This week on HST

HST Programs: June 2 - June 8, 2008

Some selected highlights

GO 11120: A Paschen-Alpha Study of Massive Stars and the ISM in the Galactic Center

A multi-wavelength composite image of the Galactic Centre (red - 90 cm radio data; green, mid-IR data; blue, X-ray)

The Galactic Centre lies in the heart of the constellation of Sagittarius, and at a distance of ~8 kiloparsecs from the Sun. Galactic nuclei are the likely end-point for mass accretion, and are generally the site of highly energetic activity; the Galactic Centre is no exception. AO near-infrared imaging has been brought to bear on this issue, resolving a number of stars within the core, close to the compact radio source Sagittarius A*. Monitoring of those sources over the last decade has shown that they are in rapid orbital motion around a very massive central object, now clearly identified as a ~3 million solar-mass black hole. Moving beyond the core, to distances of tens of parsecs from centre, observations have revealed molecular gas and young star forming regions. The aim of the present proposal is to use the NICMOS NIC3 camera to survey a ~32 x 13 arcminute region (~75 pc x 30 pc) in two narrowband filters: F187N, ceentred on the Paschen alpha line; and F190N, providing the continuum flux. These obbservations will provide a detailed map of the star forming activity within the Galactic nucleus.

GO 11148: High Contrast Imaging of Dusty White Dwarfs

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typcail targets0, they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years. The present program will use coronagraphic imaging with NICMOS to search for direct evidence of the suspected disks.

GO 11201: Systemic and Internal motions of the Magellanic Clouds: Third Epoch Images

The Large Magellanic Cloud (upper left) with the Small Magellanic Cloud (right) and the (foreground) Galactic globular cluster47 Tucanae

The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are the most massive satellites of the Milky Way galaxy. The orbital motions of these systems can be used to probe the mass distribution of Milky Way, and backtracking the orbits can shed light on how the three systems have interacted, In particular, the well known Magellanic Stream, stretching between the two Clouds, is thought to be a product either of interactions between the Clouds, or of ram-stripping of gas from the LMC on its last passage through the Plane of the Milky Way. The present program builds on observations obtained at two epochs with the now-defunct (but perhaps soon to be revived) ACS High Resolution Camera (ACS/HRC). The previous programs targeted known QSOs lying behind the Clouds; the QSOs serve as fixed reference points for absoltue astrometry of the numerous foreground LMC/SMC stars. First epoch observations were made in late 2002 (GO 9462), with the follow-up imaging in late 2004 (GO 10130). The tangential motions of the Clouds amount to only a few milliarcseconds, but the high spatial resolution and high stability of HST imaging makes such measurements possible, even with only a 2-year baseline. Surprisingly, the initial results suggest that the 3-D motions of both clouds are much higher than expected, suggesting either that the LMC/SMC/MW is either dynamically very young, or unbound. The present program will use WFPC2 to obtain third-epoch data in the same fields, providjng a crucial test of the initial results

GO 11311: The high-amplification microlensing event OGLE-2007-BLG-224: a substellar lens in the Galactic disk or a low-mass stellar lens in the halo

Microlensing light curve produced by a stellar lens with an appropriately placed planetary companion

Gravitational lensing is a consequence of general relativity. 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 serve as lenses to amplify the flux from background sources. Bohdan Paczynski pointed out in the mid-1980s that this effect offers 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. OGLE-2007-BLG-224/MOA-2007-BLG-163 is the formal designation for a microlensing event, towards the Galactic Bulge, that occured on May 12 2007. The event is remarkable in that the lensed star brightened by a factor of 3700, or almost 8 magnitudes (see the Ogle summary page ). This is the highest magnification event yet detected. The background source is a G dwarf, and the duration measured for the event indicate that the foreground lens has significant proper motion, suggesting that it is either a nearby brown dwarf or a more distant halo star. HST observations were obtained at the time, and the current observations will be matched against those data to determine whether the lens is now discernible. If it is not, then the lens is likely to be a brown dwarf.