This week on HST

HST Programs: May 5 - May 11, 2008

Some selected highlights

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 targets a further 25 stars, with the expectation of detecting between 4 and 6 new debris disks. Originally, the observations were scheduled for the ACS coronagraph; following the ACS failure in January 2007, a subset of the targets aree being onserved with the NICMOS coronagraph.

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 11195: Morphologies of the Most Extreme High-Redshift Mid-IR-luminous Galaxies II: The `Bump' Sources

HST images of interacting ultra-luminous IR galaxies

Luminous infrared galaxies (LIRGs) are systems that have total luminosities exceeding 1011.4 LSun, with most of the energy emitted at wavelengths longward of 10 microns. Many (perhaps most) of these galaxies are interacting or merging disk galaxies, with the excess infrared luminosity generated by warm dust associated with the extensive star formation regions. Many systems also exhibit an active nucleus, and may be in the process of evolving towards an S0 or elliptical merger remnant. Until recently, very few candidate such systems were known at high redshifts; consequently, analyses and investigations of their origins had to rely on observations of low- and moderate-redshift analogues. The team leading this HST proposal have used a combination of mid-infrared (24 micron) and near-infrared observations to identify tens of candidates, and Spitzer follow-up spectroscopy has confirmed that many lie at redshifts 2 < z < 2.5. The sources appear to fall into two broad categories: thopse with bright 24-micron fluxes with power-law spectral energy distributions and SiO absorption at mid-IR wavelengths; and "bump" sources, with an SED that peaks near 1.6 microns (rest wavelength) and PAH absorption. The brighter sources were targeted in a Cycle 15 program; the present program uses NICMOS and WFPC2 to obtain high sensitivity, high angular-resolution observations of 22 of the "bymp" systems.

GO 11513: The afterglow and host galaxy of GRB 080319: the first "naked eye" burst

Artist's impression of a GRB in action

Gamma ray bursts are described colloquially as the biggest bangs since the Big Bang. Originally detected by US spy satellites in the 1960s, these short-lived bursts of high energy radiation resisted characterisation for over 30 years. It is only within the last decade that the Galactic vs. Extragalactic debate on their origins has been setled in favour of the latter. Generically, gamma ray bursts are believed to originate in the death throes of an extremely massive star, as it collapses to form either a black hole or a highly magnetised neutron star. Most occur at moderate to high redshifts, and the optical flashes reach magnitudes between 12th and 15th at their peak. However, GRB 080319B , detected by the SWIFT satellite at 06:12 UT on March 19th 2008 (one of four bursts detected on that date), shattered all previous records. Optical imaging by the "Pi of the sky" wide-field monitoring survey shows that the source reached a peak brightness of mV~5.5 , remaining above the naked-eye threshold for ~30 seconds. The parent galaxy lies at redshift z~0.94, so the peak brightness was close to 1017 solar luminosities. The burst has faded significantly, but may still detected by HST in its previous set of observations in mid-April; the current observations will resolve detailed structure in the host galaxy.