This week on HST

HST Programs: May 19 - May 25, 2008

Some selected highlights

GO 11107: Imaging of Local Lyman Break Galaxy Analogs: New Clues to Galaxy Formation in the Early Universe

Mosaic of HST images of M82, the best-known starburst galaxy

Current Big Bang cosmological models predict that the universe should have undergone a global re-ionisation at redshifts between 6 and 20. The first generation of stars is generally tapped as the most likely source of the ionising radiation, perhaps enhanced through merger-stimulated starbursts. Direct observations of those galaxies are not possible at present; the James Webb Space Telescope is expected to open up observations of these systems. In consequence, there is considerable interest in identifying galaxies at lower redshifts that could serve as analogues for the z>6 systems. Over the last few years, the Galaxy Evolution Explorer (GALEX) has proved an important new tool in identifying candidate objects. GALEX has conducted an all-sky survey at ultraviolet wavelengths, and has uncovered sigificant numbers of UV luminous galaxies at low and moderate redshifts. Many of these galaxies are starbursts, undergoing substantial outbursts of star formation. These galaxies have been categorised as "compact UV luminous galaxies" (UVLGs). These appear to be galaxies that are undergoing small-scale mergers, leading to extensive dissipation and vigorous star formation. The present program is using the ACS/SBC prism and WFPC2 to obtain ultraviolet spectra and R-band images of 31 systems, probing the star formation history and its variation with environment.

GO 11117: The Search for Atmospheric Water in the Transiting Planet HD189733b

Key events in a planetary transit

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). Previous observations with HST have been used to determine accurate radii for HD 189733b (e.g. GO 10923 ); the present program aims to search for evidence of water absorption by differencing NICMOS narrowband images taken before, during and after primary transit.

GO 11156: Monitoring Active Atmospheres on Uranus and Neptune

Voyager 2 image of Neptune

Uranus and Neptune are the two "ice giants" of the Solar System, lying at average distances of 19 and 30 AU from the Sun. At those distances, their atmospheres are subject to much lower solar irradiance than Jupiter or Saturn and, as a result, they tend to show fewer large-scale disturbances. Even so, Neptune showed modest activity in the southern hemisphere between 2000 and 2003, although the disturbances have been less prominent in recent years. Uranus is unique among the major bodies in that it rotates on its side. With a polar obliquity is 98o degrees, its equator is close to perpendicular to the ecliptic plane. Consequently, as it circles the Sun, each pole spends almost half of a Uranian "year" (or 42 terrestrial years) hidden from the Sun in total darkness.On December 7th of this year, Uranus will pass through its equinox, with Sun passing overhead at the Uranian equator. This configuration will provide the first opportunity in modern times for us to view the entire northern hemisphere of the planet, and our first chance to view how the planetary atmosphere reacts to the change from night to day. The present SNAPSHOT program is using WFPC2 to monitor the appearance of these two planets, acquiring narrowband optical images that both record the overall structure and probe the atmosphere at different depths.

GO 11233: Multiple Generations of Stars in Massive Galactic Globular Clusters

NGC 2808, a globular cluster with multiple stellar populations

Globular clusters are remnants of the first substantial burst of star formation in the Milky Way. With typical masses of a few x 105 solar masses, distributed among several x 106 stars, the standard picture holds that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. Until recently, the only known exception to this rule was the cluster Omega Centauri, which is significantly more massive than most clusters and has both double main sequence and a range of metallicities among the evolved stars. Omega Cen has been joined by at least one more cluster, NGC 2808, which shows evidence for three distinct branches to the main sequence. The origin of this feature is notknown, but it may be significant that NGC 2808 is also one of the more massive clusters, and might therefore be able to survive several burst of star formation (or, conversely, be the product of a multi proto-globular merger). The present program aims to use WFPC2 to obtain high-precision photometry of other massive globulars, such as NGC 1851, M80 and M13.