This week on HST

HST Programs: August 28 - September 3 2017

Selected highlights

GO 14163: Honing Type Ia Supernovae as Distance Indicators, Exploiting Environmental Bias for H0 and w.

Supernova in M101

Supernovae have long attracted the attention of both amateur and professional astronomers as a means of studying the violent eruption and death of massive stars and degenerates. However, in the last decade they have also acquired considerable importance as distance indicators, tracing the expansion of the universe to redshifts well beyond the reach of more conventional yardsticks, such as cepheids, and providing a key underpinning for the hypothesised existence of dark energy. Understanding the supernovae themselves, and, in particular, their progenitors, is key to accurately interpreting their luminosities and distances. Recent observations have suggested that there may be a correlation between the brightnesses of Type Ia supernovae and some characteristics of their local environment; specifically, supernovae found in close proximity to star formation appear to be sub-luminous. The present SNAP program uses the UVIS camera on WFC3 to image the locations of supernovae that have been used to map the Hubble flow, and will use the UV-to-optical flux ratio to characterise the local environment, potentially offering a means of tightening the distribution in the Hubble diagram.

GO 14679: Extremely Metal Poor Galaxies with HST/COS: Completing the Groundwork for JWST<

The extremely metal-poor galaxy, UM 133, at z~005

In standard cosmologies, big bang nucleosynthesis is responsible for the production of hydrogen, helium and a trace abundance of lithium. The first stars - and the first galaxies - formed from this near-pristine material, and stellar nucleosynthesis accounts for the production of heavier elements (`metals', in astronomical parlance). As stars evolve and die, they return processed material to the interstellar medium, enriching the metal content and increasing the metallicity of subsequent stellar generations. Thus, within the Milky Way, the stars in the stellar halo (age 11-13 Gyrs) are 10 to 1000 times more metal-poor than the Sun's neighbours in the Galactic Disk. Spectroscopic observations of galaxies at redshifts z~6-8 in the (restframe) ultraviolet show prominent emission lines due to ionised carbon (CIII] and CIV) indicating a highly ionising radiation field. Observations of local galaxies suggest that those conditions are suggestive of a relatively low metallicity, certainly less than one-tenth solar. However, the requisite observations of more metal-poor systems are scarce. This program aims to remedy that defect through observations of six metal-poor (< one tenth solar) galaxies at low redshifts, 0.0017 < z < 0.003. These systems can serve as templates for the high signal-to-noise spectroscopic observations that JWST will provide for high-redshift (z > 8) systems.

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.

GO 14784: HAZMAT: Habitable Zones and M dwarf Activity across Time

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. Low mass, low luminosity M dwarfs are the most populous stars in the galaxy, and systems where the habitable zone lies close to the parent star, with a correspondingly higher vulnerability. M dwarf activity varies with mass/temperature, peaking at mid-M types. Activity also varies with age, declining relatively rapidly in early-type M dwarfs and persisting at later types. The present program aims to statistically characterise those variations, and infer how they might affect planetary companions, through ultraviolet spectroscopic observations of stars spannign a range of age and mass. The observations are being taken in the far- and near-UV using the Cosmic Origins Spectrograph.