This week on HST

HST Programs: November 12 - November 18, 2007

Program Number Principal Investigator Program Title Links
10583 Chris Stubbs, Harvard University Resolving the LMC Microlensing Puzzle: Where Are the Lensing Objects ? Abstract
10766 Andreas Zezas, Smithsonian Institution Astrophysical Observatory A Deep X-ray Survey of the Small Magellanic Cloud Abstract
10832 Brian M. Patten, Smithsonian Institution Astrophysical Observatory Solving the microlensing puzzle: An HST high-resolution imaging approach Abstract
10854 Karl Stapelfeldt, Jet Propulsion Laboratory Coronagraphic Imaging of Bright New Spitzer Debris Disks II Abstract
10872 Harry Teplitz, California Institute of Technology Lyman Continuum Emission in Galaxies at z=1.2 Abstract
11103 Harald Ebeling, University of Hawaii A Snapshot Survey of The Most Massive Clusters of Galaxies Abstract
11113 Keith S. Noll, Space Telescope Science Institute Binaries in the Kuiper Belt: Probes of Solar System Formation and Evolution Abstract
11116 Steven H. Saar, Smithsonian Institution Astrophysical Observatory Exploring the Early FUV History of Cool Stars: Transition Regions at 30 Myr Abstract
11130 Luis Ho, Carnegie Institution of Washington AGNs with Intermediate-mass Black Holes: Testing the Black Hole-Bulge Paradigm, Part II Abstract
11134 Karen Knierman, University of Arizona WFPC2 Tidal Tail Survey: Probing Star Cluster Formation on the Edge Abstract
11143 Andrew J. Baker, Rutgers the State University of New Jersey NICMOS imaging of submillimeter galaxies with CO and PAH redshifts Abstract
11146 Daniela Calzetti, University of Massachusetts The Role of Stellar Feedback in Galaxy Evolution Abstract
11155 Marshall D. Perrin, University of California - Berkeley Dust Grain Evolution in Herbig Ae Stars: NICMOS Coronagraphic Imaging and Polarimetry Abstract
11157 Joseph H. Rhee, University of California - Los Angeles NICMOS Imaging Survey of Dusty Debris Around Nearby Stars Across the Stellar Mass Spectrum Abstract
11163 Paula Szkody, University of Washington Accreting Pulsating White Dwarfs in Cataclysmic Variables Abstract
11175 Sandra M. Faber, University of California - Santa Cruz UV Imaging to Determine the Location of Residual Star Formation in Galaxies Recently Arrived on the Red Sequence Abstract
11178 William M. Grundy, Lowell Observatory Probing Solar System History with Orbits, Masses, and Colors of Transneptunian Binaries Abstract
11191 Wei-Hao Wang, Associated Universities, Inc. NICMOS Imaging of a z>4 High-Redshift Ultraluminous Submillimeter Source Abstract
11199 Lee W. Hartmann, University of Michigan A Hard Look at Stellar Disks at the Epoch of Planet Formation Abstract
11202 Leon Koopmans, Kapteyn Astronomical Institute The Structure of Early-type Galaxies: 0.1-100 Effective Radii Abstract
11204 Kevin Luhman, The Pennsylvania State University Imaging Circumstellar Disks and Envelopes around Proto-Brown Dwarfs Abstract
11211 George Fritz Benedict, University of Texas at Austin An Astrometric Calibration of Population II Distance Indicators Abstract
11215 Scott Anderson, University of Washington New Sightlines for the Study of Intergalactic Helium: Dozens of High-Confidence, UV-Bright Quasars from SDSS/GALEX Abstract
11233 Giampaolo Piotto, Universita di Padova Multiple Generations of Stars in Massive Galactic Globular Clusters Abstract
11312 Graham Smith, University of Birmingham The Local Cluster Substructure Survey (LoCuSS): Deep Strong Lensing Observations with WFPC2 Abstract

Some selected highlights

GO 10583: Resolving the LMC Microlensing Puzzle: Where Are the Lensing Objects ?

Microlensing light curve produced by a stellar lens with an appropriately placed planetary companion Gravitational lensing is a consequence of general relativity, and the effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. These wide-field imaging surveys target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. Statistical analysis, however, strongly suggests that both the distribution of event durations and the overall number of lenses are inconsistent with a dark matter component. So what are objects doing the lensing? This program aims to answer that question by using WFPC2 to obtain follow-up images of LMC lensed stars that were detected in the initial MACHO survey. Over a decade has elapsed since the lensing event, sufficient time, in at least some cases, for differential motion to separate lens and background star. Thus HST observations can set limits on the fraction of these events that might be produced by ordinary stars in the Galactic disk or halo.

GO 11113: Binaries in the Kuiper Belt: Probes of Solar System Formation and Evolution

A composite of HST images of the Kuiper Belt binary, WW31 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 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. This proposal will use WFPC2 imaging of known KBOs to identify new binary systems.

GO 11211: An Astrometric Calibration of Population II Distance Indicators

Measuring trigonometric parallax Trigonometric parallax measurement remains the fundamental method of determining distances to astronomical objects. The best ground-based parallax measurements can achieve accuracies of ~1 milliarcsecond, comparable with the typical accuracies achieved by the ESA Hipparcos astrometric satellite. This level of accuracy allows us to obtain reliable distances and luminosities for main sequence stars, subgiants, red giants and even a number of metal poor subdwarfs. However, with an effective distance limit of 100-150 parsecs, the sampling volume includes less than a handful of rarer, shorter-lived celestial objects. In particular, there are no RR Lyraes or Cepheids, two of the principal calibrators in the extragalactic distance scale. There is only one instrument currently available that can achieve astrometry of higher accuracy - the Fine Guidance Sensors (FGS) on HST. The present team used the FGS to measure a parallax of 3.82+/10.2 milliarseconds for RR Lyrae, the nearest star of its type. this corresponds toa distance of 262 parsecs. The present program aims to improve the calibration by extending observations to four more relatively nearby RR Lyraes (XZ Cyg, UV Oct, RZ Cep and SU Dra) and two Pop II Cepheids (Kappa Pav and VY Pyx).

GO 11233: Multiple Generations of Stars in Massive Galactic Globular Clusters

NGC 2808, a globular cluster with multiple stellar populations Globular clusters are remnants of the first substantial burst of star formation in the Milky Way. With typical masses of a few x 10M5 solar masses, distributed among several x 106 stars, the standard picture holds that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. Until recently, the only known exception to this rule was the cluster Omega Centauri, which is significantly more massive than most clusters and has both double main sequence and a range of metallicities among the evolved stars. Omega Cen has been joined by at least one more cluster, NGC 2808, which shows evidence for three distinct branches to the main sequence. The origin of this feature is notknown, but it may be significant that NGC 2808 is also one of the more massive clusters, and might therefore be able to survive several burst of star formation (or, conversely, be the product of a multi proto-globular merger). The present program aims to use WFPC2 to obtain high-precision photometry of other massive globulars, such as NGC 1851, M80 and M13.

Past weeks:
page by Neill Reid, updated 7/11/2007