This week on HST

HST Programs: December 21 - December 27, 2008

Selected highlights

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 will 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 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%. Identifying those modes requires collecting close to ~1012 (one thousand billion, or one trillion) photons. To achieve this, HST will take advantage of the fact that HD 17156 lies in the Continuous Viewing Zone (CVZ) at this time of year, and will stare exclusively at that star from late on December 21st through to January 1st 2009, a span of 148 orbits.