This week on HST

HST Programs: March 24 - March 30, 2014

Selected highlights

GO 12980: Absolute Measurement of the Cosmic Near-Infrared Background Using Eclipsed Galilean Satellites as Occulters

Jupiter and the Galilean satellite Ganymede

The Cosmic Infrared Background is generally conjectured to represent the diffuse, redshifted light from star formation early in the post-recombination Universe. It provides an important link between the resolved structure that we see today and the primordial fluctuations measured by the cosmic mcirowave background. Measuring the CIB, however, is not a straightforward task, since there are several other sources of infrared radiation that dominate the measured fluxes, notably stars at near-infrared wavelengths, the zodiacal light at mid-infrared wavelengths and emission from Galactic cirrus in the far infrared. The present program proposes a novel mean of isolating the near-infrared contribution from one of those components, the zodiacal light. The WFC3 IR camera will be used to observe the two of the Galilean satellites during the period when they have entered the jovian shadow, and are therefore under a solar eclipse, but are still visible from Earth. The overwhelming majority of the zodiacal light is contributed by scattered light from dust particles between us and Jupiter; the Galilean satellites obscure any contribution to the near-infrared background from sources that lie beyond Jupiter's orbit, including contributions from the CIB. If the latter contributions are significant, then one would expect to see reduced flux (ie dark spots) in the satellite locations. The present observations target Europa during an eclipse on April 8.

GO 13404: Mutual Orbits and physical Properties of Binary Transneptunian Objects

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 KBOs are binary (including Pluto, one of the largest known KBOs, regardless of whether one considers it a planet or not). This is a surprisingly high fraction, given the difficulties involved in forming such systems and the relative ease with which they can be disrupted. It remains unclear whether these systems formed from single KBOs (through collisions or 3-body interactions) as the Kuiper Belt and the Solar System have evolved, or whether they represent the final tail of an initial (much larger) population of primordial binaries. These issues can be addressed, at least in part, through deriving a better understanding of the composition of KBOs - and those properties can be deduced by measuring the orbital parameters for binary systems. The present proposal aims to use HST WFC3 observations to map the orbits of six binary systems. Those observations will be ued to determine the orbital period and semi-major axis and the total system mass, while the mid-infrared properties (measured by Spitzer) allow an assessment of the surface area/diameters; combining these measurements gives an estimate of the mean density.

GO 13457: Accurate Mass Determination of the Nearby Old White Dwarf Stein 2051B through Astrometric Microlensing

A rather spectacular version of black hole lensing.

Gravitational lensing is a consequence of general relativity. Its effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. Those wide-field imaging surveys have target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. The duration of each event depends on several factors, including the tangential motion of the lens and its mass. Long-term events are generally associated with a massive lens. Duration alone is not sufficient to identify a lens as a black hole - a source with very low tangential motion relative to the Sun can produce the same effect. However, microlensing not only leads to flux amplification, but also to small astrometric motions, caused by the appearance and disappearance of features in the lensed light. Those motions serve as a mass discriminant - higher mass lenses produce larger amplitude motions. This program aims to capitalise on this fact by measuring the positional deflection of a background stars introduced by the close passage of the high proper-motion white dwarf, Stein 2051B (also known as Gliese 169.1B, G175-34B or LHS 27; the companion star is an M4 red dwarf at ~40 AU separation). Lying at distance of only 5.5 parsecs, Stein 2051B has a surface temperature of ~7200K and is therefore a DC, too cool to show absorption features due to hydrogen and therefore not accessible to mas measurement techniques such as gravitatiolnal redshifts. The expected signal during the stellar encounter (i.e. the deflection of the background star) is approximately 3 millarcseconds, and therefore well within HST's astrometric capabilities.

GO 13472: The Hubble Constant to 1%? STAGE 4: Calibrating the RR Lyrae PL relation at H-Band using HST and Gaia Parallax Stars

RR Lyrae's light curve at visible wavelengths

The classical cosmic distance scale rests on a series of distance indicators that step outwards from the Milky Way, establishing reliable measurements to ever more distant galaxies. Cephids have long been the prime calibrators in this process, but other pulsating variables, notably Mira AGB long-period variables and RR Lyrae variables, also make significant contributions. RR Lyrae variables are evolved, near-solar-mass stars that are passing through the instability strip where it crosses the horizontal branch. With periods of 0.5 to 1.5 days, they have long served as distance indicators for old stellar populations (Baade's Population II). They have been known in the Galactic field and in Galactic globular clusters for over 150 years, and they are also present in the older stellar populations of the dwarf spheroidal Galactic satellites. Cluster (or dsph) RR Lyraes are particularly interesting, since their metallicities and ages can be deduced from analysis of the colour-magnitude diagrams for those systems. They are significantly less luminous than Cepheids, nonetheless, near-infrared photometric monitoring has demonstrated that these stars delineate a period-luminosity relation at those wavelengths that has the potential to establish distances to better than 1.5% accuracy. The absoltue calibration of that relationship, however, rests on only 4 nearby RR Lyraes with trigonometric parallax measurements. The present program aims to add to the sample of astrometricall well-observed RR Lyraes by using spatial scanning on WFC3 to determine accurate parallaxes for a sample of Galactic variables lying at distances up to several kpc from the Sun. Spatial scanning enables astrometry to an acuracy of ~40 microarcseconds, offering the prospect of distances accurate to 4% for individual stars, and an overall distance scale calibration accurate to better than 3%%.