This week on HST

HST Programs: May 8 - May 14, 2006

Some selected highlights

GO 10145: Physical parameters of the upper atmosphere of the extrasolar planet HD209458b

An artist's impression of "Osiris" (HD 209458b)

HD 209458 is G0, near-solar metallicity main-sequence dwarf lying 50 parsecs from the Sun. Like (at least) 10% of solar-type stars, it harbours a planetary system, and like (at least) 3% of solar-type stars, it has a `hot Jupiter', orbiting the parent star at a distance of 0.045 AU in a period of 3.52474859 days; HD 209758b's main claim to fame is that it was the first transiting planetary system detected. Transiting planetary systems have proven a goldmine of information on the intrinsic properties of extrasolar giant planets. The orbital inclination is known, and the mass (0.69 MJ), radius (1.32 RJ) and mean density are determined directly. Moreover, HST and Spitzer can be used to probe the planetary atmosphere, by either searching for enhanced absorption during the primary eclipse (HD 209458b transits HD 209458), or by searching for reduced (mid-infrared) emission as HD 209458 eclipses the hot Jupiter. The present proposal aims to build on previous HST observations that detected enhanced Lyman alpha, OI and CII absorption, generally ascribed to gaseous mass-loss (evaporation) from the heated planetary atmosphere. This proposal will use the UV prism on ACS to obtain spectra of HD 209458 with the HRC and SBC detectors. The observations will be made during primary eclipse (transit), with the aim of detecting additional species, such as Fe II and Mg II.

GO 10487: A Search for Debris Disks in the Coeval Beta Pictoris Moving Group

WFPC2 image of Beta Pic's disk

Over the last few years, there has been growing recognition of the presence of a number of young stellar associations, each consisting of 20-50 stars, within the Solar Neighbourhood. Beta Pictoris (or Gl 219), a relatively young (~20 Myrs old) A5 dwarf, lying at a distance of just under 20 parsecs form the Sun, is the `flagship' member of one of those groups.. The IRAS satellite originally singled out this A star as unusual, detecting significant excess radiation over the expected photospheric fluxes at mid-infrared wavelengths; Smith & Terrile (1984) used ground-based coronagraphic observations to show that this was due re-radiation from a substantial debris disk. A further 30 stars are associated with this young group, with spectral types ranging from A to late M. Several of these stare are known to also have dusty disks, including the binary HR 7329 (A0 + M7) and the nearby active M1 dwarf, AU Mic (Gl 803). The present proposal aims to extend the sample, using the ACS/HRC in coronagraphic mode to search for debris disks around 21 members of the moving group.

GO 10551: Gamma-Ray Bursts from Start to Finish: A Legacy Approach A

The life history of a gamma-ray burst

Gamma ray bursts are described colloquially as the biggest bangs since the Big Bang. Originally detected by US spy satellites in the 1960s, these short-lived bursts of high energy radiation resisted characterisation for over 30 years. It is only within the last decade that the Galactic vs. Extragalactic debate on their origins has been setled in favour of the latter. Generically, gamma ray bursts are believed to originate in the death throes of an extremely massive star, as it collapses to form either a black hole or a highly magnetised neutron star. Most occur at moderate to high redshifts. With the identification of optical counterparts to increasing number of bursts, different patterns of behaviour are emerging, indicating progenitors with a range of intrinsic properties. Gamma ray bursts are, by their nature, unpredictable; thus, follow-up observations are made in Target of Opportunity mode. This proposal triggers with the detection of an appropriate burst by the Swift satellite, and couples multicolour HST ACS imaging with ground-based radio and optical follow-up observations to track its evolution over the following ~300 days.

GO 10554: Globular Cluster Systems of Elliptical Galaxies in Low Density Environments

The giant elliptical, M87, (which is not in a low density environment) has an extensive population of associated globular clusters.

Globular clusters are key remnants of the first major episode of star formation in the Milky Way Galaxy. Other galaxies, both spirals and ellipticals, have their own associated populations of globulars; the clusters associated with M31 are well resolved (HST and, to a lesser extent, ground-based observations have resulted in colour-magnitude diagrams), while hundreds of globulars have been identified in M87, the core galaxy in the Virgo cluster. It is likely that those systems, too, are fossils from the earliest stages of galaxy formation. This proposal focuses on low luminosity elliptical galaxies in low density environments. Previous studies suggest that these clusters show a bimodal colour distribution, perhaps indicative of multiple ages and multiple formation epochs. ACS imaging will be used to identify associated globulars, determine the cluster luminosity function and undertake more detailed investigation of the colour distribution.