This week on HST

HST Programs: April 13 - April 19, 2009

Selected highlights

GO 11113: Binaries in the Kuiper Belt: Probes of Solar System Formation and Evolution

A composite of HST images 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 will use WFPC2 imaging of known KBOs to identify new binary systems.

GO 11298: Calibrating Cosmological Chronometers: White Dwarf Masses

HST image of the white dwarf companion to Sirius - which isn't a target of the present program

White dwarfs are the evolutionary end point for most stars with masses less than ~7 MSun. These compact degenerate objects lack any internal heat source, and therefore gradually cool from their initial temperatures of ~100,000-200,000K. As they cool, the luminosity decreases from Mbol ~ 2-3 (for the immediate post-PN object) to Mbol ~ 17 (for 10-12 Gyr-old Galactic halo white dwarfs). The rate of cooling can be predicted using sophisticated models of white dwarf interiors. These models show that the rates are mass dependent, but the overwhelming majority of field white dwarfs are expected to have masses in the range 0.6-0.7 MSun, reflecting the steep slope to the IMF above ~1 MSun (high mass stars are rare, so high mass remnants, like Sirius B, are also rare). Confirming that hypothesis demands reliable mass measurements for individual white dwarfs. Fortunately, a number of white dwarfs are known in binary systems, and a subset of those systems are close enough to each other and to the Sun that their orbits can be mapped (and hence their dynamical masses determined) using the Fine Guidance Sensors on HST. The present program targets 4 white dwarf/white dwarf binary systems.

GO 11977: WFPC2 12-Year Proper Motions of Two Galactic Analogs of the SN1987A Rings

Ground-based imaging of the massive binary system, RY Scuti

Supernova 1987A erupted in the Large Magellanic Cloud before HST's launch, but observations since the mid-1990s have revealed extremely interesting phenomena, notably the illumination of circumstellar material that must have been ejected from the star many years prior to the explosion. SN1987A's progenitor was Sanduleak -69o202, a massive blue supergiant, that had probably undergone excursions into the red supergiant regime within the last 10,000 years or so. It is likely that much of the mass loss occurred during thi phase of evolution. The present program targets two high-mass Galactic stars that may well be analogues of SK -69o202: RY Scuti, a massive eclipsing binary system at a distance of ~2 kpc; and Sher 25, a blue supergiant in the massive, young cluster, NGC 3603, some 6 kpc from the Sun. Both stars are surrounded by compact ring nebulae, and Sher 25 also shows evidence for bipolar outflows. Both stars were observed over a decade ago using WFPC2; the present observations will allow an estimate of angular motion which, combined with radial velocity data, should provide a better estimate of the distances, and of the likely future evolution of these systems.

GO 11982: Spanning the Reionization History of IGM Helium: a Large and Efficient HST Spectral Survey of Far-UV-Bright Quasars

GALEX image of the nearby spiral, M81

The reionisation epoch for intergalactic helium is thought to occur somewhere between redshifts 3 and 4. Observations with the GALEX satellite, a NASA small explorer-class mission equipped with a 50-cm diameter telescope, are proving critical in testing this hypothesis through the identification of UV bright quasars in the appropriate redshift range. Galex was launched on 28th April 2003, and continues to operate more than 30 months beyond its nominal lifetime, conducting ultraviolet imaging and low-resolution grism spectroscopy at far-UV (125-175 nm) and near-UV (175-280 nm) wavelengths. Past HST programs by this research have used the ACS/SBC to target sources identified by cross-referencing GALEX against SDSS catalogues of moderate (1 < z < 3) and high redshift (z > 3.1) quasars. 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. The present program will use the ACS/SBC PR120L prism for spectroscopy of 40 QSOs with redshifts in the range 3.1 < z < 5.1.