This week on HST

HST Programs: January 1 - January 7, 2007

Some selected highlights

GO 10610: Astrometric masses of extrasolar planets and brown dwarfs

An artist's impression of the planets circling the M dwarf, Gl 876

The overwhelming majority of extrasolar planetary systems have been identified through radial velocity monitoring, and the detection of the reflex motion of the parent star as it orbits the common center of mass of the system. Just as radial velocities measure the stellar "wobble" introduced along the line of sight, so high precision astrometry can be used to measure motion in the plane of the sky. Combining these data gives the full three-dimensional motions of the system, and a direct measure of the mass of the planetary companion. The Fine Guidance Sensors on HST are the only system currently capable of making observations at the required sub-milliarcscond accuracy, and has already been used for astrometry of four systems, including the M dwarf Gl 876. The current GO program pursues observations of six planetary hosts; the final set of FGS observations of HD 47536 is scheduled over the next week.

GO 10624: Solving the mystery of short-hard 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 50 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. The aim of this program is to focus on the short/hard bursters. This is a Target of Opportunity project, triggered by the detection of a suitable candidate by the Swift satellite, followed by the rapid (< 3 days detection of an optical afterglow. The aim is to obtain ACS observations (weither WFC or HRC) within 5-6 days of the burst, and use the resultant images to characterise the underlying host galaxy.

GO 10816: The Formation History of Andromeda's Extended Metal-Poor Halo

HST ACS image of the outer bulge/inner halo resgions of M31

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 advent on the ACS on HST, it has become possible to resolve main sequence late-F and G dwarfs in M31. This permits observations that probe stars with luminosities below the turnoff of the Galactic halo population, and substantial effort has been devoted to this program in recent cycles. The initial results suggested that M31's halo might be very different from our own - specifically, the data indicated the preence of a significant number of stars with both intermediate age (6-8 Gyrs) and intermediate metallicity ([Fe/H] ~ -0.5), rather than the >10 Gyrs and -5 < [Fe/H] < -1.5 values derived for the Galactic halo. However, all previous HST observations were at radial distances of less than 30 kpc from the M31 centre, and it has become clear that those data were contaminated by the M31 Bulge (or "spheroid" component). Recent observations indicate that a more traditional halo component dominates at larger radii. The present program aims to confirm that hypothesis and derive reliable age, metallicity and density estimates. The ACS will be used with the F606W and F814W filters to image several fields lying in the far-flung outskirts of M31, at distances of 22 to 35 kpc from the nucleus.

GO 10882: Emission Line Snapshots of 3CR Radio Galaxies

Extended structure in the radio galaxy, 3C 75

The Third Cambridge Catalogue of Radio Sources was compiled originally in 1959 from observations at 159 MHz made by the Cambridge Radio Astronomy Group using the Cambridge interferometer (Edge et al, Mem RAS, 68, 37). The addition of observations at 178 MHz led to the revised catalogue, the 3CR (Bennett, 1962, Mem RAS, 68, 137). This catalogue was the main basis for the Fanaroff-Riley classification scheme (FR-Is are double-lobed sources with small separation between the peaks; FR-IIs have a separation that exceeds half the largest size of the source), and includes many of the brightest known radio galaxies. As such, the 3CR sources have remained extremely important observational targets for understanding the nature and structure these highly energetic sources. Over the past several cycles, many 3CR galaxies have been observed at a variety of wavelengths by HST. Those observations have revealed new optical jets, dust lanes, face-on disks with optical jets, besides point-like nuclei whose properties support FR-I/BL Lac unified schemes. The present program is using the ramp filters on the ACS/WFC to obtain images covering the H-alpha and O[III]5007 emission lines, probing the star formation characteristics of these radio galaxies.