This week on HST

HST Programs: May 26 - June 1, 2008

Cycle 17 TAC results released on May 28, 2008

Some selected highlights

GO 10487: A Search for Debris Disks in the Coeval Beta Pictoris Moving Group

WFPC2 image of Beta Pic's disk

Over the last few years, there has been growing recognition of the presence of a number of young stellar associations, each comprising 20-50 stars and brown dwarfs, passing through the Solar Neighbourhood. Beta Pictoris (or Gl 219), a relatively young (~20 Myrs old) A5 dwarf, lying at a distance of just under 20 parsecs form the Sun, is the `flagship' member of one of those groups.. The IRAS satellite originally singled out this A star as unusual, detecting significant excess radiation over the expected photospheric fluxes at mid-infrared wavelengths; Smith & Terrile (1984) used ground-based coronagraphic observations to show that this was due re-radiation from a substantial debris disk. A further 30 stars are associated with this young group, with spectral types ranging from A to late M. Several of these stare are known to also have dusty disks, including the binary HR 7329 (A0 + M7) and the nearby active M1 dwarf, AU Mic (Gl 803). The present proposal aims to extend the sample by using coronagraphy to search for debris disks around 21 members of the moving group. The original program used the ACS/HRC; following the ACS failure, observations were rescheduled with the NICMOS coronagraph.

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 11195: Morphologies of the Most Extreme High-Redshift Mid-IR-luminous Galaxies II: The `Bump' Sources

HST images of interacting ultra-luminous IR galaxies

Luminous infrared galaxies (LIRGs) are systems that have total luminosities exceeding 1011.4 LSun, with most of the energy emitted at wavelengths longward of 10 microns. Many (perhaps most) of these galaxies are interacting or merging disk galaxies, with the excess infrared luminosity generated by warm dust associated with the extensive star formation regions. Many systems also exhibit an active nucleus, and may be in the process of evolving towards an S0 or elliptical merger remnant. Until recently, very few candidate such systems were known at high redshifts; consequently, analyses and investigations of their origins had to rely on observations of low- and moderate-redshift analogues. The team leading this HST proposal have used a combination of mid-infrared (24 micron) and near-infrared observations to identify tens of candidates, and Spitzer follow-up spectroscopy has confirmed that many lie at redshifts 2 < z < 2.5. The sources appear to fall into two broad categories: thopse with bright 24-micron fluxes with power-law spectral energy distributions and SiO absorption at mid-IR wavelengths; and "bump" sources, with an SED that peaks near 1.6 microns (rest wavelength) and PAH absorption. The brighter sources were targeted in a Cycle 15 program; the present program uses NICMOS and WFPC2 to obtain high sensitivity, high angular-resolution observations of 22 of the "bymp" systems.

GO 11227: The orbital period for an ultraluminous X-ray source in NGC1313

Artist's impression of an ultraluminous x-ray source

Ultraluminous X-ray sources have luminosities that exceed the Eddington luminosity for stellar-mass black holes by one or two orders of magnitude. These systems are found in the outer regions of nearby galaxies, mainly actively star-forming systems. None are known within the Milky Way galaxy. There are currently several models for their origin, including beamed emission from conventional black holes and accretion onto intermediate-mass black holes (IMBH). The latter objects are hypothesised to have masses between 100 and a few thousand MSun, sufficiently low that they can avoid spiraling in to the galactic centres. In both cases, the black hole is hypothesised to lie in a binary system. with the stellar companion providing the accreted material. The present proposal aims to differentiate between those models through photometric of monitoring several sources, notably the extremely active source X-2 in the starburst galaxy, NGC 1313. The aim is to search for periodicities that might establish the orbital period, combine those data with radial velocity measurements, and hence obtain a dynamical estimate for the black hole mass.