This week on HST

HST Programs: August 23, 2010 - August 29, 2010

Selected highlights

GO 11630 Monitoring active Atmospheres on Uranus and Neptune

Nicmos image of aurorae on Uranus

The atmospheres of the gas giant planets in the solar system are dynamic entities that can exhibit dramatic changes over a variety of timescales. Those changes are most apparent in Jovian atmosphere, which displays a wide variety of bands and spots, reflecting complex meteorological phenomena (see, e.g., previous ACS observations of the upper atmosphere and of the new little red spot ). This is not surprising since Jupiter atmosphere receives the highest input of solar energy. However, secular variations are also evident in the atmospheres of the outer planets, albeit usually at a more subtle level. The present program builds on past HST programs (see Program GO 10534 ) that have monitored atmospheric changes in the two outermost gas giants, Uranus and Neptune. Both exhibit long-term seasonal variations, whose origins are not yet well understood; both are capable of generating dark spots - phenomena that are presumably related to Jupiter's Great Red Spot and Saturn's Great White Spot. The present observations use a variety of filters on Wide-Field Camera 3 (notably the quad methane filter) to probe conditions are a variety of levels within the planetary atmospheres.

GO 11707: 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 11721: Verifying the Utility of Type Ia Supernovae as Cosmological Probes: Evolution and Dispersion in the Ultraviolet Spectra

Recent supernova in M100

Supernovae are the most spectacular form of stellar obituary. In recent years, these celestial explosions have acquired even more significance through the use of Type Ia supernovae as distance indicators in mapping the `dark energy' acceleration term of cosmic expansion. However, while there are well-established models for the two main types of supernovae (runaway fusion on the surface of a white dwarf in a binary system for Type Ia, or detonation of the core in Type II), some uncertainties remain as to the uniformity of the events. Consequently, there is potential for systematic bias in the distance estimates. One of the questionmarks comes from spectroscopy of a number of supernovae at intermediate redshift (z~0.5) that appears to show a substantial dispersion in properties at UV wavelengths. The present program aims to probe this issue by using STIS to obtain UV spectra of nearby supernovae, and therefore examining the detailed behaviour in the local universe.

GO 12331: Massive Star CSI: Has The Progenitor of SN2008S Vanished?

Supernova 2008S in NGC 6946

SN2008S was discovered in the relatively nearby spiral galaxy NGC 6946 on February 1 2008 by the UK amateur astronomer, Ron Arbour. As a relatively bright object, it received extensive attention from both the amateur and professional astronomical communities. The overall light curve showed relatively slow photometric evolution, similar to past supernovae SN 1998S and SN 1979C, and led to its identification as a Type IIn. A possible progenitor has been identified from Spitzer imaging, with no eviodence for an optical counterpart in deep ground-based imaging. This indicates that the supernova was embedded within a very dusty environment. The peak brightness was significantly fainter than expected, and there have been suggestions that this may represent a new class of supernova. It is not clear whether the progenitor has survived the explosion, or been disrupted. The present program couples Spitzer IRAC and HST/WFC3-IR observations, with the aim of either characterising the progenitor or demonstrating its destruction.