Andrew Gould

We present metallicity estimates for seven open clusters based on spectrophotometric indices from moderate-resolution spectroscopy. Observations of field giants of known metallicity provide a correlation between the spectroscopic indices and the metallicity of open cluster giants. We use chi^2 analysis to fit the relation of spectrophotometric indices to metallicity in field giants. The resulting function allows an estimate of the target-cluster giants' metallicities with an error in the method of pm0.08 dex. We derive the following metallicities for the seven open clusters: NGC 1245, [m/H]=-0.14pm0.04; NGC 2099, [m/H]=+0.05pm0.05; NGC 2324, [m/H]=-0.06pm0.04; NGC 2539, [m/H]=-0.04pm0.03; NGC 2682 (M67), [m/H]=-0.05pm0.02; NGC 6705, [m/H]=+0.14pm0.08; NGC 6819, [m/H]=-0.07pm0.12. These metallicity estimates will be useful in planning future extra-solar planet transit searches since planets may form more readily in metal-rich environments.Comment: 38 pages, including 12 figures. Accepted for publication in A

We analyze 166 spheroid subdwarfs (6.5 < M_V < 14.5) found in 53 fields observed with the Wide Field Planetary Camera on the Hubble Space Telescope. The fields cover 221 arcmin² over a wide range of directions. The spheroid luminosity function (LF) is inconsistent at about the 3 σ level with the local spheroid LF of Dahn et al. even when the normalization of the latter is corrected to take account of the latest data on spheroid kinematics. The difference may reflect systematic errors in one of the two studies or features of the spheroid spatial distribution that are not included in the simplest models. The mass function, which shows no obvious structure, can be represented by a power law, dN/d ln M ∝ M^α, with α = 0.25 ± 0.32 over the mass range 0.71 M_☉ > M > 0.09 M_☉. The spheroid therefore does not contribute significantly to microlensing unless the mass function changes slope dramatically in the substellar range. The total local mass density of spheroid stars (including remnants and unseen binary companions) is ρ ~ 6.4 × 10^-5 M_☉ pc^-3, with an uncertainty of about 50%. The power-law indices α = 0.25 for the spheroid and α = 0.44 for the disk (both uncorrected for binaries) are similar to those of globular clusters of moderate-to-high metallicity.

We analyze 166 spheroid subdwarfs $(6.50.09 Msun. The spheroid therefore does not contribute significantly to microlensing unless the mass function changes slope dramatically in the substellar range. The total local mass density of spheroid stars (including remnants and unseen binary companions) is \rho ~ 6.4E-5 Msun/pc^3, with an uncertainty of about 50%. The power-law indices \alpha=0.25 for the spheroid and \alpha=0.44 for the disk (both uncorrected for binaries) are similar to those of globular clusters of moderate to high metallicity.

We show that it is possible to image the structure of a distant quasar on scales of 1 AU by constructing a telescope that uses a nearby dwarf star as its "primary lens" together with a satellite-borne "secondary." The image produced by the primary is magnified by ~10⁶ in one direction but is contracted by 0.5 in the other and therefore contains highly degenerate one-dimensional information about the two-dimensional source. We discuss various methods for extracting information about the second dimension including "femtolens interferometry" where one measures the interference between different parts of the one-dimensional image with each other. Assuming that the satellite could be dispatched to a position along a star-quasar line of sight at a distance r from the Sun, the nearest available dwarf-star primary is likely to be at ~30 pc (r/45 AU)^-2. The secondary should consist of a one-dimensional array of mirrors extending ~170 m to achieve 1 AU resolution or ~350 m to achieve 1 AU resolution.

The statistics of gravitationally lensed quasars with multiple images in the 0.1''-7'' range have been measured in various surveys. Little is known, however, about lensed-quasar statistics at larger image separations, which probe masses on the scale of galaxy clusters. We extend the results of the HST Snapshot Survey for Lensed Quasars to the 7''-50'' range for a sub-sample of 76 quasars that is free of known selection effects. Using a combination of multicolor photometry and spectroscopy, we show that none of the point sources in the entire field of view of the HST observations of these quasars are lensed images. Large-separation quasar lensing is therefore not common. We carry out a detailed calculation of the expected statistics of large-separation lensing for this quasar sample, incorporating realistic input for the mass profiles and mass function of galaxy clusters. We find that the observational null results are consistent with the expected effect of galaxy clusters, even if these have existed in their present form and number since z of about 2. The rarity of large-separation lensed quasars can rule out some extreme scenarios, e.g. that the mass-function of clusters has been severely underestimated, or that large mass concentrations that are not associated with galaxies (i.e. ``failed'' clusters) are common. The rareness of wide lensing also sets limits on the cosmological constant that are independent of limits derived from galaxy lensing. The lensing statistics of larger quasar samples can probe the structure, number, and evolution of clusters, as well as the geometry of space.Comment: LaTex, ApJ, submitte

During some gravitational lensing events, the lens transits the disk of the star. This event causes a shift in the apparent radial velocity of the star proportional to the star's rotation speed. The magnification of such an event is different from that expected for a point source. By measuring both effects, one can determine the rotation parameter v sin i. The method is especially useful for K-giant stars because these stars have turbulent velocities that are typically large compared with their rotation speed. By making a series of radial velocity measurements, one can typically determine v sin i to the same accuracy as the individual radial velocity measurements. There are approximately 10 microlensing transit events per year that would be suitable for these measurements.

We propose that the relative variability on short timescales of the multiple images of a lensed quasar, after removal of the time delay, may be caused by hot spots or other moving structures in the accretion disk crossing microlens caustics caused by stellar-mass objects in the lensing galaxy. Such variability has been reported in the two images of 0957+561. The short durations would be caused by the high rotation speed of the disk (v/c ~ 0.1), rather than by planetary mass objects in the slowly moving (v/c ~ 10^-3) lens. This interpretation could be confirmed by finding periodicity, or correlations of the spectral and flux variations caused by the Doppler effect in the disk. We also propose another signature of stationary accretion disks (with no intrinsic variability): the gradient of the magnification over the accretion disk should cause a relative color change between the images whose sign and amplitude are correlated with the time derivative of the flux difference between the images. Other color terms induced by the radial variation of disk colors are of second order in the magnification gradient. The methods proposed here can be used first to verify that accretion disks near supermassive black holes are the source of the continuum radiation from quasars, and then to study them.

Microlensing is one of the most promising methods of reconstructing the stellar mass function down to masses even below the hydrogen-burning limit. The fundamental limit to this technique is the presence of unresolved binaries, which can, in principle, significantly alter the inferred mass function. Here we quantify the fraction of binaries that can be detected using microlensing, considering specifically the mass ratio and separation of the binary. We find that almost all binary systems with separations greater than b ~ 0.4 of their combined Einstein ring radius are detectable assuming a detection threshold of 3%. For two M dwarfs, this corresponds to a limiting separation of 1 AU. Since very few observed M dwarfs have companions at separations 1 AU, we conclude that close binaries will probably not corrupt the measurements of the mass function. We find that the detectability depends only weakly on the mass ratio. For those events for which individual masses can be determined, we find that binaries can be detected down to b ~ 0.2.