This week on HST

HST Programs: April 6 - April 12, 2009

Selected highlights

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 11981: FUV imaging survey of Galactic open clusters

The Eagle Nebula and the M16 star cluster, one of the galactic clusters imaged in this program

Concensus holds that the overwhelming majority of stars within the disk of the Milky Way formed in high density star clusters, comparable with the Orion Nebula Cluster or (pictured here) M16. At young ages, these massive clusters are characterised by the presence of hot, high luminosity O and B-type stars which photoionise gas and generate strong stellar winds. Ultraviolet imaging, especially imaging in the far-UV, is an extremely effective means of probing both the distribution of ionised gas (through continuum emission) and of dust (by scattering) within and around these clusters. In addition, measurement of the UV spectral energy distributions of individual young stars can probe the extent and state of circumstellar disks, which generate ultraviolet emission. In addition to probing the cluster environment, these observations can be used to search for binary and multiple systems. The present proposal will use WFPC2 to survey six young and intermediate-age clusters, including M87, M16 and NGC 6281.

GO 11990: Determining the Physical Properties of Comet 103P/Hartley 2 in Support of NASA's EPOXI Mission

An artist's concept of the Deep Impact spacecraft flying past a comet. Credit: NASA/JPL/UMD/Pat Rawlings

Sometimes even space missions can go green: NASA's EPOXI mission (Extrasolar Planet Observation and Deep Impact Extended Investigation) was launched in January 2005 as Deep Impact. The original mission involved a successful encounter with Comet Tempel 1 on 4 July 2005, during time an impactor was "dropped" onto the cometary nucleus, ejecting cometary material that was imaged by the main vehicle. Since the spacecraft was still operational, with significant fuel, the decision was taken to use gravitational encounters (with Earth) to target another comet. The initial choice was Comet Boethin, a periodic comet on a ~22 year orbit; however, extensive surveys failed to reveral any trace of the comet, and the suspicion is that it has been disrupted. Consequently, EPOXI was re-targeted at Comet 103P/Hartley 2, discovered on UK Schmidt plates by Malcolm Hartley in 1986 and subsequently recovered through a variety of ground-based observations. The comet will make a close approach to Earth in 2010, possibly reaching naked eye brightness, and the EPOXI encounter is targeted for November 4 in that year. One of the critical pieces of information required in planning the mission is the rotational period of the nucleus. Cometary nuclei ae asymmetrical, so accurate photometry can be used determine the period. The present proposal will use WFPC2 on HST to determine the light curve, and hence map the rotation of the nucleus.