This week on HST

Ground-based (Palomar) image of the HD 32297 circumstellar debris disk

It is now well established that planet formation occurs within circumstellar disks.The past decade has seen the identification of many examples of such phenomena around young stars, notably through infrared observations by the Spitzer space telescope. The general indications are that the planet-formation process occurs rapidly, with the disks largely dissipating within 10-15 million years of formation. Key questions that remain, however, include the rate of gas depletion within the disks, the extent to gas survives within the debris disks and the abundance distribution of any residual gaseous materials. Most examples of circumstellar disks are found in stars in young star-forming associations, but there are a number among isolated, young stars in the Solar neighbourhood. One such is the relatively nearby (d~140 pc) A star, HD 32297, where a near edge-on disk has been identified through ground-based imaging. The present HST program aims to take advantage of that orientation by using STIS of the the primary star to search for absorption features introduced by circumstellar material along the line of sight.

GO 11700: Bright Galaxies at z>7.5 with a WFC3 Pure Parallel Survey

The ACS optical/far-red image of the Hubble Ultra Deep Field

Galaxy evolution in the early Universe is a discipline of astronomy that has been transformed by observations with the Hubble Space Telescope. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance, and the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS). The highest redshift objects found in the UDF have redshifts approaching z~7. Pushing to larger distances, and greater ages, demands observatons at near-infrared wavelengths, as the characteristics signatures of star formation are driven further redward in the spectrum. Wide Field Camera 3, installed in Servicing Mission 4, is well suited to these observations, and a number of programs are in place in Cycle 17 that address these issues. Indeed, WFC3 is employed in pure parallel mode by several programs. These take advantage of other science programs, usually with COS, that involve 2-5 orbit pointings on sources at high galactic latitude. The WFC3 pointing is unplanned, since it depends on the orientation adopted for the prime observations, but 2-5 orbits of IR imaging can reach galaxies at redshifts exceeding z=7 (potentially even z~8) in high latitude fields. This is one of two such programs in the cycle 17 portfolio.

GO 12065: 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.

GO 12433: Coordinated HST, Venus Express, and Venus Climate Orbiter Observations of Venu

Artist's impression of the ESA mission Venus Express in orbit around Venus

Ground-based amateur astronomers are always warned never to look directly at the Sun with a telescope (or, come to thast, with the naked eye). The same consideration holds for HST; indeed, HSt is normally unable to observe any targets that lie within 50 degrees of the Sun (the solar avoidance zone). This has obviously restricted HST's ability to provide information on objects in the inner Solar System, but there are a few exceptional occasions when it is possible to (legally) work around the observing constraints. One case arises when Venus is at its maximum elongation from the Sun. At that time, the angular separation is ~45 degrees, so the planet is still within the solar avoidance zone. However, it is possible to obtain observations by taking advantage of HST's location in low-Earth orbit. During each 90-minute orbit, the Earth occults the Sun and HST enters "night". If Venus is in the appropriate elongation, then Venus will appear from behing the Earth before the next HST "sunrise". If HST is placed in the appropriate orientation, it can obtain ~5 minutes observations before slewing away to avoid the Sun. HST used this technique to observe Venus in 1997, and the same techniques are being used in the current cycle to obtain STIS ultraviolet spectra to complement in-situ observations being made the ESA's Venus Express satellite.