This week on HST

HST Programs: December 7 - December 13, 2008

Some selected highlights

GO 11110: Searching for Lyman alpha Emission from FUSE Lyman Continuum Candidates

The starburst galaxy, M82

Ultraviolet light plays an important role in ionising the interstellar and intergalactic medium. Hot, massive stars are the one of the principal sources of UV flux in the present universe. It is therefore important to understand the efficiency with which that light manages to escape from star forming regions, particularly as a means of estimating the relative improtance of star formation as an ionising source in the early universe. The present program targets a number of star forming galaxies that have been observed by the Far Ultraviolet Satellite Explorer (FUSE) and detected in the Lyman continuum (shortward of 912 Angstroms). The ACS/SBC camera is being used to image those systems at Lyman alpha; ground-based imaging has already been used to map the Balmer line emission. All of these data will be combined in models that will be used to estimate the global effect of star formation on the surrounding gaseous environment.

GO 11704: The Ages of Globular Clusters and the Population II Distance Scale

Hubble Heritage image of the globular cluster, M15

Globular clusters are the oldest structures within the Milky Way that are directly accessible to observation. They are relatively simple systems, with relatively simple colour-magnitude diagrams (albeit with some complexities adduced from recent HST observations, see GO 11233 ). Matching those CMDs against theoretical models allows us to set constraints on the age of the oldest stars in the Galaxy, and hence on the age of the Milky Way and the epoch of galaxy formation. However, the accuracy of those age determinations rest crucially on the accuracy of the cluster distance determinations. The clusters themselves lie at distances of several kpc at best, and tens of kpc at worst; thus, direct trigonometric parallax measurements must await microacrsecond astrometric missions. The classical method of deriving distances is main sequence fitting - using nearby stars, with similar chemical abundances and accurate parallax measurements, to map out the main sequence in absolute units, and then scaling the clusetr data to fit. The problem with this method is that metal-poor subdwarfs are rare, so even Hipparcos was only able to obtain accurate distances to a handful of stars. The present program aims to improve the distance measurements by using the Fine Guidance Sensors on HST to determine sub-millarcsecond trigonometric parallaxes to 9 subdwarfs, almost doubling the sample available for MS fitting.

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%.