This week on HST

HST Programs: June 26 - July 2, 2017

Selected highlights

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 14650: A simple definitive test for chloride salts on Europa

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, analyses of observations taken by the Space Telescope Imaging Spectrograph (STIS) and Wide-Field Camera 3 on Hubble indicated the presence of an extended cloud of Lyman-alpha emission, strongly suggesting that Europa's oceans may be vaporising into space. Evaporation on Europa's surface could leave residual trace elements of the underlying seas. Recent observations suggest that chloride salts, rather than sulphate salts, may be the prevalent constituent, and the present program aims to use STIS to scan the surface and look for characteristics spectral signatures at 4300 and 8300 Angstroms.

GO 14704: A Year in the Whirlpool

The Whirlpool galaxy, M51

NGC 5194, the Whirlpool galaxy, is a grand design spiral galaxy with an interacting lower-mass companion, NGC 5195, lying at a distaince of ~7 Mpc from the Milky Way. Originally catalogued as the 51st obect in Charles Messier's list of non-comets, its spiral nature was first discerned by visual observations using the Earl of Rosse's Leviathan of Parsontown. With its clearly deefined spiral structure, M 51 is a prime target for star formation investigations. The present program aims to probe the star formation history by mapping the number and distribution of asymptotic giant branch long period variable (AGB LPV) stars. or miras. These LPV have periods ranging from ~150 days to more than 500 days, and there are well determined correlations between the periodicity and the mass (and hence age) of the individual stars. with such long periods, obervations need to be spaced over a correspondingly long time frame, and the present program schedules observations at 34 epochs over a full year.

GO 14767: The Panchromatic Comparative Exoplanetary Treasury Program

Artist's impression of the GJ 1214 system

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 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 100 from ground-based observations, most detected through wide-field photometric surveys, while the high-sensitivity data provided by Kepler has added a further 1000+ confirmed systems and ~2000 additional candidates. Transiting systems not only provide an accurate measure of the planetary radius (at least relative to the parent star), they also provide us with an opportunity to probe the atmospheric composition through spectroscopy during the transit. Hubble has made significant inroads in this area, while Spitzer has contributed measurements of planetary emissivity through observations during and after eclipse. The James Webb Space Telescope has the capability to revolutionise our knowledge in this field through highly sensitiive observations at near and mid-infra red wavelengths. The present program aims to lay the foundation for those programs by using Hubble to compile multiwavelength (UV to near-IR) spectroscopic observations of 20 exoplanets. The targets are all gas giants, ranging from super-jovian masses to neptunian masses. The observations will be obtained with the Space Telescope Imaging Spectrograph and the near-infrared grisms on Wide Field Camera 3.