This week on HST

HST Programs: February 16 - February 22, 2009

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 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 requires ~1012 photons - undoubtedly the highest signal-to-noise observation yet attempted by HST), then these 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 11966: The Recent Star Formation History of SINGS Galaxies

Spitzer IRAC image of the one-armed spiral, NGC 4725; the 5.8 and 8 micron data (red) highlight warm dust clouds

SINGS is the Spitzer Infrared Nearby Galaxy Survey, a comprehensive imaging and spectroscopic study of 75 nearby galaxies (D < 30 Mpc). The program combines Spitzer mid-infrared observations (With IRAC, MIPS and the IRS) with optical data from ground-based telescope and HST, radio observations and UV imaging with GALEX. The overall goalm of the program is to map the distribution of gas and dust in these nearby systems, and hence probe the nature and distribution of physical processes within the ISM. Many of the SINGS targets lack high-resolution ultraviolet imaging, and the present communtiy program was allocated Director's Discretionary Time of HST for WFPC2 F336W (U-band) and (in a few cases) F656N (H-alpha) imaging of a subset of the sample. The data will be of particular use in resolving and providing reliable age estimates for individual star clusters.

GO 11974: High-resolution imaging for 9 very bright, spectroscopically confirmed, group-scale lenses

Strong galaxian lens from the SLACS survey

Gravitational lensing provides a powerful method of tracing the mass distribution in both galaxy clusters and individual galaxies, while at the same time amplifying the light from background galaxies to allow detailed investigation of their properties. This proposal will obtain WFPC2 observations of a 9 strong lenses, identified from the ground-based Sloan Digital Sky Survey. By themselves, the SDSS data have neither sufficient resolution nor sensitivity to map the detailed structure of the lens, and hence the underlying galaxian potential. The aim is to combine the WFPC2 data with detailed models to measure the mass distribution and light profiles of the lensing galaxies, and constrain the morphological properties of the background lensed galaxy.

GO 11984: Imaging Saturn's High Latitude 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 reaches its equinox, with the Sun moving into the northern Saturnian hemisphere, and Earth crosses the ring plane in mid-September. As a result, it is now possible to view both Saturnian poles simultaneously. This proposal aims to use the ACS/SBC to study aurorae in the both hemispheres, comparing their relative characteristics against observations from previous years. In particular, the southern aurorae tend to occur in an oval whose centre is displaced from the south (rotational) pole. Does the northern hemisphere exhibit a similar phenomenon? These observations are correlated with in situ measurements by the Cassini spacecraft.