This week on HST

HST Programs: February 28, 2011 - March 6, 2011

Selected highlights

GO 12178: Spanning the Reionization History of IGM Helium: a Highly Efficient Spectral Survey of the Far-UV-Brightest Quasars

GALEX image of the nearby spiral, M81

The reionisation epoch for intergalactic helium is thought to occur somewhere between redshifts 3 and 4. Observations with the GALEX satellite, a NASA small explorer-class mission equipped with a 50-cm diameter telescope, are proving critical in testing this hypothesis through the identification of UV bright quasars in the appropriate redshift range. Galex was launched on 28th April 2003, and continues to operate more than 30 months beyond its nominal lifetime, conducting ultraviolet imaging and low-resolution grism spectroscopy at far-UV (125-175 nm) and near-UV (175-280 nm) wavelengths. Past HST programs by this research have used the ACS/SBC to target sources identified by cross-referencing GALEX against SDSS catalogues of moderate (1 < z < 3) and high redshift (z > 3.1) quasars. These sources can serve as effective probes of the ionisation state of the intergalactic medium at intervening redshifts. In particular, analysis of the He II Lyman-alpha absorption will shed light on the epoch of reionisation of intergalactic helium, generall placed between redshifts 3 and 4. The present program will use the ACS/SBC PR120L prism for spectroscopy of 40 QSOs with redshifts in the range 3.1 < z < 5.1.

GO 12161: Accretion in Close Pre-Main-Sequence Binaries

Wide-field image, from NOAO, of the young binary star, T Tauri, and its immediate environs

Young stars retain dense, dust and gas-rich circumstellar disks during the earliest stages of their formation and evolution. Observations of clusters and young associations suggest that these disks are present for at least the first 10 Myrs of the star's existence during which time the stars are generally given the T tauri classification. During this phase, there is substantial accretion of material onto the central star, leading to heating of the inner regions of the accretion disk, and significant emission at ultraviolet and X-ray wavelengths. Close binary stars are no exception to this paradigm, but the dynamics of the system tend to strongly truncate the individual circumstellar disks, but also lead to the formation of a large, circumbinary disk of material surrounding both stars. Previous HST programs (e.g. GO 10840 ; GO 11616 ) have used the STIS and the ACS/SBC to investigate accretion processes at FUV wavelengths in(primarily) single T Tauria. The present program will use COS to extend those investigations to the known binaries DQ Tau and UZ Tau E. COS will be used to measure the emission profiles of an extensive number of lines, probing opacities, temperatures and densities in the disk and outflow regions, and testing models for the magnetic structrue and accretion flows in young binary systems.

GO 12181: The Atmospheric Structure of Giant Hot Exoplanets

Probing the atmosphere of a transiting exoplanet

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Since that date, the number of known transiting exoplanet systems has grown to more than 100, most detected through wide-field photometric surveys with the Kepler satellite providing the highest sensitivity dataset. These transiting systems are invaluable, since they not only provide unambiguous measurements of mass and diameter, but they also provide an opportunity to probe the atmospheric structure by differencing spectra taken during and between primary secondary transit. Such observations are best done from space: indeed, the only successful atmospheric observations to date have been with HST and Spitzer. The present program aims to set these measurements on a systematic basis by targeting 13 transiting exoplanets. The WFC3-IR G141 grism will be used to search for characteristic near-infrared spectral features in those systems.

GO 12210: SLACS for the Masses: Extending Strong Lensing to Lower Masses and Smaller Radii

ACS images of galaxy-galaxy Einstein ring lenses from the Sloan survey

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 primarily to probe the mass distribution of galaxy clusters, using theoretical models to analyse the arcs and arclets that are produced by strong lensing of background galaxies, and the large-scale mass distribution, through analysis of weak lensing effects on galaxy morphologies. Gravitational lensing can also be used to investigate the mass distribution of individual galaxies. Until recently, the most common background sources were quasars. Galaxy-galaxy lenses, however, offer a distinct advantage, since the background source is extended, and therefore imposes a stronger constraints on the mass distribution of the lensing galaxy than a point-source QSO. The Sloan Digital Sky Survey is a powerful tool for identifying candidate galaxy-galaxy lenses, and has provided targets for HST imagiung programs in several previous cycles. The presentprogram is using HST-ACS imaging to survey a further 135 strong lens candidates. The HST data will verufy the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.