This week on HST

HST Programs: March 30 - April 5, 2009

Program Number Principal Investigator Program Title Links
11593 Michael C. Liu, University of Hawaii Dynamical Masses of the Coolest Brown Dwarfs Abstract
11603 Jennifer Andrews, Louisiana State University and A & M College A Comprehensive Study of Dust Formation in Type II Supernovae with HST, Spitzer and Gemini Abstract
11704 Brian Chaboyer, Dartmouth College The Ages of Globular Clusters and the Population II Distance Scale Abstract
11788 George Fritz Benedict, University of Texas at Austin The Architecture of Exoplanetary Systems Abstract
11944 Douglas R. Gies, Georgia State University Research Foundation Binaries at the Extremes of the H-R Diagram Abstract
11974 Sahar S. Allam, Fermi National Accelerator Laboratory High-resolution imaging for 9 very bright, spectroscopically confirmed, group-scale lenses Abstract
11975 Francesco R. Ferraro, Universita de Bologna UV light from old stellar populations: a census of UV sources in Galactic Globular Clusters Abstract
11977 Nathan Smith, University of California - Berkeley WFPC2 12-Year Proper Motions of Two Galactic Analogs of the SN1987A Rings Abstract
11982 Scott F. Anderson, University of Washington Spanning the Reionization History of IGM Helium: a Large and Efficient HST Spectral Survey of Far-UV-Bright Quasars Abstract
11985 Geoffrey C. Clayton, Louisiana State University and A&M College Polarimetric WFPC2 Imaging of the Dust Torus around the Born-Again Star V605 Aquilae Abstract
11986 Julianne Dalcanton, Univ. Washington Completing HST's Local Volume Legacy Abstract
11987 Michael W. Regan, Space Telescope Science Institute The Recent Star Formation History of SINGS Galaxies Abstract
11994 Mario Livio, Space Telescope Science Institute HST Participation in the IYA 100 Hours of Astronomy Abstract

Selected highlights

GO 11593: Dynamical Masses of the Coolest Brown Dwarfs

Epsilon Indi Bab, the binary brown dwarf companion of the nearby K dwarf Brown dwarfs are objects that form like stars, but lack sufficient mass to drive the central temperature above a few million degrees, and therefore never succeed in igniting core hydrogen fusion. Discovered almost 15 years ago, these objects initialy have surface temperatures of ~3,500K, but cool rapidly and move through spcetral types M, L and T. Following their discovery, considerable theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L to type T and beyond. This transition marks the emergence of methane as a dominant absorber at near-infrared wavelengths. Current models suggest this transition occurs at ~1400-1200K, and that the spectral changes are at least correlated with, and perhaps driven by, the distribution and properties of dust layers ("clouds"). The overall timescales associated with this process remains unclear. Mass is a crucial factor in mapping those changes, but mass is also the most difficult quantity to measure in a reliable fashion. The present proposal aims to tackle this issue through astrometry of ultracool binary systems, deriving the orbits and hence dynamical masses. Initially designed for ACS, the current observations are being made with WFPC2, and the binary system SDSSJ092615.38+584720.9 will be imaged in the coming week.

GO 11704: The Ages of Globular Clusters and the Population II Distance Scale

Hubble Heritage image of the globular cluster, M15 Globular clusters are the oldest structures within the Milky Way that are directly accessible to observation. They are relatively simple systems, with relatively simple colour-magnitude diagrams (albeit with some complexities adduced from recent HST observations, see GO 11233 ). Matching those CMDs against theoretical models allows us to set constraints on the age of the oldest stars in the Galaxy, and hence on the age of the Milky Way and the epoch of galaxy formation. However, the accuracy of those age determinations rest crucially on the accuracy of the cluster distance determinations. The clusters themselves lie at distances of several kpc at best, and tens of kpc at worst; thus, direct trigonometric parallax measurements must await microacrsecond astrometric missions. The classical method of deriving distances is main sequence fitting - using nearby stars, with similar chemical abundances and accurate parallax measurements, to map out the main sequence in absolute units, and then scaling the clusetr data to fit. The problem with this method is that metal-poor subdwarfs are rare, so even Hipparcos was only able to obtain accurate distances to a handful of stars. The present program aims to improve the distance measurements by using the Fine Guidance Sensors on HST to determine sub-millarcsecond trigonometric parallaxes to 9 subdwarfs, almost doubling the sample available for MS fitting.

GO 11977: WFPC2 12-Year Proper Motions of Two Galactic Analogs of the SN1987A Rings

Ground-based imaging of the massive binary system, RY Scuti Supernova 1987A erupted in the Large Magellanic Cloud before HST's launch, but observations since the mid-1990s have revealed extremely interesting phenomena, notably the illumination of circumstellar material that must have been ejected from the star many years prior to the explosion. SN1987A's progenitor was Sanduleak -69o202, a massive blue supergiant, that had probably undergone excursions into the red supergiant regime within the last 10,000 years or so. It is likely that much of the mass loss occurred during thi phase of evolution. The present program targets two high-mass Galactic stars that may well be analogues of SK -69o202: RY Scuti, a massive eclipsing binary system at a distance of ~2 kpc; and Sher 25, a blue supergiant in the massive, young cluster, NGC 3603, some 6 kpc from the Sun. Both stars are surrounded by compact ring nebulae, and Sher 25 also shows evidence for bipolar outflows. Both stars were observed over a decade ago using WFPC2; the present observations will allow an estimate of angular motion which, combined with radial velocity data, should provide a better estimate of the distances, and of the likely future evolution of these systems.

GO 11982: Spanning the Reionization History of IGM Helium: a Large and Efficient HST Spectral Survey of Far-UV-Bright 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.

Past weeks:
page by Neill Reid, updated 30/3/2009