This week on HST

HST Programs: September 29 - October 5, 2014

Selected highlights

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, G105-30 (also known as LHS 1838, WD 0618+067 and GJ 3392). Lying at distance of about 22 parsecs, G105-30 is a late-type DA white dwarf (DA8.5). Spectroscopic analysis of the Balmer line profiles indicates a surface temperature of ~5940 K and a mass of ~1.0 solar masses (Bergeron et al, 2001). The white dwarf passed within ~0.08 arcseconds of a 19th magnitude star in June 2012. The expected signal during the stellar encounter (i.e. the deflection of the background star) is approximately 2 millarcseconds, and therefore well within HST's astrometric capabilities. The present observation finishes the observing sequence, with the white dwarf now more than an arcsecond from the bakground star.

GO 13678: The Fifth and Final Epoch

ESO image of the material surrounding the long-period Cepheid, RS Puppis

Cepheid variable stars have been the prime extragalactic distance indicator since Henrietta Leavitt's discovery of the period-luminosity relation described by Cepheids in the Small Magellanic Cloud. It was Hubble's identification of Cepheids in NGC 6822 that finally established that at least some nebulae were island universes. Cepheids and the extragalactic distance scale figure largely in HST's history, notably through the Hubble Constant Program, one of the initial Key Projects. HST has since observed Cepheids in more than 30 galaxies. Establishing a Galactic sample with reliable distance determinations is obviously crucial to this process. Long period Cepheids, with pulsation periods in excess of 25 days, play a key role, since they are more luminous, easier to detect and can be observed with higher photometric accuracy in distant galaxies. The present program builds on the Cycle 21 program GO 13334 (The Longest Period Cepheids, a bridge to the Hubble Constant) and aims to add to the sample of well-observed Galactic Cepheids by using spatial scanning on WFC3 to determine accurate parallaxes for nine Cepheids at distances up to 4 kpc from the Sun. Spatial scanning enables astrometry to an acuracy of ~40 microarcseconds, offering the prospect of distances accurate to 4% for individual Cepheids, and an overall distance scal calibration accurate to ~1%.Program GO 13334 acquired observations at 4 epochs for each star; the present program adds a fifth, solidifying the parallax determination by eliminating the potential for binary motion and resolving correlations between parallax and proper motion.

GO 13692: Orbits and Physical Properties of Four 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 four 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 13794: Seasonal Dependence of the Escape of Water from the Martian Atmosphere

HST imaging of Mars - high clouds are visible in the upper atmosphere

Mars has been in the news recently for several reasons. The rover Curiousity continus to explore the surface; it is seeing the close passage of the Oort cloud comet, Comet 2013A Siding Spring, discovered by Robert McNaught last year; and this week sees the arrival of MAVEN, the Mars Atmosphere and Volatile EvolutioN planetary probe launched in 2013 and scheduled for orbit insertion on September 21. MAVEN's mission is to study the tenuous Martian atmosphere, using the onboard instruments to probe the upper atmosphere and determine its rate of escape. Hubble offers some limited opportunities to tackle the same questions, and the current progam aims to complement MAVEN's in situ measurements through ultraviolet imaging and spectroscopy with the Advanced Camera for Surveys' Solar Blind channel (ACS/SBC) and the Space Telescope Imagign Spectrograph. Those observations can detect emission from hydrogen and oxygen, and previous observations have revealed significant changes with the martian seasons. The present observations are part of a series that aim to characterise the atmospheric structure as Mars passes through its seasonal changes. In addition, these observations, taken while most of the Martian satellites have been powered down for the Comet Siding Spring encounter, offer the prospect of testing whether the comet's tail might have some detectable impact on the extended Martian atmosphere.