This week on HST

HST Programs: June 26 - July 2, 2006

Some selected highlights

GO 10143: Ultracool companions to the nearest L dwarfs

NICMOS images of the ultracool L/T binary, 2MASS J22521073-1730134

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 potential formation mechanisms for these very low-mass objects. In particular, there have been suggestions that these are "stars interrupted" - cores that were ejected from the natal cocoon of the parent molecular cloud before they could accrete sufficient material to reach the hydrogen-burning mass limit. One means of testing this hypothesis is by studying the binary properties, since dynamical ejection is likely to disrupt wider, weakly bound systems. So far, the observations show that utlracool binaries are indeed preferentially found with small separations (<15 AU) - although not quite as small as theory predicts. Interestingly, almost all of the known systems also have components with near-equal mass. The current program is using NICMOS to survey the 80 L-dwarf systems known to lie within 20 parsecs of the Sun.

GO 10173: Multicolour snapshot observations of 3C Radio Galaxies

Hubble WFPC2 planetary camera images of the central regions of several 3C radio galaxies

The Third Cambridge Catalogue of Radio Sources was compiled originally in 1959 from observations at 159 MHz made by the Cambridge Radio Astronomy Group using the Cambridge interferometer (Edge et al, Mem RAS, 68, 37). The addition of observations at 178 MHz led to the revised catalogue, the 3CR (Bennett, 1962, Mem RAS, 68, 137). This catalogue was the main basis for the Fanaroff-Riley classification scheme (FR-Is are double-lobed sources with small separation between the peaks; FR-IIs have a separation that exceeds half the largest size of the source), and includes many of the brightest known radio galaxies. As such, the 3CR sources have remained extremely important observational targets for understanding the nature and structure these highly energetic sources. Over the past several cycles, many 3CR galaxies have been observed at a variety of wavelengths by HST. Those observations have revealed new optical jets, dust lanes, face-on disks with optical jets, besides point-like nuclei whose properties support FR-I/BL Lac unified schemes. The present programs aim to use NICMOS and the ACS/HRC to extend coverage to near-infrared and ultraviolet wavelengths, searching for dust and regions of star formation in the central regions.

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 10534 Active Atmospheres on Uranus and Neptune

Nicmos image of aurorae on Uranus

The atmospheres of the gas giant planets in the solar system are dynamic entities that can exhibit dramatic changes over a variety of timescales. Those changes are most apparent in Jovian atmosphere, which displays a wide variety of bands and spots, reflecting complex meteorological phenomena (see, e.g., previous ACS observations of the upper atmosphere and of the new little red spot ). This is not surprising since Jupiter atmosphere receives the highest input of solar energy. However, secular variations are also evident in the atmospheres of the outer planets, albeit usually at a more subtle level. The present program aims to monitor atmospheric changes in the two outermost gas giants, Uranus and Neptune. Both exhibit long-term seasonal variations, whose origins are not yet well understood; both are capable of generating dark spots - phenomena that are presumably related to Jupiter's Great Red Spot and Saturn's Great White Spot. The present observations use a variety of filters on ACS and WFPC2 (notably the 619 nm methane filter) to probe conditions are a variety of levels within the planetary atmospheres.