This week on HST

HST Programs: September 25 - October 1, 2006

ACS is currently unavailable for observations

Some selected highlights

GO 10879: A search for planetary-mass companions to the nearest L dwarfs - completing the survey

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 ultracool 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 complete a high-resolution imaging survey of the 80 L-dwarf systems known to lie within 20 parsecs of the Sun.

GO 10780: The Unusual Afterglow and Host Galaxy of the Short GRB 060121

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. In particular, there are strong indications that the bursts can be grouped within two broad classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts. The long/soft bursts appear to originate in the collapse of very massive stars, while the short/hard bursts are coalescing binary systems (probably pairs of netron stars or black holes). This Director's Discretionary Time proposal focuses on the short/hard burst GRB 060121 which was significantly fainter than the norm, and which also has a faint host galaxy. This may indicate the presence of significant dust obscuration, which would be unexpected for this type of object.

GO 11000: Evaporating Disks

Artist's impression of an Orion proplyd irradiated by an O star

It is generally agreed that the overwhelming majority of stars in the Galactic disk formed within fairly dense environments, comparable with the Orion Nebula Cluster. Planetary systems form within the circumstellar disks that envelope young stars during the first 5-10 million years of their life. These protoplanetary disks (proplyds) have been identified from HST observations of several young star-forming regions, including Orion and Carina. Those active star formation regions are sufficiently massive that the high-mass end of the stellar mass function extends beyond 10 solar masses, and the cluster therefore includes a significant component of luminous early-type B and O stars. Those stars have temperatures exceedng 20,000K, and are therefore extremely bright at ultraviolet wavelengths, particularly shortward of Lyman-alpha. They are therefore a strong source of ionising radiation, and have the abiity to influence the evolution of circumstellar disks around lower mass stars in the cluster; indeed, low-mass stars that wander too close may find their disks disrupted, with consequent implications for planet formation. The aim of the present proposal is to combine NICMOS and Spitzer/IRS observations to study the gaseous components of proplyds in IC2396 and NGC 2264.