This week on HST

HST Programs: April 3 - April 9, 2017

Selected highlights

GO 14114: A Wide-Field WFC3 Imaging Survey in the COSMOS Field

The full COSMOS field

Hubble has made significant contributions in many science areas, but galaxy formation, assembly and evolution is a topic that has been transformed by the series of deep fields obtained over the past 20 years. The largest area survey to date was conducted in Cycles 12 & 13,when the Advanced Camera for Surveys was used to obtain single filter (F814) images of 2 square degrees in 640 orbits. The resultant dataset, the COSMOS survey, has been surveyed subsequently from both ground and space, with data spanning all wavelengths from X-ray through optical and infrared to the sub-millimetre and millimetre regimes. The HST data are important in providing data with exquisite angular resolution, as well as depth, providing important morphological information on galaxies in the field. Multi-wavelength data at the same resolution can provide important information on star formation history and evolution, and the spatial distribution of dust. Additional HST observations have been obtained for subsets of the field (eg the CANDELs field), but the large areal span has made full coverage overly expensive. That is, until a new observing technique was devised for HST that enables observations of multiple distinct fields in a single orbit. In standard the observing scenario, HST moves to a field, then acquries a guidestar to ensure acurate pointing and stability during the observation; each guide star acquisition takes 4-5 minutes, and HST is generally restricted to 2 pointings at most. In the new mode, HST makes the initial guide-star acquisition, but then offsets to new fields. The telescope drifts without a guide star, but this can be dealt with for near-infrared wavelength imaging; the HST detectors use multiple non-destructive reads, and the telescope drifts by a very limited amount in the time between reads. The net result is that it is possible to cover the COSMOS fields with H-band observations in 57 orbits, adding high angular resolution near-infrared data to compleemnt the ACS I-band data.

GO 14633: A SNAP UV Spectroscopic Study of Star-Planet Interactions

SOHO image of an extremely strong solar flare

Stellar activity, whether through flares and coronal mass ejections, has the potential to affect the ability of life to evolve and survive on otherwise habitable planets in stellar systems. High energy radiation can lead to significant mutations, and particle ejections can actually strip the atmospheres of unfortunate planets. Previous HST programs have focused on low mass M dwarfs, the most populous stars in the galaxy, and systems where the habitable zone lies close to the parent star, with a correspondingly higher vulnerability. Those observations suggest evidence for interactions between the stellar transition region and the planets, with a correlation between the presence of high temperature emission lines (N V, C IV, Si IV) and the planetary mass and orbit i.e. suggestive of planetary interactions driving conditions in the stellar corona. The present SNAP program expand obsevations to higher-mass K dwarfs and solar-type G dwarfs that are known to harbour exoplanets, searching for similar correlations. The Cosmic Origins Spectrograph will be used to obtain UV spectra (1150-1450 Angstroms), providing a broad sampling of the range of activity levels among these dwarfs.

GO 14756: Hubble 2020: Outer Planet Atmospheres Legacy (OPAL) Program

Global map of Jupiter from OPAL's 2015 observations

The gas giants and ice giants in the outer system have extended, dynamic gaseous atmospheres that show a range of phenomena reflecting the underlying composition and cloud structure. These are generally driven by solar insolation, with the consequence that the frequency, scale and range of features diminishes from Jupiter through saturn and Uranus to Neptune. Monitoring the changes in the wide variety of features in these atmospheres can provide insight into the velocity structrue and the energy sources. Hubble OPAL program was established in 2014 to support this type of monitoring campaign. The program targets all four outer planets for imaging with a wide range of broad and narrow-band filters on Wide Field Camera 3. The observations are spaced over two consecutive rotations, providing full longitudinal coverage for each planet.

GO 14811: The Grand Finale : probing the origin of Saturn's aurorae with HST observations simultaneous to Cassini polar measurements

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. They are a common phenomenon 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. Saturn passed through its equinox in August 2009, and in the succeeding years the north pole has tilted more and more towards the Sun. As a consequence, we now have an excellent view of the polar regions. Moreover, the Cassini probe has moved into a polar orbit, that takes it regularly across the auroral regiosn. The present program is using STIS to obtain time-tagged FUV images during those passages, enabling a direct comparison of the large-scale structure with in situ measurements by the Cassini spacecraft.