Sloan Fellows

About: Sloan Fellows is a based out in . It is known for research contribution in the topics: Galaxy & Star formation. The organization has 55 authors who have published 253 publications receiving 35008 citations. The organization is also known as: Sloan Fellows.

Soft X-Ray Absorption by High-Redshift Intergalactic Helium

Abstract: The Lyα absorption from intergalactic, once-ionized helium (He II) has been measured with the Hubble Space Telescope in four quasars over the last few years in the redshift range 24 < z < 32 These observations have indicated that the He II reionization may not have been completed until z 28 and that large fluctuations in the intensity of the He II-ionizing background were present before this epoch The detailed history of He II reionization at higher redshifts is, however, model-dependent and difficult to determine from these observations, since the intergalactic medium (IGM) can be completely optically thick to Lyα photons when only a small fraction of the helium remains as He II In addition, finding quasars in which the He II Lyα absorption can be observed becomes increasingly difficult at higher redshift owing to the large abundance of hydrogen Lyman limit systems It is pointed out here that He II in the IGM should also cause detectable continuum absorption in the soft X-rays The spectrum of a high-redshift source seen behind the IGM when most of the helium was He II should recover from the He II Lyman continuum absorption at an observed energy of ~01 keV Galactic absorption will generally be stronger, but not by a large factor; the intergalactic He II absorption can be detected as an excess over the expected Galactic absorption from the 21 cm H I column density In principle, this method allows a direct determination of the fraction of helium that was singly ionized as a function of redshift if the measurement is done on a large sample of high-redshift sources over a range of redshifts

The origin of the negative torque density in disk-satellite interaction

Abstract: Tidal interaction between a gaseous disk and a massive orbiting perturber is known to result in angular momentum exchange between them. Understanding astrophysical manifestations of this coupling such as gap opening by planets in protoplanetary disks or clearing of gas by binary supermassive black holes (SMBHs) embedded in accretion disks requires knowledge of the spatial distribution of the torque exerted on the disk by a perturber. Recent hydrodynamical simulations by Dong et al (2011) have shown evidence for the tidal torque density produced in a uniform disk to change sign at the radial separation of $\approx 3.2$ scale heights from the perturber's orbit, in clear conflict with the previous studies. To clarify this issue we carry out a linear calculation of the disk-satellite interaction putting special emphasis on understanding the behavior of the perturbed fluid variables in physical space. Using analytical as well as numerical methods we confirm the reality of the negative torque density phenomenon and trace its origin to the overlap of Lindblad resonances in the vicinity of the perturber's orbit - an effect not accounted for in previous studies. These results suggest that calculations of the gap and cavity opening in disks by planets and binary SMBHs should rely on more realistic torque density prescriptions than the ones used at present.

AN INDEPENDENT MEASUREMENT OF THE INCIDENCE OF Mg ii ABSORBERS ALONG GAMMA-RAY BURST SIGHT LINES: THE END OF THE MYSTERY?

Abstract: In 2006, Prochter et al. reported a statistically significant enhancement of very strong Mg ii absorption systems intervening the sight lines to gamma-ray bursts (GRBs) relative to the incidence of such absorption along quasar sight lines. This counterintuitive result has inspired a diverse set of astrophysical explanations (e.g., dust, gravitational lensing) but none of these has obviously resolved the puzzle. Using the largest set of GRB afterglow spectra available, we reexamine the purported enhancement. In an independent sample of GRB spectra with a survey path three times larger than Prochter et al., we measure the incidence per unit redshift of ≥1 A rest-frame equivalent width Mg ii absorbers at z ≈ 1 to be l(z) = 0.18 ± 0.06. This is fully consistent with current estimates for the incidence of such absorbers along quasar sight lines. Therefore, we do not confirm the original enhancement and suggest those results suffered from a statistical fluke. Signatures of the original result do remain in our full sample (l(z) shows an ≈1.5 enhancement over l(z)_(QSO)), but the statistical significance now lies at ≈90% c.l. Restricting our analysis to the subset of high-resolution spectra of GRB afterglows (which overlaps substantially with Prochter et al.), we still reproduce a statistically significant enhancement of Mg ii absorption. The reason for this excess, if real, is still unclear since there is no connection between the rapid afterglow follow-up process with echelle (or echellette) spectrographs and the detectability of strong Mg ii doublets. Only a larger sample of such high-resolution data will shed some light on this matter.

Kepler-63b: A Giant Planet in a Polar Orbit around a Young Sun-like Star

Abstract: We present the discovery and characterization of a giant planet orbiting the young Sun-like star Kepler-63 (KOI-63, $m_{\rm Kp} = 11.6$, $T_{\rm eff} = 5576$ K, $M_\star = 0.98\, M_\odot$). The planet transits every 9.43 days, with apparent depth variations and brightening anomalies caused by large starspots. The planet's radius is $6.1 \pm 0.2 R_{\earth}$, based on the transit light curve and the estimated stellar parameters. The planet's mass could not be measured with the existing radial-velocity data, due to the high level of stellar activity, but if we assume a circular orbit we can place a rough upper bound of $120 M_{\earth}$ (3$\sigma$). The host star has a high obliquity ($\psi$ = $104^{\circ}$), based on the Rossiter-McLaughlin effect and an analysis of starspot-crossing events. This result is valuable because almost all previous obliquity measurements are for stars with more massive planets and shorter-period orbits. In addition, the polar orbit of the planet combined with an analysis of spot-crossing events reveals a large and persistent polar starspot. Such spots have previously been inferred using Doppler tomography, and predicted in simulations of magnetic activity of young Sun-like stars.

Dynamical Evolution of Thin Dispersion-Dominated Planetesimal Disks

Abstract: We study the dynamics of a vertically thin, dispersion-dominated disk of planetesimals with eccentricities and inclinations (normalized in Hill units) satisfying , . This situation may be typical (even if only temporarily) for, e.g., a population of protoplanetary cores in the end of the oligarchic phase of planet formation. In this regime of orbital parameters, planetesimal scattering has an anisotropic character and strongly differs from scattering in thick () disks. We derive analytical expressions for the planetesimal scattering coefficients and compare them with numerical calculations. We find significant discrepancies in the inclination scattering coefficients obtained by the two approaches and ascribe this difference to the effects not accounted for in the analytical calculation: multiple scattering events (temporary captures, which may be relevant for the production of distant planetary satellites outside the Hill sphere) and distant interaction of planetesimals prior to their close encounter. Our calculations show that the inclination of a thin, dispersion-dominated planetesimal disk grows exponentially on a very short timescale implying that (1) such disks must be very short-lived and (2) planetesimal accretion in this dynamical phase is insignificant. Our results are also applicable to the dynamics of shear-dominated disks switching to the dispersion-dominated regime.