This week on HST

HST Programs: June 18 - June 24, 2007

Some selected highlights

GO 10504 Characterizing the Sources Responsible for Cosmic Reionization

A 3-D model of cosmic reionisation at redshift z=5.81

The standard cartoon for the evolution of the Universe envisages three distinct phases following the Big Bang: the `recombination era', spanning the first few hundred thousand years, where the ionised hydrogen and helium plasma generated by the Big Bang gives way to cooler, neutral atomic gas; the `dark ages', lasting ~500,000 years, where the Universe is populated by neutral absorbing gas; and `cosmic reionisation', where the first episodes of star and black hole formation lead to photoionisation of neutral hydrogen, and much increased transparency in the inter(proto-)galactic medium. A key question for cosmological investigations is the identity of the cosmic reionisers - were quasars (i.e. massive black holes) primarily responsible, or were more conventional star clusters, or Population III objects, the key ingredient? The aim of this proposal is to use gravitational lensing by massive clusters to probe the universe at redshifts beyond z=6. Combining NICMOS and ACS imaging with mid-infrared observations with Spitzer, the goal is to identify a representative sample of luminous objects at redshifts 6 < z < 10, laying the foundation for future more detailed investigations with JWST.

GO 10810: The Gas Dissipation Timescale: Constraining Models of Planet Formation

HST ACS image of the face-on debris disk around the nearby G dwarf, HD 107146

While much debate has raged in recent months over exactly how to define a planet, there is very little debate in the astronomical community about where planets form: they form in circumstellar disks. During the earliest stages of their existence, the disks are dusty, gas-rich and high opacity; for example, see NICMOS images of T Tauri stars and IRAS sources and current HST proposals 10540 and 10864. After only ~10 million years, however, most of the gas dissipates, leaving a young planetary system with a rich content of dust, rocks, planetoids and planets. This period corresponds to the high bombardment phase in earth's history, when the Moon was formed. To the outside observer, the dusty disk has low surface brightness, and is much less prominent than the gaseous disk. HST can image these disks via scattered light at near-infrared and, in a few cases, optical wavelengths - probably the most spectacular example is Beta Pic (see the recent HST ACS images ). Recent models suggest that, while these debris disks have a much lower gaseous content than classic T Tauri disks, they should retain some gas content, which can be crucial in influencing planet formation. The aim of the present program is to use the prism on the UV-sensitive ACS Solar Blind Channel SBC) to search for molecular hydrogen absorption at 1600 Angstroms. The program targets 11 systems with ages between 10 and 50 Myrs, including two members of the TW Hydrae association.

GO 10915: ACS Nearby Galaxy Survey

SIRTF image of NGC 2976 (from the SINGS program)

Colour-magnitude diagrams derived from photometric surveys have proven invaluable in achieving an understanding of the main properties of the galactic stellar populations. Large ground-based telescopes allowed extension of this type of analysis to the principal satellites of the Milky Way and, to a limited extent, the Andromeda spiral. With the advent of HST, particularly following SM3B and the installation of the Advance Camera for Surveys, those fundamental CMD studies can be extended to higher-density star fields, fainter magnitudes and intrinsically lower luminosity stars. Until recently, studies have concentrated on nearer Local Group galaxies; the ambitious aim of the current program is to conduct a systematic survey of all major star-forming galaxies within ~3.5Mpc of the Milky Way, together with a number of galaxies in the M81 group at a distance of ~4 Mpc. The program includes a total of 45 galaxies, ranging from massive spiral systems to dwarf galaxies. Initially, observations were made using the wide-field camera on ACS, sampling selected fields in the wide-V (F606W) and I (F814W) passbands. Following the ACS failure in January 2007, the program has been re-designed, focusing on the larger galaxies within ~3.5 Mpc. Observations of IC 5152, are scheduled during the coming week.

GO 11072: Measuring the Physical Properties of the first two WASP transiting extrasolar planets

Artist's impression of a planetary transit & a partial light curve

Transiting extrasolar planets offer particularly valuable insight into the structure of these non-Solar System gas giants. Besides providing direct measures of mass (with no complications for v sin(i)) and radius (from accurate time-series photometry), spectroscopic observations obtained during either transit or planetary eclipse can probe the atmospheric structure and chemical composition. The first such systems, including HD 209658b and HD 189733b, were originally discovered as radial velocity variables, and only subsequently identified as transits based on follow-up photometry. However, the last few years have seen the instigation of almost a dozen wide-field photometric surveys that are searching specifically for such systems - and those surveys are starting to bear fruit. The present HST program aims to obtaina time series of NICMOS narrowband images of two exoplanet hosts identified by the Wide Angle Search for Planets (WASP) project. Like most wide-field surveys, this program uses a battery of mini-telescopes (or large camera) to rapidly scan selected areas of the celestial sphere. The original data have sufficient photometric accuracy to allow the detection of transiting planets (which typically produce photometric signatures of depth 0.5-1%). The higher-precision follow-up observations with HST will allow more accurate determinations of the eclipse depth, and hence stronger constraints on the planetary radii.