This week on HST

HST Programs: December 31, 2007 - January 6, 2008

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 11289: SL2S - The Strong Lensing Legacy Survey

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 CFHT Legacy survey provides a powerful tool for identifying candidate galaxy-galaxy lenses. Optical ground-based imaging, even from Hawaii, cannot match the results from a 2.4-metre telescope in orbit. Thus, the present program is using WFPC2 imaging to verify the nature of those candidates. The high resolution images can then be analysed to model the underlying mass distribution.

GO 11290: Dynamical Masses and Third Bodies in the Sirius System

HST image of Sirius A and B (epoch 2005)

Sirius B is the hot white dwarf companion to the Dog Star. Lying at a distance of only 2.6 parsecs (~8.6 l.y.; the fifth nearest star system and the nearest white dwarf), Sirius was originally identified as a double star in the early 1840s, when Friedrich Bessel's astrometry of the primary revealed residual orbital motion, with a ~50-year period. The companion remained undetected until 1862, when Alvin Clark finally resolved Sirius B through visual observations with the 18.5 inch refractor that he had recently installed in Dearborn Observatory. There have been persistent suggestions that the system includes a third companion; many rest on ancient descriptions of the system as red (which likely stem from atmospheric reddening when the star is at low altitudes, rather than a recent red giant phase). Recent observations with HST offer the tantalising suggestion of orbital perturbations that might stem from a much lower mass companion. This program aims to use HST for long-term astrometric monitoring, with the aim of verifying or refuting this hypothesis.

GO 11299: Calibrating the Mass-Luminosity Relation at the End of the Main Sequence

The MV-mass relation for low-mass stars (from T. Henry)

The mass-luminosity relation remains one of the key underpinnings of stellar astrophysics, notably in probing the grey area that separates hydrogen-burning stars from cooling-powered brown dwarfs. The calibration of thsi relation rests on observations of binary systems, primarily eclipsing binaries at masses above 1 MSun, and primarily astrometric binaries at sub-solar masses. In the latter case, reliable mass determinations require orbital measurements of extremely high accuracy, which, in turn, requires high precision astrometry over at least one orbital period. The Fine Guidance Sensors on HST have proven invaluable in this regard, since they allow sub-milliarcsecond accuracy astrometry of binary systems with sub-arcsecond separations; in other words, HST allows measurement of nearby, low-mass binaries with periods short enough to allow completion of the observations in significantly less than an astronomer's lifetime. The current program is using the FGS to monitor six close binary systems. Observations are scheduled of Gliese 54 (also known as LHS 1208). Lying at a distance of ~7.3 parsecs, this system consists of two near-equal luminosity (i.e. mass) M2.5 dwarfs.