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Andrew Gould

Publications and source records attributed to Andrew Gould.

At least 253 records · Page 14Linked to original sources
arXiv2011

SDSS-III: Massive Spectroscopic Surveys of the Distant Universe, the Milky Way Galaxy, and Extra-Solar Planetary Systems

Building on the legacy of the Sloan Digital Sky Survey (SDSS-I and II), SDSS-III is a program of four spectroscopic surveys on three scientific themes: dark energy and cosmological parameters, the history and structure of the Milky Way, and the population of giant planets around other stars. In keeping with SDSS tradition, SDSS-III will provide regular public releases of all its data, beginning with SDSS DR8 (which occurred in Jan 2011). This paper presents an overview of the four SDSS-III surveys. BOSS will measure redshifts of 1.5 million massive galaxies and Lya forest spectra of 150,000 quasars, using the BAO feature of large scale structure to obtain percent-level determinations of the distance scale and Hubble expansion rate at z<0.7 and at z~2.5. SEGUE-2, which is now completed, measured medium-resolution (R=1800) optical spectra of 118,000 stars in a variety of target categories, probing chemical evolution, stellar kinematics and substructure, and the mass profile of the dark matter halo from the solar neighborhood to distances of 100 kpc. APOGEE will obtain high-resolution (R~30,000), high signal-to-noise (S/N>100 per resolution element), H-band (1.51-1.70 micron) spectra of 10^5 evolved, late-type stars, measuring separate abundances for ~15 elements per star and creating the first high-precision spectroscopic survey of all Galactic stellar populations (bulge, bar, disks, halo) with a uniform set of stellar tracers and spectral diagnostics. MARVELS will monitor radial velocities of more than 8000 FGK stars with the sensitivity and cadence (10-40 m/s, ~24 visits per star) needed to detect giant planets with periods up to two years, providing an unprecedented data set for understanding the formation and dynamical evolution of giant planet systems. (Abridged)

astro-ph.IM↗
arXiv2011

The Mass and Radius of the Neutron Star in the Bulge Low-Mass X-ray Binary KS 1731-260

Measurements of neutron star masses and radii are instrumental for determining the equation of state of their interiors, understanding the dividing line between neutron stars and black holes, and for obtaining accurate statistics of source populations in the Galaxy. We report here on the measurement of the mass and radius of the neutron star in the low-mass X-ray binary KS 1731-260. The analysis of the spectroscopic data on multiple thermonuclear bursts yields well-constrained values for the apparent angular area and the Eddington flux of the source, both of which depend in a distinct way on the mass and radius of the neutron star. The binary KS 1731-260 is in the direction of the Galactic bulge, allowing a distance estimate based on the density of stars in that direction. Making use of the Han & Gould model, we determine the probability distribution over the distance to the source, which is peaked at 8 kpc. Combining these measurements, we place a strong upper bound on the radius of the neutron star, R <= 12 km, while confining its mass to M <= 1.8 M_sun.

astro-ph.GA↗
arXiv2011

The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III

The Sloan Digital Sky Survey (SDSS) started a new phase in August 2008, with new instrumentation and new surveys focused on Galactic structure and chemical evolution, measurements of the baryon oscillation feature in the clustering of galaxies and the quasar Ly alpha forest, and a radial velocity search for planets around ~8000 stars. This paper describes the first data release of SDSS-III (and the eighth counting from the beginning of the SDSS). The release includes five-band imaging of roughly 5200 deg^2 in the Southern Galactic Cap, bringing the total footprint of the SDSS imaging to 14,555 deg^2, or over a third of the Celestial Sphere. All the imaging data have been reprocessed with an improved sky-subtraction algorithm and a final, self-consistent photometric recalibration and flat-field determination. This release also includes all data from the second phase of the Sloan Extension for Galactic Understanding and Evolution (SEGUE-2), consisting of spectroscopy of approximately 118,000 stars at both high and low Galactic latitudes. All the more than half a million stellar spectra obtained with the SDSS spectrograph have been reprocessed through an improved stellar parameters pipeline, which has better determination of metallicity for high metallicity stars.

astro-ph.IM↗
arXiv2011

Gravitational Pulse Astronomy

Thompson 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 pulses that are well approximated by parabolic encounters. I show that it is impossible to detect these with searches based on standard assumptions of circular binaries, nor could they be detected by analogs of the repeating-pulse searches that have been carried out at higher frequencies, even if these were modified to barycentric time as a function of putative sky position. Rather, new search algorithms are required that take account of the intrinsic 3-body motion of the source as well as the motion of the Earth. If these eccentric binaries account for even a modest fraction of the observed SN rate, then there should be of order 1 pulse every 20 seconds coming from within 1 kpc, and there should be of order 200 detectable sources in this same volume. I outline methods of identifying these sources both to remove this very pernicious background to other signals, and to find candidate SN Ia progenitors, and I sketch practical methods to find optical counterparts to these sources and so measure their masses and distances.

astro-ph.SR↗
arXiv2010

Anthropic Argument for Three Generations

The 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 for, and there are strong (though not air-tight) arguments that there are no more than three generations. The question is: why exactly N_gen=3? I argue that three generations is a natural prediction of the multiverse theory, provided one adds the additional, quite reasonable assumption that N_gen in a randomly realized universe is a steeply falling function of number. In this case N_gen > 2 to permit CP violation (and so baryogenesis and thus physicists) and N_gen < 4 to avoid highly improbable outcomes. I thereby make a testable anthropic-principle prediction: that when a theory of randomly realized N_gen is developed, the probability will turn out to be steeply falling in N_gen.

hep-ph↗
arXiv2010

Post-Decadal White Paper: A Dual-Satellite Dark-Energy/Microlensing NASA-ESA Mission

A 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 designs. The two satellites, one led by ESA and the other by NASA, would be explicitly designed to perform complementary functions of a single, dual-satellite dark-energy/microlensing ``mission''. One would be a purely optical wide-field camera, with large format and small pixels, optimized for weak-lensing, which because of its simple design, could be launched by ESA on relatively short timescales. The second would be a purely infrared satellite with marginally-sampled or under-sampled pixels, launched by NASA. Because of budget constraints, this would be launched several years later. The two would complement one another in 3 dark energy experiments (weak lensing, baryon oscillations, supernovae) and also in microlensing planet searches. Signed international agreements would guarantee the later NASA launch, and on this basis equal access of both US and European scientists to both data sets.

astro-ph.IM↗
arXiv2010

SEGUE-2 Limits on Metal-Rich Old-Population Hypervelocity Stars In the Galactic Halo

We present new limits on the ejection of metal-rich old-population hypervelocity stars from the Galactic center (GC) as probed by the SEGUE-2 survey. Our limits are a factor of 3-10 more stringent than previously reported, depending on stellar type. Compared to the known population of B-star ejectees, there can be no more than 30 times more metal-rich old-population F/G stars ejected from the GC. Because B stars comprise a tiny fraction of a normal stellar population, this places significant limits on a combination of the GC mass function and the ejection mechanism for hypervelocity stars. In the presence of a normal GC mass function, our results require an ejection mechanism that is about 5.5 times more efficient at ejecting B-stars compared to low-mass F/G stars.

astro-ph.GA↗
arXiv2009

A Puzzle Involving Galactic Bulge Microlensing Events

We study a sample of 16 microlensed Galactic bulge main sequence turnoff region stars for which high dispersion spectra have been obtained with detailed abundance analyses. We demonstrate that there is a very strong and highly statistically significant correlation between the maximum magnification of the microlensed bulge star and the value of the [Fe/H] deduced from the high resolution spectrum of each object. Physics demands that this correlation, assuming it to be real, be the result of some sample bias. We suggest several possible explanations, but are forced to reject them all,and are left puzzled. To obtain a reliable metallicity distribution in the Galactic bulge based on microlensed dwarf stars it will be necessary to resolve this issue through the course of additional observations.

astro-ph.GA↗
arXiv2009

The Effect of Differential Limb Magnification on Abundance Analysis of Microlensed Dwarf Stars

Finite 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 the same as when it is unmagnified, and measurements of the atmospheric parameters and abundances will be affected. The accuracy of abundances measured from spectra taken during microlensing events has become important recently because of the use of highly magnified dwarf stars to probe abundance ratios and the abundance distribution in the Galactic bulge. In this paper, we investigate the effect of finite source effects on spectra by using magnification profiles motivated by two events to synthesize spectra for dwarfs between 5000K to 6200K at solar metallicity. We adopt the usual techniques for analyzing the microlensed dwarfs, namely, spectroscopic determination of temperature, gravity, and microturbulent velocity, relying on equivalent widths. We find that ignoring the finite source effects for the more extreme case results in errors in Teff < 45K, in log g of <0.1 dex and in microturbulent velocity of <0.1 km/s. In total, changes in equivalent widths lead to small changes in atmospheric parameters and changes in abundances of <0.06 dex, with changes in [FeI/H] of <0.03 dex. For the case with a larger source-lens separation, the error in [FeI/H] is <0.01 dex. This latter case represents the maximum effect seen in events whose lightcurves are consistent with a point-source lens, which includes the majority of microlensed bulge dwarfs published so far.

astro-ph.GA↗
arXiv2009

A Second Method to Photometrically Align Multi-Site Microlensing Light Curves: Source Color in Planetary Event MOA-2007-BLG-192

At present, microlensing light curves from different telescopes and filters are photometrically aligned by fitting them to a common model. We present a second method based on photometry of common field stars. If two spectral responses are similar (or the color of the source is known) then this technique can resolve important ambiguities that frequently arise when predicting the future course of the event, and that occasionally persist even when the event is over. Or if the spectral responses are different, it can be used to derive the color of the source when that is unknown. We present the essential elements of this technique and apply it to the case of MOA-2007-BLG-192, an important planetary event for which the system may be a terrestrial planet orbiting a brown dwarf or very low mass star. The refined estimate of the source color that we derive here, V-I=2.36 +- 0.03, will aid in making the estimate of the lens mass more precise.

astro-ph.EP↗
arXiv2009

Chemical Composition of Faint (I~21 mag) Microlensed Bulge Dwarf OGLE-2007-BLG-514S

We 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 ([Fe/H] = 0.33 +/- 0.15) places this star at the upper end of the bulge giant metallicity distribution. Using a K-S test, we find a 1.6% probability that the published microlensed bulge dwarfs share an underlying distribution with bulge giants, properly accounting for a radial bulge metallicity gradient. We obtain abundance measurements for 15 elements and perform a rigorous error analysis that includes covariances between parameters. This star, like bulge giants with the same metallicity, shows no alpha enhancement. It confirms the chemical abundance trends observed in previously analyzed bulge dwarfs. At supersolar metallicities, we observe a discrepancy between bulge giant and bulge dwarf Na abundances.

astro-ph.SR↗
arXiv2009

Limits on Unresolved Planetary Companions to White Dwarf Remnants of 14 Intermediate-Mass Stars

We present Spitzer IRAC photometry of white dwarf remnants of 14 stars with M = 3-5 Msol. We do not detect mid-infrared excess around any of our targets. By demanding a 3 sigma photometric excess at 4.5 micron for unresolved companions, we rule out planetary mass companions down to 5, 7, or 10 M_J for 13 of our targets based on the Burrows et al. (2003) substellar cooling models. Combined with previous IRAC observations of white dwarf remnants of intermediate-mass stars, we rule out \geq 10 M_J companions around 40 white dwarfs and \geq 5 M_J companions around 10 white dwarfs.

astro-ph.SR↗
arXiv2009

Spectroscopic Confirmation of the Pisces Overdensity

We present spectroscopic confirmation of the "Pisces Overdensity", also known as "Structure J", a photometric overdensity of RR Lyrae stars discovered by the Sloan Digital Sky Survey (SDSS) at an estimated photometric distance of ~85kpc. We measure radial velocities for 8 RR Lyrae stars within Pisces. We find that 5 of the 8 stars have heliocentric radial velocities within a narrow range of -87 km/s < v < -67 km/s, suggesting that the photometric overdensity is mainly due to a physically associated system, probably a dwarf galaxy or a disrupted galaxy. Two of the remaining 3 stars differ from one another by only 9 km/s, but it would be premature to identify them as a second system.

astro-ph.GA↗
arXiv2009

Lyman-alpha Emission From Cosmic Structure I: Fluorescence

We present predictions for the fluorescent Lyman-alpha emission signature arising from photoionized, optically thick structures in Smoothed Particle Hydrodynamic (SPH) cosmological simulations of a Lambda-CDM universe using a Monte Carlo Lyman-alpha radiative transfer code. We calculate the expected Lyman-alpha image and 2-dimensional spectra for gas exposed to a uniform ultraviolet ionizing background as well as gas exposed additionally to the photoionizing radiation from a local quasar, after correcting for the self-shielding of hydrogen. As a test of our numerical methods and for application to current observations, we examine simplified analytic structures that are uniformly or anisotropically illuminated. We compare these results with recent observations. We discuss future observing campaigns on large telescopes and realistic strategies for detecting fluorescence owing to the ambient metagalactic ionization and in regions close to bright quasars. While it will take hundreds of hours on the current generation of telescopes to detect fluorescence caused by the ultraviolet background (UVB) alone, our calculations suggest that of order ten sources of quasar-induced fluorescent Lyman-alpha emission should be detectable after a 10 hour exposure in a 10 arcmin^2 field around a bright quasar. These observations will help probe the physical conditions in the densest regions of the intergalactic medium as well as the temporal light curves and isotropy of quasar radiation.

astro-ph.CO↗
arXiv2009

OGLE-2005-BLG-071Lb, the Most Massive M-Dwarf Planetary Companion?

We combine all available information to constrain the nature of OGLE-2005-BLG-071Lb, the second planet discovered by microlensing and the first in a high-magnification event. These include photometric and astrometric measurements from Hubble Space Telescope, as well as constraints from higher order effects extracted from the ground-based light curve, such as microlens parallax, planetary orbital motion and finite-source effects. Our primary analysis leads to the conclusion that the host of Jovian planet OGLE-2005-BLG-071Lb is an M dwarf in the foreground disk with mass M= 0.46 +/- 0.04 Msun, distance D_l = 3.3 +/- 0.4 kpc, and thick-disk kinematics v_LSR ~ 103 km/s. From the best-fit model, the planet has mass M_p = 3.8 +/- 0.4 M_Jup, lies at a projected separation r_perp = 3.6 +/- 0.2 AU from its host and so has an equilibrium temperature of T ~ 55 K, i.e., similar to Neptune. A degenerate model less favored by Δχ^2 = 2.1 (or 2.2, depending on the sign of the impact parameter) gives similar planetary mass M_p = 3.4 +/- 0.4 M_Jup with a smaller projected separation, r_\perp = 2.1 +/- 0.1 AU, and higher equilibrium temperature T ~ 71 K. These results from the primary analysis suggest that OGLE-2005-BLG-071Lb is likely to be the most massive planet yet discovered that is hosted by an M dwarf. However, the formation of such high-mass planetary companions in the outer regions of M-dwarf planetary systems is predicted to be unlikely within the core-accretion scenario. There are a number of caveats to this primary analysis, which assumes (based on real but limited evidence) that the unlensed light coincident with the source is actually due to the lens, that is, the planetary host. However, these caveats could mostly be resolved by a single astrometric measurement a few years after the event.

astro-ph↗
arXiv2009

Clues to the Metallicity Distribution in the Galactic Bulge: Abundances in MOA-2008-BLG310 and MOA-2008-BLG311

We present abundance analyses based on high dispersion and high signal-to-noise ratio Magellan spectra of two highly microlensed Galactic bulge stars in the region of the main sequence turnoff with Teff ~ 5650 K. We find that MOA-2008-BLG310 has [Fe/H]= +0.41 (+,- 0.09 dex) and MOA-2008-BLG311 has +0.26 (+, - 0.09 dex). The abundance ratios for the ~20 elements for which features could be detected in the spectra of each of the two stars follow the trends with [Fe/H] found among samples of bulge giants. Combining these two bulge dwarfs with the results from previous abundance analysis of four other Galactic bulge turnoff region stars, all highly magnified by microlensing, gives a mean [Fe/H] of +0.29dex. This implies that there there is an inconsistency between the Fe-metallicity distribution of the microlensed bulge dwarfs and that derived by the many previous estimates based on surveys of cool, luminous bulge giants, which have mean [Fe/H] ~ -0.1 dex. A number of possible mechanisms for producing this difference are discussed. If one ascribes this inconsistency to systematic errors in the abundance analyses, we provide statistical arguments suggesting that a substantial systematic error in the Fe-metallicity for one or both of the two cases, bulge dwarfs vs bulge giants, is required which is probably larger than can realistically be accommodated.

astro-ph.SR↗