This week on HST

HST Programs: August 7 - August 13, 2006

Some selected highlights

GO 10539: Coronagraphic Imaging of Bright New Spitzer Debris Disks

HST image of the face-on debris disk in the G2 dwarf, HD 107146

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 scattered light from these disks in the presence of the bright parent stars. To date, only a handful of systems have such observations. The present program aims to expand the sample by using Spitzer observations to search for nearby stars with excess flxu at mid-infrared wavelengths - a signature of thermal radiation from circumstellar dust. The present proposal will use the NICMOS coronagraph to search for scattered light from those disks in the near-infrared.

GO 10546: The filaments of NGC 1275

A composite image of NGC 1275

NGC 1275 is the central galaxy in the relatively nearby (~80 Mpc) Perseus galaxy cluster. It has long been known as an unusual system: it is an original Seyfert galaxy (from Carl Seyfert's 1943 paper); in 1954, Baade & Minkowski identified it as the optical counterpart of Perseus A, one of the first extragalactic radio sources (it is also 3C 84); and it is the brightest extraglactic X-ray source, originally detected by Uhuru in 1971. Optically, NGC 1275 has a complex structure, with dust lanes superimposed on an elliptical-like structure. Baade & Minkowski originally suggested that we might be witnessing a galaxy collision, and subsequent observations generally support this hypothesis. There is evidence for substantial star formation, as well as a central black hole that is probably responsible for the strong radio and X-ray emission. Most recently, narrowband images have revealed an extensive network of gaseous filaments that extend more than 100 kpc from the nucleus (see the Hubble Heritage site ). The present proposal aims to us ACS on HST to image NGC 1275 and its immediate surroundings using several filters that sample both continuum radiation and line emission from H-alpha and O II. The observations will be combined with a deep (1 Megasecond) Chandra image, probing the physical mechanisms present in the filamentary structure.

GO 10552: The distance of the Orion Nebula Cluster

The Orion Nebula (from the HST Treasury program)

The Orion Nebula is well known as the nearest example of a relatively massive star-forming region. The Orion Nebula Cluster (ONC) includes several thousand members, ranging from the 15-20 Solar mass O-type stars in the Trapezium to brown dwarfs with masses as low as ~10 times that of Jupiter. The ONC has been observed intensively from ground and space, including surveys by all three current Great Observatories: HST, Spitzer and Chandra. perhaps surprisingly, the actual distance of the cluster is known to an accuracy of no better than ~15%. With a distance of ~500 parsecs, Orion lies beyond the reach of the ESA Hipparcos astrometric mission, and with a highly variable sky background (due to the nebular emission lines), the cluster is not readily accessible to standard ground-based parallax techniques. However, the recent Treasury survey of the ONC by HST revealed a handful of regions where the nebular dust is sufficiently low density (and HST is sufficiently sensitive) that we can actually see through the nebula, and pick out background galaxies. Those galaxies can serve as highly accurate reference points, and the present program aims to use these objects to obtain the first trigonometric parallax measurements for ONC members.

GO 10860: The largest Kuiper Belt Objects

An artist's impression of the view from Sedna

The Kuiper Belt lies beyond the orbit of Neptune, extending from ~30 AU to ~50 AU from the Sun, and includes at least 70,000 objects with diameters exceeding 100 km. Setting aside Pluto, the first trans-Neptunian objects were discovered in the early 1990s. Most were relatively modest in size, with diameters of a few hundred km and photometric properties that suggested an icy composition, similar to Pluto and its main satellite, Charon. Over the last three years, however, a handful of substantially larger bodies have been discovered, with diameters of more than 1000 km; one of the objects, 2003 UB313, is comparable in size to Pluto (2320 km.).Both HST and the Spitzer infrared space telescope have played an important role in these recent discoveries, measuring the angular diameter and the albedo over a wide range of wavelengths. The aim of the present set of observations is to target ~20 trans-Neptunian objects, using red (F606W) images and low-resolution spectra, obtained with the High Resolution Camera on the Advanced Camera for Surveys, to probe their size and chemical composition.