This week on HST

HST Programs: June 17 - June 23, 2013

Selected highlights

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 12998: STIS Coronagraphy of Bright New Debris Disks from the WISE All-Sky Survey

HST image of the face-on debris disk in the G2 dwarf, HD 107146

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The structure of those disks is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Analysis of the rangle of morphological structure in these systems provides insight into the distribution of properties of planetary systems. HST currently provides almost the only means of achieving the high-contrast required for the detection of scattered light from these disks in the presence of the bright parent stars. While many such systems have been observed, only a relatively small number of disks have been imaged successfully at visual or near-infrared wavelengths. The present program aims to expand the sample by targetting six solar-type stars that are known to have circumstellar disks based on mid-infrared observations with WISE, the Wide-field Infrared Survey Explorer. This sample constitutes the brightest new discoveries, and therefore the most likely candidates for detection by Hubble. The present proposal will use the occulting bar on the Space Telescope Imaging Spectrograph to carry out coronagraphic imaging at visual wavelengths.

GO 13180: Search for a Transit of Alpha Centauri Bb, the First Earth-mass Exoplanet Orbiting a Sun-like Star

An artist's impression of the hypothetical terrestial companions of Alpha Cen B

Alpha Centauri is the closest stellar system to the Sun, lying at a distance of ~1.3 parsecs. Alpha cen is a triple system, comprising two solar-type stars, Alpha Cen A and B, spectral types G2 and K1, in relatively close proximity together with the M5 dwarf, Proxima Centauri, at a separation of 15,000 AU. As our nearest neighbours (Proxima is actually the closest star to the Sun) and among the brightest stars in the sky, the solar-type stars in Alpha Cen have been the target of numerous searches for planetary companions. Specifically, both stars have been subjected to extensive radial velocity monitoring by the Geneva planet-search team using the HARPS echelle spectrograph mounted on the 3.6-metre telescope at ESO's La Silla Observatory. In October 2012, the team announced the discovery of systematic radial velocity variations of amplitude of +/- 51 cm/sec and a period of 3.2 days; assuming moderate inclinations, this correponds to a terrestrial-mass planet (M~0.93 MEarth) in a orbit with semi-major axis 0.04 AU. These observations are on the edge of current detection techniques, and some controversy remains over whether the detection is confirmed or not. Clearly, this detection would not correspond to a habitable planet - the separation from the parent star is approximately one-tenth the radius of Mercury's orbit, leading to exceedingly high temperatures on the planetary surface. Nonetheless, the discovery of a terrestrial planetary companion around our nearest neighbour has strong implications (psychological as much as statistical) for the frequency of such systems. Assuming its existence, there is a 10% chance that the planet transits the host star. HST is the only facility that is currently capable of detecting such a planet in transit. The present observations are timed to coincide with the appropriate orbital phase.

GO 13400:The Surprising Ejecta Geometry of Recurrent Nova T Pyx

Artist's impression of the recurrent nova, RS Oph (by David Hardy)

Recurrent novae are generally agreed to be close binary systems, comprising a white dwarf and a companion main sequence star that is overflowing its Roche lobe, leading to period transfers of mass onto the white dwarf surface. The mass transfer episode triggers nuclear ractions, which lead the star increasing significantly in it luminosity. T Pyxidis is one such system, and it exhibited fairly regular outbursts every 20 years between its discovery, in 1890, and 1966. Since then, however, it has been dormant, a prolonged period of quiescence that led to suggestions, earlier this year, that it might either be headed for hibernation, or in the process of accumulating sufficient mass to trigger a type Ia supernova explosion (in about 1 million years). Perhaps prompted by these suggestions (a la Monty Python Mary Queen of Scots radio sketch), T Pyxidis erupted into activity on or around April 15th 2011. HST obtained a series of multi-wavelength narrowband images of the illuminated ejecta over the past two years, tracking the evolution of the expanding ejecta. The present program builds on those observations by using WFC3 and STIS to map the spatial distribution and kinematics of the evolving remnent.