This week on HST

HST Programs: December 4 - December 10, 2006

Some selected highlights

GO 10793: A Survey for Supernovae in Massive High-Redshift Clusters

Images of supernova 2005cs in M51; this supernova happens to be a (slightly unusual) Type II

Supernovae are the most spectacular form of stellar obituary. In recent years, these celestial explosions have acquired even more significance through their use as distance indicators in mapping out the `dark energy' acceleration term of cosmic expansion. However, while there are well-established models for the two main types of supernovae - runaway fusion on the surface of a white dwarf in a binary system (type Ia), and implosion of the core of a massive star (type II) - substantive questions remain regarding the expected supernova rates, and the potential variation in those rates as a function of look-back time. This proposal aims to address that question through observations of galaxy clusters with redshifts in the range 0.5 < z < 0.9. Candidate supernovae are identified by ACS imaging in the F814W or F775W, with follow-up Keck spectroscopy used to determine the redshift of the parent galaxy.

GO 10840: The FUV fluxes of Tauri stars in the Taurus molecular cloud

Schematic view of a T Tauri star

The T Tauri stage of evolution occurs early in a star's lifetime, within ~10 Myrs of its birth, when it still retains a dense, dust and gas-rich circumstellar disk. During this phase, there is significant accretion of material onto the central star. This leads to heating of the inner regions of the accretion disk, and significant emission at ultraviolet and X-ray wavelengths. The aim of this proposal is to use the both the High Resolution Camera (HRC) and the Solar Blind Channel (SBC) on the Advanced Camera for Surveys (ACS) to study the ultraviolet energy distribution of a number of T Tauris spanning a range of ages. The HRC will be used to obtain images and low-resolution (prism) spectra at near-UV wavelengths (2000 - 3000 Angstroms), while the SBC will provide comparable data at Far-UV wavelengths. The present observations target DP Tauri, a ~1.7 Myr-old classical T Tauri star in the Taurus molecular cloud complex.

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 10886: The Sloan Lens ACS Survey: Towards 100 New Strong Lenses

ACS images of galaxy-galaxy Einstein ring lenses from the Sloan survey

Gravitational lensing is a consequence the theory of general relativity. Its importance as an astrophysical tool first became apparent with the realisation (in 1979) that the quasar pair Q0957+561 actually comprised two lensed images of the same background quasar. In the succeeding years, lensing has been used primarily to probe the mass distribution of galaxy clusters, using theoretical models to analyse the arcs and arclets that are produced by strong lensing of background galaxies, and the large-scale mass distribution, through analysis of weak lensing effects on galaxy morphologies. Gravitational lensing can also be used to investigate the mass distribution of individual galaxies. Until recently, the most common background sources were quasars. Galaxy-galaxy lenses, however, offer a distinct advantage, since the background source is extended, and therefore imposes a stronger constraints on the mass distribution of the lensing galaxy than a point-source QSO. The Sloan sky survey provides a powerful tool for identifying candidate galaxy-galaxy lenses; this program is using HST-ACS imaging to verify the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.