This week on HST

HST Programs: May 9, 2011 - May 15, 2011

Selected highlights

GO 12169: The frequency and chemical composition of planetary debris discs around young white dwarfs

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. The present program aims to address this question by using COS to obtain UV observations of young white dwarfs, probing correlations with progenitor mass and examining the detailed composition of the accreted materials.

GO 12235: The energy of auroral electrons at Saturn and the associated atmospheric heating

Saturnian aurorae

Planetary aurorae are stimulated by the influx of charged particles from the Sun, which travel along magnetic field lines and funnel into the atmosphere near the magnetic poles. Aurorae therefore require that a planet has both a substantial atmosphere and a magnetic field. They are a common phenomenon on Earth, sometimes visible at magnetic latitudes more than 40 degrees from the pole, and have also been seen on Jupiter, Saturn, Uranus and Neptune. In August 2009, Saturn reached its equinox, with the Sun moving into the northern Saturnian hemisphere, and Earth crossed the ring plane in mid-September. As a result, it is now possible to view both Saturnian poles simultaneously. Saturn has been the target of a number of programs over the past 2-3 cycles, primarily using the ACS/SBC to obtain ultraviolet imaging data. The present program aims to build on those analyses by employing STIS to obtain FUV spectroscopy of active regions near the Saturnian auroral poles. The observations are being coordinated with UVIS measurements taken {\sl in situ} by the interplanetary probe, Cassini. The primary aims are to determine the altitude of the auroral features, and map the temperature distribution within the atmosphere at high latitudes.

GO 12273: Mass of the Local Group from Proper Motions of Distant Dwarf Galaxie

The dwarf galaxy, Leo A, as imaged by the Subaru telescope

M31 and the Milky Way are the two largest members of the Local Group, with masses of ~4 x 1011 and ~1011 MSun, respectively. As such, they dominate the system dynamics; M33 and the LMC are the next largest systems, with masses lower by a factor of 10. Radial velocity measurement show that M31 and the Milky Way are converging at a velocity of ~125 km/sec; however, interpreting that result in cosmological terms requires a better understanding of the total mass of the Local Group. Using a variety of techniques, current estimates range over a factor of 5, from ~1.3 x 1012 MSun to ~5.6 x 1012 MSun. T%he present program aims to apply stronger constraints to this fundamental value by measuring proper motions for four dwarf galaxies that lie towards the edge of the local group: Cetus, Leo A, Tucana and the Sagittarius Dwarf Irregular. First epoch observations with the ACS/WFC are already available in the archive for these four systems. The present program will build on those, obtaining new I-band (F814W) observations with the ACS/WFC, while simulateously using the WFC3-UVIS camera in parallel to obtain deep B (F475W) and I (F814W) colour-magnitude data for these low-mass systems.

GO 12283: WISP - A Survey of Star Formation Across Cosmic Time

A region of massive star formation

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. The present program builds on the ability of HST to carry out parallel observations, using more than one instrument. While the Cosmic Origins Spectrograph is focused on obtaining ultraviolet spectra of unparalleled signal-to-noise, this program uses the near-infrared grisms mounted on the Wide-Field Camera 3 infrared channel to obtain low resolution spectra between 1 and 1.6 microns of randomly-selected nearby fields. The goal is to search for emission lines characteristic of star-forming regions. In particular, these observations are capable of detecting Lyman-alpha emission generated by star formation at redshfits z > 5.6. A total of up to 40 "deep" (4-5 orbit) and 20 "shallow" (2-3 orbit) fields will be targeted in the course of this observing campaign.