This week on HST

HST Programs: December 14 - December 20, 2008

Some selected highlights

GO 10877: A Snapshot Survey of the Sites of Recent, Nearby Supernovae

A recent supernova in M100

Supernovae mark the (spectacular) evolutionary endpoint for a subset of stellar systems. Standard models predict that they originate from massive stars and (probably) close binaries with a compact (WD, neutron star) component, but there are still some questions remaining over whether we fully understand the range of possible progenitors. The last decade has seen the development of a number of large-scale programs, usually using moderate-sized telescopes, that are dedicated to monitoring (relatively nearby galaxies, searching for new supernovae. This program obtains follow-up images of recent supernovae, concentrating on systems within 20 Mpc of the Milky Way. The observations are taken well after maximum, with the aim of using the high spatial resolution of WFPC2 to identify the fading remnant and perhaps determine its origin.

GO 11945: Asteroseismology of Extrasolar Planet Host Stars

Sun-like stars undergo a variety of low-level pulsations driven by internal instabilities. Asteroseismology uses those pulsations to study the internal structure of stars. Not surprisingly, those oscillations were first discovered in the Sun. In the early 1960s, Robert Leighton used the 60-foot solar tower on Mt. Wilson to obtain spectroheliograms of the Sun, narrowband images centred on Zeeman-split lines that showed the velocity structure across the surface; those data revealed periodic variations with P~296 seconds, the 5-minute solar oscillations. Detecting such variations require extemely high signal-to-noise; nonetheless, observations have been extended to a handful of other stars. In particular, ESA's COROT mission has detected recently pulsations in three F-type stars. The present program aims to use the Fine Guidance Sensors on HST to measure the pulsational modes in the star HD 17156, an 8th magnitude G-type subgiant at a distance of ~ 78 parsecs from the Sun. The crucial characteristic of this star is that it harbours a planetary system where at least the innermost hot Jupiter, HD 17156b, transits the host star. Those transits provide a measure of the stellar radius, and hence the mean density. If multiple pulsational modes are detected with the FGS (which will require ~1012 photons - undoubtedly the highest signal-to-noise observation yet attempted by HST), then those data will provide an entirely independent measurement of the internal density structure, and can determine the stellar age to an accuracy of 5-10%.

GO 11957: Hubble Investigation of Asteroid 21 Lutetia in Support of the Rosetta Mission Flyby

A comparison between fly-by images of the asteroid Gaspra and the two martian satellites, Phobos (lower) and Deimos (upper)

ESA's Rosetta mission aims to make the first ever landing on a cometary nucleus. As such, it is a follow-up to the Giotto mission, which encountered, imaged and was sand-blasted by Comet Halley during the latter's 1986 perihelion passage. Rosetta was launched on 2 March 2004, and will rendezvous with Comet 67P/Churyumov-Gerasimenko in early 2014. After an extensive period of mapping the comet, Rosetta will release the lander, named Philae (the island in the Nile river where the Rosetta stone was found), which will land on the comet's surface and transmit back data on the chemical composition. On its way to Comet 67P/Churyumov-Gerasimenko, Rosetta will make a number of fly-bys of Earth and Mars, boosting the orbital energy, and will encounter two asteroids: 2867 Steins (September 5, 2008) and 21 Lutetia (July 10, 2010). The current HST observations are in preparation for the second encounter. Lutetia is a main-belt asteroid with an estimated diameter of 100 kilometres and a potentially unusual composition: although classified as a metallic (type M) asteroid, the spectral characteristics are somewhat unusual. Deep HST imaging with WFPC2 will be used to search for dust debris and for low-mass companions.