This week on HST

HST Programs: December 3 - December 9, 2007

Some selected highlights

GO 11210: 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 11213: Distances to Eclipsing M Dwarf Binaries

Artist's impression of a cool binary system

Eclipsing binaries are stellar systems where the orbital plane lies in the line of sight, leading to the components undergoing mutual eclipses. These systems are extremely powerful probes of stellar properties, since (given the appropriate radial velocity measurements) they permit direct measurement of both stellar masses and radii. Accurate distances can also be derived from these systems. These results are particularly interesting for stars near the bottom of the main sequence, approaching the hydrogen buyrning limit. The present program aims to use the Fine Guidance Sensors on HST to determine sub-milliarcsecond trigonometric parallaxes for five M-dwarf binaries: YY Gem, GU Boo, CM Dra, NSVS0103 and TRES-HER0-R

GO 11312: The Local Cluster Substructure Survey (LoCuSS): Deep Strong Lensing Observations with WFPC2

Combined optical and X-ray (Chandra) image of a lensing galaxy cluster

Gravitational lensing supplies a powerful method of tracing the mass distribution in galaxy clusters; at the same time, the amplified the light from background galaxies provides a means of probing the early stages of galaxy formation. These measurements are particularly effective when X-ray imaging data are also available, allowing direct measurement of the mass density and distribution of the hot intracluster medium. This snapshot proposal aims to use the Wide Field Camera on ACS to observe the central regions of low redshift (0.15 < z < 0.3) clusters with the requisite Chandra observations. The HST images will allow the resolution of lensed arcs in the cluster cores (due to strong lensing) and characterisation of weak-lensing distortions of the image profiles of faint background galaxies. The frequency and detailed distribution (size, multiplicity, redshifts) of the strong lens systems sets strong constraints on the total mass content, and its structure, in the central regions of low-redshift clusters. Those results, in turn, constrain cluster evolution, and offer insight into likely schemes for studying dark energy at higher redshifts.

GO 11361: Hubble Heritage Observations of Mars at 2007 Opposition

HST images of Mars oppositions over the last decade

Mars lies at an average distance of 228 million kilometres, or 1.52 AU, from the Sun, and has an orbital period of 687 days. As a result, it comes into opposition with Earth (i.e. the Sun, Earth and Mars lie along a straight line) once every 780 days, or approximately every 2 years. At that time, Mars makes its closest approach, but the actual distance varies significantly from opposition to opposition since the Martian orbit has significant eccentricity (e=0.093, as compared with e=0.007 for Earth). HST has been systematically observing Mars at opposition since February 1995, 15 months after WFPC2 was installed in Servicing Mission 1. At that time, the planet lay at a distance of 101 million kilometres from Earth and subtended a diameter less than 14 arcseconds. Succeeding oppositions were at increasingly smaller separations, until the opposition of August 28 2003, when Mars was only 55.8 million miles away with an angular diameter of 25.1 arcseconds, its closest approach since 57,617 BCE (but we won't have to wait quite so long - Mars passes even closer on August 28 2287). As in past years, HST will take a series of images of Mars as it approaches opposition, which, this year, will be at 19:47 UT (2:47 pm EST) on Christmas Eve. At that time, Mars will lie at a distance of 88.2 million kilometres with a diameter of 15.2 arcseconds.