This week on HST

HST Programs: May 29 - June 4, 2006

Some selected highlights

GO 10145: Physical parameters of the upper atmosphere of the extrasolar planet HD209458b

"Osiris" (HD 209458b) find itself too much in the sun

HD 209458 is G0, near-solar metallicity main-sequence dwarf lying 50 parsecs from the Sun. Like (at least) 10% of solar-type stars, it harbours a planetary system, and like (at least) 3% of solar-type stars, it has a `hot Jupiter', orbiting the parent star at a distance of 0.045 AU in a period of 3.52474859 days; HD 209758b's main claim to fame is that it was the first transiting planetary system detected. Transiting planetary systems have proven a goldmine of information on the intrinsic properties of extrasolar giant planets. The orbital inclination is known, and the mass (0.69 MJ), radius (1.32 RJ) and mean density are determined directly. Moreover, HST and Spitzer can be used to probe the planetary atmosphere, by either searching for enhanced absorption during the primary eclipse (HD 209458b transits HD 209458), or by searching for reduced (mid-infrared) emission as HD 209458 eclipses the hot Jupiter. The present proposal aims to build on previous HST observations that detected enhanced Lyman alpha, OI and CII absorption, generally ascribed to gaseous mass-loss (evaporation) from the heated planetary atmosphere. This proposal will use the UV prism on ACS to obtain spectra of HD 209458 with the HRC and SBC detectors. The observations will be made during primary eclipse (transit), with the aim of detecting additional species, such as Fe II and Mg II.

GO 10514: Kuiper Belt Binaries: Probes of Early Solar System Evolution

Composite HST image of the Kuiper Belt binary, WW31

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known KBOs are binary (including Pluto, one of the largest known KBOs, regardless of whether one considers it a planet or not). This is a surprisingly high fraction, given the difficulties involved in forming such systems and the relative ease with which they can be disrupted. It remains unclear whether these systems formed from single KBOs (through collisions or 3-body interactions) as the Kuiper Belt and the Solar System have evolved, or whether they represent the final tail of an initial (much larger) population of primordial binaries. This proposal aims to use ACS/HRC images of known KBOs toidentify new binary systems.

GO 10573 Globular Clusters in the Direction of the Inner Galaxy

The bulge globular cluster, M19

Globular clusters have served as important probes of Galactic structure since the early years of the twentieth century, when Harlow Shapley used their spatial distribution as evidence for his `Large Galaxy' model. Their ages, chemical abundances and kinematics provide insight into the formation and early evolution of the Milky Way. A substantial number of globulars lie in the inner Galaxy, many of which are probably associated with the Bulge, rather than the halo (as originally proposed by Robert Zinn in 1980). The line of sight towards these systems passes through the heart of the Galactic disk, and most are subject to significant reddening by foreground dust. This can prove a severe hindrance to reliable interpretation on the colour magnitude diagrams. This proposal will obtain ACS/WFC observations in the F435W, F555W and F814W filters, and will use the resultant B, V and I data to map the differential extinction across 27 Bulge globular clusters. All of these clusters are inc highly crowded fields, so the ACS observations are necessary to derive accurate CMDs and determine reliable intrinsic properties.

GO 10842 A Cepheid Distance to the Coma Cluster

NGC 4911, from DSS scans of POSS II IIIaJ plate material

Cepheid variable stars have been the prime extragalactic distance indicator since Henrietta Leavitt's discovery of the period-luminosity relation described by Cepheids in the Small Magellanic Cloud. It was Hubble's identification of Cepheids in NGC 6822 that finally established that at least some nebulae were island universes. Cepheids and the extragalactic distance scale figure largely in HST's history, notably through the Hubble Constant Program, one of the initial Key Projects. Hubble has accumulated WFPC2 and NICMOS observations of Cepheids in 31 galaxies. All of those galaxies lie within 25 Mpc; thus, both the Key Project's derivation of H0 = 72 +/- 8 km/sec/Mpc and the competing value, H0 = 56 +/- 7 km/sec/Mpc, (an offset of 1.5 sigma), rely on secondary indicators to take measurements to the far-field Hubble flow. The aim of the present project is to use the higher sensitivity and higher resolution of ACS to push the Cepheid measurements to the Coma cluster. The program will target two spiral galaxies in the cluster, NGC 4911 and NGC 4921. If Coma lies at a distance of 100 Mpc ( (m-M)=35.0), then long-period Cepheids (P~50 days) have mean apparent magnitudes of V~29 - challenging observatons even for ACS. The present set of observations is designed to optimise the observing strategy in anticipation of the full obsrving program, which will be executed in Spring 2007.

GO 10989: 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, which has an entourage of at least three planetary-mass companions. The current GO program pursues observations of six planetary hosts, and FGS observations of three targets, HD 136118, HD 145675 (14 Her) and HD 168443, are scheduled over the next week.