This week on HST

HST Programs: November 30 - December 6, 2009

Selected highlights

GO 11570: Narrowing in on the Hubble Constant and Dark Energy

A GALEX image of Messier 106 (NGC 4258), one of the galaxies targeted in this program

The Hubble constant remains a key parameter in understanding cosmology and the evolution of the Universe. Refining measurements of H0 therefore still represents a vital means of probing the nature of dark energy. The present program aims to tackle this question by laying a firmer foundation to the SNe Ia distance scale. The WFC3 IR camera will be used to identify and characterise Cepheid variables in eight relatively nearby galaxies that have hosted Type Ia SNe. Cepheids have signficantly lower amplitude at near-infrared wavelengths, and the measured magnitudes are less subject to uncertainties due to foreground reddening and variations in metallicity. As a consequence, determining the mean apparent magnitude, and hence the period/apparent magnitude relation, is substantially more straightforward than at optical wavelengths. Matching the observed relation against reference stars from the LMC allows a more reliable determination of the distance to the parent galaxy, and hence a firmer zeropoint for the SNe Ia distance scale. The aim is to reduce the level of systematics in determinations oif H0 to the 3 percent level.

GO 11666: Chilly Pairs: A Search for the Latest-type Brown Dwarf Binaries and the Prototype Y Dwarf

NICMOS images of the ultracool L/T binary, 2MASS J22521073-1730134; the northern component, notably fainter at F160W, is the T dwarf.

Ultracool dwarfs are defined as having spectral types later than M7, and therefore include the recently discovered L and T dwarfs. They encompass the lowest mass stars (masses < ~0.1 MSub) and sub-stellar mass brown dwarfs, with surface temperatures ranging from ~2500K (~M7) to <700K (late-type T dwarfs). Following their discovery over a decade ago, considerable theoretical attention has focused on the evolution of the intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L to type T. This point marks the emergence of methane as a dominant absorber at near-infrared wavelengths. Current models suggest the transition occurs at ~1400-1200K, and that the spectral changes are at least correlated with, and perhaps driven by, the distribution and properties of dust layers ("clouds") within the atmosphere. The overall timescales associated with the process remain unclear. The present proposal aims to tackle this issue through identifying, and characterising, ultracool binary systems with extremely cool components. Since these systems are almost certainly coeval, the relative spectral energy distributions of the two components can be used to set constraints on evolutionary models. More than 80 ultracool binary systems are currently known; almost all have relatively small linear separations (<15 AU), and components with mass ratios close to one. The present program targets 27 ultracool dwarfs with spectral types in the range T5 to T9, and will use WFC3 IR observations to search for previously unrecognised close, faint companions.

GO 11724: Direct Age Determination of the Local Group dE Galaxies NGC 147 and NGC 185

An image of the Andromeda galaxy and the dwarf elliptical NGC 205 taken on the Canada-france-Hawaii telescope using the CFH12K camera

The Andromeda galaxy, M31, is the nearest large spiral to the Milky Way system, and, like the Milky Way, it is acccompanied by a large number of much smaller, satellite galaxies. The majority of those systems are dwarf spheroidals, similar to Milky Way satellites like Sextans, Draco and Leo. M31 lacks any moderately massive irregular galaxies as companions, like the LMC and SMC, but it does posses the four nearest examples of dwarf elliptical systems: NGC 147, NGC 185, NGC 205 (M110) and NGC 221 (M32). The latter two systems lie too close to the body of M31 itself to allow detailed investigation of its constituent populations, but NGC 147 and NGC 185 lie at large projected distances. Both of these galaxies have been studied extensively by ground- and space-based telescopes. Those observations strongly suggest that both galaxies are dominated by old stellar populations, although it remains unclear whether there is a significant contribution from intermediate age stars, as are present in some dwarf spheroidal satellites of the Milky Way. The present proposal will use ACS and WFC3 to obtain deep imaging of fields in both galaxies, with the aim of deriving deep, accurate colour-magnitude diagrams that will reach below the main-sequence turnoff and can be used to probe the detailed star formation history.

GO 11790: HST/FGS Astrometric Search for Young Planets Around Beta Pic and AU Mic

The exozodiacal disk around Beta Pic

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The disk structure is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Two of the most prominent and best characterised disks are those acompanying the A-type star Beta Pictoris, and the nearby M dwarf, AU Mic. Both stars are young, with age estimates ranging from 8 to 20 million years for beta Pic and 5 to 10 million years for AU Mic. beta Pic's disk, in particular, shows evidence for significant sub-structure that has been attributed to planetary companions. The present HST program aims to use the Fine Guidance Sensors to obtain high precision astrometry of these two young stars, and search for systematic astrometric residuals due to the hypothesised planetary companions.