This week on HST

HST Programs: July 23 - July 29, 2007

Some selected highlights

GO 11081:RR Lyrae stars in M31 Globular Clusters: How did the M31 Spiral Galaxy Form?

A section of the Andromeda spiral, imaged as part of the Lowell Local Group Survey

RR Lyrae variables are evolved, near-solar-mass stars that are passing through the instability strip where it crosses the horizontal branch. With periods of 0.5 to 1.5 days, they have long served as distance indicators for old stellar populations (Baade's Population II). RR Lyrae variables have been known in the Galactic field and in Galactic globular clusters for over 150 years, and they are also present in the older stellar populations of the dwarf spheroidal Galactic satellites. Cluster (or dsph) RR Lyraes are particularly interesting, since their metallicities and ages can be deduced from analysis of the colour-magnitude diagrams for those systems. The first RR Lyraes in neighbouring M31 were identified in the mid-1980s by Pritchet & van den Bergh, using CCD images taken by the Canada France Hawaii Telescope. Those RR Lyraes were field stars, members of the M31 halo population. RR Lyraes in M31 globulars have proved more elusive, largely due to the high star density and consequent image crowding. HST observations, however, finally revealed RR Lyraes in several of the brighter M31 clusters. The present program aims to capitalise on those discoveries, using WFPC2 imaging to monitor, and derived light curves for, RR Lyraes in six M31 clusters.

GO 11100: Two new `bullets' for MOND: revealing the properties of dark matter in massive merging clusters

1E0657-56, the original Bullet cluster: Chandra observations of the X-ray emission are shown in red; the inferred dark matter distribution is shown in blue.

The Bullet cluster, 1E0657-56, was publicised in a press release in August 2006, during the Prague IAU meeting. Originally identified through the strong X-ray emission, the "cluster" actually comprises two galaxy clusters that are undergoing a collision. The X-ray emission, generated by hot gas, is offset from the galaxian cluster members - as expected, given that the intercluster baryonic material in the two clusters will interact dissipationally during the collision. Crucially, however, the dark matter distributions, as determined from weak lensing, remain centred on the galaxies. This runs contrary to the predictions of modified gravity theories, such as MOND. The present program aims to use WFPC2 to obtain deep images of two further cluster mergers, MACSJ0025.4-1222 and MACSJ2243.3-0935, and will use weak lensing analysis to determine the dark matter distributions in those two systems.

GO 11118: Investigating Near-Equinox Atmospheric Change on Uranus

Hubble ACS image of Uranus from 2004

Uranus is unique among the major bodies in the Solar System in that it rotates on its side. Unlike the other planets, Uranus has a polar obliquity of 98o degrees, so its equator is close to perpendicular to the ecliptic plane. Consequently, as Uranus circles the Sun, each pole spends almost half of a Uranian "year" (or 42 terrestrial years) hidden from the Sun in total darkness. On December 7th of this year, Uranus will pass through its equinox, with Sun passing overhead at the Uranian equator. This configuration will provide the first opportunity in modern times for us to view the entire northern hemisphere of the planet, and our first chance to view how the planetary atmosphere reacts to the change from night to day. This program will use NICMOS and WFPC2 to acquire optical and near-infrared images, probing the atmosphere at different depths for changes in structure related to the changes in solar irradiance.

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.