This week on HST

HST Programs: October 22 - October 28, 2007

Some selected highlights

GO 11170: UV Imaging of the Martian Corona and the Escape of Hydrogen

The Martian atmosphere

Mars possesses a very thin atmosphere that consists mainly of carbon dioxide (~95%). The other major constituents are nitrogen (~2.7%) and argon (~1.6%), with oxygen and water vapour contributing trace amounts. The surface pressure is only 7 millibars, or less than 1% of Earth's current atmosphere. This tenuous body is the remnant of a much denser envelope that was gradually dispersed primarily through interactions with solar radiation and particles in the solar wind. Those interactions still take place, generating an extended corona of very low density gas, including hydrogen from dissociated water vapour. The aim of the present program is to use the Solar Blind Channel on the ACS to search for coronal Lyman alpha emission. The observatons are co-ordinated with in situ measurementsby the SPICAM instrument on Mars Express.

GO 11194: Beyond the Bullet: Direct Detection of Dark Matter in Merging Galaxy Clusters

1E0657-56, the original Bullet cluster: Chandra observations of the X-ray emission are shown in red; the inferred dark matter distribution is shown in blue.

The Bullet cluster, 1E0657-56, was publicised in a press release in August 2006, during the Prague IAU meeting. Originally identified through the strong X-ray emission, the "cluster" actually consists of two galaxy clusters that are undergoing a collision. The X-ray emission, generated by hot gas, is offset from the galaxian cluster members - as expected, given that the intercluster baryonic material in the two clusters will interact dissipationally during the collision. Crucially, the dark matter distributions, as determined from weak lensing, remain centred on the galaxies. 1E0657-56, however, is only one cluster, and there is the possibility, however remote, that a conspiracy of circumstances and chance alignments has given rise to the observed characteristics. The present program aims to set that concern at rest through WFPC2 observatons of a second cluster, Abell 1758.

GO 11229: The Search for Evolution of Emission from Dust in Supernovae with HST and Spitzer

Supernova 1987A in the LMC, an HST image acquired by WFPC2 in 1998

Type II supernovae are produced by core collapse in stars exceeding ~7 solar masses. These massive stars support a complex series of nuclear reactions on the main sequence, ranging from hydrogen fusiuon near the surface to silicon reactions that build up the iron core. This produces an internal onion-ring structure, with a wide range of chemical species that are blasted into the interstellar medium when the core exceeds the Chandrasekhar mass an implodes. These supernovae are widely hypothesised as a potent source of interstellar dust, with the material condesning within the cooling ejecta. However, observations have so far provided only lukewarm support for these theoretical expectations. This proposal combines optical and near-infrared HST observations of recent type II supernovae, using WFPC2 and NICMOS, respectively, with mid-infrared imaging with IRAC and MIPS on Spitzer. The aim is to use the multiwavelength imaging to probe the evolution of the dust content in the surrounding ejecta. NICMOS observations of SN2006BC in NGC 2397.

GO 11203: A Search for Circumstellar Disks and Planetary-Mass Companions around Brown Dwarfs in Taurus

Stars and brown dwarfs forming from a collapsing molecular cloud (from one of the simulations constructed by Matthew Bate)

Understanding how and why brown dwarfs form is a key part of the larger issue of understanding the overall star formation process. The end points are obvious: we start with a large cloud of molecular gas, a giant molecular cloud (GMC) typically encompassing 105< to 106 solar masses of material, and end up with a collection from a few hundred to several thousand discrete objects, with masses that can range from (perhaps) a few Jupiter masses to 100 solar masses. The detail covered by the intermediate stages is less well understood. Brown dwarfs pose a particular problem because their final masses are so much smaller than the Jeans mass for a typical GMC. One potential mechanism, originating primarily with Matthew Bate and Ian Bonnell, envisages brown dwarfs truly as failed stars - objects that were ejected from star clusters before they could accrete sufficient material to reach stellar masses. That model makes several predictions, including a low binary fraction (<5%) and little in the way of circumstellar material. The present program aims to test the latter prediction by surveying very low-mass stars and brown dwarfs in the Taurus star-forming region, using WFPC2 far-red imaging to search for evidence for circumstellar disks.