This week on HST

HST Programs: October 30 - November 5, 2006

Some selected highlights

GO 10624: Solving the mystery of short-hard gamma-ray bursts

An artist's impression of a gamma-ray burst

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two 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). The first optical counterpart to a gamma ray burst was identified in 1998, allowing confirmation of their extragalactic nature, and, since then, more than 50 bursts have been detected at X-ray wavelengths, and half that number detected at either optical or radio wavelengths; all of these detections are long/soft bursts. The aim of this program is to focus on the short/hard bursters. This is a Target of Opportunity project, triggered by the detection of a suitable candidate by the Swift satellite, followed by the rapid (< 3 days detection of an optical afterglow. The aim is to obtain ACS observations (weither WFC or HRC) within 5-6 days of the burst, and use the resultant images to characterise the underlying host galaxy.

GO 10843: Deep Imaging of Extremely Metal-Poor Galaxies

ACS HST image of I Zwicky 18, the archetypical metal-poor dwarf galaxy

In standard cosmologies, big bang nucleosynthesis is responsible for the production of hydrogen, helium and a trace abundance of lithium. The first stars - and the first galaxies - formed from this near-pristine material, and stellar nucleosynthesis accounts for the production of heavier elements (`metals', in astronomical parlance). As stars evolve and die, they return processed material to the interstellar medium, enriching the metal content and increasing the metallicity of subsequent stellar generations. Thus, within the Milky Way, the stars in the stellar halo (age 11-13 Gyrs) are 10 to 1000 times more metal-poor than the Sun's neighbours in the Galactic Disk. However, a small number of dwarf galaxies are known that seem to have lain fallow, avoiding star formation and self-enrichment, for almost a Hubble time. The classic example is the dwarf galaxy I Zwicky 18, with a metallicity of [M/H]~-1.2; analyses of recent HST ACS observations suggest an age of only 500 Myrs for the dominant stellar population. Nonetheless, the non-primodial abundances indicates that these systems have experienced previous star formation. The present program targets another dwarf system, CGCG 269-049, for deep ACS observations with the aim of searching for an underlying population of evolved stars, produced by a prior star formation episode (or episodes), and searching for dynamical features and young star clusters.

GO 10860: The largest Kuiper Belt Objects

The view from sedna: an artist's impression

The Kuiper Belt lies beyond the orbit of Neptune, extending from ~30 AU to ~50 AU from the Sun, and includes at least 70,000 objects with diameters exceeding 100 km. Setting aside Pluto, the first trans-Neptunian objects were discovered in the early 1990s. Most are relatively modest in size, with diameters of a few hundred km and photometric properties that suggested an icy composition, similar to Pluto and its main satellite, Charon. Over the last three years, however, a handful of substantially larger bodies have been discovered, with diameters of more than 1000 km; one of the objects, 2003 UB313, is comparable in size to Pluto (2320 km.). These recent results, of course, were one of the main stimuli for the IAU's recent revision of Pluto's status from planet to `dwarf planet' - an issue that still remains contentious in some quarters. However, regardless of spats over nomenclature, it is clear that these objects (ice planets?) are a significant component in the outer Solar System. Both HST and the Spitzer infrared space telescope have played an important role in these recent investigations, measuring the angular diameter of the larger KBOs, and the albedo over a wide range of wavelengths. The aim of the present set of observations is to target ~20 trans-Neptunian objects, using red (F606W) images and low-resolution spectra, obtained with the High Resolution Camera on the Advanced Camera for Surveys, to probe their size and chemical composition.

GO 10922: Searching for Signs of a Double Generation of Stars in Galactic Globular Clusters

The complex globular cluster, Omega Centauri

Omega Centauri is one the most intriguing members of the galactic globular cluster system. With a mass of ~5 x 106 MSun, Omega Cen is about ten times more massive than the typical large globular. In addition, it has long been known to show a substantial range in chemical abundance, unlike the homogeneous characteristics of other clusters. Finally, both ground-based (CTIO observations by Lee et al, 1999) and HST (Piotto & collaborators, 2004) imaging have revealed significant complexity in the colour-magnitude diagram, suggestive of several distinct episodes of star formation. In particular, the HST observations suggest the presence of two distinct main sequences, with different chemical composition and age. These features are more characteristic of a dwarf galaxy than a single star-burst globular cluster, and there have been repeated suggestions over the years that the cluster we call Omega Cen is the stripped core, the product of a merger with the Milky Way. The present observational program will build on the previous HST investigations, using ACS to obtain high-precision photometry of main sequence stars, allowing a detailed assessment of the relative composition and age of the two major components.