This week on HST

HST Programs: September 10 - September 16, 2007

Some selected highlights

GO 10840: The FUV fluxes of Tauri stars in the Taurus molecular cloud

Schematic view of a T Tauri star

The T Tauri stage of evolution occurs early in a star's lifetime, within ~10 Myrs of its birth, when it still retains a dense, dust and gas-rich circumstellar disk. During this phase, there is significant accretion of material onto the central star. This leads to heating of the inner regions of the accretion disk, and significant emission at ultraviolet and X-ray wavelengths. The aim of this proposal is to use combine observations with WFPC2 and with the Solar Blind Channel (SBC) on the Advanced Camera for Surveys (ACS) to study the ultraviolet energy distribution of a number of T Tauris spanning a range of ages. The WFPC2 will be used to obtain images at near-UV wavelengths (2000 - 3000 Angstroms), while the SBC will provide comparable data at Far-UV wavelengths. The present observations target DO Tauri and GK Tauri, ~2 Myr-old classical T Tauri stars in the Taurus molecular cloud complex.

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.

GO 11178: Probing Solar System History with Orbits, Masses, and Colors of 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 will use the Planetary camera on WFPC2 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 11225: The Wavelength Dependence of Accretion Disk Structure

The first Einstein cross, the gravitational lensed QSO, G2237+0305

Gravitational lensing is a consequence the theory of general relativity. Its importance as an astrophysical tool first became apparent with the realisation (in 1979) that the quasar pair Q0957+561 actually comprised two lensed images of the same background quasar. In the succeeding years, lensing has been used to probe the mass distributions on a variety of scales: of galaxies (primarily via multiply-imaged quasars); of galaxy clusters (arcs and arclets); and at the largest scales (weak lensing). However, lensing can also provide insight on the small-scale properties of the object being lensed. In a lensed QSO, the light from the QSO follows different paths to produce the separate images; each of those paths has a different length; consequently, flux variations in the source show up at different times in the separate images. The present program aims to take advantage of this property to probe the structure of the accretion disks surrounding the central black hole in a number of lensed QSOs. The program will combine ultraviolet observations with the ACS/SBC on HST with Chandra X-ray data. Studying the variation as a function of wavelength should probe the accretion disk structure, since light from the inner regions are expected to dominate at shorter wavelengths, while the outer regiosn dominate at longer wavelengths.