This week on HST

HST Programs: September 7 - September 13, 2015

Selected highlights

GO 13667: Observations of the Pluto System during the New Horizons Encounter

New Horizon images of Pluto and Charon

Pluto, one of the largest members of the Kuiper Belt and, until recently, the outermost planet in the solar system, is the primary target of NASA's New Horizons Mission and has been the subject of a range of supporting HST programs over the past few years. Those observing programs have added four moons to the Pluto system. James Christy originally identified the largest moon, Charon, in 1978 from photographic plates Charon has a diameter of ~1200 km, or almost half that of Pluto itself. In 2005, Hubble added two small moons, christened Nix and Hydra, approximately 5,000 times fainter than Pluto itself, implying diameters as small as ~30-50 km if the surface composition is similar. More recent observations, in support of New Horizons mission, aimed to use WFC3 to search for faint rings due to dust particles that might jeopardise the space craft and require a course correction. While no rings were detected unequivocally, two small satellite, christened "P4" and "P5", have been discovered. Both are significantly fainter than Nix and Hydra, and may well be as small as 10-13 km in size. There is also some evidence that might point to the presence of a debris ring within Charon's orbit. Most recently, Hubble carried out an extensive imaging survey to identify KBOs lying beyond Pluto that might seve as targets for an extended new Horizons mission. Two such candidates were identified, and Hubble will be conducting follow up observations to further constrain their orbits. The present observations are in direct support of the primary New Horizons mission. WFC3 is being used to provide a comprehensive set of direct imaging and grism observations that provide reference data to give a broader context for interpreting the spectacular New Horizons data compiled during the highly successful July 14th system fly-by. The spacecraft is now 70 million kilometers beyond Pluto, some 33.4 AU from the Sun.

GO 13682: Distances and stellar populations of seven low surface brightness galaxies in the field of M101

The spiral galaxy, M101

M 101 is a grand design spiral galaxy, viewed face-on and lying at a distance of ~6 Megaparsecs. Similar in size to the Milky Way, the system is relatively isolated from other major galaxies. However, like the Milky Way, it has a number of lower-mass satellite galaxies, including NGC 5204, NGC 5474 and Holmberg IV. The galaxy has recently been targeted for observations with the Dragonfly telescope, a cluster of 10 400-mm Canon lenses optimised to detect low-durface brightness objects. Those observations have revealed seven such objects. If associated with M101, the galaxies are very similar to the low-surface brightness dwarfs found within the Milky Way system. The present program aims to obtain ACS images of the systems, recolving individual stars and hence determining their distance through the location of the tip of the red giant branch.

GO 13691: CHP-II: The Carnegie Hubble Program to Measure Ho to 3% Using Population II

RR Lyrae's light curve at visible wavelengths

The classical cosmic distance scale rests on a series of distance indicators that step outwards from the Milky Way, establishing reliable measurements to ever more distant galaxies. Cepheids have long been the prime calibrators in this process, but other pulsating variables, notably Mira AGB long-period variables and RR Lyrae variables, also make significant contributions, while stellar population characteristics, such as the location of the tip of the red giant branch (TRGB), also play a role. RR Lyrae variables are evolved, near-solar-mass stars that are passing through the instability strip where it crosses the horizontal branch. With periods of 0.5 to 1.5 days, they have long served as distance indicators for old stellar populations (Baade's Population II). They have been known in the Galactic field and in Galactic globular clusters for over 150 years, and they are also present in the older stellar populations of the dwarf spheroidal Galactic satellites. Cluster (or dsph) RR Lyraes are particularly interesting, since their metallicities and ages can be deduced from analysis of the colour-magnitude diagrams for those systems. They are significantly less luminous than Cepheids, nonetheless, near-infrared photometric monitoring has demonstrated that these stars delineate a period-luminosity relation at those wavelengths that has the potential to establish distances to better than 1.5% accuracy. Distances to stellar populations can also be derived from measuring the location of the TRGB, marking the point in intermediate- and low-mass star evolution where core temperatures are raised to the point thatn helium ignites in the triple-alpha reaction, and the star evolves rapidly onto the horizontal branch. The present programs combines deep optical/far-red/near-IR imaging of moderately distant galaxies that have hosted Type Ia supernovae and of nearby galaxies with RR Lyraes with WFC3-IR observations of 4 individual Galactic variables with trigonometric parallaxes to cosmntruct a distance ladder that is independent of the Cepheid calibration. The oevrall goal is to define H0 to 3% accuracy.

GO 14241: Cloud Atlas: Vertical Cloud Structure and Gravity in Exoplanet and Brown Dwarf Atmospheres

Ground-based imaging the the very low-mass brown dwarf binary, 2MASS1207

Recent years have seen the discovery of numerous extrasolar planets. Initially, most were discovered through radial velocity monitoring; more recently the superb photometric properties of the Kepler mission have led to the discovery of numerous transiting systems. Lagging behind in numbers, but offering the greatest prospect for probing physical conditions, is direct imaging. A handful of systems with resolved planetary companions have been discovered, most notably the multi-planet system around the nearby A-type star, HR 8799, and 2MASS1207B, the planetary-mass wide companion to the brown dwarf, 2MASS1207A, a member of the TW Hydrae association. These systems are expected to have gross properties that are similar to the Solar System gas giants, particularly atmospheric cloud structure. but they are much more massive and therefore have higher gravities. The present program aims to gain insight into their structure by comparing resolved exoplanets and higher mass, but similar temperature, brown dwarfs. Observations of isolated brown dwarfs have shown evidence for systematic variations in brightness, possibly due to dust within the atmosphere forming clouds, perhaps giving the dwarf's surface a banded appearance, similar to Jupiter. The clouds themselves may appear and disappear over relatively short timescales, leading to photometric variations at particular wavelengths. Past programs have used both Spitzer and HST to monitor spectral variability in a number of systems. The present program will use Wide-field Camera 3 to obtain time-series near-infrared grism spectra of ten brown dwarfs and high-mass exoplanets. Variations in those spectra map the atmospheric cloud structure in each system, and those variations can be examined for correlations with the mass/gravity of the parent object.