This week on HST

HST Programs: November 3 - November 9, 2014

Selected highlights

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%%.

GO 13498: HST Frontier Fields - Observations of MACSJ0717.5+3745

The Frontier Fields cluster, MACSJ0717.5 +3745

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. The present program builds on the highly successful CLASH program,which used 17-colour ACS/WFC3 images to map 25 galaxy clusters, tracing the mas profile and the dark matter distribution. in addition, the observations identified several lensed galaxies at redshifts that enter the JWST domaine, with the most distant object lying at a redshift z~11, within a few hundred million years of the Big Bang. The Frontier Fields program is a large-scale Director's Discretionary program that capitalises on the latter characteristic by targeting 4-6 strong-lensing galaxy clusters for very deep optical and near-infrared imaging. WFC3 and ACS will be used to observe the clusters, with simultaneous imaging obtained in parallel of a nearby "blank" field. Since the observations need to made at a specific orientation, they are being taken in two sets, ~6 months apart, alternating between detectors. MACSJ0717.5+3745 at z=0.545 is the third target: at this first epoch of observation, the cluster is being imaged with WFC3-IR, with ACS being used to obtain optical data on the nearby blank field; the second epoch observations switch cameras, with ACS on the cluster and WFC3-IR on the parallel field.

GO 13679: Europa's Water Vapor Plumes: Systematically Constraining their Abundance and Variability

The HST imaging of a potential water plume around Europa's south pole superimposed on an image of the satellite

Europa is the smallest, and the most intriguing, of the four Galilean satellites of Jupiter. With a diameter of 3139 km, Europa is almost twice the size of Earth's moon and significantly larger than Mercury. In 1957, Gerard Kuiper commented that both infrared spectroscopy and the optical colours and albedo suggested that Jovian satellite II (Europa) is covered "by H2O snow". Images taken by the Voyager space probes in the late 1970s (see left) reveal a smooth surface, with only a handful of craters larger than a few kilometres. These features are consistent with a relatively young, icy surface. Subsequent detailed investigations by the Galileo satellite strongly suggest that a substantial body of liquid water, heated by tidal friction, underlies a 5 to 50 km thick icy crust. The presence of this subterranean (subglacial?) ocean clearly makes Europa one of the two most interesting astrobiology targets in the Solar System. Most recently, analysis of observations taken by the Space Telescope imaging Spectrograph (STIS) on Hubble indicated the presence of an extended cloud of Lyman-alpha emission near the polar regions while Europa was furthest in its orbit from Jupiter, strongly suggesting that Europa's oceans may be vaporising into space.Follow-up observations on two further occasions earlier in 2014 failed to detect any emission, suggesting that the emission is either sporadic or periodic; in the latter case, the emission might be related to the location of Europa within its orbit and the consequent tidal strain imposed by Jupiter. The present program aims to address this question through a methodical series of observatons designed to image Europa at a progressive series of orbital locations. The program uses STIS to search for H and O auroral emissions at UV wavelengths\ and will aim to map the distribution of emission at different phases of the Europan orbit..

GO 14036: Post-equinox Uranus aurorae during a strong magnetosphere-solar wind shock interaction

Nicmos image of aurorae on Uranus

The atmospheres of the gas giant planets in the solar system are dynamic entities that can exhibit dramatic changes over a variety of timescales. In addition to changes within the atmospheres themselves, due the formation and dissipation of storms, these systems can exhibit auroral activity. Planetary aurorae are stimulated by the influx of charged particles from the Sun, which travel along magnetic field lines and funnel into the atmosphere near the magnetic poles. Aurorae therefore require that a planet has both a substantial atmosphere and a magnetic field. Aururae are common phenomena on Earth, sometimes visible at magnetic latitudes more than 40 degrees from the pole, and have also been seen on Jupiter, Saturn, Uranus and Neptune. The Uranian aurorae are much less intense than those on the inner gas giants, and were first detected by Voyager 2 during its flyby in 1986 and have rarely been detected since. In 1986, Uranus was oriented almost pole-on to Earth, allowing observations of only one hemisphere. Now, 25 years later, Uranus has completed more than a quarter of its 84-year-period orbit, and passed through the equator-on ring plane crossing in May-August of 2007 (see Program GO 10870 ). As a result, we now have clear access to both the northern and southern polar regions. In 2011 and 2012, HST observatiosn were triggered to coincide with Uranus interacting with known structure in the solar wind, and those observations succeeded in detecting aurorae. The present program aims to repeat this process, targeting Uranus for observation as it encounters another solar wind event. The present program will use STIS to image the planet over the course of 7 orbits.