Joshua N. Winn

A super-Earth transiting a nearby low-mass star

A decade ago, the detection of the first transiting extrasolar planet provided a direct constraint on its composition and opened the door to spectroscopic investigations of extrasolar planetary atmospheres. Because such characterization studies are feasible only for transiting systems that are both nearby and for which the planet-to-star radius ratio is relatively large, nearby small stars have been surveyed intensively. Doppler studies and microlensing have uncovered a population of planets with minimum masses of 1.9–10 times the Earth’s mass (M⊕), called super-Earths. The first constraint on the bulk composition of this novel class of planets was afforded by CoRoT-7b (refs 8, 9), but the distance and size of its star preclude atmospheric studies in the foreseeable future. Here we report observations of the transiting planet GJ 1214b, which has a mass of 6.55M⊕ and a radius 2.68 times Earth’s radius (R⊕), indicating that it is intermediate in stature between Earth and the ice giants of the Solar System. We find that the planetary mass and radius are consistent with a composition of primarily water enshrouded by a hydrogen–helium envelope that is only 0.05% of the mass of the planet. The atmosphere is probably escaping hydrodynamically, indicating that it has undergone significant evolution during its history. The star is small and only 13 parsecs away, so the planetary atmosphere is amenable to study with current observatories.

Measurement of Spin-Orbit Alignment in an Extrasolar Planetary System

We determine the stellar, planetary, and orbital properties of the transiting planetary system HD 209458 through a joint analysis of high-precision radial velocities, photometry, and timing of the secondary eclipse. Of primary interest is the strong detection of the Rossiter-McLaughlin effect, the alteration of photospheric line profiles that occurs because the planet occults part of the rotating surface of the star. We develop a new technique for modeling this effect and use it to determine the inclination of the planetary orbit relative to the apparent stellar equator (λ = -4o.4 ± 1o.4), and the line-of-sight rotation speed of the star (v sin /_★ = 4.70 ± 0.16 km s^(-1)). The uncertainty in these quantities has been reduced by an order of magnitude relative to the pioneering measurements by Queloz and collaborators. The small but nonzero misalignment is probably a relic of the planet formation epoch, because the expected timescale for tidal coplanarization is larger than the age of the star. Our determination of v sin /★ is a rare case in which rotational line broadening has been isolated from other broadening mechanisms.

Transit Photometry of the Core-dominated Planet HD 149026b

We report g, V, and r photometric time series of HD 149026 spanning predicted times of transit of the Saturn-mass planetary companion, which was recently discovered by Sato and collaborators. We present a joint analysis of our observations and the previously reported photometry and radial velocities of the central star. We refine the estimate of the transit ephemeris to Tc = (2,453,527.87455) + (2.87598)N (HJD). Assuming that the star has a radius of 1.45 ± 0.10 R☉ and a mass of 1.30 ± 0.10 M☉, we estimate the planet radius to be (0.726 ± 0.064)RJup, which implies a mean density of 1.07 g cm-3. This density is significantly greater than predicted for models that include the effects of stellar insolation and in which the planet has only a small core of solid material. Thus, we confirm that this planet likely contains a large core and that the ratio of core mass to total planet mass is more akin to that of Uranus and Neptune than to either Jupiter or Saturn.

PKS 1830?211: A Face-on Spiral Galaxy Lens

ABSTRACTWe present new Hubble Space Telescope images of the gravitational lensPKS 1830–211, which allow us to characterize the lens galaxy and update thedetermination of the Hubble constant (H 0 ) from this system. The I-band im-age shows that the lens galaxy is a face-on spiral galaxy with clearly delineatedspiral arms. The southwestern image of the background quasar passes throughone of the spiral arms, explaining the previous detections of large quantities ofmolecular gas and dust in front of this image. The lens galaxy photometry isconsistent with the Tully-Fisher relation, suggesting the lens galaxy is a typi-cal spiral galaxy for its redshift. The lens galaxy position, which was the mainsource of uncertainty in previous attempts to determine H 0 , is now known pre-cisely. Given the current time delay measurement and assuming the lens galaxyhas an isothermal mass distribution, we compute H 0 = 44 ± 9 km s −1 Mpc −1 for an Ω m = 0.3 flat cosmological model. We describe some possible systematicerrors and how to reduce them. We also discuss the possibility raised by Courbinet al. (2002), that what we have identified as a single lens galaxy is actually aforeground star and two separate galaxies.Subject headings: quasars: individual (PKS 1830–211)—gravitational lensing—cosmology: distance scale

Obliquity Tides on Hot Jupiters

Obliquity tides are a potentially important source of heat for extrasolar planets on close-in orbits. Although tidal dissipation will usually reduce the obliquity to zero, a nonzero obliquity can persist if the planet is in a Cassini state, a resonance between spin precession and orbital precession. Obliquity tides might be the cause of the anomalously large size of the transiting planet HD 209458b.

Planets in Stellar Clusters Extensive Search. III. A Search for Transiting Planets in the Metal-rich Open Cluster NGC 6791*

We have undertaken a long-term project, Planets in Stellar Clusters Extensive Search (PISCES), to search for transiting planets in open clusters. In this paper we present the results for NGC 6791, a very old, populous, metal-rich cluster. We have monitored the cluster for over 300 hr, spread over 84 nights. We have not detected any good transiting planet candidates. Given the photometric precision and temporal coverage of our observations and the current best estimates for the frequency and radii of short-period planets, the expected number of detectable transiting planets in our sample is 1.5. We have discovered 14 new variable stars in the cluster, most of which are eclipsing binaries, and present high-precision light curves spanning 2 years for these new variables and also the previously known variables.

The Orbit and Occultations of KH 15D

The unusual flux variations of the pre-main-sequence binary star KH 15D have been attributed to occultations by a circumbinary disk. We test whether or not this theory is compatible with newly available data, including recent radial velocity measurements, CCD photometry over the past decade, and photographic photometry over the past 50 years. We find the model to be successful, after two refinements: a more realistic motion of the occulting feature and a halo around each star that probably represents scattering by the disk. The occulting feature is exceptionally sharp edged, raising the possibility that the dust in the disk has settled into a thin layer and providing a tool for fine-scale mapping of the immediate environment of a T Tauri star. However, the window of opportunity is closing, as the currently visible star may be hidden at all orbital phases by as early as 2008.

The Unusual Optical Afterglow of the Gamma-Ray Burst GRB 021004: Color Changes and Short-Timescale Variability*

We report UBVRI observations of the optical afterglow of the gamma-ray burst GRB 021004. We observed significant (~10%-20%) deviations from a power-law decay on several timescales, ranging from a few hours down to 20-30 minutes. We also observed a significant color change starting ~1.5 days after the burst, confirming the spectroscopic results already reported by Matheson et al. We discuss these results in the context of several models that have recently been proposed to account for the anomalous photometric behavior of this event.

The Reddest Quasars. II. A Gravitationally Lensed FeLoBAL Quasar

We report the discovery of a z = 2.65 low-ionization iron broad absorption line quasar, FIRST J100424.9+122922, which is gravitationally lensed by a galaxy at z ≈ 0.95. The object was discovered as part of a program to find very red quasars by matching the FIRST radio survey with the Two Micron All Sky Survey in the near-infrared. J100424.9+122922 is the second lensed system to be found in this program, suggesting that many gravitational lenses are probably missed from conventional optical quasar surveys. We have made a simple lens model and a rough estimate of the reddening in the immediate environment of the quasar which suggests that the quasar is intrinsically very luminous and is accreting at close to the Eddington limit of its ~109 M⊙ black hole. The lensing galaxy has a small amount of dust, which is responsible for some excess reddening observed in the fainter image of the quasar, but is otherwise a fairly typical massive elliptical galaxy. We model the selection effects working against the detection of red quasars in both lensed and unlensed samples. We show that these selection effects are very effective at removing even lightly reddened high-redshift quasars from magnitude-limited samples, whether they are lensed or not. This suggests that the red quasar population in general could be very large, and in particular the class of iron broad absorption line quasars of which J100424.9+122922 is a member may be much larger than their rarity in magnitude-limited samples would suggest.

PMN J1632?0033: A New Gravitationally Lensed Quasar

We report the discovery of a gravitationally lensed quasar resulting from our survey for lenses in the southern sky. Radio images of J1632-0033 with the Very Large Array and the Australia Telescope Compact Array exhibit two compact, flat-spectrum components with separation 147 and flux density ratio 13.2. Images with the Hubble Space Telescope reveal the optical counterparts to the radio components and also the lens galaxy. An optical spectrum of the bright component obtained with the first Magellan telescope reveals quasar emission lines at redshift 3.42. Deeper radio images with the Multi-Element Radio Linked Interferometry Network and the Very Long Baseline Array reveal a faint third radio component located near the center of the lens galaxy, which is either a third image of the background quasar or faint emission from the lens galaxy.