This week on HST

HST Programs: August 4 - August 10, 2008

Some selected highlights

GO 11072: Measuring the Physical Properties of the first two WASP transiting extrasolar planets

Artist's impression of a planetary transit & a partial light curve

Transiting extrasolar planets offer particularly valuable insight into the structure of these non-Solar System gas giants. Besides providing direct measures of mass (with no complications for v sin(i)) and radius (from accurate time-series photometry), spectroscopic observations obtained during either transit or planetary eclipse can probe the atmospheric structure and chemical composition. The first such systems, including HD 209658b and HD 189733b, were originally discovered as radial velocity variables, and only subsequently identified as transits based on follow-up photometry. However, the last few years have seen the instigation of almost a dozen wide-field photometric surveys that are searching specifically for such systems - and those surveys are starting to bear fruit. The present HST program aims to obtaina time series of NICMOS narrowband images of two exoplanet hosts identified by the Wide Angle Search for Planets (WASP) project. Like most wide-field surveys, this program uses a battery of mini-telescopes (or large camera) to rapidly scan selected areas of the celestial sphere. The original data have sufficient photometric accuracy to allow the detection of transiting planets (which typically produce photometric signatures of depth 0.5-1%). The higher-precision follow-up observations with HST will allow more accurate determinations of the eclipse depth, and hence stronger constraints on the planetary radii.

GO 11110: Searching for Lyman alpha Emission from FUSE Lyman Continuum Candidates

The starburst galaxy, M82

Ultraviolet light plays an important role in ionising the interstellar and intergalactic medium. Hot, massive stars are the one of the principal sources of UV flux in the present universe. It is therefore important to understand the efficiency with which that light manages to escape from star forming regions, particularly as a means of estimating the relative improtance of star formation as an ionising source in the early universe. The present program targets a number of star forming galaxies that have been observed by the Far Ultraviolet Satellite Explorer (FUSE) and detected in the Lyman continuum (shortward of 912 Angstroms). The ACS/SBC camera is being used to image those systems at Lyman alpha; ground-based imaging has already been used to map the Balmer line emission. All of these data will be combined in models that will be used to estimate the global effect of star formation on the surrounding gaseous environment.

GO 11134: WFPC2 Tidal Tail Survey: Probing Star Cluster Formation on the Edge

ACS image of the spectacular tidal tail of the Tadpole galaxy, Arp 188

Gravitational interactions between close galaxy pairs produce tidal tails, which can often be sites of extensive star formation in young, gas-rich spiral systems. Within the Milky Way, we know that most stars are born in fairly dense clusters, comparable with the Orion Nebula Cluster, and observatios of nearby sprials (M31, M33) suggest that the same situation pervades in those systems. However, observations suggest that star clusters tend to be rare in tidal tails. The present program is using WFPC2 to obtain R (F606W) and I (F814W) band observations of 12 tidal tail systems, with the aim of quantifying the contribution made by star clusters to star formation in these sparse environments.

GO 11136: Resolving Ultracool Astrophysics with Brown Dwarf Binaries

NICMOS images of the ultracool L/T binary, 2MASS J22521073-1730134; the northern component, notably fainter at F160W, is the T dwarf.

Ultracool dwarfs are defined as having spectral types later than M7, and therefore include the recently discovered L and T dwarfs. They encompass the lowest mass stars (masses < ~0.1 MSub) and sub-stellar mass brown dwarfs, with surface temperatures ranging from ~2500K (~M7) to ~700K (late-type T dwarfs). Following their discovery over a decade ago, considerable theoretical attention has focused on the evolution of the intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L to type T. This point marks the emergence of methane as a dominant absorber at near-infrared wavelengths. Current models suggest this transition occurs at ~1400-1200K, and that the spectral changes are at least correlated with, and perhaps driven by, the distribution and properties of dust layers ("clouds"). The overall timescales associated with this process remains unclear. The present proposal aims to tackle this issue through observations of ultracool binary systems. Since these systems are almost certainly coeval, the relative spectral energy distributions of the two components can be used to set constraints on evolutionary models. More than 40 ultracool binary systems are currently known; almost all have relatively small linear separations (<15 AU), and components with mass ratios close to one. The present program targets 13 systems with spectral types near the L/T boundary.