This week on HST

HST Programs: September 27, 2010 - October 3, 2010

Selected highlights

GO 11572: Characterizing Atmospheric Sodium in the Transiting hot-Jupiter HD189733b

An artist's impression of the hot Jupiter circling a solar-type star

HD 198733 is a 7th magnitude G5 dwarf that lies at a distance of ~20 parsecs from the Sun, in the direction of the constellation of Vulpecula. Like many other nearby solar-type stars, HD 189733 has an associated planetary system, including a hot Jupiter, a ~1.15 MJ gas giant with an orbital period of 2.12 days. Most significantly, that inner planet transits the central star, making HD 189733 the closest transiting system found so far. Transiting systems offer a potential gold-mine for extrasolar planetary studies, since not only is the orbital inclination well defined, but the diameter (and hence the average density) is directly measureable form the eclipse depth, while the atmospheric composition can be probed through line absorption or re-radiated thermal flux. The results from these measurments can be used to test, and improve, theoretical models of extrasolar planets. These observations are best done from space (indeed, the only successful atmospheric observations to date have been with HST and Spitzer). HD 189733 has already been studied extensively with HST using the grism on the Advanced Camera for Surveys and, at near-infrared wavelengths, NICMOS. However, it's discovery postdated the STIS electronic failure, so it has never been studied at even moderate spectral resolution. The present observations will use the G750M grating on STIS to monitor three transits, and the data will be analysed for the presence of enhanced sodium absorption, as previously detected in the original bright transiting planet, HD 209458b.

GO 11644: A dynamical-compositional survey of the Kuiper belt: a new window into the formation of the outer solar system

The architecture of the outer Solar System

The Kuiper Belt lies beyond the orbit of Neptune, extending from ~30 AU to ~50 AU from the Sun, and includes at least 70,000 objects with diameters exceeding 100 km. Setting aside Pluto, the first trans-Neptunian objects were discovered in the early 1990s. Most are relatively modest in size, with diameters of a few hundred km and photometric properties that suggested an icy composition, similar to Pluto and its main satellite, Charon. Over the last three years, a handful of substantially larger bodies have been discovered, with diameters of more than 1000 km; one of the objects, 2003 UB313, is comparable in size to Pluto (2320 km.). At the same time, ground-based surveys, such as the Deep Ecliptic Survey, the Canada-France Ecliptic plane Survey and the Palomar Quest Survey, scanned the ecliptic for fainter, lower-mass objects, with the aim of using their properties to assess the likely chemical composition and dynamical history of the early Solar System. The present program will use Wide Field Camera 3 to push up to 2 magnitudes fainter than these ground-based studies, providing reliable estimates of compositions for a representative sample of KBOs.

GO 12181: The Atmospheric Structure of Giant Hot Exoplanets

Probing the atmosphere of a transiting exoplanet

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Since that date, the number of known transiting exoplanet systems has grown to more than 100, most detected through wide-field photometric surveys with the Kepler satellite providing the highest sensitivity dataset. These transiting systems are invaluable, since they not only provide unambiguous measurements of mass and diameter, but they also provide an opportunity to probe the atmospheric structure by differencing spectra taken during and between primary secondary transit. Such observations are best done from space: indeed, the only successful atmospheric observations to date have been with HST and Spitzer. The present program aims to set these measurements on a systematic basis by targeting 13 transiting exoplanets. The WFC3-IR G141 grism will be used to search for characteristic near-infrared spectral features in those systems.

GO 12381: Imaging the Crab nebula while it is flaring in gamma-rays

The Crab Nebula

Messier 1, the Crab Nebula, presents one of astronomy's iconic images. The remnant of a bright supernova observed in 1054 by Arabian and Chinese astronomers, the Crab was first recorded in 1731 by the English astronomer, John Bevis, 37 years before Messier compiled his catalogue of non-comets. The energy source for the gaseous emission is the neutron star that lies in the centre of nebulosity, and was one of the first pulsars to be identified. The Crab is also a source of high energy emission, including gamma rays. Recently (September 19-21), the gamma ray flux was observed to double in strength by both the Fermi and Agile spacecrafts. Follow-up high-energy observations are being obtained with the Integral, Swift and Chandra observatories. The HST observations will be used to study the detailed structure of the complex knots and filaments, to search for further insight into the nature of this event.