This week on HST

HST Programs: October 15 - October 21, 2012

Selected highlights

GO 12492: The Nature of the Binary Companions to the Blue Straggers in the Old Open Cluster NGC 188

The old open cluster, NGC 188

Blue stragglers are found in old open clusters and globular cluster systems. Most appear to lie on the main sequence, but in a location `up and to the right' of the main-sequence turnoff; that is, these stars have higher mass than the turnoff stars, and, if coeval, ought to have evolved through the giant branches to become white dwarfs. The resolution of this paradox appears to lie in stellar mergers; blue stragglers represent the remnants of violent dynamical processes that can only occur within high density star clusters. Recent models identify three possible formation routes for these stars: mass transfer in binary systems; stellar collisions through dynamical encounters; and mergers of close pairs within triple systems. Recent observations of the old galactic open cluster, NGC 188, have shown that many of the blue stragglers in this system are binary systems today, with wide companions on long period orbits. A key test for the different formation lies in determining the nature of those wider companions: if mass transfer is the mechanism, the companions should all be whikte dwarfs; if the blue stragglers formed as mergers within triple systems, the wide companions are still likely to be unevolved main-sequence stars. The present prorgam aims to test these mdoels by using the ACS/SBC to obtain high-fidelity UV photometry, searching for the UV upturn that would represent a hot, white dwarf companion.

GO 12891: Search For Binaries Among Ultra-Slow Rotating Trojans, Hildas, and Outer Main Belt Asteroids

Preliminary orbital determination for the KBO WW31, based on C. Veillet's analysis of CFHT observations; the linked image shows the improved orbital derivation, following the addition of HST imaging

The Solar System includes a number of regions occupied by numerous small solid bodies, notably the main asteroid belt, between the orbits of Mars and Jupiter, and the Edgeworth-Kuiper Belt, beyond the orbit of Neptune. More than 96,000 bodies have been catalogued in the former region, including the larger (few hundred km diameter) minor planets like Ceres, Pallas, Juno and Vesta. The main belt asteroids fall into three main categories: carbonaeous (C-type), silicate (S-type) and metal-rich (M-type). A handful of objects have recently been detected showing cometary-like outbursts, suggestive of the presence of volatiles (eg asteroid 596 Scheila); these are classed as "main belt comets". A subset of the main belt asteroids have been captured by Jupiter into orbits that lead or trail Jupiter itself by ~ 60 degrees. these are the Trojan asteroids. The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distances of ~50 AU from the Sun. Over 500 KBOs (or trans-Neptunian objects, TNOs) are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. The origins of these smaller bodies remains a subject of some debate. In particular, it remains unclear where these bodies formed within the protoplanetary disk. The present proposal aims to probe this question by searching for binary systems among the Outer Main Belt and Trojan asteroids. Once orbits are determined, binary asteroids provide a means of determing the mass, density and hence the composition of these objects. A comparison between the properties of systems in the inner Solar System and in the Kuiper Belt should provide insight on whether a common origin is a reasonable hypothesis.

GO 12940: The Unevolved Massive Star Content of the Magellanic Clouds

Hubble WFC3 imaging of the Tarantula Nebula within 30 Doradus in the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are the two most massive satellite galaxies in the Milky Way system. The LMC is likely a low-mass irregular spiral system, with mass ~1010 solar masses, while the SMC is an irregular system, mass~few x 109 solar masses. Both are gas-rich, with extensive on-going star formation. In consequence, our external perspective provides an excellent opportunity for probing the star formation process, particularly the formation of massive stars in young clusters, which are frequently difficult to observe within the Milky way itself. Identifying the most massive stars in such systems can be challening, since the spectral energy distribution peaks in then ultraviolet, therefore negating the efficacy of optical photometry as a temperature indicator. The present program will use the WFC3-UVIS camera to obtain short ultraviolet (F225W) images of 23OB associations in the LMC and SMC. Those data will be combined with extensive ground-based imaging to identify the high-termperature O-type stars for subsqequent follow-up spectroscopy.

GO 13017: UV Spectroscopy of Lyman Break Galaxy Analogs: A Local Window on the Early Universe

Mosaic of HST images of M82, the best-known starburst galaxy

Current Big Bang cosmological models predict that the universe should have undergone a global re-ionisation at redshifts between 6 and 20. The first generation of stars is generally tapped as the most likely source of the ionising radiation, perhaps enhanced through merger-stimulated starbursts. Candidate galaxies at high redshift have been identified by searching for "dropouts": objects where the flux decreases significantly in the photometric passband associated with the Lyman break at a particular redshift. Deep observations with WFC3 have extended coverage to redshifts 7 and 8. However, detailed observations of such systems are not yet possible. Consequently, there is great interest in identifying galaxies at lower redshifts that could serve as analogues for the z>6 systems. Over the last few years, the Galaxy Evolution Explorer (GALEX) has proved an important new tool in identifying "Lyman Break Analogues". GALEX has conducted an all-sky survey at ultraviolet wavelengths, and has uncovered sigificant numbers of UV luminous galaxies at low and moderate redshifts. The present prorgam builds on a Cycle 17 program (GO 11727) that used the Cosmic Origins Spectrograph to characterise a small sample of these systems. The observations revealed conenctions between high velocity (>1000 km/sec) supernova driven winds, complex Lyman-alpha profiles, non-uniform distributions of HI gas and the presence of interstellar absortion lines that, taken together, suggest that the Lyman continuum radiation from these star-forming systems is not confined within the galaxies. With the extension of the GALEX UV survey, the present program targets additional systems to further probe the global characteristcis of these systems.