This week on HST

HST Programs: March 5 - March 11, 2007

Some selected highlights

GO 10782: Quit winking: Jupiter opens its other eye

ACS imaging of the "Little Red Spot" from April, 2006

In the late 1930s, bright white clouds expanded and encircled Jupiter's southern hemisphere in a band near 33 degrees south planetographic latitude. Those clouds collapsed into three large anticyclonic storms, later named the White Ovals, that were second in size only to the Great Red Spot. In the mid to late 1990s these storms suddenly approached each other very closely, resulting in the 1998 merger of two of the storms. In 2000, the third oval also merged, leaving one remaining large White Oval. Early in 2006, amateur observers noted that the oval appeared to be changing in appearance and turning red. While small red spots do appear on Jupiter from time to time, they usually form as a colored spot, cloud over, and become white, rather than the opposite. Using the High Resolution Camera (HRC) on ACS, HST obtained a series of images of the `Little Red Spot' in early 2006. The multiwavelength observations probed the vertical structure of the storm, investigating possible mechanisms that might have prompted these changes. The current program complements those observations, using WFPC2 to investigate the long term evolution of the strom system.

GO 10847: Coronagraphic Polarimetry of HST-Resolved Debris Disks

HST ACS/HRC coronagraphic images of debris disks around two nearby stars

The last decade has seen the identification of debris disks around a number of stars in the Solar neighbourhood. The coronagraphic capabilities available on HST have been particularly useful in this regards, since the lower background present in space-based observations offers significant advantages in detecting these low surface brightness features. Those observations are complemented by mid-infrared surveys (notably by Spitzer) of denser, warmer disks in young stars. The latter observations detect thermal emission from the dust grains, permitting some constraints to be set on the chemical composition. The present program aims to use polarimetric observations of debris disks to probe the distribution of grain sizes and the density distribution of dust in those somewhat older systems. Observations of the A0 star, HD 32297, and the G3 dwarf, HD 61005, are scheduled in the coming week.

GO 10989: Astrometric masses of extrasolar planets and brown dwarfs

Artist's impression of one of the planets circling Epsion eridani (from Nova Celestia )

The overwhelming majority of extrasolar planetary systems have been identified through radial velocity monitoring, and the detection of the reflex motion of the parent star as it orbits the common center of mass of the system. Just as radial velocities measure the stellar "wobble" introduced along the line of sight, so high precision astrometry can be used to measure motion in the plane of the sky. Combining these data gives the full three-dimensional motions of the system, and a direct measure of the mass of the planetary companion. The Fine Guidance Sensors on HST are the only system currently capable of making observations at the required sub-milliarcscond accuracy, and has already been used for astrometry of several systems, including the M dwarf Gl 876 and, most recently, Epsilon Eridani. The current GO program pursues observations of six planetary hosts, and FGS observations of HD 168443 and HD 38529, which has at least two planetary companions, are scheduled over the next week.

GO 11083: The Structure, Formation and Evolution of Galactic Cores and Nuclei

A wide-field view of the Virgo cluster

The Virgo cluster, lying at a distance of ~20 MPc, is the nearest large galaxy cluster. The cluster embraces more than 2,000 galaxies, with masses ranging from ~3 x1012 MSun for the central giant elliptical, M87, to ~109 MSun dwarf systems, predominantly ellipticals but with a smattering of irregulars. In Cycle 11, the same proposal used the Advanced Camera for Surveys to observe more than 100 elliptical and S0 galaxies in Virgo, obtaining deep images in the SDSS g (green) and z (far-red) passbands. These observations show a systematic variation with luminosity in the surfacve brightness profile. Giant ellipticals have a relatively flat brightness profile in the central regions, while dwarfs tends to have compact nuclei. The aim of the current proposal is to use ultraviolet (WFPC2/F225W) and near-infrared (NIC1/F160W) images of the central regions to probe star formation and the star formation history on scales of 0.1-1.0 arcsecond (10-100 parsecs).