arXiv subjects

Andrew Gould

Publications and source records attributed to Andrew Gould.

At least 361 records · Page 20Linked to original sources

arXiv1998

Systematics of RR Lyrae Statistical Parallax III: Apparent Magnitudes and Extinctions

We sing the praises of the central limit theorem. Having previously removed all other possible causes of significant systematic error in the statistical parallax determination of RR Lyrae absolute magnitudes, we investigate systematic errors from two final sources of input data: apparent magnitudes and extinctions. We find corrections due to each of ~0.05 mag, i.e., ~1/2 the statistical error. However, these are of opposite sign and so roughly cancel. The apparent magnitude system that we previously adopted from Layden et al. was calibrated to the photometry of Clube & Dawe. Using Hipparcos photometry we show that the Clube & Dawe system is ~0.06 mag too bright. Extinctions were previously pinned to the HI-based map of Burstein & Heiles. We argue that A_V should rather be based on new COBE/IRAS dust-emission map of Schlegel, Finkbeiner & Davis. This change increases the mean A_V by ~0.05 mag. We find M_V=0.77 +/- 0.13 at [Fe/H]=-1.60 for a pure sample of 147 halo RR Lyraes, or M_V=0.80 +/- 0.11 at [Fe/H]=-1.71 if we incorporate kinematic information from 716 non-kinematically selected non-RR Lyrae stars from Beers & Sommer-Larsen. These are 2 and 3 sigma fainter than recent determinations of M_V from main sequence fitting of clusters using Hipparcos measurements of subdwarfs by Reid and Gratton et al. Since statistical parallax is being cleared of systematic errors and since the chance of a >2 sigma statistical fluctuation is <1/20, we conclude that these brighter determinations may be in error. In the course of three papers, we have corrected 6 systematic errors whose absolute values total 0.20 mag. Had these, contrary to the expectation of the central limit theorem, all lined up one way, they could have resolved the conflict in favor of the brighter determinations. In fact, the net change was only 0.06 mag.

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arXiv1998

Isolating Red Giant Stars in M31's Elusive Outer Spheroid

Deep UBRI images of a 15'x15' field in the M31 galaxy's outer spheroid obtained with the KPNO 4-m telescope, and Keck 10-m telescope+LRIS I-band images, are used to isolate candidate red giant branch stars located at R=19 kpc (projected) along the minor axis. These stars are distinguished from the more numerous distant field galaxies by isolating objects whose colors are consistent with the long but narrow locus occupied by RGB stars in UBRI color space and whose angular sizes are consistent with the stellar PSF (FWHM~0.6"-1.5"). We carry out the same analysis of data on a comparison field with a similar Galactic latitude to the M31 halo field. The color-magnitude diagram of comparison field objects is well described by a superposition of foreground Galactic dwarf stars against a backdrop of faint blue field galaxies, while the M31 halo field contains a clear excess of faint red objects (I~20-23, B-I~2-3.5) in addition to these two components. The CMD location of this population of faint red objects is as would be expected for RGB stars at the distance of M31. The surface density of RGB candidates in the R=19 kpc M31 halo field is consistent with the findings of two recent HST studies. The data indicate that M31's stellar halo is much denser/larger than that of the Galaxy: (ρ_M31/ρ_MW) (Λ/1.5)^(-ν) ~ 10, where Λis the ratio of M31's and the Galaxy's radial scale lengths and ν=-3.8 is the assumed power law index of the density profile. The color and slope of the RGB in M31's outer halo are suggestive of a relatively metal-rich population, [Fe/H] >~ -1, in agreement with the HST measurements.

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arXiv1998

No Death Star -- For Now

A star passing within $\sim 10^4 \au$ of the Sun would trigger a comet shower that would reach the inner solar system about 0.18 Myr later. We calculate a prior probability of ~0.4% that a star has passed this close to the Sun but that the comet shower has not yet reached the Earth. We search the HIPPARCOS catalog for such recent close-encounter candidates and, in agreement with Garcia-Sanchez et al. (1997), find none. The new result reported in this Letter is an estimation of the completeness of the search. Because of the relatively bright completeness limit of the catalog itself, V~8, the search is sensitive to only about half the stars that could have had such a near encounter. On the other hand, we show that the search is sensitive to nearly all of the past encounters that would lead to a major shower in the future and conclude that it is highly unlikely that one will occur during the next 0.5 Myr.

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arXiv1998

The Spheroid Luminosity and Mass Functions From HST Star Counts

We analyze 166 spheroid subdwarfs $(6.5 M >0.09 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 ρ~ 6.4E-5 Msun/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.

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arXiv1998

Spectrophotometric Resolution of Stellar Atmospheres with Microlensing

Microlensing is a powerful tool for studying stellar atmospheres because as the source crosses regions of formally infinite magnification (caustics) the surfaceof the star is resolved, thereby allowing one to measure the radial intensity profile, both photometrically and spectroscopically. However, caustic crossing events are relatively rare, and monitoring them requires intensive application of telescope resources. It is therefore essential that the observational parameters needed to accurately measure the intensity profile are quantified. We calculate the expected errors in the recovered radial intensity profile as a function of the unlensed flux, source radius, spatial resolution the recovered intensity profile, and caustic crossing time for the two principle types of caustics: point-mass and binary lenses. We demonstrate that for both cases there exist simple scaling relations between these parameters and the resultant errors. We find that the error as a function of the spatial resolution of the recovered profile, parameterized by the number of radial bins, increases as $N_R^{3/2}$, considerably faster than the naive $N_R^{1/2}$ expectation. Finally, we discuss the relative advantages of binary caustic-crossing events and point-lens events. Binary events are more common, easier to plan for, and provide more homogeneous information about the stellar atmosphere. However, a sub-class of point-mass events with low impact parameters can provide dramatically more information provided that they can be recognized in time to initiate observations.

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arXiv1998

The Hipparcos Proper Motions in Support of the Short RR Lyrae Distance Scale

In this paper we investigate whether a misestimate of proper motions could have been a source of substantial systematic errors in the statistical parallax determination of the absolute magnitude of RR Lyrae stars. In an earlier paper, we showed that the statistical parallax method is extremely robust and rather insensitive to various systematic effects. The main potential problem with this method would therefore arise from systematically bad observational inputs, primarily radial velocities and proper motions. In that paper, we demonstrated that the radial velocities have not been systematically misestimated. Here we turn our attention to proper motions. We compare three different catalogs of proper motions --- Lick, Hipparcos and the one compiled by Wan et al. (WMJ). We find that the WMJ catalog is too heterogeneous to be a reliable source. We analyze the sample of 165 halo RR Lyrae stars with either Lick or Hipparcos proper motions. For the stars with both Lick and Hipparcos proper motions we use the weighted means of reported values. Various possible biases are investigated through vigorous Monte Carlo simulations and we evaluate small corrections due to Malmquist bias, anisotropic positions of the stars on the sky, and non-Gaussian distribution of stellar velocities. The mean RR Lyrae absolute magnitude is M_V = 0.74 +/- 0.12 at the mean metallicity of the sample <[Fe/H]> = -1.60, only 0.01 mag brighter than the value obtained in the previous study which did not incorporate Hipparcos proper motions. The faint absolute magnitudes of RR Lyrae stars confirmed by this analysis gives strong support to the short distance scale.

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arXiv1998

Microlens Parallax Asymmetries Toward the LMC

If 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, if the lenses are in the LMC, the parallax effects should be negligible. A ground-based search for such parallax asymmetries would therefore clarify the location of the lenses. A modest effort (2 hours per night on a 1 m telescope) could measure 15 parallax asymmetries over 5 years and so marginally discriminate between the halo and the LMC as the source of the lenses. A dedicated 1 m telescope would approximately double the number of measurements and would therefore clearly distinguish between the alternatives. However, compared to satellite parallaxes, the information extracted from ground-based parallaxes is substantially less useful for understanding the nature of the halo lenses (if that is what they are). The backgrounds of asymmetries due to binary-source and binary-lens events are estimated to be approximately 7% and 12% respectively. These complicate the interpretation of detected parallax asymmetries, but not critically.

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arXiv1998

Mathematics of Statistical Parallax and the Local Distance Scale

We 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 into various systematic effects. We apply our analytic formalism to the RR Lyrae sample of Layden et al. The velocity distribution of RR Lyrae stars is highly non-Gaussian, with kurtoses K_π= 2.04, K_θ= 3.22 and K_z = 4.28 in the three principal directions, but this has almost no effect on either the best fit or the uncertainty of the luminosity determination. Indeed, our principal result is that the statistical parallax method is extremely robust in the face of all systematic effects that we considered. Our analysis, applied to the Layden et al. RR Lyrae sample, strictly confirms the majority of their results. The mean RR Lyrae absolute magnitude is M_V = 0.75 +/- 0.13 at the mean metallicity of the sample <[Fe/H]> = -1.61, compared to M_V = 0.71 +/- 0.12 obtained by Layden et al. Most of the difference is due to Malmquist bias which was not considered in previous studies. We also analyze a semi-independent non-kinematically selected sample of stars with metallicities at the [Fe/H] <= 1.5 taken from Layden et al. and Beers & Sommer-Larsen and obtain M_V = 0.79 +/- 0.12 at <[Fe/H]> = -1.79. Additionally, this analysis yields measurements of the radial bulk motion (4 +/- 10 km/s) and vertical bulk motion (0 +/- 6 km/s) of the halo relative to the Local Standard of Rest.

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arXiv1997

The Proper Motion of NGC 6522 in Baade's Window

We have detected seven stars with a common proper motion which are located within 2.5 arcminutes of the globular cluster NGC 6522 in the Baade's Window field of the Galactic bulge. We argue that these stars are members of the cluster, and derive a weighted mean proper motion and heliocentric radial velocity of mu_l = 1.4 +/- 0.2 mas/yr, mu_b = -6.2 +/- 0.2 mas/yr, v = -28.5 +/- 6.5 km/s. We rederive the distance to NGC 6522 (0.91 +/- 0.04 R_0, where R_0 is the Galactocentric distance) and metallicity ([Fe/H] = -1.28 +/- 0.12), making use of recent revisions in the foreground extinction toward the cluster (A_V = 1.42 +/- 0.05). We find the spatial velocity of the cluster and conclude that the cluster stays close to the Galactic center, and may have experienced significant bulge/disk shocking during its lifetime.

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arXiv1997

LMC Microlenses: Dark or Luminous?

Zhao 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 which, they believe, could account for most of the observed microlensing optical depth. Here I show that a large-area surface-brightness map made by de Vaucouleurs constrains any such structure to one of four possibilities. Either 1) it does not account for a significant fraction of the observed microlensing, 2) it covers the inner ~3 degrees of the LMC but does not extend beyond ~5 degrees from the LMC center, 3) it is smooth on scales of ~15 degrees in both transverse directions or 4) it has a stellar mass-to-light ratio which exceeds by a factor >10 that of known stellar populations. The second and third possibilities would not be expected to apply to tidal debris. The last merely rephrases the dark-matter problem in a new form.

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arXiv1997

Pixel Lensing Search For Bright Microlensing Events and Variables in the Galactic Bulge

We describe a new method to search for gravitational microlensing toward the Galactic bulge that employs a small camera rather than a conventional telescope and probes new regions of parameter space. The small aperture (~65 mm) permits detection of stellar flux variations corresponding to magnitudes 7 15. The large pixel size (~10") and ~(6 deg)^2 field of view allows observation of the entire bulge with a few pointings. With this large pixel size (and with the even larger 30" PSF that we advocate) most bulge stars are unresolved, so one is in the regime of pixel lensing: microlensing and other forms of stellar variation are detected from the difference of pixel counts in successive images. We identify three principal uses of such a search. First, the observations are analogous to normal pixel lensing observations of the bulge of M31, but are carried out under conditions where the detected events can be followed up in detail. This permits crucial checks on the systematics of the M31 searches. Second, the search gives a complete inventory of bright bulge variables. Third, "extreme microlensing events" (EMEs) can be found in real time. EMEs are events with maximum magnifications A_max~200 which, if they were observed intensively from two observatories, could yield the mass, distance, and speed of the gravitational lens. The instrumentation required to carry out the observations is inexpensive. The observations could be made in parallel with existing microlensing searches and/or follow-up observations. The data reduction is much simpler than in ordinary pixel lensing because the PSF can be fixed by the optics and so does not vary with atmospheric conditions.

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arXiv1997

Non-Acceleration of Sgr A*: Implications for Galactic Structure

We show that observations by Backer and collaborators over the past two decades constrain the time derivative of the proper motion of Sgr A* to be less than 0.14 mas yr^{-2}. Using this result and a preliminary measurement by Eckart and Genzel of sigma ~500 kms for the velocity dispersion of the star cluster within 0.2" of Sgr A*, we derive the following implications. First, if the nuclear star cluster is dominated by a massive black hole, then either Sgr A* is that black hole or it orbits the black hole with a radius less than 3 AU. Second, even if the star cluster does not contain a massive black hole, Sgr A* is constrained to move slower than 20 kms (1 sigma) relative to the center of mass of the cluster. The Galactocentric distance is therefore R_0=7.5 +/- 0.7 kpc, independent of the nature of Sgr A*. These error bars could be substantially reduced by future observations. If they are, it will also be possible to probe the motion of the nuclear star cluster relative to the center of mass of the Galaxy at the ~4 kms level.

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arXiv1997

Upper Limit to the Distance to the Large Magellanic Cloud

We 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 obtain an upper limit to the distance to the Large Magellanic Cloud μ_lmc < 18.37 +/- 0.04. This result is very close to Gould's original value, and contradicts claims that the new spectra measurements lead to a significant increase in the distance estimate and a substantial increase in the statistical errors. We present new evidence (partly provided by A.P.S. Crotts) that the ring is intrinsically elliptical, with axis ratio b/a ~ 0.95. For an elliptical ring, μ_lmc < 18.44 +/- 0.05. We elaborate on Gould's original argument that the upper limit is robust, i.e., that it is only weakly sensitive to unmodeled aspects of the ring such as its hydrodynamic properties and ionization history.

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arXiv1997

The Zero Point of Extinction Toward Baade's Window

We measure the zero point of the Stanek (1996) extinction map by comparing the observed (V-K) colors of 206 K giant stars with their intrinsic (V-K)_0 colors as derived from their Hβindices. We find that the zero point of the Stanek map should be changed by ΔA_V = -0.10 +/- 0.06 mag, obtaining as a bonus a three-fold reduction of the previous statistical error. The most direct way to test for systematic errors in this determination would be to conduct a parallel measurement based on the (V-K) colors of RR Lyraes (type ab).

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arXiv1997

Microlensing: Current Results and Future Prospects

The 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 with stars that should have been seen. Toward the bulge, the total mass is consistent with that given by dynamical estimates of the bulge mass, but the observed timescales seem to require that a large fraction of the bulge is made of brown dwarfs. I discuss possible experiments to resolve these puzzles. I also discuss several applications of microlensing to non-dark-matter problems including searching for planets and imaging the black hole at the center of a quasar with 10^{-10} arcsec resolution.

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arXiv1997

Cepheid Luminosity Versus Galaxy Rotation Speed: L ~ v^{0.7}

The 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 of Tully-Fisher could in principle produce this effect, we argue that such a miscalibration is very unlikely. The other possible explanation is that the inferred Cepheid luminosity is correlated with the rotation speed v (and hence W) of its parent galaxy: L ~ v^{0.7}. This proportionality is superposed on the well-established relation between period and luminosity.) Such a dependence would be expected if Cepheid luminosity is correlated with metallicity, since galaxies with deeper potential wells tend to retain more of their metals. It would induce a discrepancy of 0.83 mag between H_0 determinations that are calibrated using M31 (log W=2.71) and the H_0 determination from SNIa, which is calibrated in galaxies with mean line width =2.23. A discrepancy of 0.83 mag corresponds to the difference between 80 km/s/Mpc and 55 km/s/Mpc, very similar to the actual discrepancy reported in the current literature.

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arXiv1996

A survey for large image-separation lensed quasars

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.

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arXiv1996

Signatures of Accretion Disks in Quasar Microlensing

We propose that relative variability on short time-scales 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 due to the high rotation speed of the disk (v/c ~ 0.1), rather than planetary mass objects in the slowly moving (v/c ~ 0.001) lens. This interpretation could be confirmed by finding periodicity, or correlations of the spectral and flux variations due to 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.

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