This week on HST

HST Programs: March 21, 2011 - March 27, 2011

Selected highlights

GO 11613: GHOSTS: Stellar Outskirts of Massive Spiral Galaxies

The (relatively) nearby barred spiral galaxy, NGC 253

The subdwarf stars that populate the Galactic halo are generally recognised as fossil remnants of the first episode of substantial star formation to afflict the Milky Way galaxy. The structure and density distribution of our own halo has been inferred partly from deep starcounts, partly from globular cluster systems and partly from the kinematics of local subdwarfs; most analyses favour a near-spherical system with density r-3.5. The aim of this proposal is to extend these studies to a number of other nearby spiral systems. ACS and WFC3/UVIS are being used to obtain SNAP observations in the F606W and F814W filters in regions within the galactic halos. The observations are capable of obtaining photometry extending 2-3 magnitudes below the tip of the red giant branch in those galaxies. The data should permit separation of the contributions from disk, thick disk and bulge, and isolation of the halo population. The observations will allow measurement of the halo metallicity distribution and an estimate of the shape of the halo in these systems.

GO 11616: The Disks, Accretion, and Outflows (DAO) of T Tau stars

Wide-field image, from NOAO, of T Tauri and its immediate environs

The T Tauri stage of evolution occurs early in a star's lifetime, within ~10 Myrs of its birth, when it still retains a dense, dust and gas-rich circumstellar disk. During this phase, there is substantial accretion of material onto the central star. This leads to heating of the inner regions of the accretion disk, and significant emission at ultraviolet and X-ray wavelengths. Previous HST programs (e.g. GO 10840 ) have used the STIS and the ACS/SBC to investigate these processes at FUV wavelengths. The present program will extend those investigations using COS, which provides more than an order of magnitude more sensitivity and resolution. The survey will target 32 T Tauri stars, including 26 "classical" T Tauris and 6 "weak-lined" T Tauris (the latter are surrounded by less disk material, and are generally believed to be at a later stage of evolution than the CTTs). COS will be used to measure the emission profiles of an extensive number of lines, probing opacities, temperatures and densities in the disk and outflow regions.

GO 12066: Through a Lens, Darkly - New Constraints on the Fundamental Components of the Cosmos

The ACS optical/far-red image of the galaxy cluster, Abell 2218; note the lensed arcs

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. The present program aims to capitalise fully on HST's imaging capabilities, utilising the refurbished Advanced Camera for Surveys and the newly-installed Wide-Field Camera 3 to obtain 14-colour imaging of 25 rich clusters. The data will be use to map the mass profiles of the clusters and probe the characteristics of the high-redshift lensed galaxies. Since ACS and WFC3 can be operated in parallel, the program will also use parallel imaging in offset fields to search for high-redshift supernovae. The present observatiosn target the cluster Abell 2261, at redshift z=0.224.

GO 12169: The frequency and chemical composition of planetary debris discs around young white dwarfs

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typcail targets0, they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years.In subsequent years, a significant number of DA white dwarfs have been found to exhibit narrow metallic absorption lines in their spectra. Those lines are generally attributed to "pollution" of the white dwarf atmospheres. Given that the diffusion time for metals within the atmospheres is short (tens to hundreds of years), the only reasonable means of maintaining such lines in ~20% of the DA population is to envisage continuous accretion from a surrounding debris disk. The present program aims to address this question by using COS to obtain UV observations of young white dwarfs, probing correlations with progenitor mass and examining the detailed composition of the accreted materials.