This week on HST

HST Programs: May 14 - May 20, 2007

Some selected highlights

GO 10870: The Ring Plane Crossings of Uranus in 2007

Images of Uranus spanning 2000 to 2004, showing the rotation of the ring plane

Like the other Solar System gas giants, Uranus not only has an extensive number of satellite moons, but also possesses a ring system. Unlike the other giant planets, Uranus has a polar obliquity of 98o degrees, so its equator is close to perpendicular to the ecliptic plane. Consequently, from our vantage point on Earth, we view the north and south poles alternately during Uranus' 84-year circling of the Sun. Midway between the polar apparitions, of course, we view Uranus' equatorial plane - and see the ring system edge-on. The next ring plane crossing will occur in May and August 2007. At this juncture, the denser and more prominent rings will almost disappear from view, providing an opportunity to search for small satellite "shepherd" moons. These moons are expected to be present, acting as gravitational delineators, defining the radial size of the individual rings. Besides searching for the shepherds, the current HST program will use the High Resolution Camera on ACS to measure the thickness of the rings, and study the colours of the recently discovered fainter rings.

GO 10877: A Snapshot Survey of the Sites of Recent, Nearby Supernovae

A recent supernova in M100

Supernovae mark the (spectacular) evolutionary endpoint for a subset of stellar systems. Standard models predict that they originate from massive stars and (probably) close binaries with a compact (WD, neutron star) component, but there are still some questions remaining over whether we fully understand the range of possible progenitors. The last decade has seen the development of a number of large-scale programs, usually using moderate-sized telescopes, that are dedicated to monitoring (relatively nearby galaxies, searching for new supernovae. This program obtains follow-up images of recent supernovae, concentrating on systems within 20 Mpc of the Milky Way. The observations are taken well after maximum, with the aim of using the high spatial resolution of WFPC2 to identify the fading remnant and perhaps determine its origin.

GO 10896: An Efficient ACS Coronagraphic Survey for Debris Disks around Nearby Stars

HST ACS image of the edge-on debris disk around the nearbt F-type dwarf, HD 139664 (g Lupi)

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The disk structure is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Analysis of the rangle of morphological structure in these systems provides insight into the distribution of properties of planetary systems. Moreover, the overall statistics provide insight into the lifetimes and evolution of these structures. The coronagraphic imaging supplied by both NICMOS and ACS on HST remains the most effective means of achieving the high-contrast necessary to detect scattered light from these disks in the presence of the bright parent stars. These observations are particularly effective in probing radii that correspond to the Kuiper belt in our solar system. However, the sample of such systems is still rather small. This program aims to build on that foundation by using the ACS coronagraph to survey young, luminous stars near the Sun. This first phase of the survey, carried out with ACS in Cycle 11, included observations of 22 bright, nearby stars, concentrating on spectral types A and F. Two of those stars, HD 53143 and HD 139664, prove to have detectable debris disks. The current, second phase of this survey will target a further 25 stars, with the expectation of detecting between 4 and 6 new debris disks.

GO 10903 Resolving the LMC Microlensing Puzzle: Where are the Lensing Objects?

HST ACS image of the MACHO microlensing source LMC5: the blue source is the LMC star, the red source the foreground Galactic M dwarf that acted as a lens

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 of the fraction of these events that originate from lenses in the LMC, from foreground disk or halo M dwarfs, or lensing events that prove to be a misclassified background supernova or GRB.