This week on HST

HST Programs: April 2 - April 8, 2012

Selected highlights

GO 12457: Through a Lens, Darkly - New Constraints on the Fundamental Components of the Cosmos

The cluster MACS J1206.2-0.47, imaged by HST as part of the CLASH program

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. The present program aims to capitalise fully on HST's imaging capabilities, utilising the refurbished Advanced Camera for Surveys and the newly-installed Wide-Field Camera 3 to obtain 17-colour imaging of 25 rich clusters. The data will be use to map the mass profiles of the clusters and probe the characteristics of the high-redshift lensed galaxies. Since ACS and WFC3 can be operated in parallel, the program will also use parallel imaging in offset fields to search for high-redshift supernovae. The present observations target the cluster RXJ2129+0005 at z=0.24.

GO 12586: Detecting Isolated Black Holes through Astrometric Microlensing

A rather spectacular version of black hole lensing.

Gravitational lensing is a consequence of general relativity. Its effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. Those wide-field imaging surveys have target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. The duration of each event depends on several factors, including the tangential motion of the lens and its mass. Long-term events are generally associated with a massive lens. Duration alone is not sufficient to identify a lens as a black hole - a source with very low tangential motion relative to the Sun can produce the same effect. However, microlensing not only leads to flux amplification, but also to small astrometric motions, caused by the appearance and disappearance of features in the lensed light. Those motions serve as a mass discriminant - higher mass lenses produce larger amplitude motions. The expected astrometric signal from a black hole lens is > 1.4 millarcseconds, just measureable with HST. This program aims to capitalise on this fact by searching for lensing by black holes in the Galactic field. The observations target long-duration lensing events in the Galactic Bulge.

GO 12660: Long-term observations of Saturn's northern 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 passed through its equinox, with the Sun moving into the northern Saturnian hemisphere, and Earth crossed the ring plane in mid-September of the same year. As a result, the main focus of attention has now shifted towards Saturn's north pole, which is becoming steadily more accessible as the planet's inclination tlts more towards us. This proposal builds on a seies of past programs (GO 11566, 11984 and 12176) and aims to use the ACS/SBC to study aurorae in the northern hemispheres, comparing their current 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? Where possible, these observations are correlated with in situ measurements by the Cassini spacecraft.

GO 12681: Search for a photodissociated evaporating ocean on the super-Earth 55 Cancri e

Artist's conception of 55 Cnc e transiting the parent star

55 Cnc, or rho1 Cnc, is a binary star system, comprising a G8 priomary and an M3.5 secondary lying a separation of close to 1,000 AU. The system lies at a distance of 12.3 parsecs, and the primary star was among the earliest of the Sun's neighbours identified as having an associated planetary system. There are at least 5 known planets: 55 Cnc b, a hot Jupiter with a period of ~14.7 days and semi-major axis 0.11 AU, discovered in 1997; 55 Cnc c and d, at separations of 0.24 and 5.74 AU, identified in 1999 from analysis of radial velocity residuals; 55 Cnc e, lying closest to the parent star at a separation of only 0.016 AU (0.74 day period); and 55 Cnc f, with a semi-major axis of 0.78 AU. The planets in this system are not coplanar; the obial inclinations of c and d differ by at least 30 degrees. The last two planets are of particular interest: 55 Cnc f likely has a mass close to that of Saturn, but happens to lie close to the habitable zone of the parent star; and 55 Cnc e is a super-Earth, with a total mass close to 8.2 times that of the earth. Moreover, the MOST satellite has shown that the last-named planet also transits the parent star. Lying at such a close separation, the surface temperature is likely to be around 3,000 K. A comparison between the transit\observations made by MOST (at visual wavelengths) and Spitzer (at mid-infrared wavelengths) suggest that the measured diameter is wavelength dependent, possible indicating that the planet is embedded in a thick, gaseous envelope, perhaps due to an evaporating ocean. The present observations aim to use STIS to obtain detailed spectrophotometric measuremets of several transits, searching for potential evidence for absorption due to Lyman-alpha within the gaseous halo.