This week on HST

HST Programs: December 18 - December 24, 2006

Some selected highlights

GO 10508: Orbits, Masses, and Densities of Three Transneptunian Binaries

Preliminary orbital determination for the KBO WW31, based on C. Veillet's analysis of CFHT observations; the linked image shows the improved orbital derivation, following the addition of HST imaging

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 (or trans-Neptunian objects, TNOs) 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. These issues can be addressed, at least in part, through deriving a better understanding of the composition of KBOs - and those properties can be deduced by measuring the orbital parameters for binary systems. The present proposal aims to use the ACS/HRC to determine the relative orbits for several known KBO binaries. Just as with binary stars, the orbital period and semi-major axis give the total system mass, while the mid-infrared properties (measured by Spitzer) allow an assessment of the surface area/diameters; combining these measurements gives an estimate of the mean density.

GO 10792: Quasars at Redshift z=6 and Early Star Formation History

SDSS 1030+0524, one of the high redshift QSOs targeted in this program

One of the major achievements of the Sloan Digital Sky Survey has been the detection of numerous high redshift quasars. These high luminosity objects serve as cosmic lamp-posts when it comes to probing star formation at such early epochs (lookback times exceeding 10 Gyrs): quasars are powered by accretion onto black holes, which are likely to form in high-mass (proto-)galaxies; those galaxies, in turn, are likely to be in high density environments (i.e. proto-clusters); thus, detailed investigations of the properties of high-redshift QSOs is likely to provide a snapshot of galaxy structure at those early epochs. The present proposal concentrates on z>6 QSOs, using the NICMOS grisms to obtain low resolution near-IR spectra that will allow measurement of the relative abundance of Fe and Mg in those objects. Magnesium is an alpha element, generated predominantly in Type II SN; Fe comes predominantly from Type I SN, which require 0.5-1 Gyrs to reach fruition; hence, this ratio provides an estimate of the time that has elapsed since the first outburst of star formation in these systems.

GO 10896: An Efficient ACS Coronagraphic Survey for Debris Disks around Nearby Stars

HST ACS image of the edge-on debris disk around the nearbt F-type dwarf, HD 139664 (g Lupi)

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 disk structure 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. Moreover, the overall statistics provide insight into the lifetimes and evolution of these structures. The coronagraphic imaging supplied by both NICMOS and ACS on HST remains the most effective means of achieving the high-contrast necessary to detect scattered light from these disks in the presence of the bright parent stars. These observations are particularly effective in probing radii that correspond to the Kuiper belt in our solar system. However, the sample of such systems is still rather small. This program aims to build on that foundation by using the ACS coronagraph to survey young, luminous stars near the Sun. This first phase of the survey, carried out with ACS in Cycle 11, included observations of 22 bright, nearby stars, concentrating on spectral types A and F. Two of those stars, HD 53143 and HD 139664, prove to have detectable debris disks. The current, second phase of this survey will target a further 25 stars, with the expectation of detecting between 4 and 6 new debris disks.

GO 10915: ACS Nearby Galaxy Survey

SIRTF image of NGC 2976 (from the SINGS program)

Colour-magnitude diagrams derived from photometric surveys have proven invaluable in achieving an understanding of the main properties of the galactic stellar populations. Large ground-based telescopes allowed extension of this type of analysis to the principal satellites of the Milky Way and, to a limited extent, the Andromeda spiral. With the advent of HST, particularly following SM3B and the installation of the Advance Camera for Surveys, those fundamental CMD studies can be extended to higher-density star fields, fainter magnitudes and intrinsically lower luminosity stars. Until recently, those studies have concentrated on Local Group galaxies; the ambitious aim of the current program is to conduct a systematic survey of all major star-forming galaxies within ~3.5Mpc of the Milky Way, together with a number of galaxies in the M81 group at a distance of ~4 Mpc. The program includes a total of 45 galaxies, ranging from massive spiral systems to dwarf galaxies. The observations are being made using the wide-field camera on ACS, sampling selected fields in the wide-V (F606W) and I (F814W) passbands. Observations of UGC 5808 and NGC 2976 are scheduled during the coming week.