This week on HST

HST Programs: May 23, 2011 - May 29, 2011

Selected highlights

GO 11616: The Disks, Accretion, and Outflows (DAO) of T Tau stars

Wide-field image, from NOAO, of T Tauri and its immediate environs

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 substantial 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. Previous HST programs (e.g. GO 10840 ) have used the STIS and the ACS/SBC to investigate these processes at FUV wavelengths. The present program will extend those investigations using COS, which provides more than an order of magnitude more sensitivity and resolution. The survey will target 32 T Tauri stars, including 26 "classical" T Tauris and 6 "weak-lined" T Tauris (the latter are surrounded by less disk material, and are generally believed to be at a later stage of evolution than the CTTs). 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.

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 12254: Helium-core White Dwarfs and Cataclysmic Variables in NGC 6752: New Clues to the Dynamical Evolution of Globular Clusters

The intermediate-metallicity cluster, NGC 6752

Lying at a distance of ~4 kpc in the southern constellation of Pavo, NGC 6752 is the third brightest globular cluster and one of the closest to the Sun. It is moderately metal-poor, with a metallicity [Fe/H] ~ -1.5, or ~3% that of the Sun. Given its proximity and the relatively low reddening due to foreground dust, NGC 6752 has received substantial observational attention since its discovery in the mid 1820s by the Scots astronomer, James Dunlop. [At the time, Dunlop was working for Sir Thomas Brisbane at Paramatta Observatory, NSW.] Ground-based and HST observations reveal a colour-magnitude diagram characterised by an extended horizontal branch, a main sequence of some modest width, diverse abundances on the giant branch and a number of white dwarfs outlining a degenerate sequence. Recent detailed observations suggest that this cluster harbours multiple stellar populations. The present program focuses on the white dwarfs, using a combination of blue and red, wide- and narrow-band imaging with the Wide-Field camera on ACS in an attempt to identify candidate binary systems, both double-degenerates and cataclysmic variables.

GO 12290: Do Rocky Extrasolar Minor Planets Have a Composition Similar to Bulk Earth?

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

White dwarf stars are the final evolutionary state of stars like the Sun. Comprised of degenerate material that compacts more than half the mass of the Sun into a radius comparable with that of the earth, white dwarfs have no internal energy source, and gradually cool and fade with time. At higher temperatures (> 10,000K), their spectra are dominated by broad lines of either hydrogen (DA white dwarfs) or helium (DBs). A handful of spectroscopic oddities, with prominent metal lines (Ca, Mg) were discovered in the 1970s and 80s. Those elements are predicted to diffuse relatively rapidly within the white dwarf atmosphere, and those objects were typically explained as resulting from interstellar accretion. However, high resolution optical spectroscopy obtained in the late 1990s revealed that a significant fraction of DA white dwarfs have narrow metallic absorption lines. Crucially, one of the stars found to have such absorption features is G29-38, the white dwarf originally tagged as harbouring a brown dwarf companion based on its showing a strong mid-infrared excess. Given the relatively high frequency of occurrence and the short residence time in the atmosphere, interstellar accretion is not an option, and those lines must be maintained by continuous accretion from a circumstellar debris disk (which contributes the IR excess in G 29-38). That debris disk, in turn, must be regularly replenished, presumably from collisions between rocky remnants of a solar system. Thus, probing the abundance distribution of the accreted material should illuminate the composition of the residual asteroidal material. The present program aims to tackle this issue by using the Cosmic Origins Spectrograph to obtain UV spectra of a sample of 6 white dwarfs known to exhibit metallic absorption lines.