tidal debris lying along the line of sight to the LMC. Zaritsky & Lin claim to have detected such a structure ... More
Microlensing: Prospects for the FutureNov 18 1996Four ongoing microlensing experiments have produced important new results but also big puzzles, the major one being that the expected classes of lenses cannot account for the observed distribution of time scales. I discuss future experiments that could ... More
Transverse Velocities of Galaxies From Microlens ParallaxesSep 01 1994The transverse velocity of a spiral galaxy can be measured to an accuracy $\sim 60\,\kms$ by making parallax observations of quasars being microlensed by stars in the disk of the galaxy. To make the measurement, a quasar must be located behind the disk ... More
Macho Parallaxes From A Single SatelliteAug 10 1994Massive Compact Objects (Machos) are currently being discovered at substantially higher rates than would be expected from standard models of known stellar populations. To determine whether they are due to non-standard distri- butions of known populations ... More
Microlensing by Kuiper, Oort, and Free-Floating PlanetsMar 11 2016Microlensing is generally thought to probe planetary systems only out to a few Einstein radii. Microlensing events generated by bound planets beyond about 10 Einstein radii generally do not yield any trace of their hosts, and so would be classified as ... More
Generalized Microlensing Effective TimescaleDec 23 2013Dec 26 2013The microlensing effective timescale t_eff=beta*t_E is used frequently in high-magnification (beta << 1) microlensing events, because it is better constrained than either the impact parameter beta or the Einstein timescale t_E separately. It also facilitates ... More
WFIRST Ultra-Precise Astrometry I: Kuiper Belt ObjectsMar 17 2014Aug 02 2014I show that the WFIRST microlensing survey will enable detection and precision orbit determination of Kuiper Belt Objects (KBOs) down to H_=28.2 over an effective area of about 17 sq.deg. Typical fractional period errors will be about 1.5% x 10^(0.4(H-28.2)) ... More
Post-Decadal White Paper: A Dual-Satellite Dark-Energy/Microlensing NASA-ESA MissionSep 30 2010A confluence of scientific, financial, and political factors imply that launching two simpler, more narrowly defined dark-energy/microlensing satellites will lead to faster, cheaper, better (and more secure) science than the present EUCLID and WFIRST ... More
Wide Field Imager in Space for Dark Energy and PlanetsFeb 12 2009A wide-field imager in space could make remarkable progress in two very different frontiers of astronomy: dark energy and extra-solar planets. Embedding such an imager on a much larger and more complicated DE mission would be a poor science-approach under ... More
Optimal Microlensing ObservationsJul 31 1998One of the major limitations of microlensing observations toward the Large Magellanic Cloud (LMC) is the low rate of event detection. What can be done to improve this rate? Is it better to invest telescope time in more frequent observations of the inner ... More
Microlens Parallax Asymmetries Toward the LMCFeb 11 1998If the microlensing events now being detected toward the Large Magellanic Cloud (LMC) are due to lenses in the Milky Way halo, then the events should typically have asymmetries of order 1% due to parallax from the reflex motion of the Earth. By contrast, ... More
chi^2 and Linear FitsOct 20 2003The mathematics of linear fits is presented in covariant form. Topics include: correlated data, covariance matrices, joint fits to multiple data sets, constraints, and extension of the formalism to non-linear fits. A brief summary at the end provides ... More
Analytic Study of Grid Star and Reference Star Selection for the Space Interferometry MissionFeb 16 2001May 18 2001Grid stars and reference stars provide the fundamental global and local astrometric reference frames for observations by the Space Interferometry Mission. They must therefore be astrometrically stable at the ~1 uas level. I present simple formulae in ... More
A New Argument Against An Intervening Stellar Population Toward the LMCFeb 26 1999Mar 10 1999Zaritsky & Lin have claimed detection of an intervening population of stars toward the Large Magellanic Cloud (LMC) which, they believe, could account for a substantial fraction of the observed microlensing events. I show that the observed time scales ... More
Extreme MicrolensingMar 27 1996Extreme microlensing events, defined as events with maximum magnification $A_\max\gsim 200$ are a potentially powerful probe of the mass spectrum and spatial distribution of objects along lines of sight toward the Galactic bulge. About 75 yr${}^{-1}$ ... More
Self-Lensing By A Stellar DiskAug 01 1994I derive a general expression for the optical depth $\tau$ for gravitational lensing of stars in a disk by Massive Compact Objects (Machos) in the same disk. For the more restricted case where the disk is self-gravitating and the stars and Machos have ... More
The Hollywood Strategy for Microlensing Detection of PlanetsAug 09 1996Follow the big stars! I review the theory of detection and parameter measurement of planetary systems by follow-up observations of ongoing microlensing events. Two parameters can generically be measured from the event itself: the planet/star mass ratio, ... More
Microlensing Search of $10^6$ QuasarsApr 26 1995By monitoring $10^6$ quasars one could search for lensing by stars and Massive Compact Halo Objects (Machos) out to redshifts $z\sim 4$. If Machos have a present cosmological density $\Omega_{L,0}=1\%$, then the expected event rate is $\Gamma\sim 200\,\yr^{-1}$. ... More
Osculating Versus Intersecting Circles in Space-Based Microlens Parallax DegeneraciesMay 16 2019I investigate the origin of arc degeneracies in satellite microlens parallax pi_E measurements with only late time data, e.g., t > t0 + t_E as seen from the satellite. I show that these are due to partial overlap of a series of osculating, exactly circular, ... More
LSST's DC Bias Against Planets and Galactic-Plane ScienceApr 11 2013An LSST-like survey of the Galactic plane (deep images every 3-4 days) could probe the Galactic distribution of planets by two distinct methods: gravitational microlensing of planets beyond the snow line and transits by planets very close to their hosts. ... More
Investigation of the Contamination of the Gould (2003) Halo SampleAug 09 2007A recent astroph posting argued that the Gould (2003a) halo sample is substantially contaminated with thick-disk stars, which would then ``wash out'' any signature of granularity in the halo velocity distribution due to streams. If correct, this would ... More
Resolution of the MACHO-LMC-5 Puzzle: The Jerk-Parallax Microlens DegeneracyNov 24 2003Mar 14 2004By extending the constant-acceleration analysis of Smith, Mao, & Paczynski to include jerk, I show that microlens parallax measurements are subject to a four-fold discrete degeneracy. The new degeneracy is characterized by a projected velocity \tilde ... More
A New Kinematic Distance Estimator to the LMCJun 01 1999The distance to the Large Magellanic Cloud (LMC) can be directly determined by measuring three of its properties, its radial-velocity field, its mean proper motion, and the position angle \phi_ph of its photometric line of nodes. Statistical errors of ... More
Microlens Parallaxes with SIRTFJul 23 1998The Space Infrared Telescope Facility (SIRTF) will drift away from the Earth at about 0.1 AU/yr. Microlensing events will therefore have different characteristics as seen from the satellite and the Earth. From the difference, it is possible in principle ... More
A New Macho Search StrategySep 26 1994I propose a radical revision in the search strategy for Massive Compact Objects (Machos) toward the Galactic bulge: monitor the entire $\sim 300$ square deg of the bulge and tune the search primarily to microlensing events of bright $M_V\lsim 2$ stars. ... More
Self-Lensing By BinariesSep 22 1994Lensing of one member of a binary by its companion is studied for several classes of binaries. For binaries in which at least one member is an ordinary (non-compact) star, the optical depths to lensing is extremely low, $\tau\lsim v_\esc^4/c^4\sim 10^{-11}$. ... More
Microlensing Events: End of the Dark Halo?Aug 17 1994I obtain an upper limit for the optical depth to microlensing toward Baade's Window of $3\times 10^{-6}$ by assuming that all of the mass of the Galaxy interior to the Sun (and not in the bulge) is in a disk. The exponential scale height of the disk is ... More
Analytic Error EstimatesJun 21 1994I present an analytic method for estimating the errors in fitting a distribution. A well-known theorem from statistics gives the minimum variance bound (MVB) for the uncertainty in estimating a set of parameters $\l_i$, when a distribution function $F(z;\l_1 ... More
Microlens Masses from 1-D Parallaxes and Heliocentric Proper MotionsAug 04 2014Sep 25 2014One-dimensional (1-D) microlens parallaxes can be combined with heliocentric lens-source relative proper motion measurements to derive the lens mass and distance, as suggested by Ghosh et al. (2004). Here I present the first mathematical anlysis of this ... More
Anthropic Argument for Three GenerationsNov 11 2010Nov 15 2010The standard model of particle physics contains N_gen=3 generations of quarks and leptons, i.e., two sets of three particles in each sector, with the two sets differing by 1 unit of charge in each. All 12 "predicted" particles are now experimentally accounted ... More
The Most Precise Extra-Galactic Black-Hole Mass MeasurementMar 04 2013I use archival data to measure the mass of the central black hole in NGC 4526, M = (4.70 +- 0.14) X 10^8 Msun. This 3% error bar is the most precise for an extra-galactic black hole and is close to the precision obtained for Sgr A* in the Milky Way. The ... More
On the Difference in Statistical Behavior Between Astrometric and Radial-Velocity Planet DetectionsJul 27 2008Astrometric and radial-velocity planet detections track very similar motions, and one generally expects that the statistical properties of the detections would also be similar after they are scaled to the signal-to-noise ratio of the underlying observations. ... More
v_\perp CMDMar 21 2004I present an Hipparcos color-magnitude diagram (CMD) that is color-coded by transverse velocity v_\perp. This illustrates the connection between the photometric and kinematic properties of various stellar populations in a manner that is particularly suitable ... More
Stellar Halo Parameters from 4588 SubdwarfsJul 31 2002Using a reduced proper motion discriminator, I obtain a sample of 4588 subdwarfs from the Revised NLTT Catalog of Salim & Gould. The ample statistics and low contamination permit much more precise determinations of halo parameters than has previously ... More
A Natural Formalism for MicrolensingJan 25 2000If the standard microlensing geometry is inverted so that the Einstein ring is projected onto the observer plane rather than the source plane, then the relations between the observables (\theta_E,\tilde r_E) and the underlying physical quantities (M,\pi_rel) ... More
Selection of Nearby Microlensing Candidates for Observation by SIMMay 11 1999I investigate the prospects for using the Space Interferometry Mission (SIM) to measure the masses of nearby stars from their astrometric deflection of more distant sources, as originally suggested by Paczynski and by Miralda-Escude. I derive an analytic ... More
Search for Intra-Cluster Machos by Pixel Lensing of M87Jan 29 1995Feb 02 1995Intra-Cluster Machos (ICMs) are a plausible candidate for at least some of the dark matter in clusters of galaxies. ICMs can be detected by searching toward M87 for ``pixel lensing'', gravitational microlensing of unresolved stars. Dedicated observations ... More
Microlensing: Current Results and Future ProspectsMar 09 1997The initial results of microlensing surveys toward the Galactic bulge and the LMC are puzzling. Toward the LMC, the total mass in MACHOs is of order half that required to explain the dark matter, but the estimated MACHO mass (~0.4 Msun) is most consistent ... More
Microlensing and the Stellar Mass FunctionApr 02 1996Traditional approaches to measuring the stellar mass function (MF) are fundamentally limited because objects are detected based on their luminosity, not their mass. These methods are thereby restricted to luminous and relatively nearby stellar populations. ... More
Limits on the Gravity Wave Background From Microlensed QuasarsJul 18 1995Aug 01 1995The paper previously submitted under this title is incorrect in that it drastically overestimates the cumulative deflection due to a gravitational wave (GW) background. Avi Loeb gives a simple argument that there can be no $(D\omega)^{1/2}$ enhancement: ... More
K Band Microlensing of the Inner GalaxyDec 20 1994Dec 21 1994Microlensing searches toward the inner galaxy $(|l|,|b|\leq 22.\hskip-2pt'5)$ have several major advantages. First, the event rate is strongly dominated by bulge-bulge lensing events where both the source and lens lie in the bulge. Second, these bulge-bulge ... More
The Supernova Ring Revisited II: Distance to the LMCJul 14 1994I derive an upper limit to the distance to SN 1987A of $\dsn<46.77\pm 0.76\,\kpc$, or a distance modulus $\mu_\sn<18.350\pm 0.035$. If SN 1987A lies in the plane of the Large Magellanic Cloud (LMC) and hence 500 pc in front of the LMC center, this implies ... More
Star Counts From HST: Implications for Dark MatterNov 22 1996Star counts made with the Hubble Space Telescope (HST) probe four populations that are important for dark matter: disk, halo, bulge, and intergalactic. The disk mass function falls for masses M<0.6 Msun in sharp contrast to the rising Salpeter function ... More
Measuring the Rotation Speed of Giant Stars From Gravitational MicrolensingNov 07 1996During some gravitational lensing events, the lens transits the face of the star. This causes a shift in the apparent radial velocity of the star which is proportional to its rotation speed. It also changes the magnification relative to what would be ... More
Theory of Pixel LensingSep 02 1995Feb 07 1996Pixel lensing, gravitational microlensing of unresolved stars, is potentially much more sensitive and much more widely applicable than is generally recognized. I give explicit expressions for the pixel noise induced by a time-variable PSF, by photometric ... More
Microlensing by Stars in the Disk of M31Mar 11 1994The optical depth to microlensing toward M31 due to known stars in the disk of M31 itself is $\tau\sim 2\times 10^{-7}e^{-r/d}$ where $d$ is the disk scale length and $r$ is the distance along the major axis. Thus, there can be significant lensing toward ... More
Tritium as an Anthropic ProbeJul 09 2012I show that if tritium were just 20 keV lighter relative to helium-3, then the current deuterium burning phase of pre-main-sequence stellar evolution would be replaced by deuterium+tritium burning. This phase would take place at the same temperature but ... More
E = mc^2 Without RelativityApr 22 2005Apr 23 2005The equivalence of mass and energy is indelibly linked with relativity, both by scientists and in the popular mind. I prove that E = mc^2 by demanding momentum conservation of an object that emits two equal electromagnetic wave packets in opposite directions ... More
Gravitational Pulse AstronomyNov 19 2010Feb 02 2011Thompson has argued that the Kozai mechanism is primarily responsible for driving white-dwarf binary mergers and so generating type Ia supernovae (SNe). If so, the gravitational wave signal from these systems will be characterized by isolated repeating ... More
An Upper Limit on the Granularity of the Local Stellar HaloApr 15 2003I use the statistical properties of 4588 nearby halo stars to show that if the local stellar halo is composed of cold streams from disrupted dwarf satellites as is predicted by popular cosmologies, then at 95% confidence there are at least 1350 such streams. ... More
Applications of Microlensing to Stellar AstrophysicsMar 30 2001Over the past decade, microlensing has developed into a powerful tool to study stellar astrophysics, especially stellar atmospheres, stellar masses, and binarity. I review this progress. Stellar atmospheres can be probed whenever the source in a microlensing ... More
Theory of MicrolensingApr 04 2000I present a somewhat selective review of microlensing theory, covering five major areas: 1) the derivation of the basic formulae, 2) the relation between the observables and the fundamental physical parameters, 3) binaries, 4) astrometric microlensing, ... More
Galactic Structure From Infrared SurveysAug 10 1998By combining the 2MASS and DENIS infrared surveys with the USNO-B proper-motion catalog, it will be possible to map the structure of the Galaxy in unprecedented detail. The key parameter that these surveys measure together, but neither measures separately, ... More
Les Microlentilles GravitationnellesMay 17 1996Je presente une revue des microlentilles gravitationnelles en quatre lecons. Dans un premier temps, je discute la theorie des microlentilles dans le contexte general des lentilles, en incluant les effets de la taille finie des sources, de la parallaxe, ... More
Two Stars Two Ways: Confirming a Microlensing Binary Lens Solution with a Spectroscopic Measurement of the OrbitJun 04 2015Light curves of microlensing events involving stellar binaries and planetary systems can provide information about the orbital elements of the system due to orbital modulations of the caustic structure. Accurately measuring the orbit in either the stellar ... More
Kepler-like Multi-Plexing for Mass Production of Microlens ParallaxesOct 15 2013Nov 07 2013We show that a wide-field Kepler-like satellite in Solar orbit could obtain microlens parallaxes for several thousand events per year that are identified from the ground, yielding masses and distances for several dozen planetary events. This is roughly ... More
Kepler planet candidates consistent with core accretionFeb 04 2011We show that the distribution of Kepler candidate planets from Borucki et al. is consistent with the predictions of the core accretion model.
Stellar Mass Function From SIM Astrometry/PhotometryJul 07 1999By combining SIM observations with ground-based photometry, one can completely solve microlensing events seen toward the Galactic bulge. One could measure the mass, distance, and transverse velocity of ~100 lenses to ~5% precision in only ~500 hours of ... More
Improved Astrometry and Photometry for the Luyten Catalog. I. Bright StarsApr 15 2002We outline the construction of an updated version of the New Luyten Two-Tenths (NLTT) catalog of high proper motion stars, which will contain improved astrometry and photometry for the vast majority of the ~59,000 stars in NLTT. The bright end is constructed ... More
Photometric Microlens Parallaxes with SIMFeb 08 1999Astrometric measurements of microlensing events can in principle determine both the "parallax" \tilde r_E and the "proper motion" \mu of an individual event which (combined with the Einstein time scale t_E) in turn yield the mass, distance, and transverse ... More
Geometry of the HST Planetary Camera FieldJul 06 1993We present a solution for the relative positions and orientations of the four CCD chips on the Hubble Space Telescope (HST) Planetary Camera (PC). An accurate solution is required when matching HST images with ground-based images or with one another. ... More
A Spectroscopic Method to Measure Macho Proper MotionsJan 12 1994A Massive Compact Halo Object (Macho) that lenses a background star will magnify different parts of the rotating stellar disk by varying amounts. The differential magnification will cause a shift in the centroid of the star's spectral lines during the ... More
The RR Lyrae Distance ScaleAug 02 1998We review seven methods of measuring the absolute magnitude M_V of RR Lyrae stars in light of the Hipparcos mission and other recent developments. We focus on identifying possible systematic errors and rank the methods by relative immunity to such errors. ... More
A High-Resolution Spectrum of the Highly Magnified Bulge G-Dwarf MOA-2006-BLG-099SJan 14 2008We analyze a high-resolution spectrum of a microlensed G-dwarf in the Galactic bulge, acquired when the star was magnified by a factor of 110. We measure a spectroscopic temperature, derived from the wings of the Balmer lines, that is the same as the ... More
A New Photometric Model of the Galactic Bar using Red Clump GiantsMar 26 2013Jun 11 2013We present a study of the luminosity density distribution of the Galactic bar using number counts of red clump giants (RCGs) from the OGLE-III survey. The data were recently published by Nataf et al. (2013) for 9019 fields towards the bulge and have $2.94\times ... More
Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised CatalogJun 18 2002We complete construction of a catalog containing improved astrometry and new optical/infrared photometry for the vast majority of NLTT stars lying in the overlap of regions covered by POSS I and by the second incremental 2MASS release, approximately 44% ... More
Classifying Luyten Stars Using An Optical-Infrared Reduced Proper Motion DiagramMay 08 2002Jul 12 2002We present a V-J reduced proper motion (RPM) diagram for stars in the New Luyten Two-Tenths (NLTT) catalog. In sharp contrast to the RPM diagram based on the original NLTT data, this optical-infrared RPM diagram shows distinct tracks for white dwarfs, ... More
Finite Source Effects in Microlensing EventsJun 17 1996The computation of the magnification of a finite source by an arbitrary gravitational lens can be reduced from a two-dimensional to a one-dimensional integral using a generalization of Stoke's theorem. For a large source lensed by a planetary-system whose ... More
Cepheid Luminosity Versus Galaxy Rotation Speed: L ~ v^{0.7}Feb 23 1997The distance modulus of a galaxy determined from Cepheids differs from its distance modulus determined from Tully-Fisher by an amount that is proportional to the galaxy's line width, W: Delta mu ~ (-1.73\pm 0.46)log(W). While a miscalibration of the slope ... More
Mathematics of Statistical Parallax and the Local Distance ScaleMar 21 1997Jan 29 1998We present a mathematical analysis of the statistical parallax method. The method yields physical insight into the maximum-likelihood determinations of the luminosity and velocity distribution and enables us to conduct a vigorous Monte Carlo investigation ... More
Statistical Determination of the MACHO Mass SpectrumApr 22 1995Jan 16 1996The mass function of 51 Massive Compact Objects (MACHOs) detected toward the Galactic bulge is statistically estimated from Einstein ring crossing times $t_{\rm e}$. For a Gaussian mass function, the best fitting parameters are $\langle \log (m/M_{\odot}) ... More
Statistics Of Microlensing Optical DepthOct 18 1994We show that the statistical error, $\sigma_{\tau}$, in estimating the optical depth, $\tau$, due to microlensing is substantially higher than the naive Poisson estimate: $\sigma_{\tau} = \sqrt{\eta / N} \tau$, where $N$ is the number of detections and ... More
Chemical Composition of Faint (I~21 mag) Microlensed Bulge Dwarf OGLE-2007-BLG-514SOct 07 2009We present a high-resolution spectrum of a microlensed G dwarf in the Galactic bulge with spectroscopic temperature T_eff = 5600 +/- 180 K. This I~21 mag star was magnified by a factor ranging from 1160 to 1300 at the time of observation. Its high metallicity ... More
Origin of the Break and Mass-Dependence in the Wide-Binary Projected-Separation DistributionOct 26 2006Oct 27 2006The distribution of Galactic-disk wide binaries shows a clear break in slope at projected separations of about r_\perp ~ 2500 AU in two basically independent surveys by Chaname & Gould and Lepine & Bongiorno. The latter also showed that the frequency ... More
Complete Parallax and Proper Motion Solutions For Halo Binary-Lens Microlensing EventsSep 29 1998Oct 01 1998A major problem in the interpretation of microlensing events is that the only measured quantity, the Einstein time scale t_E, is a degenerate combination of the three quantities one would like to know, the mass, distance, and speed of the lens. This degeneracy ... More
Revised NLTTJul 17 2002Revised NLTT (New Luyten Catalogue of Stars With Proper Motions Larger than Two Tenths of an Arcsecond) contains improved astrometry and new optical/infrared photometry for the vast majority of NLTT stars lying in the overlap of regions covered by POSS ... More
Searching for Failed Supernovae With Astrometric BinariesNov 27 2001Stars in the mass range 8 Msun<M<30 Msun are thought to end their lives as luminous supernovae that leave behind a neutron star. However, if a substantial fraction of these stars instead ended as black-hole remnants, without producing a supernova (a `failed' ... More
Sgr A* ``Visual Binaries'': A Direct Measurement of the Galactocentric DistanceDec 16 1998Dec 16 1998We present a new geometrical method for measuring the distance to the Galactic center (R_0) by solving for the Keplerian orbit of individual stars bound to the black hole associated with the Sgr A* from radial velocity and proper motion measurements. ... More
Monte Carlo Simulations of Macho Parallaxes From a SatelliteJul 25 1995Three ongoing microlensing experiments have found more candidate events than expected from the known stars. These experiments measure only one parameter of the massive compact halo objects (machos), the magnification time scale of the events. More information ... More
New Hipparcos-based Parallaxes for 424 Dim StarsAug 29 2003We present a catalog of 424 common proper motion companions to Hipparcos stars with good (>3 sigma) parallaxes, thereby effectively providing new parallaxes for these companions. Compared to stars in the Hipparcos catalog, these stars are substantially ... More
Disk and Halo Wide Binaries from the Revised Luyten Catalog: Probes of Star Formation and MACHO Dark MatterJul 24 2003Sep 17 2003We present a catalog of 1147 candidate common proper motion binaries selected from the revised New Luyten Two-Tenths Catalog. Among these, we identify 999 genuine physical pairs using the measured proper-motion difference and the relative positions of ... More
Astrometric Resolution of Severely Degenerate Binary Microlensing EventsJan 11 2000We investigate whether the "close/wide" class of degeneracies in caustic-crossing binary microlensing events can be broken astrometrically. Dominik showed that these degeneracies are particularly severe because they arise from a degeneracy in the lens ... More
Einstein Radii from Binary Lensing EventsApr 05 1996Jun 26 1996We show that the Einstein ring radius and transverse speed of a lens projected on the source plane, $\hat{r}_{\rm e}$ and $\hat{v}$, can be determined from the light curve of a binary-source event, followed by the spectroscopic determination of the orbital ... More
Galactic versus Extragalactic Pixel Lensing Events toward M31Feb 19 1996Jun 26 1996A new type of gravitational microlensing experiment toward a field where stars are not resolved is being developed observationally and theoretically: pixel lensing. When the experiment is carried out toward the M31 bulge area, events may be produced both ... More
Finding Planets Around White Dwarf Remnants of Massive StarsNov 15 2007Dec 05 2007Planet frequency shows a strong positive correlation with host mass from the hydrogen-burning limit to M ~ 2Msun. No search has yet been conducted for planets of higher-mass hosts because all existing techniques are insensitive to these planets. We show ... More
Upper Limit to the Distance to the Large Magellanic CloudMay 07 1997May 19 1997We remeasure the ``light echo'' times to the near and far side of the ring around SN 1987A using the method of Gould (1995) and the new reductions of the original UV spectra by Sonneborn et al. (1996). Under the assumption that the ring is circular, we ... More
Nearby Microlensing Events - Identification of the Candidates for the SIMSep 28 1999Apr 03 2000The Space Interferometry Mission (SIM) is the instrument of choice when it comes to observing astrometric microlensing events where nearby, usually high-proper-motion stars (``lenses''), pass in front of more distant stars (``sources''). Each such encounter ... More
Empirical Calibration of Hubble Space Telescope WFPC2 F606W and F814W PhotometrySep 30 1998Using the ground-based V and I photometry of a sample of stars from the Groth Strip, we obtain the first empirical calibration of the F606W and F814W Hubble Space Telescope WFPC2 filters for 0.5<V-I<4.5. We present results in the form of corrections that ... More
Weak Lensing By Nearby StructuresFeb 22 1994Weak gravitational lensing due to nearby structures, such as the Coma cluster, and the Local Supercluster can be expected to polarize images of distant galaxies by ${\cal O}(0.2%\Omega)$ with coherence over scales of tens of square degrees. The Sloan ... More
Millisecond Pulsars as Probes of Mass Segregation in the Galactic CenterDec 19 2001Jan 31 2002We propose a simple test for the existence of a cluster of black hole remnants around Sgr A* that is based on a small sample of any type of Galactic Center objects, provided they are substantially less massive than the black holes and constitute part ... More