This week on HST

HST Programs: July 18, 2011 - July 24, 2011

Selected highlights

GO 12036: Accretion flows and winds in pre-main sequence 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 extends those investigations using COS, which provides more than an order of magnitude more sensitivity and resolution. The observations focus on two well-known stars: BP Tau, spectral type K5, a member of the Taurus-Auriga association; and RU Lupi, a mid-G type member of the Lupus star forming cloud. Both stars lie at distances of ~140 parsecs, are known to harbour circumstellar disks, and have ages less than ~10 myrs. The COS observations will be used to probe the temperature distribution, flow velocities and column densities in the disk and outflow regions.

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 imaging programs in several previous cycles. The presentprogram is using HST-ACS imaging to survey a further 135 strong lens candidates. The HST data will verify the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.

GO 12251: The first characterization of a super-earth atmosphere

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 700, with the ovewhelming majority stemming from Kepler's high-sensitivity, high-fidelity survey of ~100 square degrees in Cygnus and the remainder are drawn from wide-field photometric surveys. Kepler's prime focus is solar-type stars, so the field surveys add an important element of diversity into planetary cataglogues. These transiting systems are invaluable not only in providing unambiguous measurements of mass and diameter, but also in providing 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 tagets one of the more unusual systems identified to date, GJ 1214b, the 6.7 Earth-mass companion of a mid-type M dwarf that lies ~13 pc from the Sun. The planet has a 1.6-day period transitting orbit, allowing an accurate measurement of its radius (6.6 Earth radii, assuming a stellar radius of 0.21 RSun). Observations with Spitzer hace suggested that the planet may have a metal-rich atmosphere. The rpesent program will use the WFC3-IR G141 grism to search for characteristic near-infrared spectral features in those systems.

GO 12322: 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.