arXiv subjects

Andrew Gould

Publications and source records attributed to Andrew Gould.

At least 307 records · Page 17Linked to original sources

arXiv2003

The End of the MACHO Era: Limits on Halo Dark Matter from Stellar Halo Wide Binaries

We simulate the evolution of halo wide binaries in the presence of MAssive Compact Halo Objects (MACHOs) and compare our results to the sample of wide binaries of Chaname & Gould (2003). The observed distribution is well fit by a single power law for angular separation, 3.5" < theta < 900", whereas the simulated distributions show a break in the power law whose location depends on the MACHO mass and density. This allows us to place upper limits on MACHO density as a function of their assumed mass. At the 95% confidence level, we exclude MACHOs with mass M > 43Msun at the standard local halo density rho_H. This all but removes the last permitted window for a full MACHO halo for masses M > 10^{-7.5}Msun.

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arXiv2003

Disk and Halo Wide Binaries from the Revised Luyten Catalog: Probes of Star Formation and MACHO Dark Matter

We 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 each binary's components on a reduced proper-motion (RPM) diagram. The RPM positions also serve to classify them as either disk main-sequence (801), halo subdwarf (116), or pairs containing at least one white dwarf (82). The disk and halo samples are complete to separations of θ=500" and θ=900", which correspond to ~0.1 pc and ~1 pc, respectively. At wide separations, both distributions are well described by single power laws, dN/dθ~ θ^{-α}: α=1.67+-0.07 for the disk and α=1.55+-0.10 for the halo. The fact that these distributions have similar slopes (and similar normalizations as well) argues for similarity of the star-formation conditions of these two populations. The fact that the halo binaries obey a single power law out to ~1 pc permits strong constraints on halo dark-matter candidates. At somewhat closer separations (10"<θ<25"), the disk distribution shows a pronounced flattening, which is detected at very high statistical significance and is not due to any obvious systematic effect. We also present a list of 11 previously unknown halo stars with parallaxes that are recognized here as companions of Hipparcos stars.

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arXiv2003

New Hipparcos-based Parallaxes for 424 Dim Stars

We 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 dimmer. The catalog includes 20 WDs and an additional 29 stars with M_V>14, the great majority of the latter being M dwarfs.

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arXiv2003

M Dwarfs from Hubble Space Telescope Star Counts. V. The I-band Luminosity Function

We derive the disk I-band luminosity function from the Zheng et al. sample of \~1400 disk M dwarfs observed with the Hubble Space Telescope. We adopt a Galactic-height-dependent color-magnitude relation to account for the metallicity gradient above the Galactic plane. The resultant I-band luminosity function peaks at M_I~9.5 and drops sharply toward M_I~10.5.

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arXiv2003

Resolving the Microlens Mass Degeneracy for Earth-Mass Planets

Of all planet-finding techniques, microlensing is potentially the most sensitive to Earth-mass planets. However, microlensing lightcurves generically yield only the planet-star mass ratio: the mass itself is uncertain to a factor of a few. To determine the planet mass, one must measure both the ``microlens parallax'' and source-lens relative proper motion. Here we present a new method to measure microlens masses for terrestrial planets. We show that, with only a modest adjustment to the proposed orbit of the dedicated satellite that finds the events, and combined with observations from a ground-based observing program, the planet mass can be measured routinely. The dedicated satellite that finds the events will automatically measure the proper motion and one projection of the ``vector microlens parallax.'' If the satellite is placed in an L2 orbit, or a highly elliptical orbit around the Earth, the Earth-satellite baseline is sufficient to measure a second projection of the vector microlens parallax from the difference in the lightcurves as seen from the Earth and the satellite as the source passes over the caustic structure induced by the planet. This completes the mass measurement.

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arXiv2003

An Upper Limit on the Granularity of the Local Stellar Halo

I 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. Moreover, no stream can contain more than 2.3% of the local stars, while average streams must contain less than 0.08%.

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arXiv2003

Stellar Contribution to the Galactic Bulge Microlensing Optical Depth

We estimate the optical depth to self-lensing by stars in the Galactic bulge using the HST star counts of Holtzman et al and Zoccali et al as extrapolated by Gould into the brown-dwarf and remnant regimes and deprojected along the line of sight using the model of Dwek et al. We find a self-lensing optical depth tau(bulge-bulge)=0.98 x 10^{-6}. When combined with the lensing of bulge stars by foreground stars in the disk, this yields tau(bulge-total)=1.63 x 10^{-6}, in reasonable agreement with the estimates of tau=2.13 +/- 0.40 x 10^{-6} and tau=1.08 +/- 0.30 x 10^{-6} based on observations of clump giants by the MACHO and EROS collaborations.

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arXiv2003

Early-type Stars: Most Favorable Targets for Astrometrically Detectable Planets in the Habitable Zone

Early-type stars appear to be a difficult place to look for planets astrometrically. First, they are relatively heavy, and for fixed planetary mass the astrometric signal falls inversely as the stellar mass. Second, they are relatively rare (and so tend to be more distant), and for fixed orbital separation the astrometric signal falls inversely as the distance. Nevertheless, because early-type stars are relatively more luminous, their habitable zones are at larger semi-major axis. Since astrometric signal scales directly as orbital size, this gives early-type stars a strong advantage, which more than compensates for the other two factors. Using the Hipparcos catalog, we show that early-type stars constitute the majority of viable targets for astrometric searches for planets in the habitable zone. We contrast this characteristic to transit searches, which are primarily sensitive to habitable planets around late-type stars.

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arXiv2003

Sgr A* Companion S0-2: A Probe of Very High-Mass Star Formation

The star S0-2, which is orbiting Sgr A* with a 15-yr period, almost certainly did not form in situ. We propose that it was injected into this close orbit by the tidal disruption of a massive-star binary, whose primary was more massive than S0-2 and at least 60 Msun. From numerical integrations we find that 1-2% of incoming binaries with closest approach equal to 130 AU leave the secondary in an orbit with eccentricity within 0.01 of that of SO-2. If additional stars are found orbiting Sgr A* with relatively short periods, they could be used to probe the formation of massive stars in the Galactocentric region, even though the massive stars themselves have long since perished.

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arXiv2002

Completeness of USNO-B for High Proper-Motion Stars

I 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 ~20mas/yr) may actually have still larger errors than tabulated.

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arXiv2002

Sensitivity of Transit Searches to Habitable Planets

Photon-limited transit surveys in V band are in principle about 20 times more sensitive to planets of fixed size in the habitable zone around M stars than G stars. In I band the ratio is about 400. The advantages that the habitable zone lies closer and that the stars are smaller (together with the numerical superiority of M stars) more than compensate for the reduced signal due to the lower luminosity of the later-type stars. That is, M stars can yield reliable transit detections at much fainter apparent magnitudes than G stars. However, to achieve this greater sensitivity, the later-type stars must be monitored to these correspondingly fainter magnitudes, which can engender several practical problems. We show that with modest modifications, the Kepler mission could extend its effective sensitivity from its current M_V=6 to M_V=9. This would not capture the whole M dwarf peak, but would roughly triple its sensitivity to Earth-like planets in the habitable zone. However, to take advantage of the huge bump in the sensitivity function at M_V=12 would require major changes.

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arXiv2002

Transit Target Selection Using Reduced Proper Motions

In searches for planetary transits in the field, well over half of the survey stars are typically giants or other stars that are too large to permit straightforward detection of planets. For all-sky searches of bright V<~11 stars, the fraction is ~90%. We show that the great majority of these contaminants can be removed from the sample by analyzing their reduced proper motions (RPMs): giants have much lower RPMs than dwarfs of the same color. We use Hipparcos data to design a RPM selection function that eliminates most evolved stars, while rejecting only 9% of viable transit targets. Our method can be applied using existing or soon-to-be-released all-sky data to stars V<12.5 in the northern hemisphere and V<12 in the south. The method degrades at fainter magnitudes, but does so gracefully. For example, at V=14 it can still be used to eliminate giants redward of V-I~0.95, that is, the blue edge of the red giant clump.

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arXiv2002

A 1" Telescope: The Optimal Approach to Bright-Star Planetary Transits

Planetary transits of bright stars, V < 10, offer the best opportunity for detailed studies of extra-solar planets, such as are already being carried out for HD209458b. Since these stars are rare, they should be searched over the entire sky. In the limits of zero read-out time, zero sky noise, and perfect optics, the sensitivity of an all-sky survey is independent of telescope aperture: for fixed detector size and focal ratio, the greater light-gathering power of larger telescopes is exactly cancelled by their reduced field of view. Finite read-out times strongly favor smaller telescopes because exposures are longer so a smaller fraction of time is wasted on readout. However, if the aperture is too small, the sky noise in one pixel exceeds the stellar flux and the field of view becomes so large that optical distortions become unmanageable. We find that the optimal aperture is about 1". A one-year survey using such a 1" telescope could detect essentially all hot-jupiter transits of V < 10 stars observable from a given site.

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arXiv2002

Stellar Halo Parameters from 4588 Subdwarfs

Using 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 been possible. The stellar halo is not moving with respect to the Local Standard of Rest (LSR) in either the vertical or radial direction, up to uncertainties of 2 km/s. This indicates that either the LSR is on a circular orbit or the Sun happens to lie very close to an extremum of the LSR's elliptical orbit. Similarly, tentative detections of vertical proper motion of Sgr A* relative to the LSR are either incorrect or they reflect real physical motion of the central black hole relative to the Galactic potential. The correlation coefficients of the halo velocity ellipsoid, which would reflect any possible misalignment between its principal axes and the cardinal directions of the Galaxy, vanish to within 2%. The halo subdwarf luminosity function peaks at M_V ~ 10.5 with a full width half maximum of about 2.5 mag.

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arXiv2002

Revised NLTT

Revised 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 I and by the 2MASS release, approximately 44% of the sky. The epoch 2000 positions are typically accurate to 130 mas, the proper motions to 5.5 mas/yr, and the V-J colors to 0.25 mag. Relative proper motions of binary components are meaured to 3 mas/yr. The false identification rate is ~1% for 11 < V < 18 and substantially less at brighter magnitudes.

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arXiv2002

Classifying Luyten Stars Using An Optical-Infrared Reduced Proper Motion Diagram

We 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, subdwarfs, and main-sequence stars. It thereby permits the identification of white-dwarf and subdwarf candidates that have a high probability of being genuine.

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arXiv2002

Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog

We 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% of the sky. The epoch 2000 positions are typically accurate to 130 mas, the proper motions to 5.5 mas/yr, and the V-J colors to 0.25 mag. Relative proper motions of binary components are measured to 3 mas/yr. The false identification rate is ~1% for 11 < V < 18 and substantially less at brighter magnitudes. These improvements permit the construction of a reduced proper motion diagram that, for the first time, allows one to classify NLTT stars into main-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We in turn use this diagram to analyze the properties of both our catalog and the NLTT catalog on which it is based. In sharp contrast to popular belief, we find that NLTT incompleteness in the plane is almost completely concentrated in MS stars, and that SDs and WDs are detected almost uniformly over the sky DEC > -33 deg. Our catalog will therefore provide a powerful tool to probe these populations statistically, as well as to reliably identify individual SDs and WDs.

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arXiv2002

Improved Astrometry and Photometry for the Luyten Catalog. I. Bright Stars

We 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 by matching NLTT stars to Hipparcos, Tycho-2, and Starnet; the faint end by matching to USNO-A and 2MASS. In this first paper, we detail the bright-end matching procedure. We show that for the majority of stars in his catalog, Luyten measured positions accurate to 1" even though he recorded his results much more coarsely. However, there is a long tail of position errors, with one error as large as 11 deg. Proper-motion errors for the stars with small position errors are 24 mas/yr (1 sigma) but deteriorate to 34 mas/yr for stars with inferior positions. NLTT is virtually 100% complete for V<11.5 and |b|>15 deg, but completeness in this magnitude range falls to about 75% at the Galactic plane. Incompleteness near the plane is not uniform, but is rather concentrated in the interval -80<l<20, where the Milky Way is brightest.

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