This week on HST

HST Programs: November 27 - December 3, 2006

Some selected highlights

GO 10508: Orbits, Masses, and Densities of Three Transneptunian Binaries

Preliminary orbital determination for the KBO WW31, based on C. Veillet's analysis of CFHT observations; the linked image shows the improved orbital derivation, following the addition of HST imaging

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 (or trans-Neptunian objects, TNOs) 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. These issues can be addressed, at least in part, through deriving a better understanding of the composition of KBOs - and those properties can be deduced by measuring the orbital parameters for binary systems. The present proposal aims to use the ACS/HRC to determine the relative orbits for several known KBO binaries. Just as with binary stars, the orbital period and semi-major axis give the total system mass, while the mid-infrared properties (measured by Spitzer) allow an assessment of the surface area/diameters; combining these measurements gives an estimate of the mean density.

GO 10578: Eclipsing Binaries in the Local Group: Calibration of the Zero-point of the Cosmic Distance Scale and Fundamental Properties of Stars in M31

Schematic of an eclipsing binary system

Placing the cosmic distance scale on a reliable foundation remains a prime concern of cosmological astrophysics. Several HST programs are directed towards this end, sampling various rungs of the distance ladder. The current program is directed towards measurements of eclipsing binaries in nearby galaxies. In principle, these systems provide a purely geometrical means of measuring distances. We know the viewing angle for these systems: close to edge-on. Consequently, absolute stellar radii can be derived by combining the radial velocity curves with the photometric eclipse timing. Given the radius and a temperature measurement, the stellar luminosities can be determined; comparing those luminosities against the apparent brightness (making due allowance for foreground reddening) gives a direct distance estimate. The prime targets of the current program are two B-type eclipsing systems in the Andromeda spiral, M31. The ACS prism will be used to obtain spectra covering wavelengths from Lyman-alpha (1150 Angstroms) to the far red (9000 Angstroms), and those data will be used to derive temperature estimates for these stars.

GO 10881: The Ultimate Gravitational Lensing Survey of Cluster Mass and Substructure

Lensing by the z=0.39 galaxy cluster Cl0024+16; the inset shows several reconstructions of the z=1.63 lensed object, based on separate analysis of the brighter lensed images in this HST image.

Gravitational lensing supplies a powerful method of tracing the mass distribution in galaxy clusters; at the same time, the amplified the light from background galaxies provides a means of probing the early stages of galaxy formation. These measurements are particularly effective when X-ray imaging data are also available, allowing direct measurement of the mass density and distribution of the hot intracluster medium. This snapshot proposal aims to use the Wide Field Camera on ACS to observe the central regions of low redshift (0.15 < z < 0.3) clusters with the requisite Chandra observations. The HST images will allow the resolution of lensed arcs in the cluster cores (due to strong lensing) and characterisation of weak-lensing distortions of the image profiles of faint background galaxies. The frequency and detailed distribution (size, multiplicity, redshifts) of the strong lens systems sets strong constraints on the total mass content, and its structure, in the central regions of low-redshift clusters. Those results, in turn, constrain cluster evolution, and offer insight into likely schemes for studying dark energy at higher redshifts.

GO 10921: Tangential Velocities of Objects in the Orion Nebula and Locating the Embedded Outflow Sources

Sections of the Orion Nebula, from WFPC2 observations

The Orion Nebula, lying at a distance of ~500 pc and with an age of ~5 Myrs, is the nearest example of a large, young star-forming region. The Orion Nebula Cluster, the main site of star formation, has a complement of approaching 10,000 stars and brown dwarfs with masses ranging from a few MJupiter to more than 10 MSun. The most massive stars, including the central O stars of the Trapezium, produce jets, shocks and outflows, powered by winds and photoionisation. HST has been observing the ONC for well over a decade years, and the initial set of WFPC2 observations provide an excellent reference set of images for measuring the relative motions of gaseous knots, shock fronts and the higher-velocity stellar cluster members. The present observational program will use both ACS and WFPC2 to obtain second-epoch images in a variety of wide- and narrow-band filters, coupled with parallel near-infrared imaging with NICMOS.

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Past weeks:

• Cycle 14 observations (from March 13 2006 to Jul 1 2006)
• Cycle 15 observations (from July 1 2006)

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page by Neill Reid, updated 20/11/2006