This week on HST

HST Programs: October 31, 2011 - November 6, 2011

Selected highlights

GO 12281: STIS coronagraphic Imaging of the Kuiper Belt Surrounding the HR 8799 Planetary System

GO 12511: Determining the Atmospheric Properties of Directly Imaged Planets

HST-NICMOS imaging of the three outer planets in the HR 8799 system

HR 8799 is bright A-type star, lying ~39 parsecs from the Sun within the constellation Pegasus. It is one of the youngest stars in the Solar Neighbourhood, with an age of roughly 30 million years, and is likely a member of the loose group known as the Columba Association. The star itself is a member of the Lambda Bootis class, a type of star that exhibits unusually weak metallic features, suggesting that the surface atmospheric layers are depleted in abundance. Possible explanations include accretion of metal-poor gas from a surrounding circumstellar disk. HR 8799's main claim to fame is as the first star to have direct imaging observations of an associated planetary system. In 2008, a Canadian team led by Christian Marois announced the discovery of three giant planet companions, denoted HR 8799b, c and d, at orbital separations from 24 to 68 AU. The discoveries were based on near-infrared adaptive-optics assisted observations with the Gemini North and Keck telescopes. The planets likely have masses between 5 and 10 times that of Jupiter. Subsequent observations revealed a fourth planet, HR8799e, lying at a projected distance of ~14.5 AU from the parent star. Moreover, an examination of data within the HST archives showed that the outer three planets were detected on NICMOS observations taken in 1998, and careful re-analysis of those data, together with subsequent observations, by a team led by Remi Soummer has resulted in strong constraints on the orbital parameters. Mid-infrared observations with Spitzer have revealed complex debris disk structure. Program GO 12281 aims to detect scattered light from the planetary disk structure through observations with the STIS coronagraph. Program GO 12511 targets the same system, but with the IR channel of Wide-Field Camera 3. The prime aim of this program is to obtain photometry of the planetary companions at a range of wavelengths between 8,000 Angstroms and 1.6 microns, and use the resultant data to constrain the spectral properties,a nd hence the likely composition of the planetary atmospheres.

GO 12286: Hubble Infrared Pure Parallel Imaging Extragalactic Survey, HIPPIES

The ACS optical/far-red image of the Hubble Ultra Deep Field

Galaxy evolution in the early Universe is a discipline of astronomy that has been transformed by observations with the Hubble Space Telescope. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance, and the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS). The highest redshift objects found in the UDF have redshifts approaching z~7. Pushing to larger distances, and greater ages, demands observatons at near-infrared wavelengths, as the characteristics signatures of star formation are driven further redward in the spectrum. Wide Field Camera 3, installed in Servicing Mission 4, is well suited to these observations, and a number of programs are in place in Cycle 17 that address these issues. Indeed, WFC3 is employed in pure parallel mode by several programs. These take advantage of other science programs, usually with COS, that involve 2-5 orbit pointings on sources at high galactic latitude. The WFC3 pointing is unplanned, since it depends on the orientation adopted for the prime observations, but 2-5 orbits of IR imaging can reach galaxies at redshifts exceeding z=7 (potentially even z~8) in high latitude fields. This is one of two such programs in the cycle 17 portfolio.

GO 12515: Probing the outer limits of a galactic halo - deep imaging of exceptionally remote globular clusters in M31

Hubble WFPC2 image of the brightest M31 globular cluster, Mayall II/G1

M31, the Andromeda galaxy, is the nearest large spiral system to the Milky Way (d ~ 700 kpc), and, with the Milky Way, dominates the Local Group. The two galaxies are relatively similar, with M31 likely the larger system; thus, Andromeda provides the best opportunity for a comparative assessment of the structural properties of the Milky Way. Moreover, while M31 is (obviously) more distant, our external vantage point can provide crucial global information that complements the detailed data that we can acquire on individual members of the stellar populations of the Milky Way. With the installation on the ACS on HST at Servicing Mission 3B, it became possible to resolve main sequence late-F and G dwarfs in M31, permitting observations that probe stars with luminosities significantly below the turnoff of the halo population. Substantial effort has been devoted over the past 5 cycles to several programs probing the field stars at radial distances of more than 30 kpc from the galactic nucleus (e.g. GO 10816 ). Those observations show that, while the inner halo of M31 has a significant omponent of intermediate metallicity stars, perhaps related to the Bulge, the outer halo is predominantly metal poor. As in our own Milky Way, globular clusters provide another means of probing the structure and characteristics of Andromeda's halo population. The present program builds on the ground-based Pan-Andromeda Archaeological Survey (PandAS) discovery of a number of candidate globular cluster systems at projected distances of more than 85 kpc from Andromeda's nucleus. The ACS WFC will be used to image 13 of those systems, and obtain a better characterisation of their overall properties.

GO 12764: The demographics of dark gamma-ray bursts

An artist's impression of a gamma-ray burst

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts. The long/soft bursts appear to originate in the collapse of very massive stars, while the short/hard bursts are coalescing binary systems (probably pairs of netron stars or black holes). The first optical counterpart to a gamma ray burst was identified in 1998, allowing confirmation of their extragalactic nature, and, since then, more than 60 bursts have been detected at X-ray wavelengths, and half that number detected at either optical or radio wavelengths; all of these detections are long/soft bursts. In the past decade, optical counterparts have been detected for many of these sources, allowing not only direct study of their characteristics, but also investigations of the properties of the underlying host galaxies. Some sources, however, do not appear to generate optical counetrparts, perhaps because the optical emission is suppressed due to substantial local dust obscuration. The absence of those data has hampered previous attempts to match those sources, and their environments, against GRBs with optical counetrparts. The present program aims to circumvent this issue by using Chandra observations to obtain accurate positions for the X-ray counterparts of such sources, and then match that astrometry against deep visual (F606W) and near-infrared (F160W) HST WFC3 images.