This week on HST

HST Programs: April 19, 2010 - April 25, 2010

Selected highlights

GO 11556: Investigations of the Pluto System

Hubble Space Telescope image of Pluto, Charon and the two new moons, Nix & Hydra

Pluto, one of the largest members of the Kuiper Belt and, until recently (still, for some), the outermost planet in the solar system, has been in the news over the last year or two. Besides the extended "planet"/"dwarf planet" debate, Pluto is the primary target of the New Horizons Mission, and Hubble observations in 2005 led to the discovery of two small moons. Together with Charon, itself only discovered in 1978, these additions make Pluto a 4-body system. Christened Nix and Hydra, the two new moons are 5,000 fainter than Pluto itself, implying diameters as small as ~30-50 km if the surface composition is similar to Pluto itself. The present program aims to better characterise these bodies through multicolour observations with WFC3. The observations are phased with a cadence that samples the different orbital periods: 6.4 days for Pluto/Charon; 24.9 days for Nix/Pluto; and 38 days for Hydra/Pluto. The goal is toidentify systematic photometric variations that might probe the rotation period, testing whether these moons are in synchronous rotation with Pluto itself.

GO 11565: A search for astrometric companions to very low-mass, Population II stars

HST image of Gliese 623AB, a nearby disk-dwarf binary system

Binary stars provide an important avenue for setting constraints on stellar evolution, particularly binary stars that consist of components with significantly different masses. The simplest means of producing stars in a dynamically bound system is during the formation process; consequently, it is reasonable to assume that the the stars are coeval and a comparison between the observed properties and predictions can constrain stellar models. Binary systems are common in stars in the Galactic disk, although the overall frequency does decrease with stellar mass, ranging from ~60-70% for G dwarfs to closer to 20% for late-type M dwarfs and ultracool dwarfs. Binaries are much rarer among the metal-poor members of the Galactic halo, with only a handful known among cool, late-type subdwarfs. The present program aims to expand the sample by using WFC3 on HST to image low-mass, metal-poor subdwarfs within 120 parsecs of the Sun, searching for close, faint companions that can be monitored for astrometric orbit determinations.

GO 11670: The Host Environments of Type Ia Supernovae in the SDSS Survey

SN 2007uy and 2008D in NGC 2770

Supernovae have long attracted the attention of both amateur and professional astronomers as a means of studying the violent eruption and death of massive stars and degenerates. However, in the last decade they have also acquired considerable importance as distance indicators, tracing the expansion of the universe to redshifts well beyond the reach of more conventional yardsticks, such as cepheids, and providing a key underpinning for the hypothesised existcen of dark energy. Understanding the supernovae themselves, and, in particular, their progenitors, is key to accurately interpreting their luminosities and distances. The present program aims to tackle that aspect of the problem by using ACS to obtain deep, high resolution images of galaxies that have harboured recent type Ia supernovae. The targets are all drawn from the Sloan Digital Sky Survey, which has uncovered more than than 500 type Ia supernovae,. The supernovae themselves are long gone from view, but the ACS data will be used to probe the stellar populations in the immediate vicinity of the explosion, and hence gain a better understanding of the likely progenitor.

GO 11702: Search for Very High-z Galaxies with WFC3 Pure Parallel

The ACS optical/far-red image of the Hubble Ultra Deep Field

Galaxy evolution in the early Universe is a discipline of astronomy that has been transformed by observations with the Hubble Space Telescope. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance, and the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS). The highest redshift objects found in the UDF have redshifts approaching z~7. Pushing to larger distances, and greater ages, demands observatons at near-infrared wavelengths, as the characteristics signatures of star formation are driven further redward in the spectrum. Wide Field Camera 3, installed in Servicing Mission 4, is well suited to these observations, and a number of programs are in place in Cycle 17 that address these issues. Indeed, WFC3 is employed in pure parallel mode by several programs. These take advantage of other science programs, usually with COS, that involve 2-5 orbit pointings on sources at high galactic latitude. The WFC3 pointing is unplanned, since it depends on the orientation adopted for the prime observations, but 2-5 orbits of IR imaging can reach galaxies at redshifts exceeding z=7 (potentially even z~8) in high latitude fields. This is one of two such programs in the cycle 17 portfolio.