This week on HST

HST Programs: July 10 - July 16, 2006

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 10624: Solving the mystery of short-hard gamma-ray bursts

An artist's impression of a gamma-ray burst

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts. The long/soft bursts appear to originate in the collapse of very massive stars, while the short/hard bursts are coalescing binary systems (probably pairs of netron stars or black holes). The first optical counterpart to a gamma ray burst was identified in 1998, allowing confirmation of their extragalactic nature, and, since then, more than 50 bursts have been detected at X-ray wavelengths, and half that number detected at either optical or radio wavelengths; all of these detections are long/soft bursts. The aim of this program is to focus on the short/hard bursters. This is a Target of Opportunity project, triggered by the detection of a suitable candidate by the Swift satellite, followed by the rapid (< 3 days detection of an optical afterglow. The aim is to obtain ACS observations (weither WFC or HRC) within 5-6 days of the burst, and use the resultant images to characterise the underlying host galaxy.

GO 10760 Black Hole X-ray Novae in M31

XMM-Newton X-ray image of the central regions of M31; the red dot marks a very active X-ray nova

X-ray novae are generally believed to originate in close binary systems that consist of a red dwarf and a highly compact companion, either a neutron star or a black hole. As in cataclysmic binaries (where the compact object is a white dwarf), orbital energy loss leads to the two components spiralling closer together, prompting Roche lobe overflow from the red dwarf onto the compact object. As that material is accreted onto the neutron star/black hole, it forms an accretion disk, releasing energy at a relatively modest level in the form of UV and X-ray radiation. X-ray outbursts occur when material from the disk accretes directly onto the NS/BH, and those outbursts can achieve luminosities that exceed 1037 erg/s. Consequently, these objects are easily visible in nearby galaxies, including both M33 (Trapezium) and M31 (Andromeda). Our bird's-eye view of those systems allows an assessment of the relative numbers of neutron star and black hole systems. The present program builds on extensive previous Chandra and HST observations of the central regions of M31, and aims to use ACS multi-colour imaging to probe active black hole X-ray novae, identified from near-contemporaneous Chandra-ACIS observations.

GO 10923 Measuring the size of the close-in transiting extrasolar planet HD 189733b

Key events in a planetary transit

HD 198733 is a 7th magnitude G5 dwarf that lies at a distance of ~20 parsecs from the Sun in the constellation of Vulpecula. Like many other nearby solar-type stars, HD 189733 has an associated planetary system, including a hot Jupiter, a ~1.15 MJ gas giant with an orbital period of 2.12 days. Most significantly, that inner planet transits the central star, making HD 189733 the closest transiting system found so far. Transiting systems offer a potential gold-mine for extrasolar planetary studies, since not only is the orbital inclination well defined, but the diameter (and hence the average density) is directly measureable form the eclipse depth, while the atmospheric composition can be probed through line absorption or re-radiated thermal flux. The results from these measurments can be used to test, and improve, theoretical models of extrasolar planets. These observations are best done from space (indeed, the only successful atmospheric observations to date have been with HST and Spitzer). The aim of the present program is to use the ACS High Resolution Camera coupled with a grism to derive very accurate measurements of the light curve during a series of transits, and obtain correspondingly accurate constraints on the planetary diameter.