This week on HST

HST Programs: March 2 - March 8, 2009

Selected highlights

GO 11972: Investigating the Early Solar System with Distant Comet Nuclei

The nucleus of Comet Borelly, as resolved by Deep Space 1

Comets are among both the least substantial and most spectacular inhabitants of the Solar System. They are believed to have played an important role in the early Solar System, delivering water to Earth shortly after its formation. Near the Sun, great comets can spawn tails that are millions of miles in length, stretching over tens of degrees. The source of that splendour is a small cometary nucleus, typically no more than 10-20 kilometres in size. The aim of the present program is to use WFPC2 to image eight dynamically new, long-period comets when they are at large distances from the Sun, and their activity levels are low. Those data will be used to estimate the sizes of the individual nuclei, providing information that can be fed into models used to predict thermal properties and outgassing behviours.

GO 11975: UV light from old stellar populations: a census of UV sources in Galactic Globular Clusters

Hubble Heritage image of the globular cluster, M15

Globular clusters are the oldest structures within the Milky Way that are directly accessible to observation. They are relatively simple systems, with relatively simple colour-magnitude diagrams (albeit with some complexities adduced from recent HST observations, see GO 11233 ). Matching those CMDs against theoretical models not only allows us to set constraints on the age of the oldest stars in the Galaxy, and hence on the age of the Milky Way and the epoch of galaxy formation, but also probes the range of properties of stellar populations at these ages. In the latter respect, a long-standing issue centres on the morphology of the the horizontal branch - specifically, the relative number of UV-bright, extreme horizontal branch stars. Such stars are believed likely to be the source of UV light in (otherwise red and dead) ellipticla galaxies, but the exact origin (or origins), and frequency, of EHB stars remains unclear. The present HST program will use WFPC2 and the ACS/SBC to map the central regions of 46 globular clusters, and carry out a census of UV-bright stars.

GO 11978: Luminous and dark matter in disk galaxies from strong lensing and stellar kinematics

Strong galaxian lens from the SLACS survey

Gravitational lensing provides a powerful method of tracing the mass distribution both within galaxy clusters and for individual galaxies. At the same time, lensing amplifies the light from background galaxies to allow detailed investigation of their properties. The present proposal builds on the extensive catalogue of strong lensing systems uncovered by the Sloan Digital Sky Survey, focusing on lenses produced by (foreground) disk galaxies. The aim is to use the lens morphology to probe the structu8re and extent of the dark matter halos of those galaxies. Previous analyses have been limited by the sparse number of such sources, and complications introduced by different viewing angles. However, with the advent of SDSS and the asociated lensing surveys, the number of lenses produced by edge-on disk galaxies has increased substantially, permitting detailed measurements not only of the morphology lensed galaxy, but also of the kinematics of the foreground lens. This program aims to obtain images of up to 20 spiral systems, spanning a range of types and disk/bulge ratios. The multicolour (F450W, F606W, F814W, or ~B/V/I) observations will be made using WFPC2.

GO 11984: Imaging Saturn's High Latitude Aurorae

Planetary aurorae are stimulated by the influx of charged particles from the Sun, which travel along magnetic field lines and funnel into the atmosphere near the magnetic poles. Aurorae therefore require that a planet has both a substantial atmosphere and a magnetic field. They are a common phenomenon on Earth, sometimes visible at magnetic latitudes more than 40 degrees from the pole, and have also been seen on Jupiter, Saturn, Uranus and Neptune. In August 2009, Saturn reaches its equinox, with the Sun moving into the northern Saturnian hemisphere, and Earth crosses the ring plane in mid-September. As a result, it is now possible to view both Saturnian poles simultaneously. This proposal aims to use the ACS/SBC to study aurorae in the both hemispheres, comparing their relative characteristics against observations from previous years. In particular, the southern aurorae tend to occur in an oval whose centre is displaced from the south (rotational) pole. Does the northern hemisphere exhibit a similar phenomenon? These observations are correlated with in situ measurements by the Cassini spacecraft.