This week on HST

HST Programs: May 28 - June 3, 2012

Selected highlights

GO 12475: Cool Star Winds and the Evolution of Exoplanetary Atmospheres

The solar corona

All stars lose mass although, in the best 1984 tradition, some lose mass more equally than others. With growing evidence for the near-ubiquity of planetary systems, increasing attention is being devoted to how mass loss varies from star to star, and how varying mass loss rates might affect the conditions on planets orbiting within the habitable zone of that system. On Earth, we have direct experience of the impact of mild solar variability, where coronal mass ejections can lead to spectracular auroral displays and satellite disruption; more energetic outbursts on other stars could lead to significant erosion of the outer atmosphere, and perhaps radiation-induced mutation of hypothetical life forms. The present proposal aims to probe one aspect of this subject by using the Space Telescope Imaging Spectrograph (STIS) to probe mass loss rates in a sample of four late-type dwarfs thata re known to ahve planetary companions. All four of these stars are known to haver ealtively low mass-loss rates. High resolution ultraviolet spectra will be used to search for emission due to Mg II and Fe II, characteristic of stellar winds; in addition, lower resolution spectra will be used to search for Lyman-alpha absorption due to hydrogen within the putative stellar winds.

GO 12478: The Mysterious Redness of Saturn's Rings

Cassini's measurements of fine structure in the Saturnian ring system

The most prominent feature in the Saturnian system is undoubtedly the multiple set of rings that encircle the planet. Originally noted by Galileo in his first telescopic observations, albeit characterised more as horns, the rings were first characterised as such by Christian Huyghens in 1655. Since then, telescopic observations of increasing power and resolution have revealed their segregation into many radial systems. The close-up imaging carried out by the Cassini satellite since its arrival in the Saturnian system in 2004 has revealed increasingly complex structures, many driven by the presence of small shepherding satellites. The ring system has long been known to be made up from numerous small icy fragments. However, the detailed composition of those fragements remains uncertain. The present program aims to probe this issue by using the Space Telescope imagign Spectrograph (STIS) to map the spectrum of the reflected light at wavelengths between 1500 and 3000 Angstroms. This region includes features due to carbon-based materials (including graphite), ammonia, water, ozone and nanophase metals, and can therefore set constraints on the underlying composition and hence the possible origin of the materials.

GO 12487: Probing Population II star formation in a z=7 galaxy

A redshift 7 galaxy candidate identified as a drop-out in the Ultradeep field

When Walter Baade resolved the bulge of the Andromeda spiral into individual stars using the Mt. Wilson 100-inch in 1944, he laid the basis for stellar population studies that culminated in the Milky Way population model outlined by Oort at the 1957 Vatican conference. That model had two main populations, the metal-rich stars that constitute Population I in the gas-rich Galactic disk, and the metal-poor stars in the galactic halo that comprised Population II. With the knowledge that stars generate metals, it made sense that the metal-poor Pop II stars were older than the (still star-forming) Pop I systems, but the fact that the Pop II stars themselves have metals (albeit at abundances from 0.1 to 0.00001 solar) implies that they were polluted by an even earlier generation of stars. Hence, Population III - a first generation of stars, postulated to form at redshifts z>6. The identification of galactic systems at those redshifts opens the door for potential examination of their properties. The present program targets the galaxy BDF-521, spectroscopically confirmed at redshift z=7.008, for rest-frame UV observations with broad- and narrowband filters on the WFC3-IR camera. Ground-based observations suggest thatt he system has an extremely sleep continuum slope, suggesting very low metallicity, and narrowband observations with the F132n filter will probe the He II 1640 emission.

GO 12576: Orbit determination for Fomalhaut b and the origin of the debris belt halo

HST-ACS image of the planetary-mass companion of Fomalhaut

Fomalhaut, or alpha Piscis Austrini, is one of the Sun's closest neighbours, an A-type star with a mass approximately twice that of the Sun and an age between 100 and 300 million years, lying at a distance of only ~7.7 parsecs. Observations with the IRAS satellite in the early 1980s revealed the presence of significant excess radiation at mid-infrared wavelengths, indicating the presence of substantial dust within a disk that is being irradiated by the luminous central star. Since then, observations of Fomalhaut and nearby stars of that ilk have led to a much more detailed characterisation of the debris disk phase. In particular, Spitzer has mapped warm dust in these systems, while HST imaging has provided exquisite resolution in reflected light. It is now recogised that debris disks are the evolutionary stage where planet formation has likely run to completion, the gas has fully dissipated but the disk remains well populated with dusty material spanning a wide range of sizes. Indeed, it is likely that this phase coincides with the heavy bombardment epoch within the Solar System. ACS imaging of Fomalhaut reveals extensive structure in the disk, notably a sharply-defined, eccentric inner edge to the disk, which led to the prediction of a ~Saturn-mass planet at that location. Subsequent ACS/HRC observations led to the identification of that planet, the first direct imaging of a "conventional" exoplanet. The detection was confirmed in 2010 by HST observations with STIS. The exoplanet is variable at optical wavelengths, suggesting that the observed flux is not only contributed by reflected light from the planetary "surface". The current program combines further STIS observations, designed to detect the planet at the current epoch and hence map the orbit, with observations with the UVIS channel on WFC3, which are designed to probe the nature of the dusty debris disk.