This week on HST

HST Programs: April 1 - March 7, 2013

Selected highlights

GO 12869: The chemical diversity of extra-solar planetary systems

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typcail targets0, they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years.In subsequent years, a significant number of DA white dwarfs have been found to exhibit narrow metallic absorption lines in their spectra. Those lines are generally attributed to "pollution" of the white dwarf atmospheres. Given that the diffusion time for metals within the atmospheres is short (tens to hundreds of years), the only reasonable means of maintaining such lines in ~20% of the DA population is to envisage continuous accretion from a surrounding debris disk. Several programs in recent cycles have used the Cosmic Origins Spectrograph to UV spectra of white dwarfs that were already known to have polluted atmospheres, providing aditional information on the chemical composition of the pollutants. Those targeted observations were supplemented by a SNAP program that aimed to identify additional candidates. The present program focuses on two white dwarfs that show unusual elemental distributions, specifically, strong C and Si absorption, elements that are not prominent in an Earth-like composition. The aim is to double the signal-to-noise of the spectral observations and better constrain the abundances of other elements, notably Fe, O and N.

GO 12900: Mapping the Methane and Aerosol Distributions within Titan's Troposphere: Complementing The Cassini/VIMS T90 Flyby of Titan

Saturn's satellite, Titan, as seen from Cassini

Saturn's largest satellite, Titan, has emerged as one of the most interesting bodies in the Solar System, in no small part because it one of the few terrestrial objects, and the only moon, that possesses a significant atmosphere. Titan's radius is 2580 km, or almost 50% larger than the Moon and ~100 km larger than Mercury. Discovered by Christian Huyghens in 1655, the existence of a substantial atmosphere was first suspected with the detection of limb darkening in the early 20th century and was confirmed by Gerard Kuiper's mid-century spectroscopic observations. In more recent years, we have had the advantage of in situ inspections, first through Voyager 1's close passage in 1981 and, over the last decade, with the observations made by the Cassini orbiter since its arrival in the saturnian system in July 2004, and by the Huyghens lander in January 2005. Those measurements show that the atmosphere is dominated by nitrogen (~98%) with the bulk of the remainder contributed by methane. The origin of that methane has not yet been established. Cassini continues to orbit Saturn, and will undertake a close fl-by of Titan on April 5th. During that close passage, Cassini's Visual and Infrared Mapping Spectrometer will be used to map the atmosphere. Those observations will be coupled with spatially-resolved HST STIS spectroscopy, which provides hgiher spectral resolution at optical wavelengths. The primary goal is to map the distribution of methane within Titan's troposphere (heights below ~40 km).

GO 12931: Ultra-Faint Galaxies at the Peak Epoch of Star Formation

HST image of the galaxy cluster, Abell 1689

Gravitational lensing provides a powerful method of tracing the mass distribution in galaxy clusters, while amplifying the light from background galaxies. The potential for using the latter property to probe the early universe is now gradually being realised with the detection of galaxies at redshifts within the reionisation era, including two galaxies with redshifts likely exceeding z+10. Models show that Abell 1689 served as an highly potent lensing system, allowing detection of galaxies at redshifts exceeding z>7. Equally significantly, the magnification introduced by the lensing at lower redshifts is sufficient that detections are not limited to the brighest galaxies at those epoch, providing insight into the details of star formation in galaxies matching the Milky Way's precursors. The present program builds on past HST observations of this cluster, which have resulted in the identification of more than 80 lensed galaxies at redshifts 1.7 < z < 2.3. The present program will use deep imaging with the F225W and F336W filters on the WFC3/UVIS camera to broaden the redshift coverage to include low-mass galaxies at edshifts between z=1 and z=3.

GO 12982: Are the Milky Way's High Velocity Clouds Fuel for Star Formation or for the Galactic Corona?

A map of the high velocity cloud systems surrounding the Milky Way (B. Wakker, U. Wisconsin).

The stellar components of the Milky Way Galaxy are well known: the disk, the central bulge and the old, metal-poor stellar halo. However, the Milky Way is also surrounded by a halo of hot, gas that is itself embedded within a much more tenuous corona of even hotter, ionised gas. Within that structure lie high velocity clouds. Originally discovered in the 1930s as absorption features in stellar spectra, these clouds have velocities that differ significantly from the rotational velocity along that line of sight, and they are generally believed to be undergoing infall into the Galaxy. The origin and nature of these systems remains uncertain, with some favouring a Galactic origin, driven by star formation and feedback between disk and halo, and others supporting their origin within the warm-hot intergalactic medium. HVCs are not self luminous, so indirect methods need to be applied to examine their characteristics. The most effective is to identify stars that lie behind individual systems and, as with their discovery in the 1930s, search the stellar spectra for signature absorption lines produced by material within the cloud. Many, indeed most, of the key absorption features lie at ultraviolet wavelengths, a spectral region that has been opened up with the installation of the Cosmic Origins Spectrograph on HST. The present observations build on a Cycle 17 program that used COS to obtain spectra of distant halo stars aligned with a subset of the known HVCs within the Milky Way. The results indicate that HVCs are streams of gas in the lower halo, ruling out several models for their formation and evolution. The current observations will probe several known features for structure closer to the Disk by using COS to target stars significantly closer to the Sun.