Alfred B. Schultz

System parameters of the transiting extrasolar planet HD 209458b

We derive improved system parameters for the HD 209458 system using a model that simultaneously fits both photometric transit and radial velocity observations. The photometry consists of previous Hubble Space Telescope STIS and FGS observations, 12 I-band transits observed between 2001 and 2003 with the Mount Laguna Obser- vatory 1 m telescope, and six Stromgrenb þ y transits observed between 2001 and 2004 with two of the Automatic Photometric Telescopes at Fairborn Observatory. The radial velocities were derived from Keck HIRES observations. The model properly treats the orbital dynamics of the system and thus yields robust and physically self-consistent solutions. Our set of system parameters agrees with previously published results, although with improved ac- curacy. For example, applying robust limits on the stellar mass of 0.93-1.20 M� ,w e fi nd 1:26RJ < Rplanet < 1:42RJ and 0:59MJ < Mplanet < 0:70MJ. We can reduce the uncertainty of these estimates by including a stel- lar mass-radius relation constraint, yielding Rplanet ¼ 1:35 � 0:07 ðÞ RJ and Mplanet ¼ 0:66 � 0:04

Rings in the Planetesimal Disk of β Pictoris

The nearby main-sequence star beta Pictoris is surrounded by an edge-on disk of dust produced by the collisional erosion of larger planetesimals. Here we report the discovery of substructure within the northeast extension of the disk midplane that may represent an asymmetric ring system around beta Pic. We present a dynamical model showing that a close stellar flyby with a quiescent disk of planetesimals can create such rings, along with previously unexplained disk asymmetries. Thus we infer that beta Pics planetesimal disk was highly disrupted by a stellar encounter in the last hundred thousand years.

First Results from the Space Telescope Imaging Spectrograph: Optical Spectra of Gliese 229B

We report the firstHubble Space TelescopeSpace Telescope Imaging Spectrograph (STIS) CCD spectroscopy of the bona fide brown dwarf Gliese 229B. The optical spectrum shows absorptions of Csi at 8944 Aand water vapor bands at 9300-9600 A ˚. Strong CaH, FeH, TiO, and VO bands observed in late M dwarfs are absent from the spectrum of Gliese 229B. The formation of dust grains may explain the absence of strong atomic lines and molecular bands of these refractory elements. The broad spectral coverage obtained helps resolve current spec- ulations about the presence of dust clouds in the atmosphere of cool brown dwarfs. We find the slope of the STIS/CCD spectrum and the lack of flux detected shortward of 8000 Astrongly supports the presence of dust hazes suspended in the photosphere of Gl 229B rather than a complete settling of the grains to regions below the photosphere. Subject headings: binaries: general — stars: individual (Gl 229B) — stars: low-mass, brown dwarfs

UMBRAS: a matched occulter and telescope for imaging extrasolar planets

We describe a 1-meter space telescope plus free-flying occulter craft mission that would provide direct imaging and spectroscopic observations of Jovian and Uranus-sized planets about nearby stars not detectable by Doppler techniques. The Doppler technique is most sensitive for the detection of massive, close-in extrasolar planets while the use of a free-flying occulter would make it possible to image and study stellar systems with planets comparable to our own Solar System. Such a mission with a larger telescope has the potential to detect earth-like planets. Previous studies of free-flying occulters reported advantages in having the occulting spot outside the telescope compared to a classical coronagraph onboard a space telescope. Using an external occulter means light scatter within the telescope is reduced due to fewer internal obstructions and less light entering the telescope and the polishing tolerances of the primary mirror and the supporting optics can be less stringent, thereby providing higher contrast and fainter detection limits. In this concept, the occulting spot is positioned over the star by translating the occulter craft, at distances of 1,000 to 15,000 kms from the telescope, on the sky instead of by moving the telescope. Any source within the telescope field-of-view can be occulted without moving the telescope. In this paper, we present our current concept for a 1-m space telescope matched to a free-flying occulter, the Umbral Missions Blocking Radiating Astronomical Sources (UMBRAS) space mission. An UMBRAS space mission consists of a Solar Powered Ion Driven Eclipsing Rover (SPIDER) occulter craft and a matched (apodized) telescope. The occulter spacecraft would be semi-autonomous, with its own propulsion systems, internal power (solar cells), communications, and navigation capability. Spacecraft rendezvous and formation flying would be achieved with the aid of telescope imaging, RF or laser ranging, celestial navigation inputs, and formation control algorithms.

Imaging planets about other stars with UMBRAS II

In this paper we discuss operational considerations for the free-flying occulter. Operations consist of maneuvering the Solar-Powered Ion-Driven Eclipsing Rover (SPIDER) between targets, alignment with the space-based telescope line of sight to the target, and stationkeeping target-to-target maneuvers need to be optimized to conserve propellant. A reasonable balance needs to be determined between target observation rate and the number of targets that are observable during mission lifetime. Velocity matching of the SPIDER with the telescope is essential to mission performance. An appropriate combination of solar electric and cold-gas thrusters provides the ability to match velocities using positional information derived from comminution and ranging between telescope, occulter and any metrology stations. Desirable features of using an external coronagraphic vehicle include the ability to obtain coronagraphic data with any instrument on the telescope-- imaging, spectroscopic, or interferometric.

A POSSIBLE COMPANION TO PROXIMA CENTAURI

We report Hubble Space Telescope observations that provide evidence for a companion to Proxima Centauri (Gl 551), the closest star to the Sun. Data acquired with the Faint Object Spectrograph (FOS), used as a coronagraphic camera, show excess light that can be interpreted as being from a substellar object within 05 of Proxima Cen. Two observations of Proxima Cen separated by 103 days indicate a point source (or a feature) superposed on the wing of the point-spread function in the FOS images. This feature moves relative to the aperture, and on the plane of the sky. Comparisons with other FOS images of stars acquired using the coronagraphic mode reveal no comparable features or evidence that this feature can be explained by any instrumental anomaly. If this feature denotes a companion to Proxima Cen, it has an apparent separation corresponding to ~0.5 AU at Proxima Cen and is ≈7 mag fainter than Proxima Cen in the bandpass of the FOS red detector. The small apparent separation could result from a highly eccentric orbit, which could project a close companion. Alternately, the small separation could imply a short (~1 yr) period. Further coronagraphic observations, using the Space Telescope Imaging Spectrograph, the Near-Infrared Camera and Multiobject Spectrometer, or some other instrumentation, are needed to verify this tentative result.

Hubble Space Telescope/Faint Object Spectrograph imaging of Wolf 424 AB: Is this binary a double brown dwarf system?

The low-mass binary system Wolf 424 AB (GJ 473 AB) was spatially resolved in an image obtained on 1996 April 16 with the imaging mode of the Hubble Space TelescopeFaint Object Spectrograph. The separation was measured to be 00 0 .03 at a position angle of 3537 . 7 The new measurement was 42 5 02 5 2 combined with previously published orbital positions to update the orbital elements and obtain an estimate of the total mass. Spectroscopy and photometry of these objects suggest they are nearly equal in mass. The derived orbital solution, using all of the observations since 1938 appropriately weighted, yields a total mass of m 1 1 of 0.143 M,, and component masses of »0.07 M,, just under the theoretical limit for a brown dwarf. An m2 alternate orbital solution derived solely from speckle observations since 1986 and the Faint Object Spectrograph image yields a total mass of 0.232 M,, placing both components well above the brown-dwarf limit. The mass depends on the value for the semimajor axis, which is controlled in the latter solution by an optical speckle observation made near the last period of maximum separation. Further high-resolution observations during the upcoming maximum separation, and through the next secondary maximum, are needed to constrain the orbit.

Post-NCS Performance of the HST NICMOS

We describe the on-orbit performance of the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) aboard the Hubble Space Telescope (HST) following the installation of the NICMOS Cooling System (NCS). NICMOS is operated at a higher temperature (~77 K) than in the previous observing 1997-1998 period (~62 K). Due to the higher operating temperature, the detector QE is higher, while the well depth is less. The spatial structure of the flat field response remained essentially unchanged. We will show the effects of operating at the higher temperature and present current NICMOS calibration images. In addition, we present an overview of on-orbit testing and report on the re-enabling of NICMOS.

Apodized Square Aperture Plus Occulter Concept for TPF

The standard approach to achieving TPF-level starlight suppression has been to couple a few techniques together. Deployment of a low- or medium-performance external occulter as the first stage of starlight suppression reduces manufacturing challenges, mitigates under-performance risks, lowers development costs, and hastens launch date for TPF. This paper describes the important aspects of a conceptual 4-metre apodized square aperture telescope system utilizing a low-performance external occulter. Adding an external occulter to such a standard TPF design provides a benefit that no other technique offers: scattered and diffracted on-axis starlight is suppressed by orders of magnitude before reaching the telescope. This translates directly into relaxed requirements on the remainder of the optical system.

The Advantages of Multiple Coronagraphic Vehicles in Occulter Missions

Free-flying external coronagraphs for space telescopes have been studied since the 1960s, but cost/ benefit analysis has not proved convincing for skeptics. A single space vehicle carrying an occulting screen can increase the contrast between a star and its orbiting extrasolar planets for a suitably designed space telescope. However, a number of advantages ensue when replacing the single occulter with a fleet of lighter-mass occulters. Target observation rates can increase in proportion to the number of coronagraphic free flying vehicles devoted to a mission. At the same time, mass requirements for individual vehicles is dramatically reduced. Mission lifetimes can at the same time potentially increase, and the risk of independent catastrophic failures onboard an occulting vehicle eliminating all science productivity for the mission are effectively vanish. Perunit vehicle costs are substantially lower than for production of a single unique craft.