This week on HST

HST Programs: August 6 - August 12, 2007

Some selected highlights

GO 10583: Resolving the LMC Microlensing Puzzle: Where Are the Lensing Objects ?

Microlensing light curve produced by a stellar lens with an appropriately placed planetary companion

Gravitational lensing is a consequence of general relativity, and the 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. These wide-field imaging surveys target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. Statistical analysis, however, strongly suggests that both the distribution of event durations and the overall number of lenses are inconsistent with a dark matter component. So what are objects doing the lensing? This program aims to answer that question by using WFPC2 to obtain follow-up images of LMC lensed stars that were detected in the initial MACHO survey. Over a decade has elapsed since the lensing event, sufficient time, in at least some cases, for differential motion to separate lens and background star. Thus HST observations can set limits on the fraction of these events that might be produced by ordinary stars in the Galactic disk or halo.

GO 10840: The FUV fluxes of Tauri stars in the Taurus molecular cloud

Schematic view of a T Tauri star

The T Tauri stage of evolution occurs early in a star's lifetime, within ~10 Myrs of its birth, when it still retains a dense, dust and gas-rich circumstellar disk. During this phase, there is significant accretion of material onto the central star. This leads to heating of the inner regions of the accretion disk, and significant emission at ultraviolet and X-ray wavelengths. The aim of this proposal is to use combine observations with WFPC2 and with the Solar Blind Channel (SBC) on the Advanced Camera for Surveys (ACS) to study the ultraviolet energy distribution of a number of T Tauris spanning a range of ages. The WFPC2 will be used to obtain images at near-UV wavelengths (2000 - 3000 Angstroms), while the SBC will provide comparable data at Far-UV wavelengths. The present observations target CX Tauri, a ~2 Myr-old classical T Tauri star in the Taurus molecular cloud complex.

GO 11156: Monitoring Active Atmospheres on Uranus and Neptune

Voyager 2 image of Neptune

Uranus and Neptune are the two "ice giants" of the Solar System, lying at average distances of 19 and 30 AU from the Sun. At those distances, their atmospheres are subject to much lower solar irradiance than Jupiter or Saturn and, as a result, they tend to show fewer large-scale disturbances. Even so, Neptune showed modest activity in the southern hemisphere between 2000 and 2003, although the disturbances have been less prominent in recent years. Uranus is unique among the major bodies in that it rotates on its side. With a polar obliquity is 98o degrees, its equator is close to perpendicular to the ecliptic plane. Consequently, as it circles the Sun, each pole spends almost half of a Uranian "year" (or 42 terrestrial years) hidden from the Sun in total darkness.On December 7th of this year, Uranus will pass through its equinox, with Sun passing overhead at the Uranian equator. This configuration will provide the first opportunity in modern times for us to view the entire northern hemisphere of the planet, and our first chance to view how the planetary atmosphere reacts to the change from night to day. The present SNAPSHOT program is using WFPC2 to monitor the appearance of these two planets, acquiring narrowband optical images that both record the overall structure and probe the atmosphere at different depths.

GO 11202 The Structure of Early-type Galaxies: 0.1-100 Effective Radii

HST 16309+8230, a disk galaxy, distorted due to gravitational lensing by a foreground elliptical

Despite their apparently simple appearance, the processes responsible for the formation and evolution of elliptical galaxies remain somewhat obscure. It is clear that most star formationin these systems must occur at early epochs, since these systems are highly gas poor at even moderate redshifts. Grabitational lensing provies one of the more important tools for investigating these systems, since it can probe the detailed form of the mass distribution, and test for the presence of sub-structure in the underlying dark matter, as predicted by some theoretical models. The present program is combining high-resolution, multi-colour HST imaging with ground-based low-resolution VLT/Keck spectroscopic observations of over 50 strong lensing systems. The resultant datasets can be used to investigate the structure of elliptical galaxies over a wide range of radii, and test the predictions of relevant theoretical models.