This week on HST

HST Programs: May 7 - May 13, 2007

Some selected highlights

GO 10800: Kuiper Belt Binaries: Probes of Early Solar System Evolution

Composite HST image of the Kuiper Belt binary, WW31

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known KBOs are binary (including Pluto, one of the largest known KBOs, regardless of whether one considers it a planet or not). This is a surprisingly high fraction, given the difficulties involved in forming such systems and the relative ease with which they can be disrupted. It remains unclear whether these systems formed from single KBOs (through collisions or 3-body interactions) as the Kuiper Belt and the Solar System have evolved, or whether they represent the final tail of an initial (much larger) population of primordial binaries. This proposal aims to use ACS/HRC images of known KBOs toidentify new binary systems.

GO 10832: Solving the microlensing puzzle: An HST high-resolution imaging approach

Microlensing light curve produced by a stellar lens with an appropriately placed planetary companion

Gravitational lensing is a consequence of general relativity. The effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. These wide-field imaging surveys target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. Statistical analysis, however, strongly suggests that both the distribution of event durations and the overall number of lenses are inconsistent with a dark matter component. So what are objects doing the lensing? This program aims to answer that question by using WFPC2 to obtain follow-up images of LMC lensed stars that were detected in the initial MACHO survey. Over a decade has elapsed since the lensing event, sufficient time, in at least some cases, for differential motion to separate lens and background star. Thus HST observations can set limits of the fraction of these events that might be produced by ordinary stars in the Galactic disk or halo.

GO 10867: SAINTS - Supernova 1987A INTensive Survey

November 2003 HST image of the SN1987A gaseous ring

SN1987A, in the Large Magellanic Cloud, is (as far as we know) the nearest supernova to the Sun since Kepler's supernova of 1604. While its eruption, in January 1987, predated HST's launch by over 3 years, the remnant has been a regular observational target. Those high resolution observations have revealed the development, and evolution, of extensive, intricate structures as the blast wave from SN1987A encounters the surrounding interstellar medium. In particular, a striking circum-remnant ring has developed, with numerous hot spots stimulated by the fastest moving debris. The present HST program continues to monitor the development of those features, using a series of observations that are co-ordinated with the Chandra X-ray Observatory. During the present cycle, the hotspots may fuse, as the shock fully enters the ring, and photons from these regions may excite previously hidden gas outside the ring, illuminating mass lost from the progenitor before the explosion. The inner debris are now well resolved, and clearly asymmetrical. Overall, these observations provide crucial insight into the earliest stages of formation of a supernova remnant.

GO 10907: New Sightlines for the Study of Intergalactic Helium: A Dozen High-Confidence, UV-Bright Quasars from SDSS/GALEX

GALEX, the Galaxy Evolution Explorer

GALEX is a NASA small explorer-class mission equipped with a 50-cm diameter telescope. Launched on 28th April 2003, it has beaten its nominal lifetime by 17 months (and counting), conducting ultraviolet imaging and low-resolution grism spectroscopy at far-UV (125-175 nm) and near-UV (175-280 nm) wavelengths. The principal science goal is investigating the star formation history of the universe, focusing particularly on star formation in nearby galaxies but also probing to higher redshifts. The present HST proposal aims to target the more distant regime. The GALEX source catalogues have been cross-referenced against SDSS catalogues of high redshift (z > 3.1) quasars, aiming to identify UV-bright QSOs. These sources can serve as effective probes of the ionisation state of the intergalactic medium at intervening redshifts. In particular, analysis of the He II Lyman-alpha absorption will shed light on the epoch of reionisation of intergalactic helium, generall placed between redshifts 3 and 4.