This week on HST

HST Programs: January 21 - January 27, 2013

Selected highlights

GO 12468: How Fast Did Neptune Migrate? A Search for Cold Red Resonant Binaries

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 Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). 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. Approximately 2% of the known TNOs are binary (including Pluto, one of the largest known TNOs, regardless of whether one considers it a planet or not). TNOs are grouped within three broad classes: resonant objects, whose orbits are in mean motion resonance with Neptune, indicating capture; scattered objects, whose current orbits have evolved through gravitational interactions with Neptune or other giant planets; and classical TNOs, which are on low eccentricity orbits beyond Neptune, with no orbital resonance with any giant planet. The latter class are further sub-divided into "hot" and "cold" objects, depending on whether the orbits have high or low inclinations with respect to the ecliptic. Cold, classical TNOs show relatively uniform characteristics, including red colours, high albedos and an extremely high binary fraction (>30%). They are believed to have formed in situ, and were therefore in place to experience the range of gravitational interactions as the giant planets migrated to their present location. As that migration occurred, subsets are expected to have been trapped in transitory resonance orbits. The present proposal aims to use HST to complete a photometric survey of all known resonant TNOs, with the goal of identifying the proportion of cold classical TNOs that have been captured. The relative number of such objects can be used to constrain models for Neptune's orbital migration in the early Solar System.

GO 12533: Escape of lyman-Alpha Photons from Dusty Starbursts

HST NICMOS image of the interacting Luminous IR Galaxy, NGC 6090

Ultraluminous infrared galaxies (LIRGs) are systems that are characterised as having luminosities that exceed 1012 LSun, with most of the energy emitted at wavelengths longward of 10 microns. Many (perhaps most) of these galaxies are interacting or merging disk galaxies, with the excess infrared luminosity generated by warm dust associated with the extensive star formation regions. Many systems also exhibit an active nucleus, and may be in the process of evolving towards an S0 or elliptical merger remnant. One of the surprising discoveries in recent years has been the extent to which Lyman-alpha ionising emission can be detected escaping from these dusty systems. The present program looks to quantify the distribution of these properties through COS observations of sixteen ULIRGs in the local universe (z~0.1). These relatively nearby systems can provide insight into the structure of these systems, and give clues to the likely behaviour at higher redshifts.

GO 12606: Verifying the White Dwarf Mass-Radius relation with Sirius B and other resolved Sirius-like systems

Artist's impression of a white dwarf matched against the Earth

All single stars, and most binary stars, with masses less than ~7 solar masses are expected to end their lives as white dwarfs - extremely compact objects made of degenerate material, compressing ~0.3 to 1.4 solar masses of material into a sphere little larger in radius than the Earth. Theoretical evolutionary models predict a broad correlation between the mass of the main-sequence star and the mass of the remnant, although there is significant scatter in the observed initial-final mass relation. The models also predict that white dwarfs should follow mass-radius relations that depend on the composition, temperature and internal structure. The present program aims to test the predictions of those models by determining accurate masses and radii for a sample of white dwarfs in resolved binary systems. STIS spectra will be used to measure accurate Balmer line profiles for these hot degenerates, and those pofiles can be analysed to yield effective temperatuers and surface gravities. Moreover, the H-beta line profile has a sharp core that allows accurate measurement of the apparent radial velocity of the system. This measured velocity has two main components: the star's peculiar velocity relative to the Sun; and the gravitational redshift induced by the high field on a degenerate white dwarf. Sicne these stars are members of wide binary systems, observations of the main sequence companion can be used to determine the former quantity and hence allow emasurement of the latter, and set constraints on the white dwarf mass.

GO 12879: A 1% Measurement of the Distance Scale with Perpendicular Spatial Scanning

HST WFPC2 image of NGC 4639, one of the Cepheid-rich spiral galaxies used to calibrate SNe Ia

The cosmic distance scale and dark energy are two key issues in modern astrophysics, and HST has played a vital role in probing both. On the one hand, HST has been involved in cosmic distance measurements since its inception, largely through the H0 Key Project, which used WFPC2 to identify and photometer Cepheids in 31 spiral galaxies at distances from 60 to 400 Mpc. On the other, HST is the prime instrument for investigating cosmic acceleration by searching for and following Type Ia supernovae at moderate and high redshift. These two cosmological parameters are directly related, and recent years have seen renewed interest in improving the accuracy of H0 with the realization that such measurements, when coupled with the improved constraints from the Cosmic Microwave Background, provide important constraints on cosmic acceleration and the nature of Dark Energy. Previous HST programs have focused on identifying and measuring light curves for cepheids in external galaxies (eg GO 10802 , GO 11570 ) or quantifying the effects of variations in intrinsic stellar parameters, such as metallicity (eg GO 10918 , GO 11297 ). The present program focuses on the Galactic Cepheids that form the foundation for the whole distance ladder, employing a revived version of an old technique to determine accurate astrometry, and hence trigonometric parallaxes and reliable distances. The technique is drift-scanning - tracking HST during the observation so that stars form trails on the detector. This mode of observations was available in the early years of HST's operations, and has been revived primarily as a means of obtaing high signal-to-noise grism spectroscolpic data of stars hosting transiting exoplanets. However, the same technique can be used in imaging mode, and the extended trails allow multiple measurements of position differences for stars in the field. The net result is a significant improvement in the relative precision of the final astrometry. The present program targets 11 Galactic cepheids and aims for astrometric accuracies of 20 micro-arcseconds.