arXiv subjects

B. Scott Gaudi

Publications and source records attributed to B. Scott Gaudi.

At least 217 records · Page 12Linked to original sources

arXiv2006

Frequency of Hot Jupiters and Very Hot Jupiters from the OGLE-III Transit Surveys Toward the Galactic Bulge and Carina

We derive the frequencies of hot Jupiters (HJs) with 3--5 day periods and very hot Jupiters (VHJs) with 1-3 day periods by comparing the planets actually detected in the OGLE-III survey with those predicted by our models. The models are constructed following Gould & Morgan (2003) by populating the line of sight with stars drawn from the Hipparcos catalog. Using these, we demonstrate that the number of stars with sensitivity to HJs and VHJs is only 4--16% of those in the OGLE-III fields satisfying the spectroscopic-followup limit of V_max<17.5. Hence, the frequencies we derive are much higher than a naive estimate would indicate. We find that at 90% confidence the fraction of stars with planets in the two period ranges is (1/310)(1^{+1.39}_{-0.59}) for HJs and (1/690)(1^{+1.10}_{-0.54}) for VHJs. The HJ rate is statistically indistinguishable from that found in radial velocity (RV) studies. However, we note that magnitude-limited RV samples are heavily biased toward metal-rich (hence, planet-bearing) stars, while transit surveys are not, and therefore we expect that more sensitive transit surveys should find a deficit of HJs as compared to RV surveys. The detection of 3 transiting VHJs, all with periods less than 2 days, is marginally consistent with the complete absence of such detections in RV surveys. The planets detected are consistent with being uniformly distributed between 1.00 and 1.25 Jovian radii, but there are too few in the sample to map this distribution in detail.

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arXiv2005

Survey for Transiting Extrasolar Planets in Stellar Systems: III. A Limit on the Fraction of Stars with Planets in the Open Cluster NGC 1245

We analyze a 19-night photometric search for transiting extrasolar planets in the open cluster NGC 1245. An automated transit search algorithm with quantitative selection criteria finds six transit candidates; none are bona fide planetary transits. We characterize the survey detection probability via Monte Carlo injection and recovery of realistic limb-darkened transits. We use this to derive upper limits on the fraction of cluster members with close-in Jupiter-radii, RJ, companions. We carefully analyze the random and systematic errors of the calculation. For similar photometric noise and weather properties as this survey, observing NGC 1245 twice as long results in a tighter constraint on "Hot Jupiter", HJ, companions than observing an additional cluster of similar richness as NGC 1245 for the same length of time as this survey. This survey observed ~870 cluster members. If 1% of stars have 1.5 RJ HJ companions, we expect to detect one planet for every 5000 dwarf stars observed for a month. To reach a ~2% upper limit on the fraction of stars with 1.5 RJ HJ companions, we conclude a total sample size of ~7400 dwarf stars observed for at least a month will be needed. Results for 1.0 RJ companions, without substantial improvement in the photometric precision, will require a small factor larger sample size.

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arXiv2005

Astrometric Microlensing Constraints on a Massive Body in the Outer Solar System with Gaia

A body in Solar orbit beyond the Kuiper belt exhibits an annual parallax that exceeds its apparent proper motion by up to many orders of magnitude. Apparent motion of this body along the parallactic ellipse will deflect the angular position of background stars due to astrometric microlensing ("induced parallax"). By synoptically sampling the astrometric position of background stars over the entire sky, constraints on the existence (and basic properties) of a massive nearby body may be inferred. With a simple simulation, we estimate the signal-to-noise for detecting such a body -- as function of mass, heliocentric distance, and ecliptic latitude -- using the anticipated sensitivity and temporal cadences from Gaia (launch 2011). A Jupiter-mass (M_Jup) object at 2000 AU is detectable by Gaia over the whole sky above 5-sigma, with even stronger constraints if it lies near the ecliptic plane. Hypotheses for the mass (~3M_Jup), distance (~20,000 AU) and location of the proposed perturber ("Planet X") which gives rise to long-period comets may be testable.

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arXiv2005

Identifying Lenses with Small-Scale Structure. II. Fold Lenses

When the source in a 4-image gravitational lens lies close to a "fold" caustic, two of the lensed images lie close together. If the lens potential is smooth on the scale of the separation between the two close images, then the difference between their fluxes should approximately vanish, Rfold = (F1-F2)/(F1+F2) \approx 0. Violations of this "fold relation" in observed lenses are thought to indicate the presence of structure on scales smaller than the separation between the close images. We study the fold relation and find it to be more subtle and rich than was previously realized. The degree to which Rfold can differ from zero for realistic smooth lenses depends not only on the distance of the source from the caustic, but also on its location along the caustic, and on the angular structure of the lens potential. Thus, it is impossible to say from Rfold alone whether observed flux ratios are anomalous or not. Instead, we must consider the full distribution of Rfold values that can be obtained from smooth potentials that reproduce the separation between the two close images and the distance to the next nearest image. We analyze the generic and specific features of this distribution, and then show that 5 of the 12 known lenses with fold configurations have flux ratio anomalies: B0712+472, SDSS 0924+0219, PG 1115+080, B1555+375, and B1933+503. Combining this with our previous analysis revealing anomalies in 3 of the 4 known lenses with cusp configurations, we conclude that at least half (8/16) of all 4-image lenses that admit generic, local analyses exhibit flux ratio anomalies indicative of small-scale structure. The fold and cusp relations do not reveal the nature of the implied small-scale structure, but do provide the formal foundation for substructure studies. [Abridged]

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arXiv2005

On the Rotation Period of (90377) Sedna

We present precise, ~1%, r-band relative photometry of the unusual solar system object (90377) Sedna. Our data consist of 143 data points taken over eight nights in October 2004 and January 2005. The RMS variability over the longest contiguous stretch of five nights of data spanning nine days is only 1.3%. This subset of data alone constrain the amplitude of any long-period variations with period P to be A<1% (P/20 days)^2. Over the course of any given 5-hour segment, the data exhibits significant linear trends not seen in a comparison star of similar magnitude, and in a few cases these segments show clear evidence for curvature at the level of a few millimagnitudes per hour^2. These properties imply that the rotation period of Sedna is O(10 hours), cannot be < 5 hours, and cannot be >10 days, unless the intrinsic light curve has significant and comparable power on multiple timescales, which is unlikely. A sinusoidal fit yields a period of P=(10.273 +/- 0.002) hours and semi-amplitude of A=(1.1 +/- 0.1)%. There are additional acceptable fits with flanking periods separated by ~3 minutes, as well as another class of fits with P ~ 18 hours, although these later fits appear less viable based on visual inspection. Our results indicate that the period of Sedna is likely consistent with typical rotation periods of solar system objects, thus obviating the need for a massive companion to slow its rotation.

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arXiv2005

The Hypervelocity Star SDSS J090745.0+024507 is a Short-Period Variable

We present high-precision photometry of the hypervelocity star SDSS J090745.0+024507 (HVS), which has a Galactic rest-frame radial velocity of v=709 km/s, and so has likely been ejected from the supermassive black hole in the Galactic center. Our data were obtained on two nights using the MMT 6.5m telescope, and is supplemented by lower precision photometry obtained on four nights using the FLWO 1.2m telescope. The high-precision photometry indicates that the HVS is a short-period, low-amplitude variable, with period P=0.2-2 days and amplitude A = 2-10%. Together with the known effective temperature of T_eff ~ 10,500 K (spectral type B9), this variability implies that the HVS is a member of the class of slowly pulsating B-type main sequence stars, thus resolving the previously-reported two-fold degeneracy in the luminosity and distance of the star. The HVS has a heliocentric distance of 71 kpc, and an age of ~0.35 Gyr. The time of ejection from the center of the Galaxy is < 100 Myr, and thus the existence of the OS constitutes observational evidence of a population of young stars in the proximity of the central supermassive black hole ~0.1 Gyr ago. It is possible that the HVS was a member of a binary that was tidally disrupted by the central black hole; we discuss constraints on the properties of the companion's orbit.

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arXiv2005

On the Size Distribution of Close-In Extrasolar Giant Planets

The precisions of extrasolar planet radius measurements are reaching the point at which meaningful and discriminatory comparisons with theoretical predictions are can be made. However, care must be taken to account for selection effects in the transit surveys that detect the transiting planets for which radius measurements are possible. Here I identify one such selection effect, such that the number of planets with radius R_p detected in a signal-to-noise ratio limited transit survey is proportional to R_p^alpha, with alpha~4-6. In the presence of a dispersion sigma in the intrinsic distribution of planet radii, this selection effect translates to bias b in the radii of observed planets. Detected planets are, on average, larger by a fractional amount b ~ alpha (sigma/ )^2 relative to the mean radius of the underlying distribution. I argue that the intrinsic dispersion in planetary radii is likely to be in the range sigma = (0.05-0.13)R_J, where the lower bound is that expected theoretically solely from the variance in stellar insolation, and the upper bound is the 95% c.l. upper limit from the scatter in observed radii. Assuming an arbitrary but plausible value of sigma/ ~10%, and thus b~6%, I infer a mean intrinsic radius of close-in massive extrasolar planets of =(1.03+/-0.01)R_J. This value reinforces the case for HD209458b having an anomalously large radius, and may be inconsistent with coreless models of irradiated giant planets.

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arXiv2004

On the Period Distribution of Close-In Extrasolar Giant Planets

Transit (TR) surveys for extrasolar planets have recently uncovered a population of ``very hot Jupiters,'' planets with orbital periods of P< 3 d. At first sight this may seem surprising, given that radial velocity (RV) surveys have found a dearth of such planets, despite the fact that their sensitivity increases with decreasing P. We examine the confrontation between RV and TR survey results, paying particular attention to selection biases that favor short-period planets in transit surveys. We demonstrate that, when such biases and small-number statistics are properly taken into account, the period distribution of planets found by RV and TR surveys are consistent at better than the 1-sigma level. This consistency holds for a large range of reasonable assumptions. In other words, there are not enough planets detected to robustly conclude that the RV and TR short-period planet results are inconsistent. Assuming a logarithmic distribution of periods, we find that the relative frequency of very hot Jupiters (VHJ: P=1-3 d) to hot Jupiters (HJ: P=3-9 d) is 10-20%. Given an absolute frequency of HJ of ~1%, this implies that approximately one star in ~500-1000 has a VHJ. We also note that VHJ and HJ appear to be distinct in terms of their upper mass limit. We discuss the implications of our results for planetary migration theories, as well as present and future TR and RV surveys.

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arXiv2004

Microlensing Characterization of Wide-Separation Planets

With their excellent photometric precision and dramatic increase in monitoring frequency, future microlensing survey experiments are expected to be sensitive to very short time-scale, isolated events caused by free-floating and wide-separation planets with mass as low as a few lunar masses. We estimate the probability of measuring the Einstein radius θ_E for bound and free-floating planets. We carry out detailed simulations of the planetary events expected in next-generation surveys and estimate the resulting uncertainty in θ_E for these events. We show that, for main-sequence sources and Jupiter-mass planets, the caustic structure of wide-separation planets with projected separations of < 20 AU substantially increases the probability of measuring the dimensionless source size and thus determining θ_E compared to the case of unbound planets. In this limit where the source is much smaller than the caustic, the effective cross-section to measure θ_E to 10% is ~25% larger than the full width of the caustic. Measurement of the lens parallax is possible for low-mass planetary events by combined observations from the ground and a satellite located in an L2 orbit; this would complete the mass measurements for such wide-separation planets. Finally, short-duration events caused by bound planets can be routinely distinguished from those caused by free-floating planets for planet-star separations < 20 AU from either the deviations due to the planetary caustic or (more often) the low-amplitude bump from the magnification due to the parent star.

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arXiv2004

Prospects for the Detection of Earth-Mass Planets

We compare potential state-of-the-art experiments for detecting Earth-mass planets around main-sequence stars using radial velocities, transits, astrometry, and microlensing. For conventionally-discussed signal-to-noise ratio (S/N) thresholds, S/N ~ 8, the last three methods are roughly comparable in terms of both the total number of planets detected and the mass distribution of their host stars. However we argue that S/N ~ 25 is a more conservative and realistic S/N threshold. We show analytically and numerically that the decline in the number of detections as a function of S/N is very steep for radial velocities, transits, and astrometry, such that the number of expected detections at S/N ~ 25 is more than an order-of-magnitude smaller than at conventional S/N thresholds. Indeed, unless Earth-mass planets are very common or are packed much closer to their parent stars than in the solar system, future searches using these methods (as they are currently planned) may not yield any reliable Earth-mass planet detections. On the other hand, microlensing has a much shallower S/N slope than the other techniques and so has much greater sensitivity at realistic S/N thresholds. We show that even if all stars have Earth-mass planets at periods of one year (and adopting other optimistic assumptions as well), the combined yield of all four techniques would be the detection of only about five such planets at S/N ~ 25.

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arXiv2004

Kepler and the Kuiper Belt

The proposed field-of-view of the Kepler mission is at an ecliptic latitude of ~55 degrees, where the surface density of scattered Kuiper Belt Objects (KBOs) is a few percent that in the ecliptic plane. The rate of occultations of Kepler target stars by scattered KBOs with radii r>10km is ~10^-6 to 10^-4 per star per year, where the uncertainty reflects the current ignorance of the thickness of the scattered KBO disk and the faint-end slope of their magnitude distribution. These occultation events will last only ~0.1% of the planned t_exp=15 minute integration time, and thus will appear as single data points that deviate by tiny amounts. However, given the target photometric accuracy of Kepler, these deviations will nevertheless be highly significant, with typical signal-to-noise ratios of ~10. I estimate that 1-20 of the 10^5 main-sequence stars in Kepler's field-of-view will exhibit detectable occultations during its four-year mission. For unresolved events, the signal-to-noise of individual occultations scales as t_exp^{-1/2}, and the minimum detectable radius could be decreased by an order of magnitude to ~1 km by searching the individual 3-second readouts for occultations. I propose a number of methods by which occultation events may be differentiated from systematic effects. Kepler should measure or significantly constrain the frequency of highly-inclined, ~10 km-sized KBOs.

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arXiv2004

The Many Possible Interpretations of Microlensing Event OGLE-2002-BLG-055

Microlensing event OGLE-2002-BLG-055 is characterized by a smooth, slightly asymmetric single-lens curve with an isolated, secure data point that is ~0.6 magnitudes brighter than neighboring points separated by a few days. It was previously suggested that the single deviant data point and global asymmetry were best explained by a planetary companion to the primary lens with mass ratio log(q)=-3 to -2, and parallax effects induced by the motion of the Earth. We revisit the interpretation of OGLE-2002-BLG-055, and show that the data can be explained by wide variety of models. We find that the deviant data point can be fit by a large number of qualitatively different binary-lens models whose mass ratios range, at the ~3-sigma level, from log(q) ~ -4 to -1. This range is consistent with a planet, brown dwarf, or M-dwarf companion for reasonable primary masses of M> 0.8 M_sun. A subset of these binary-lens fits consist of a family of continuously degenerate models whose mass ratios differ by an order-of-magnitude, but whose light curves differ by <2% for the majority of the perturbation. The deviant data point can also explained by a binary companion to the source with secondary/primary flux ratio of ~1%. This model has the added appeal that the global asymmetry is naturally explained by the acceleration of the primary induced by the secondary. The binary-source model yields a measurement of the Einstein ring radius projected on source plane of \hat r_E=1.87 +/- 0.40 AU. OGLE-2002-BLG-055 is an extreme example that illustrates the difficulties and degeneracies inherent in the interpretation of weakly perturbed and/or poorly sampled microlensing light curves.

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arXiv2004

Microlensing of Elliptical Sources by Fold Caustics

We consider microlensing of an elliptical source crossing a fold caustic. We derive a simple expression for the light curve of a source with uniform surface brightness that is accurate to third order in the ellipticity e (yielding errors of <1% for e<0.3). We consider caustic crossings of a rotating star, an oblate giant extrasolar planet, and an inclined standard thin accretion disk, and find the following results. (1) If most stars have rotation periods similar to chromospherically-active (i.e. spotted) giant stars in the bulge, than ~15% should be sufficiently oblate (e>0.25) to produce detectable (>1%) deviations with current observations. The form of the deviation due to ellipticity is qualitatively similar to that due to limb-darkening. Thus stellar oblateness may have to be taken into account in interpretations of precise limb-darkening measurements with microlensing. (2) Giant planets will generally not produce a detectable oblateness signal, either because they rotate too slowly (if they are close-in and tidally locked), or because they are too faint (if they are farther away from the parent star). (3) There is a near-degeneracy between ellipticity and position angle for equal-area sources with scale-free intensity profiles and elliptical isophotes. This degeneracy results in a factor of ~2 uncertainty in the measurement of the scale-length of a standard thin accretion disk using observations of a single, linear fold crossing quasar microlensing event in passbands probing the outer parts of the disk. Higher-frequency observations can reduce this uncertainty.

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arXiv2003

Survey for Transiting Extrasolar Planets in Stellar Systems: I. Fundamental Parameters of the Open Cluster NGC 1245

We derive fundamental parameters for the old, rich open cluster NGC 1245 using BVI photometry from the MDM 1.3m and 2.4m telescopes. Based on detailed isochrone fitting, we find NGC 1245 has a slightly sub-solar metallicity, [Fe/H]=-0.05 +- 0.03 (statistical) +- 0.08 (systematic), and an age of 1.04 +- 0.02 +- 0.09 Gyr. In contrast to previous studies, we find no evidence for significant differential reddening. We determine an extinction of A_V=0.68 +- 0.02 +- 0.09 mag and a distance modulus of (m-M)o=12.27 +- 0.02 +- 0.12 mag, which corresponds to a distance of 2.8 +- 0.2 Kpc. We derive a logarithmic mass-function slope for the cluster of a=-3.12 +- 0.27, where a Salpeter slope is a=-1.35. Fits to the radial surface-density profile yield a core radius of r_c=3.10 +- 0.52 arcmin (2.57 +- 0.47 pc). NGC 1245 is highly relaxed and contains a strongly mass segregated population. The mass function for the inner cluster has a very shallow slope, b=-0.56 +- 0.28. In contrast, the outer periphery of the cluster is enriched with low mass members and devoid of high mass members out to the tidal radius, r_t=20 arcmin (16.5 pc). Based on the observed surface-density profile and an extrapolated mass function, we derive a total cluster mass, M=1300 +- 90 +- 170 M_(solar).

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arXiv2003

Identifying Lenses with Small-Scale Structure. I. Cusp Lenses

The inability of standard models to explain the flux ratios in many 4-image gravitational lenses has been cited as evidence for significant small-scale structure in lens galaxies. That claim has generally relied on detailed lens modeling, so it is both model dependent and somewhat difficult to interpret. We present a more robust and generic method for identifying lenses with small-scale structure. For a close triplet of images associated with a source near a cusp caustic, the sum of the signed magnifications should approximately vanish. We derive realistic upper bounds on the sum, and argue that lenses with flux ratios that significiantly violate the bounds can be said to have structure in the lens potential on scales smaller than the image separation. Five observed lenses have such flux ratio ``anomalies'': B2045+265, 1RXS J1131-1231, and SDSS J0924+0219 have strong anomalies; B0712+472 has a strong anomaly at optical/near-IR wavelengths and a marginal anomaly at radio wavelengths; and RX J0911+0551 appears to have an anomaly, but this conclusion is subject to uncertainties about octopole modes in early-type galaxies. Analysis of the cusp relation does not identify the known anomaly in B1422+231, so methods that are more sophisticated (and less generic) than the cusp relation may be necessary to uncover flux ratio anomalies in some systems. Although these flux ratio anomalies might represent milli- or micro-lensing, we cannot identify the cause; we can only conclude that the lenses have significant structure in the potential on scales smaller than the separation between the images. Additional arguments must be invoked to specify the nature of this small-scale structure. [Abridged]

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arXiv2003

Extremely Close-In Giant Planets from Tidal Capture

Planets that form around stars born in dense stellar environments are subject to dynamical perturbations from other stars in the system. These perturbations will strip outer planets, forming a population of free-floating planets, some of which will be tidally captured before they evaporate from the system. For systems with velocity dispersion of 1 km/s, Jupiter-mass planets can be captured into orbits with periods of 0.1-0.4 days, which are generally stable over a Gyr, assuming quadratic suppression of eddy viscosity in the convective zones of the host stars. Under this assumption, and that most stars form several massive planets at separations 5-50 AU, about 0.03% of stars in rich, mature open clusters should have extremely close-in tidally captured planets. Approximately 0.005% of field stars should also have such planets, which may be found in field searches for transiting planets. Detection of a population of tidally-captured planets would indicate that most stars formed in stellar clusters. In globular clusters, the fraction of stars with tidally-captured planets rises to 0.1% -- in conflict with the null result of the transit search in 47 Tuc. This implies that, if the quadratic prescription for viscosity suppression is correct, planetary formation was inhibited in 47 Tuc: less than one planet of Jupiter-mass or greater (bound or free-floating) formed per cluster star. Less than half of the stars formed solar-system analogs. Brown dwarfs can also be captured in tight orbits; the lack of such companions in 47 Tuc in turn implies an upper limit on the initial frequency of brown dwarfs in this cluster. However, this upper limit is extremely sensitive to the highly uncertain timescale for orbital decay, and thus it is difficult to draw robust conclusions about the low-mass end of the mass function in 47 Tuc.

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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

Periodic Flux Variability of Stars due to the Reflex Doppler Effect Induced by Planetary Companions

Upcoming space-based photometric satellites offer the possibility of detecting continuum flux variability at the micro-magnitude level. We show that the Doppler flux variability induced by the reflex motion of stars due to planetary companions has an amplitude of (3-alpha)K/c, where K is the reflex radial velocity amplitude and alpha is the logarithmic slope of source spectral flux in the observed frequency band. For many of the known close-in planetary systems with periods less than ~0.2 years, the periodic Doppler variability is of order a micromagnitude, and is significant relative to the variability caused by reflected light from the planetary companion. For companions with periods greater than ~0.2 years, the Doppler signal is larger than the reflected light signal. We show that the future photometric satellites should reach the sensitivity to detect this Doppler variability. In particular, the Kepler satellite should have the photon-noise sensitivity to detect at a signal-to-noise ratio > 5, all planets with minimum mass greater than 5 Jupiter masses, and periods less than 0.1 years around the 10,000 main-sequence stars with spectral types A through K and apparent magnitude V<12 in its field-of-view.

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