This week on HST

HST Programs: April 27 - May 3, 2009

Selected highlights

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 11973: SAINTS - Supernova 1987A INTensive Survey

November 2003 HST image of the SN1987A gaseous ring

SN1987A, in the Large Magellanic Cloud, is (as far as we know) the nearest supernova to the Sun since Kepler's supernova of 1604. While its eruption, in January 1987, predated HST's launch by over 3 years, the remnant has been a regular observational target since the installation of COSTAR at the first servicing mission. Those high resolution observations have revealed the development, and evolution, of extensive, intricate structures as the blast wave from SN1987A encounters the surrounding interstellar medium. In particular, a striking circum-remnant ring has developed, with numerous hot spots stimulated by the fastest moving debris. The present HST program continues to monitor the development of those features, using a series of observations that are co-ordinated with the Chandra X-ray Observatory. Over the past few cycles, the hotspots are fusing as the shock fully enters the ring, and photons from these regions are exciting previously hidden gas outside the ring, illuminating mass lost from the progenitor before the explosion. The inner debris are now well resolved, and clearly aspherical. Overall, these observations provide crucial insight into the earliest stages of formation of a supernova remnant.

GO 11988: Searching for Intermediate Mass Black Holes in Globular Clusters via Proper Motions

The formation, and subsequent evolution, of intermediate-mass black holes (IMBHs) is a process of considerable importance for understanding the development of dense stellar clusters, and of active galactic nuclei at high redshifts. IMBHs are generally expected to originate in globular-cluster-mass structures; hence, one might expect such objects to be present in some present-day Galactic clusters, particularly those that have undergone dynamical core collapse. One means of testing for their presence is by probing the velocity dispersion of stars in the cluster core. The present program builds on previous observations by targeting five Galactic globulars (M15, NGC 6681, NGC 362, NGC 6624 and NGC 7078) that were observed using the ACS/HRC in Cycle 13. The present set of WFPC2 observations will provide motions with sufficient accuracy that it should be possible to detect or rule out the presence of a central massive object in these clusters.

GO 12000: The Natural Coronagraph of R Coronae Borealis

Artist's impression of R Corona Borealis and its immediate environment

R Coronae Borealis is a well known variable star, a favourite target for amateur astronomers, and the protype of its class. Originally identified in 1796 by the English amateur astronomer, Edward Pigott, R CrB typically resides close to 6th magnitude, barely visible to the naked eye from a dark site, but is subject to irregular, and precipitous, fading that can reduce the brightness by more than 8 magnitudes. The variability is generally attributed to the presence of substantial dust within the stellar atmosphere This type of star is highly evolved with a hydrogen deficient atmosphere, generally thought to have been produced either through the final helium flash on the red giant branch, or as a merger of a binary white-dwarf system. Such systems represent a short-lived evolutionary phase, and are correspondingly rare. Recently, R CrB faded by over 9 magnitudes, the deepest decline ever recorded. The present proposal aims to take advantage of the much reduced brightness of the central star to obtain deep WFPC2 images. Those high resolution images offer the prospect of probing structure in the immediate stellar environment, and potentially discriminating between the two competing formation scenarios.