This week on HST

HST Programs: April 21 - April 27, 2008

Some selected highlights

GO 10852: Coronagraphic Polarimetry with NICMOS: Dust grain evolution in T Tauri stars

NICMOS coronagraphic images of GM Aurigae, showing the circumstellar disk

The T Tauri phase 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. It is generally agreed that at least giant planet formation occurs during this phase, terminating when the gas dissipates to leave a dusty debris disk. The properties of the resultant planets are likely to depend strongly on the properties of the dust within the circumstellar disk. This program aims to combine coronagraphy with the polarimetric capabilities of NICMOS, HST's near-infrared camera, to study the size distribution of dust particles within the disks surrounding a representative sample of young stellar objects. The forthcoming set of observations will target GM Aurigae, a ~0.8 solar-mass star with an age between 2 and 10 million years that has a substantial circumstellar disk. Multiwavelength measurements strongly suggest that the disk, which likely has a full diameter of ~600 AU, has a central gap, radius ~4 AU; that gap may well have been cleared by a jovian-mass planet.

GO 11082: NICMOS Imaging of GOODS: Probing the Evolution of the Earliest Massive Galaxies, Galaxies Beyond Reionization, and the High Redshift Observational Universe

ACS images of a section of the GOODS fields

The Great Observatories Origins Deep Survey, is a large-scale program that is designed to probe galaxy formation and evolution at redshifts from z~1 to z~6. GOODS covers two ~150 sq. arcminute fields, one centred on the Hubble Deep Field and the second on the Chandra Deep Field South, and combines deep optical/far-red imaging (F435W, F606W, F775W and F850LP filters) using ACS on HST with deep IRAC (3.6 to 8 micron) and MIPS (25 micron) imaging with Spitzer. Chandra data are also available for the bulk of the field. The present program is supplementing those data with near-infrared H-band (F160W) imaging with NICMOS, coupled with parallel ultraviolet observations using the ACS/SBC. The prime goals include identifying candidate galaxies and active nuclei at redshifts exceeding z=7; probing the star formation rate in dusty galaxies; and, via the SBC observations, examining UV leakage from star-forming systems as a function of redshift.

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 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 105 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.