This week on HST

HST Programs: December 28, 2008 - January 4, 2009

Selected highlights

GO 11236: Did Rare, Large Escape-Fraction Galaxies Reionize the Universe?

Lyman alpha image of the radio galaxy, 4C41.17

In Big Bang cosmology, the early history of the unverise is characterised by three distinct phases: the initial expansion, during which time Big Bang nucleosynthesis occurs, and the universe cools from its initial exceedingly high temperatures; recombination, which occurs at a redshift z~1,100 (or an age of ~400,000 years), when the Universe was cool enough to allow neutral hydrogen to become dominant, leading to high opacity and the cosmic microwave background; and reionisation, when energy sources reionised hydrogen, reducing the opacity of the intergalactic medium and restoring transparency. Reionisation is generally believed to have occurred at a redshift between z~6 and z~20, with the ionising sources either (or both) the first generation of stars (Population III starbursts) and/or proto-quasars. The IGM remains ionised thereafter. A key issue in developing an understanding of this process is assessing how readily starburst-generated Lyman-alpha emission escapes from galaxies, and how starbursts contribute to reionisation at intermediate redshifts. This proposal aims to quantify this issue by targeting a large sample of starburst galaxies at redshifts z~0.7. the galaxies all lie within the region covered by the COSMOS survey, and will be observed at ultraviolet wavelengths using the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC).

GO 11788: The Architecture of Exoplanetary Systems

Artist's impression of a young planetary system

Immanuel Kant is generally credited with first proposing that the planets in the Solar System coalesced from a flat, rotating disk formed by the Solar Nebula. Direct confirmation of that process only came in the early 1990s, when millimetre-wave interferometers were able to detect molecular gas in Keplerian rotation around a handful of nearby young stars. Since then, there have been numerous other observations, including Hubble's images of proplyds (protoplanetary disks) in the Orion Cluster, and Hubble and Spitzer observations of edge-on disks in other young stars. One of the clear selling points of the Solar Nebula disk model is that it appears to offer a natural path to forming planets with coplanar orbits, matching (most of) our observations of the Solar System. On the other hand, as our knowledge of exoplanetary systems has accumulated over the last decade, it has become clear that dynamical interactions may play a very important role in the evolution of these systems. In particular, disk/planet interactions are generally regarded as responsible for the inward migration of gas giants to form hot Jupiters in <3 day period orbits. Planet-planet interactions could lead to significant changes in orbital inclination. Radial velocity planet searches are uncovering more and more multi-planet systems. This program focuses the high precision of HST's astrometric detectors, the Fine Guidance Sensors, on four of those systems. The aim is to complement the existing radial velocity measurements with sub-milliarcsecond precision astrometry, allowing determination of the true orbital paths - specifically, the relative inclination - of the low-mass objects in these systems.

GO 11945: Asteroseismology of Extrasolar Planet Host Stars

Sun-like stars undergo a variety of low-level pulsations driven by internal instabilities. Asteroseismology uses those pulsations to study the internal structure of stars. Not surprisingly, those oscillations were first discovered in the Sun. In the early 1960s, Robert Leighton used the 60-foot solar tower on Mt. Wilson to obtain spectroheliograms of the Sun, narrowband images centred on Zeeman-split lines that showed the velocity structure across the surface; those data revealed periodic variations with P~296 seconds, the 5-minute solar oscillations. Detecting such variations require extemely high signal-to-noise; nonetheless, observations have been extended to a handful of other stars. In particular, ESA's COROT mission has detected recently pulsations in three F-type stars. The present program will use the Fine Guidance Sensors on HST to measure the pulsational modes in the star HD 17156, an 8th magnitude G-type subgiant at a distance of ~ 78 parsecs from the Sun. The crucial characteristic of this star is that it harbours a planetary system where at least the innermost hot Jupiter, HD 17156b, transits the host star. Those transits provide a measure of the stellar radius, and hence the mean density. If multiple pulsational modes are detected with the FGS then those data will provide an entirely independent measurement of the internal density structure, and can determine the stellar age to an accuracy of 5-10%. Identifying those modes requires collecting close to ~1012 (one thousand billion, or one trillion) photons. To achieve this, HST will take advantage of the fact that HD 17156 lies in the Continuous Viewing Zone (CVZ) at this time of year, and will stare exclusively at that star from late on December 21st through to January 1st 2009, a span of 148 orbits.