arXiv subjects

Andrew Gould

Publications and source records attributed to Andrew Gould.

At least 343 records · Page 19Linked to original sources

arXiv1999

Can Heavy WIMPs Be Captured by the Earth?

If weakly interacting massive particles (WIMPs) in bound solar orbits are systematically driven into the Sun by solar-system resonances (as Farinella et al. have shown is the case for many Earth-crossing asteroids), then the capture of high-mass WIMPs by the Earth would be affected dramatically because high-mass WIMPs are captured primarily from bound orbits. WIMP capture would be eliminated for M_x>630 GeV and would be highly suppressed for M_x>~150 GeV. Annihilation of captured WIMPs and anti-WIMPs is expected to give rise to neutrinos coming from the Earth's center. The absence of such a neutrino signal has been used to place limits on WIMP parameters. At present, one does not know if typical WIMP orbits are in fact affected by these resonances. Until this question is investigated and resolved, one must (conservatively) assume that they are. Hence, limits on high-mass WIMP parameters are significantly weaker than previously believed.

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arXiv1999

Stellar Mass Function From SIM Astrometry/Photometry

By 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 SIM time. Among the numerous applications are 1) measurement of the mass functions (MFs) of the bulge and disk 2) measurement of the relative normalizations of the bulge and disk MFs (and so their relative contribution to the Galactic potential), 3) measurement of the number of bulge white dwarfs and neutron stars (and so the initial MF well above the present turnoff). SIM astrometric measurements are simultaneously photometric measurements. SIM astrometry determines the angular size of the Einstein ring on the sky, and comparison of SIM and ground-based photometry determines the size of the Einstein ring projected onto the observer plane. Only by combining both of these measurements is it possible to completely solve the microlensing events.

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arXiv1999

Measuring the Remnant Mass Function of the Galactic Bulge

I 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. Neutron stars and black holes can be identified individually, while white dwarfs are detected statistically from the sharp peak in their mass function near M~ 0.6Msun. This peak is expected to be more than twice as high as the `background' of main-sequence microlenses. I estimate that of order 20% of the ~400 bulge microlensing events detected to date are due to remnants, but show that these are completely unrecognizable from their time scale distribution (the only observable that `normal' microlensing observations produce). To resolve the white-dwarf peak, the SIM mass measurements must be accurate to ~5%, substantially better than is required to measure the mass function of the more smoothly distributed main sequence. Nevertheless, SIM could measure the masses of about 20 bulge remnants in 500 hours of observing time.

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arXiv1999

A New Kinematic Distance Estimator to the LMC

The 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 ϕ_ph of its photometric line of nodes. Statistical errors of 2% are feasible based on proper motions obtained with any of several proposed astrometry satellites, the first possibility being the Full-Sky Astrometric Mapping Explorer (FAME). The largest source of systematic error is likely to be in the determination of ϕ_ph. I suggest two independent methods to measure ϕ_ph, one based on counts of clump giants and the other on photometry of clump giants. I briefly discuss a variety of methods to test for other sources of systematic errors.

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arXiv1999

Selection of Nearby Microlensing Candidates for Observation by SIM

I 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 expression for the total observing time T_tot required to measure the masses of a fixed number of stars to a given precision. I find that T_tot ~ r_max^{-2}, where r_max is the maximum radius to which candidates are searched, or T_tot ~ μ_min^2, where μ_min is the minimum proper motion to which candidates are searched. I show that T_tot can be reduced by a factor 4 if source availability is extended from V_s=17 to V_s=19. Increasing r_max and V_s and decreasing μ_min all require a significantly more agressive approach to finding candidates. A search for candidates can begin by making use of the Luyton proper motion catalog together with the USNO-A2.0 all-sky astrometric catalog. However, a thorough search would require the all-sky USNO-B proper-motion catalog which is not yet available. The follow-up observations necessary to prepare for the mission will become more difficult the longer they are delayed because the candidate pairs are typically already within 1" and are getting closer.

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arXiv1999

A Precision test of Hipparcos systematics towards the Hyades

We propose and apply a test that can detect any systematic errors in the Hipparcos parallaxes towards the Hyades cluster at the level of 0.3 mas. We show that the statistical parallax method subsumes the classical moving cluster methods and provides more accurate and robust estimates of the distance and the first two moments of the velocity distribution of the Hyades cluster namely, its bulk space velocity and the velocity dispersion tensor. We predict the parallaxes of Hyades cluster members using the common cluster space velocity derived from the statistical parallax method and their individual Hipparcos proper motions. We show that the parallaxes determined in this manner (pi_pm) are consistent at the 1-sigma level with the parallaxes (pi_orb) of three Hyades spectroscopic binary systems with orbital solutions. We find that <(pi_pm - pi_orb)> = 0.52 +- 0.47 mas, where the error is dominated by the errors in the orbital parallaxes. A reduction in this error would allow a test of the systematic errors in the Hipparcos parallaxes at the 0.3 mas level. If the Hyades distance scale is fixed by Hipparcos parallaxes, then its bulk velocity in equatorial coordinates is (Vx,Vy,Vz) = (-5.70 +- 0.20, 45.62 +- 0.11, 5.65 +- 0.08)km/s, its velocity dispersion is 320 +- 39 m/s, and the distance modulus to the centroid of our sample of 43 cluster members is 3.34 +- 0.02 mag.

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arXiv1999

Correlated errors in Hipparcos parallaxes towards the Pleiades and the Hyades

We show that the errors in the Hipparcos parallaxes towards the Pleiades and the Hyades open clusters are spatially correlated over angular scales of 2 to 3 deg, with an amplitude of up to 2 mas. This correlation is stronger than expected based on the analysis of the Hipparcos catalog. We predict the parallaxes of individual cluster members, pi_pm, from their Hipparcos proper motions, assuming that all cluster members have the same space velocity. We compare pi_pm with their Hipparcos parallaxes, pi_Hip, and find that there are significant spatial correlations in pi_Hip. We derive a distance modulus to the Pleiades of 5.58 +- 0.18 mag using the radial-velocity gradient method. This value, agrees very well with the distance modulus of 5.60 +- 0.04 mag determined using the main-sequence fitting technique, compared with the value of 5.33 +- 0.06 inferred from the average of the Hipparcos parallaxes of the Pleiades members. We show that the difference between the main-sequence fitting distance and the Hipparcos parallax distance can arise from spatially correlated errors in the Hipparcos parallaxes of individual Pleiades members. Although the Hipparcos parallax errors towards the Hyades are spatially correlated in a manner similar to those of the Pleiades, the center of the Hyades is located on a node of this spatial structure. Therefore, the parallax errors cancel out when the average distance is estimated, leading to a mean Hyades distance modulus that agrees with the pre-Hipparcos value. We speculate that these spatial correlations are also responsible for the discrepant distances that are inferred using the mean Hipparcos parallaxes to some open clusters. Finally, we note that our conclusions are based on a purely geometric method and do not rely on any models of stellar isochrones.

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arXiv1999

A New Argument Against An Intervening Stellar Population Toward the LMC

Zaritsky & 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 of these events imply that if such an intervening population were composed of ordinary stars that gave rise to a significant fraction of the microlensing events, then it could not be associated with the LMC. I present two independent statistical arguments which together essentially rule out such a chance alignment of unassociated structures. On the other hand, if the intervening structure is associated with the LMC, I show that of order half the mass in this structure is in substellar objects, which would make it unlike any known stellar population.

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arXiv1999

Photometric Microlens Parallaxes with SIM

Astrometric measurements of microlensing events can in principle determine both the "parallax" \tilde r_E and the "proper motion" μof an individual event which (combined with the Einstein time scale t_E) in turn yield the mass, distance, and transverse velocity of the lens. We show, however, that the parallax measurements are generically several orders of magnitude less precise than the proper-motion measurements. Fortunately, astrometric measurements by the Space Interferometry Mission (SIM) are simultaneously **photometric** measurements, and since SIM will be in solar orbit, these allow SIM to be used as a classical (photometric) parallax satellite. We show that SIM photometric parallaxes are of comparable precision to its astrometric proper-motion measurements. For I=15 bulge stars, complete solutions with about 5% accuracy in mass, distance, and transverse velocity can be obtained from about 5 hours of observation, 100 to 10,000 times shorter than would be required for a purely astrometric solution of similar precision. Thus, it should be possible to directly measure the mass functions of both the bulge and the inner disk (including both dark and luminous objects) with only a few hundred hours of SIM observations.

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arXiv1998

Faint star counts with HST

We describe a program of star counts in the range 19 < I < 26 made with the WFPC cameras aboard the Hubble Space Telescope. Red (V-I > 1.0) stars at these magnitudes are primarily disk and spheroid M dwarfs. The stars are found both on dedicated images as part of the parallel program and by using appropriate archive data. We measure the faint end of the luminosity functions of the disk and spheroid (i.e. stellar halo). We measure the low mass end of the mass function and show that M dwarfs do not dominate the total disk or spheroid mass. We place strong I-band constraints on the amount of halo dark matter in the form of low mass stars (such as M dwarfs or cool white dwarfs). The disk and spheroid contribute only a minor amount of optical depth toward the Magellanic clouds.

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arXiv1998

Sgr A* ``Visual Binaries'': A Direct Measurement of the Galactocentric Distance

We 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. We identify three stars to which the method may be applied, and show that 1-5 % accuracy of R_0 can be expected after 15 years of observing, and 0.5-2 % after 30 years of observing, depending on what the orbital parameters of these three stars turn out to be. Combining the measurements of the three stars with favorable orbital parameters leads to even more precise values. In the example that we present, such combined solution yields 4 % accuracy already by the year 2002. All these estimates assume that annual position measurements will continue to be made with the 2 mas precision recently reported by Ghez et al. The precision of the distance measurement is relatively insensitive to the radial velocity errors, provided that the latter are less than 50 km/s. Besides potentially giving an estimate of R_0 that is better than any currently in use, the greatest advantage of this method is that it is free from systematic errors.

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arXiv1998

Complete Parallax and Proper Motion Solutions For Halo Binary-Lens Microlensing Events

A 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 can be partly broken by measuring either a "parallax" or a "proper motion" and completely broken by measuring both. Proper motions can easily be measured for caustic-crossing binary-lens events. Here we examine the possibility (first discussed by Hardy & Walker) that one could also measure a parallax for some of these events by comparing the light curves of the caustic crossing as seen from two observatories on Earth. We derive analytic expressions for the signal-to-noise ratio of the parallax measurement in terms of the characteristics of the source and the geometry of the event. For Galactic halo binary lenses seen toward the LMC, the light curve is delayed from one continent to another by a seemingly minuscule 15 seconds (compared to t_E ~ 40 days). However, this is sufficient to cause a difference in magnification of order 10%. To actually extract complete parallax information (as opposed to merely detecting the effect) requires observations from three non-collinear observatories. Parallaxes cannot be measured for binary lenses in the LMC but they can be measured for Galactic halo binary lenses seen toward M31. Robust measurements are possible for disk binary lenses seen toward the Galactic bulge, but are difficult for bulge binary lenses.

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arXiv1998

Empirical Calibration of Hubble Space Telescope WFPC2 F606W and F814W Photometry

Using 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 need to be applied to the two synthetic calibrations currently in use. Both calibrations are found to require corrections to zero points and color-terms.

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arXiv1998

Anomaluos RR Lyrae (V-I)_0 colors in Baade's Window

We compare (V-I)_0-(V-K)_0 color-color and (V-I)_0-log P period-color diagrams for Baade's Window and local RRab Lyrae stars. We find that for a fixed log P the Baade's Window RR Lyrae stars are ~0.17 magnitudes redder in (V-I)_0 than the local RR Lyrae stars. We also show that there is no such effect observed in (V-K)_0. We argue that an extinction misestimate towards Baade's Window is not a plausible explanation of the discrepancy. Unlike Baade's Window RR Lyrae stars, the local ones follow a black-body color-color relation and are well approximated by theoretical models. We test two parameters, metallicity and surface gravity, and find that their effects are too small to explain the (V-I)_0 discrepancy between the two groups of stars. We do not provide any explanation for the anomalous (V-I)_0 behavior of the Baade's Window RR Lyrae stars. We note that a similar effect for clump giant stars has been recently reported by Paczynski and we caution that RR Lyrae stars and clump giants, often used as standard candles, can be subject to the same type of systematics.

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arXiv1998

Galactic Structure From Infrared Surveys

By 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, is t_*=r_*/v_\perp, the time it takes a star moving at transverse speed v_\perp to cross its own radius r_*. This parameter cleanly separates white dwarfs, main sequence stars, and giants, and simultaneously separates disk from spheroid stars. The infrared photometry then yields photometric parallaxes for the individual stars, while the proper motions give kinematic information. Hence, it becomes possible to measure the physical and velocity distributions of the known Galactic components and to identify new ones. The analysis of such a huge data set poses a major technical challenge. I offer some initial ideas on how to meet this challenge.

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arXiv1998

The RR Lyrae Distance Scale

We 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. For the three most robust methods, statistical parallax, trigonometric parallax, and cluster kinematics, we find M_V (at [Fe/H] = -1.6) of 0.77 +/- 0.13, 0.71 +/- 0.15, 0.67 +/- 0.10. These methods cluster consistently around 0.71 +/- 0.07. We find that Baade-Wesselink and theoretical models both yield a broad range of possible values (0.45-0.70 and 0.45-0.65) due to systematic uncertainties in the temperature scale and input physics. Main-sequence fitting gives a much brighter M_V = 0.45 +/- 0.04 but this may be due to a difference in the metallicity scales of the cluster giants and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67 +/- 0.13 and is potentially very robust, but at present is too new to be fully tested for systematics. If the three most robust methods are combined with Walker's mean measurement for 6 LMC clusters, V_{0,LMC} = 18.98 +/- 0.03 at [Fe/H] = -1.9, then mu_{LMC} = 18.33 +/- 0.08.

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arXiv1998

Optimal Microlensing Observations

One 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 high surface-brightness fields, or in covering new, less populated outer fields? How would a factor 2 improvement in CCD sensitivity affect the detection efficiency? Would a series of major (factor 2--4) upgrades in telescope aperture, seeing, sky brightness, camera size, and detector efficiency increase the event rate by a huge factor, or only marginally? I develop a simplified framework to address these questions. With observational resources fixed at the level of the MACHO and EROS experiments, the biggest improvement (factor ~2) would come by reducing the time spent on the inner ~25 deg^2 and applying it to the outer ~100 deg^2. By combining this change with the characteristics of a good medium-size telescope (2.5 m mirror, 1" point spread function, thinned CCD chips, 1 deg^2 camera, and dark sky), it should be possible to increase the detection of LMC events to more than 100 per year (assuming current estimates of the optical depth apply to the entire LMC).

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arXiv1998

Microlens Parallaxes with SIRTF

The 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 to measure v-tilde, the transverse velocity of the lens projected onto the observer plane. Since v-tilde has very different values for different populations (disk, halo, Large Magellanic Cloud), such measurements could help identify the location, and hence the nature, of the lenses. I show that the method previously developed by Gould for measuring such satellite parallaxes fails completely in the case of SIRTF: it is overwhelmed by degeneracies which arise from fact that the Earth and satellite observations are in different band passes. I develop a new method which allows for observations in different band passes and yet removes all degeneracies. The method combines a purely ground-based measurement of the "parallax asymmetry" with a measurement of the delay between the time the event peaks at the Earth and satellite. In effect, the parallax asymmetry determines the component of v-tilde in the Earth-Sun direction, while the delay time measures the component of v-tilde in the direction of the Earth's orbit.

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