Showing papers by "Sloan Fellows published in 2002"

Orbital Perturbations of Transiting Planets: A Possible Method to Measure Stellar Quadrupoles and to Detect Earth-Mass Planets

Abstract: The recent discovery of a planetary transit in the star HD 209458, and the subsequent highly precise observation of the transit light curve with Hubble Space Telescope, is encouraging to search for any phenomena that might induce small changes in the light curve. Here we consider the effect of the quadrupole moment of the parent star and of a possible second planet perturbing the orbit of the transiting planet. Both of these cause a precession of the orbital plane and of the periastron of the planet, which result in a long-term variation of the duration and the period of the transits. For a transiting planet at 0.05 AU, either a quadrupole moment similar to that of the Sun or the gravitational tug from an Earth-like planet on an orbit of semimajor axis ~0.2 AU and a relative inclination near the optimal 45° would cause a transit duration time derivative of ~1 s yr-1.

Chandra Imaging of the X-Ray Nebula Powered by Pulsar B1509?58

Abstract: We present observations with the Chandra X-Ray Observatory of the pulsar wind nebula (PWN) powered by the energetic young pulsar B1509-58. These data confirm the complicated morphology of the system indicated by previous observations, and in addition reveal several new components to the nebula. The overall PWN shows a clear symmetry axis oriented at a position angle 150" +- 5" (north through east), which we argue corresponds to the pulsar spin axis. We show that a previously identified radio feature matches well with the overall extent of the X-ray PWN, and propose the former as the long-sought radio nebula powered by the pulsar. We further identify a bright collimated feature, at least 4' long, lying along the nebula's main symmetry axis; we interpret this feature as a physical outflow from the pulsar, and infer a velocity for this jet of greater than 0.2c. The lack of any observed counterjet implies that the pulsar spin axis is inclined at -30" to the line of sight, contrary to previous estimates made from lower resolution data. We also identify a variety of compact features close to the pulsar. A pair of semicircular X-ray arcs lie 17" and 30" to the north of the pulsar; the latter arc shows a highly polarized radio counterpart. We show that these features can be interpreted as ion compression wisps in a particle-dominated equatorial flow, and use their properties to infer a ratio of electromagnetic to particle energy in pairs at the wind shock 0.005, similar to that seen in the Crab Nebula. We further identify several compact knots seen very close to the pulsar; we use these to infer cr < 0.003 at a separation from the pulsar of 0.1 pc.

A Test of the Collisional Dark Matter Hypothesis from Cluster Lensing

Abstract: Spergel & Steinhardt proposed the possibility that dark matter particles are self-interacting as a solution to two discrepancies between the predictions of cold dark matter models and observations: First, the observed dark matter distribution in some dwarf galaxies has large, constant-density cores, as opposed to the predicted central cusps; and second, small satellites of normal galaxies are much less abundant than predicted. The dark matter self-interaction would produce isothermal cores in halos and expel the dark matter particles from dwarfs orbiting in large halos. Another consequence of the model is that halos should become spherical once most particles have interacted. Several observations show that the mass distribution in relaxed clusters of galaxies is elliptical. Here, I discuss in particular gravitational lensing in the cluster MS 2137-23, where the ellipticity of the dark matter distribution can be measured to a small radius, r ~ 70 kpc, suggesting that most dark matter particles in clusters outside this radius do not collide during the characteristic age of clusters. If true, this implies that any dark matter self-interaction with a cross section independent of velocity is too weak to have affected the observed density profiles in the dark-matter dominated dwarf galaxies, or to have facilitated the destruction of dwarf satellites in galactic halos. If sx is the cross section and mx the mass of the dark matter particle, then sx/mx < 10-25.5 cm2 GeV-1.

The QMAP and MAT/TOCO Experiments for Measuring Anisotropy in the Cosmic Microwave Background

Abstract: We describe two related experiments that measured the anisotropy in the cosmic microwave background (CMB). QMAP was a balloon-borne telescope that flew twice in 1996, collecting data on degree angular scales with an array of six high electron mobility transistor-based amplifiers (HEMTs). QMAP used an interlocking scan strategy to directly produce high signal-to-noise ratio CMB maps over a limited region of sky. The QMAP gondola was then refitted for ground-based work as the MAT/TOCO experiment. Observations were made from 5200 m on Cerro Toco in Northern Chile in 1997 and 1998 using time domain beam synthesis. MAT/TOCO measured the rise and fall of the CMB angular spectrum, thereby localizing the position of the first peak to lpeak = 216 ± 14. In addition to describing the instruments, we discuss the data selection methods, check for systematic errors, and compare the MAT/TOCO results to those from recent experiments. The previously reported data are updated to account for a small calibration shift and corrected to account for a small contribution from known sources of foreground emission. The resulting amplitude of the first peak for 160 < l < 240 is δTpeak = 80.9 ± 3.4 ± 5.1 μK, where the first uncertainty is statistical and the second is from calibration.

Self-Absorption of Ionizing Radiation and Extended Narrow-Line Emission in High-Redshift Quasi-stellar Objects

Abstract: We calculate the H I column density of self-absorption in quasi-stellar objects (QSOs) predicted in a model where the QSOs are located in the same halos that contain the gas in damped Lyα absorption systems. The model is parameterized by the probability P0 that any halo has an active QSO. We assume that the QSOs ionize the gas but do not expel or heat it beyond the photoionization effect. When clumping of the gas is ignored, the derived H I column densities produce negligible Lyman limit absorption even in the lowest luminosity QSOs with an optical depth of ~10% for luminosity L = 0.01LB* when P0 = 10-2. We also compute the He II Lyman limit self-absorption, which is slightly higher but still small. The self-absorption is increased if the gas is highly clumped; for P0 = 10-2, a clumping factor of ~10 results in a significantly reduced overall emissivity of H I Lyman limit photons from QSOs and affects the predicted intensity of the ionizing background. A clumping factor of ~5 is sufficient to reduce the He II Lyman limit emissivity and to delay the predicted epoch of He II reionization. The presence of the gas associated with damped absorption systems around QSOs can also be detected from the narrow Lyα emission line, which should have an angular extent of 01-1'' in typical high-redshift QSOs.