This week on HST

HST Programs: March 14 - March 20, 2016

Selected highlights

GO 13665: Exploring the Diversity of Exoplanet Atmospheres in the Super-Earth Regime

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. With the added numbers, observations have pushed detections to lower and lower masses, and it is now clear that the most common type of planet is the "super-Earth" - planets with masses that are several (3-6) times that of Earth and radii 2-4 times larger than Earth. One of the earliest examples is the planet circling the M dwarf, GJ 1214. Such planets have no obvious analogue in the Solar System, and the measured masses and diameters might reflect a range of interior structurees: large rocky bodies with relatively thin atmospheres; dense cores surrounded by a steam atmosphere; or "mini-Neptunes", with rock or ice cores surrounded by extended hydrogen or helium atmospheres. The present program aims to probe the diversity of these systems by using the G141 grism on Wide-Field Camera 3 to obtain time-resolved scanning observations of five transiting systems. The goal is to obtain data that will clearky distinguish between large scaleheight, hydrogen-dominated atmospheres and a more compact, steam-dominated systems.

GO 14181: A Snapshot WFC3 IR Survey of Spitzer/Hershel-Identified Protostars in Nearby Molecular Clouds

An image of the Orion Nebula superimposed on the 13CO map of Orion A (from this link ).

Giant molecular cloud complexes serve as nurseries for star formation. Deeply embedded in dust and gas, young stars are generally extremely difficult to detect at optical wavelengths. Consequently, these complexes have been subject to extensive scrutiny at near- and mid-infrared wavelengths, initially through ground-based observing campaigns and more recently by the Spitzer and Herschel space missions. Those observations have resulted in the identification of numerous embedded sources, young stellar objects (YSOs) that are still accreting from the surrounding molecular gas .he present proposal aims to follow up on those discoveries by obtaining WFC3-IR SNAPs of candidate protostars in several molecular cloud complexes. These observations will provide an excellent complement to Spitzer and Herschel since, while HST cannot offer either the same areal coverage or sensitivity at mid-infrared wavelegths, the imaging has a resolution close to 0.1 arcsecond, an order of magnitude higher than the Spitzer images. The observations are therefore capable of detecting very faint companions, with luminosities consistent with sub-stellar masses, as well as identifying jets and outflows associated with the star formation process. The present program is using the F160W filter to obtain H-band images and determine the true nature of these objects.

GO 14327: See Change: Testing time-varying dark energy with z>1 supernovae and their massive cluster hosts

HST/ACS images of a supernova in a galazy at z=1.2

The last few years of the twentieth century saw a revolution in cosmology, with the measurement of the acceleration term in expansion at high redshifts and the identification of dark energy as a major cosmological component. The overall significance of this result has been recognised through the award of the Nobel prize and, most recently, the Fundamental Physics Breakthrough Prize to Perlmutter, Riess and Schmidt and their respective teams. Type Ia supernovae are the prime yardstick for measuring the rate of expansion at moderate and high redshifts. The seminal work in this field was carried out with ground-based telescopes, but Hubble offers almost the only way of obtaining reliable post-maximum photometry of these objects to determine the full shape of the light curve. Many previous HST supernovae programs have concentrated on field galaxies, but applying appropriate corrections for in situ reddening by dust remains an issue in these systems, while the overall SNe detection rates are relatively low at high redshifts. The present program takes a different tack, and aims to minimise the uncertainties by searching for supernovae in massive, high-redshift clusters. The expectation is that the majority of detections lie within dust-poor elliptical galaxies; moreover, supernova rates may be higher. The program will obtain ACS observations of ten of the most massive galaxy clusters lying at redshifts 1.1 < z < 1.75.

GO 14487: Light Echoes and the Progenitor of SN 2016adj in Cen A

SN 2016adj in Cen A (Eric Zbinden, 10 Feb 2016)

Type II supernovae are generally believed to be originate in the implosion of massive stars. As such, they provide insight into both the final stages of stellar evolution, and how stars enrich the interstellar medium. It is important to understand the distribution of intrinsic properties of these exploding stars, and whether those properties, particularly luminosity, correlate with other parameters, such as metallicity. Relatively nearby supernovae that can be probed in detail are crucial to this purpose. Astronomers were therefore delighted with the discovery of a supernova in the relatively nearby galaxy, NGC 5128 (d~3 to 3.5 Mpc). Also known as Centaurus A, the galaxy is characterised by a strong dust lane, peppered with blue stars, generally agreed to reflect the product of a merger event between an elliptical and smaller dusty system. The supernova, designated SN2016adj, was discovered within Cen A's dust lane by the Backyard Observatory Supernova Search in February 2016. Spectroscopy indicates that this is a type IIB system, initially showing strong hydrogen emission, but with neutral helium appearing over the course of time. The present HST program aims to identify the progenitor by obtaining precise astrometry of the supernova, and matching against previous observations. In addition, the observatios will probe the immediate environments of the supernova by using imaging with the WFC3 UVIS camera to search for evidence of light echoes - reflections of the original explosion produced by interstellar features in the vicinity of the supernova.