This week on HST

HST Programs: April 30 - May 6, 2007

Some selected highlights

GO 10468: Jupiter's Upper Stratospheric Hazes Probed with Ganymede

Ganymede and Jupiter, as imaged by Cassini on 3 December 2000

Over the next week, Hubhle will use WFPC2 to take a series of images of Ganymede, catching the satellite as it slips into occultation behind Jupiter. The observations, in five different passbands spanning the ultraviolet (250 nm) to the far red (892 nm), will probe the upper layers of the parent planet's atmosphere, and permit investigation of the structure of haze within the Jovian stratosphere. Until now, studies of the creation, growth and motions of these layers has been limited to theoretical analyses; the WFPC2 images will provide the first empirical measurements of the aerosols responsible for the high haze.

GO 10802: SHOES-Supernovae, HO, for the Equation of State of Dark energy

WFPC2 image of NGC 4639, one of the spirals targeted in this program

The cosmic distance scale and dark energy are two key issues in modern astrophysics, and HST has played a vital role in probing both. On the one hand, HST has been involved in cosmic distance measurements since its inception, largely through the H0 Key Project, which used WFPC2 to identify and photometer Cepheids in 31 spiral galaxies at distances from 60 to 400 Mpc. On the other, HST is the prime instrument for investigating cosmic acceleration by searching for and following Type Ia supernovae at moderate and high redshift. These two cosmological parameters are directly related, and recent years have seen renewed interest in improving the accuracy of H0 with the realization that such measurements, when coupled with the improved constraints from the Cosmic Microwave Background, provide important constraints on cosmic acceleration and the nature of Dark Energy. The present HST program combines observations that are designed to tackle both questions. NICMOS is being used to observe known Cepheids in several Key Project spirals that have hosted Type Ia supernovae; the near-infrared data will provide more accurate distance estimates for those galaxies, tying together the Cepheid and SN Ia distance scales. Before its untimely demise, the ACS/WFC was being used for parallel observations designed to search for high-redshift supernovae. While the ACS observations are no longer possible, NICMOS monitoring of the Cepheids continues.

GO 10849: Imaging scattered light from debris disks discovered by Spitzer around 21 sun-like stars

Hubble coronagraphic images of debris disks around two nearby stars, a young M dwarf and a solar-type star

Young stars have circumstellar disks, forming due to the conservation of angular momentum as young protostars accrete material from the parent molecular cloud. Initially gas rich T Tauri-like systems, the gas within the disk is largely removed over the first few Myrs, leaving a residual dust-dominated debris disk, perhaps (probably?) populated by number of gas giant and proto-terrestrial planets. The dust disks dissipate with time, becoming more difficult to detect as the star ages and eventually evolving to low density components like the solar zodiacal disk. Over the last few years, Spitzer has revolutionised studies of this field, providing sensitive observations in the thermal mid-infrared that are well tuned to finding the older debris disks around nearby stars. The current program has identified 21 nearby solar-type stars with excess mid-infrared radiation, and aims to use the NICMOS coronagraph to image the debris disks in the scattered light at near-infarred wavelengths.

GO 10855: The Near-IR Spectra and Thermal Emission of Hot Jupiters

Mid-IR photometry by Spitzer of the transiting system, HD 209458, showing the flxu decrease as the planet is eclipsed by the primary star.

Since the 1995 announcement of the discovery of 51 Peg b, over 225 extrasolar planets have been discovered in more than 200 stellar systems. This catalogue includes 16 transiting systems, a particularly valuable subset: not only do these systems eliminate the uncertainties in the orbital inclination, thereby providing unambiguous mass estimates, but photometry and spectroscopy at optical and near-infrared wavelengths during the primary and secondary eclipses can probe the planetary atmospheres. To date, HST STIS observations have been used to detect sodium absorption in the first known transiting planet, HD 209458b, while Spitzer has succeeded in detecting re-radiated mid-IR emission from HD 209458b and HD 189733b. This program will use NICMOS to target the latter systems, the brightest known transiting exoplanet systems. The aim is to obtain grism spectroscopy during primary eclipse (when the planet transits the star) and secondary eclipse (when the planet is hidden). The NICMOS spectra will cover a spectral region that includes strong water absorption, and the goal of the program is to difference the spectra at primary and secondary eclipse and search for possible signatures of watre within the planetary atmosphere.