This week on HST

HST Programs: November 5 - November 11, 2012

Selected highlights

GO 12471: The Bottom of the Iceberg: Faint z~2 Galaxies and the Enrichment of the IGM

WFC3 grism imaging of a section of the GOODS-North field

Observations with the Hubble telescope have impacted many science areas, but probably none more so than investigations of galaxy formation and evolution in the high redshift universe. Deep field images, starting with the oroginal HDF and progressing through the HDF-South, GOODS-North and South, the Ultra-Deep Field and the infrared UDF, have been coupled with observations at other wavelengths from space and ground to probe galaxy structure at redshifts extending to z=9 and beyond. Most analyses of galaxy evolution as a function of redshift favour the general conclusion that the global star formation rate peaked at redshifts in the range z=2 to 3. Extensive star formation in proto-galaxies at those redshifts leads to extensive gaseous outflows and strong interactions. The present program aims to build on past research by using the spectroscopic capabilities offered by the WFC3-IR grism to survey fields centred on 15 high-redshift QSOs whose spectra have been used to probe the line-of-sight structure of the intergalactic medium. The grism observations will permit dientification of fainter galaxies that may be associated with the strong absorbers detected in the QSO spectra.

GO 12870: The mass and temperature distribution of accreting white dwarfs

An accreting white dwarf starn in a close binary system

Supernovae are the most spectacular form of stellar obituary. Since B2FH, the physical processes underlying their eruptive deaths have been known to play a key role in populating the ISM with metals beyond the iron peak. More recently, these celestial explosions have acquired even greater significance through the use of Type Ia supernovae as distance indicators in mapping the `dark energy' acceleration term of cosmic expansion. However, while there are well-established models for the two main types of supernovae (runaway fusion on the surface of a white dwarf in a binary system for Type Ia, or detonation of the core in Type II), some significant uncertainties remain concerning the physical details of the disruption, and, potentially, the overall uniformity of these events. Consequently, there is potential for systematic bias in the distance estimates. The present program aims to set constraints on the various mechanisms associated with white dwarf stars by investigating the rotational properties of over 40 degenerate companions in catclysimic variable systems. All of these systems are in the process of accreting material from the companion star, as the latter voerflows its Roche lobes. The program aims to obtain ultraviolet spectra with the Cosmic origins Spectrograph, probing both the spin rates and the orbital parameters. Over the next decade, these data may lead to the determinaton of reliable masses for both stars once accurate parallax measurements become available from Gaia.

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 12929: COS Observations of Pulsating DB White Dwarfs

The surface-temperature map on a pulsating white dwarf (Figure by Mike Montgomery U. Texas group)

White dwarfs are compact, electron-degenerate remnants that represent the final evolutionary stage for stars less massive than ~7 Msun. White dwarfs emerge from planetary nebulae with extremely high surface temperatures, but with no central energy source, they simply cool like a brick. As they cool, the spectral energy distribution and the spectral characteristics evolve with time. Most white dwarfs have thin hydrogen envelopes, and are therefore have strong Balmer-series absorption lines in the optical at temperatures above ~8,000 degrees (DA white dwarfs). A sizeable minority, however, have helium envelopes, and spectra dominated by helium lines; these are DB white dwarfs. Both DA and DB white dwarfs evolve through regions of atmospheric instability as they cool - instability strips, analagous to those populated by Cepheids and RR Lyraes in the hydrogen-burning regime. White dwarfs undergo non-radial pulsations are high-order gravity modes, driven by opacity variations in the stellar interior. In the case of DB white dwarfs, the region falls near surface temperatures of ~19,000K. Determing accurate effective temperatures is crucial to constraining the models developed for these highly compact objects. The present program aims to use UV spectra obtaiend with' the Cosmic Origins Spectrograph to refine temperatures for 6 pulsating DB white dwarfs.