This week on HST

HST Programs: February 25 - March 2, 2008

Some selected highlights

GO 11107: Imaging of Local Lyman Break Galaxy Analogs: New Clues to Galaxy Formation in the Early Universe

Mosaic of HST images of M82, the best-known starburst galaxy

Current Big Bang cosmological models predict that the universe should have undergone a global re-ionisation at redshifts between 6 and 20. The first generation of stars is generally tapped as the most likely source of the ionising radiation, perhaps enhanced through merger-stimulated starbursts. Direct observations of those galaxies are not possible at present; the James Webb Space Telescope is expected to open up observations of these systems. In consequence, there is considerable interest in identifying galaxies at lower redshifts that could serve as analogues for the z>6 systems. Over the last few years, the Galaxy Evolution Explorer (GALEX) has proved an important new tool in identifying candidate objects. GALEX has conducted an all-sky survey at ultraviolet wavelengths, and has uncovered sigificant numbers of UV luminous galaxies at low and moderate redshifts. Many of these galaxies are starbursts, undergoing substantial outbursts of star formation. These galaxies have been categorised as "compact UV luminous galaxies" (UVLGs). These appear to be galaxies that are undergoing small-scale mergers, leading to extensive dissipation and vigorous star formation. The present program is using the ACS/SBC prism and WFPC2 to obtain ultraviolet spectra and R-band images of 31 systems, probing the star formation history and its variation with environment.

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 11157: NICMOS Imaging Survey of Dusty Debris Around Nearby Stars Across the Stellar Mass Spectrum

The debris disk around beta Pic

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The structure of those disks is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Analysis of the rangle of morphological structure in these systems provides insight into the distribution of properties of planetary systems. HST currently provides the only means of achieving the high-contrast required for the detection of these scattered light disks in the presence of the bright parent stars. The present proposal is using NICMOS to target 22 nearby stars that have a strong mid-infrared excess, based on combining optical, near-infrared and mid-infrared IRAS observations. Most of the targets are early-type stars, with spectral types ranging from late-B to mid-G.

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.