This week on HST

HST Programs: November 4 - November 10, 2013

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 ecliopse on April 8.

GO 13113: Energy Dependent X-ray Microlensing and the Structure of Quasars

The first Einstein cross, the gravitational lensed QSO, G2237+0305

Gravitational lensing is a consequence the theory of general relativity. Its importance as an astrophysical tool first became apparent with the realisation (in 1979) that the quasar pair Q0957+561 actually comprised two lensed images of the same background quasar. In the succeeding years, lensing has been used to probe the mass distributions on a variety of scales: of galaxies (primarily via multiply-imaged quasars); of galaxy clusters (arcs and arclets); and at the largest scales (weak lensing). However, lensing can also provide insight on the small-scale properties of the object being lensed. In a lensed QSO, the light from the QSO follows different paths to produce the separate images; each of those paths has a different length; consequently, flux variations in the source show up at different times in the separate images. The present program aims to take advantage of this property to probe the structure of the accretion disks surrounding the central black hole in a number of lensed QSOs by combining multi-epoch X-ray observations with the Chandra satellite with UV imaging with HST's WFC3/UVIS camera. Studying the variation as a function of wavelength will probe the accretion disk structure, since light from the inner regions are expected to dominate at shorter wavelengths, while the outer regions dominate at longer wavelengths.

GO 13297: The HST Legacy Survey of Galactic Globular Clusters: Shedding UV Light on Their Populations and Formation

Hubble image of the metal-poor globular cluster, M15

Globular clusters are members of the Galactic halo population, representing remnants from the first extensive period of star formation in the Milky Way. As such, the properties of the 106 to 107 stellar constituents can provide crucial insight into the earliest stages of galaxy formation. Until recently, conventional wisdom was that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. One of the most surprising disoveries in recent years is the realisation that this simple picture no longer holds. Up until about 5 years ago, the only known counter-example to convention was the cluster Omega Centauri, which is significantly more massive than most clusters and has both a complex main sequence structure and a range of metallicities among the evolved stars. High precision photometric observations with HST has demonstrated that Omega Cen is far from unique, with multiple populations evident in numerous other clusters, including NGC 2808, NGC 1851, 47 Tuc and NGC 6752. Multiple populations have also been discerned in a number of clusters in the Magellanic clouds. Sustaining multiple bursts of star formation within these systems demands that they retain gas beyond the first star forming event, which appears to set a requirement that these clusters were significantly more massive during their epoch of formation; put another way, the current globulars may represent the remnant cores of dwarf galaxy-like systems. That, in turn, implies that the stars ejected from those systems make a significant contribution to the current galactic halo. One of the most effective means of identifying and studying multi-population clusters is combining high-precision photometry over a wide wavelength range, particularly extending to UV wavelengths. Sixty-five globular clusters already have R/I (F606W, F814W) Hubble imaging and photometry thanks to the Cycle 14 program, An ACS Survey of Galactic Globular Clusters (GO 10775). The present program aims to build on those data by adding UV/blue observations using the F275W, F336W and F438W filters on the WFC3-UVIS camera. The colorus derived from these filters enable characterisation of the C, N and O abundances of the component stellar populations in these systems.

GO 13467: Follow The Water: The Ultimate WFC3 Exoplanet Atmosphere Survey

Probing the atmosphere of a transiting exoplanet

The first exoplanet, 51 Peg b, was discovered in 1995 through high-precision radial velocity measurements. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Since that date, the number of known transiting exoplanet systems has grown to more than 400 in over 300 planetary systems, with the overwhelming majority identified by the Kepler satellite, which has also contributed close to 3,000 additional (very strong) candidates. As these observations have accumulated,the broad diversity of exoplanet systems has become increasingly apparent. Transiting systems are invaluable, since they provide not only unambiguous measurements of mass and diameter, but also an opportunity to probe the atmospheric structure by differencing spectra taken during and between primary secondary transit. Such observations are best done from space: indeed, while high-precision ground-based observations have succeeded in constraining atmospheric properties in a few systems, the only successful detections of atmospheric features to date have been with HST and Spitzer. HST capabilities have been enhanced in the last few years with addition of spatial scanning, moving the target star over the chip in a controlled fashion during an observation. This allows observers to accumulate images or spectra of substantially higher signal-to-noise, a crucial advantage if one is looking for flux differences of elss than 1 part in 104. Past programs have accumulated observations of over a dozen exoplanets, using STIS at optical wavelengths and WFC3 in the near-infrared. The present program targets eight exoplanet systems with a diverse range of properties: HD 209458b,GL 3470b, HAT-P-26b, WASP-12b, WASP-18b, WASP-43b, WASP-80b and WASP-19b. The WFC3-IR G141 grism will be used to search for the characteristic near-infrared spectral features due to water in the amospheres of these exoplanets.