This week on HST

HST Programs: June 19 - June 25, 2006

Some selected highlights

GO 10496: Decelerating and Dustfree: Efficient Dark Energy Studies with Supernovae and Clusters

HST images of intermediate redshift supernovae

The last few years of the twentieth century saw a revolution in cosmology, with the measurement of the acceleration term in expansion at high redshifts and the identification of dark energy as a major cosmological component. Type Ia supernovae are the prime yardstick for measuring the rate of expansion at moderate and high redshifts, and Hubble offers almost the only way of obtaining reliable post-maximum photometry of these objects to determine the full shape of the light curve. Most previous HST supernovae programs have concentrated on field galaxies, such as those illustrated here, but applying appropriate corrections for in situ reddening by dust remains an issue in these systems. The present program aims to minimise the uncertainties by searching for supernovae in massive, high-redshift clusters, with the expectation that the majority of detections lie within dust-poor elliptical galaxies. ACS survey observations of eight clusters are scheduled for the coming week, together with follow-up NICMOS observations of a supernova detected in previous ACS images.

GO 10512 Search for Binaries Among Faint Jupiter Trojan Asteroids

An artist's impression of the binary asteroid, Patroclus

Binary stars have been known for well over 300 years, but it is only within the last decade that it has been clear that asteroids are also often found in pairs. This is somewhat unexpected, since the low mass of asteroids leads to very low binding energy between the components. Both the formation mechanism(s?) and the stability of current systems remain unclear. The present program is using the HRC on ACS to carry out a snapshot survey of faint Trojan asteroids, which reside at the stable L4 and L5 Lagrange points on Jupiter's orbit. Several larger Trojans are known to be binary (e.g. Patroclus, see this link for more information), but data remain spare for the fainter members of this population. The collision environment among the Trojans is similar to that of Main Belt asteroids, but the composition is likely to be very different; thus, sampling the binary fraction over a significant range in mass should help us understand the relative importance of collisional and binary formation mechanisms.

GO 10525: Characterizing the Near-UV Environment of M Dwarfs: Implications for Extrasolar Planetary Searches and Astrobiology

SOHO image of an extremely strong solar flare

M dwarfs - at least, the subset of M dwarfs known as flare stars - are renowned for possessing extremely active chromospheres and coronae. Their discovery as highly variable objects happened largely by chance. Willem Luyten had noticed in 1924 that certain M dwarfs showed spectroscopic variability, with the occasional appearance of emission lines, while in the early 1940s van Maanen commented that two late-type dwarfs, Gl 412B (WX UMa) and Gl 285 (YZ CMi), had brightened by over a magnitude on a handful of parallax plates. The crucial observations came in 1948, when E.F Carpenter noticed that the fainter component of a wide binary system had brightened by more than 3 magnitudes in a matter of minutes. In the succeeding 50 years, these stars have been subjected to extensive observations, particularly at optical and X-ray wavelengths, and the underlying physical processes are relatively well understood. However, most attention has focused on the more active flare stars, and we still have a relatively uncertain grasp on the flare frequency among less active stars. This issue has acquired increased importance as more attention has been devoted to the potential of M dwarfs as planetary hosts. The habitable zones lie much closer to the parent star, and planets are correspondingly vulnerable to detrimental effects from enhanced UV radiation, particularly short-wavelength UV-C. This proposal uses ACS HRC prism to obtain low-resolution near-UV spectra of nearby M dwarfs, providing a broad sampling of the range of activity levels among these low mass dwarfs.

GO 10632: Searching for galaxies at z>6.5 in the Hubble Ultra Deep Field

The Hubble Ultra-Deep Field (the UDF) is the deepest image so far obtained of the Cosmos. The original program comprised 412 orbits directed at a single ACS field within the Chandra Deep Field-South (CDF-S) GOODS area. Those 412 orbits were divided among four filters - F435W (56 orbits), F606W (56 orbits), F775W (150 orbits), and F850LP (150 orbits). A further 144 orbits were devoted to a 3x3 grid of F110W (J) and F160W (H) NICMOS images covering the same field.(GO program 9803). NICMOS was also employed in parallel with the ACS observations to obtain deep J- and H-band images of adjacent fields. The present proposal devotes 204 orbits to new observations of the prime and parallel fields, but with HST inverted; thus, the program will obtain ACS imaging (in F606W, F775W and F750LP) of the parallel fields, and deeper NICM (F110W, F160W) data for part of the original ACS UDF. The primary goal of the program is identify candidate very high redshift (6.5 < z < 8.5) sources, using photometric redshifts derived by combining the optical and near-infrared photometric catalogues.

Note: ACS suspended due to side 1 electronics failure on June 19 2006.