This week on HST

HST Programs: March 22, 2010 - March 28, 2010

Program Number Principal Investigator Program Title Links
11149 Eiichi Egami, University of Arizona Characterizing the Stellar Populations in Lyman-Alpha Emitters and Lyman Break Galaxies at 5.7 Abstract
11516 James Green, University of Colorado at Boulder COS-GTO: Cold ISM Abstract
11520 James Green, University of Colorado at Boulder COS-GTO: QSO Absorbers, Galaxies and Large-scale Structures in the Local Universe Abstract
11524 James Green, University of Colorado at Boulder COS-GTO: WARM AND HOT ISM IN AND NEAR THE MILKY WAY Abstract
11548 S. Thomas Megeath, University of Toledo NICMOS Imaging of Protostars in the Orion A Cloud: The Role of Environment in Star Formation Abstract
11550 James Green, University of Colorado at Boulder COS-GTO: X-Ray Binaries Abstract
11555 Alexander Brown, University of Colorado at Boulder Transition Region and Chromospheric Activity on Low Metallicity Arcturus Moving Group `Alien' Dwarfs Abstract
11567 Charles R. Proffitt, Computer Sciences Corporation Boron Abundances in Rapidly Rotating Early-B Stars. Abstract
11568 Seth Redfield, Wesleyan University A SNAPSHOT Survey of the Local Interstellar Medium: New NUV Observations of Stars with Archived FUV Observations Abstract
11588 Raphael Gavazzi, CNRS, Institut d'Astrophysique de Paris Galaxy-Scale Strong Lenses from the CFHTLS survey Abstract
11592 Nicolas Lehner, University of Notre Dame Testing the Origin(s) of the Highly Ionized High-Velocity Clouds: A Survey of Galactic Halo Stars at z>3 kpc Abstract
11604 David J. Axon, Rochester Institute of Technology The Nuclear Structure of OH Megamaser Galaxies Abstract
11626 Philip Massey, Lowell Observatory Searching for the Upper Mass Limit in NGC 3603, the Nearest Giant H II Region Abstract
11643 Ann Zabludoff, University of Arizona A Timeline for Early-Type Galaxy Formation: Mapping the Evolution of Star Formation, Globular Clusters, Dust, and Black Hole Abstract
11644 Michael E. Brown, California Institute of Technology A dynamical-compositional survey of the Kuiper belt: a new window into the formation of the outer solar system Abstract
11651 Carole A. Haswell, Open University Is the atmosphere of the hottest known transiting exoplanet evaporating? Abstract
11654 Robert P. Kirshner, Harvard University UV Studies of a Core Collapse Supernova Abstract
11663 Mark Brodwin, Harvard University Formation and Evolution of Massive Galaxies in the Richest Environments at 1.5 < z < 2.0 Abstract
11666 Adam J. Burgasser, Massachusetts Institute of Technology Chilly Pairs: A Search for the Latest-type Brown Dwarf Binaries and the Prototype Y Dwarf Abstract
11679 Craig L. Sarazin, The University of Virginia Probing The Globular Cluster / Low Mass X-ray Binary Connection in Early-type Galaxies At Low X-ray Luminositie Abstract
11687 Thomas R. Ayres, University of Colorado at Boulder SNAPing Coronal Iron Abstract
11704 Brian Chaboyer, Dartmouth College The Ages of Globular Clusters and the Population II Distance Scale Abstract
11707 Kailash Sahu, Space Telescope Science Institute Detecting Isolated Black Holes through Astrometric Microlensing Abstract
11709 David Bersier, Liverpool John Moores University Stretching the diversity of cosmic explosions: The supernovae of gamma-ray bursts Abstract
11715 Howard E. Bond, Space Telescope Science Institute The Luminous Galactic Cepheid RS Puppis: A Geometric Distance from its Nested Light Echoes Abstract
11719 Julianne Dalcanton, University of Washington A Calibration Database for Stellar Models of Asymptotic Giant Branch Stars Abstract
11732 C. S. Kochanek, The Ohio State University Research Foundation The Temperature Profiles of Quasar Accretion Disks Abstract
12011 Rachel A. Osten, Space Telescope Science Institute Magnetic Heating of the Outer Atmospheres of Very Low Mass Dwarfs Abstract
12016 Carol A. Grady, Eureka Scientific Inc. The Stars and Edge-on Disks of PDS 144: An Intermediate-Mass Analog of Wide T Tauri Multiple Stars Abstract

Selected highlights

GO 11651: Is the atmosphere of the hottest known transiting exoplanet evaporating?

Artist's conception of atmospheric ablation on a hot jupiter Among the most surprising discoveries of the latter part of the twentieth century was the finding that exoplantery systems were not simple copies of the Solar System architecture. Specifically, Jovian-mass gas giants were a relatively common occurrence at sub-Mercurian separations from sun-like stars. Characterised as "hot jupiters", the hottest jupiter is the ~1.4 MJup planet in a 1.1 day period orbit around a fairly anonymous 11th magnitude G dwarf. This is a transiting system, identified by the WASP team and designated as WASP-12b, so the orbital inclination is known, and the absolute semi-major axis is only 0.023 AU, less than one-tenth the radius of Mercury's orbit. Consequently, WASP-12b experiences substantial heating, and is estimated to have a temperature of ~2500K (comparable with an M7/M8 dwarf). At these temperatures, the expectation is that the planetary atmosphere will be subject to extensive evaporation, potentially leading to a substantial gas tail. Previous HST programs have searched for spectral signatures of this feature, and evidence has been found for such features. Some of these claims have been challenged, however. As the hottest jupiter, WASP 12-b is a prime target for testing for evaporation, and the present proposal aims to use COS to search for a gaseous coma.

GO 11666: Chilly Pairs: A Search for the Latest-type Brown Dwarf Binaries and the Prototype Y Dwarf

NICMOS images of the ultracool L/T binary, 2MASS J22521073-1730134; the northern component, notably fainter at F160W, is the T dwarf. Ultracool dwarfs are defined as having spectral types later than M7, and therefore include the recently discovered L and T dwarfs. They encompass the lowest mass stars (masses < ~0.1 MSub) and sub-stellar mass brown dwarfs, with surface temperatures ranging from ~2500K (~M7) to <700K (late-type T dwarfs). Following their discovery over a decade ago, considerable theoretical attention has focused on the evolution of the intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L to type T. This point marks the emergence of methane as a dominant absorber at near-infrared wavelengths. Current models suggest the 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") within the atmosphere. The overall timescales associated with the process remain unclear. The present proposal aims to tackle this issue through identifying, and characterising, ultracool binary systems with extremely cool components. Since these systems are almost certainly coeval, the relative spectral energy distributions of the two components can be used to set constraints on evolutionary models. More than 80 ultracool binary systems are currently known; almost all have relatively small linear separations (<15 AU), and components with mass ratios close to one. The present program targets 27 ultracool dwarfs with spectral types in the range T5 to T9, and will use WFC3 IR observations to search for previously unrecognised close, faint companions.

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 cluster 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 11707: Detecting Isolated Black Holes through Astrometric Microlensing

A rather spectacular version of black hole lensing. Gravitational lensing is a consequence of general relativity. Its 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. Those wide-field imaging surveys have target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. The duration of each event depends on several factors, including the tangential motion of the lens and its mass. Long-term events are generally associated with a massive lens. Duration alone is not sufficient to identify a lens as a black hole - a source with very low tangential motion relative to the Sun can produce the same effect. However, microlensing not only leads to flux amplification, but also to small astrometric motions, caused by the appearance and disappearance of features in the lensed light. Those motions serve as a mass discriminant - higher mass lenses produce larger amplitude motions. The expected astrometric signal from a black hole lens is > 1.4 millarcseconds, just measureable with HST. This program aims to capitalise on this fact by searching for lensing by black holes in the Galactic field. The observations target long-duration lensing events in the Galactic Bulge.

Past weeks:
page by Neill Reid, updated 19/2/2010