This week on HST

HST Programs: September 21 - September 27, 2015

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 13677: 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 13779: The Faint infrared Grism Survey (FIGS)

Grism spectra from the CANDELS program

One of the exciting capabilities offered by the post-SM4 Hubble Telescope is multi-object, low-resolution, near-infrared spectroscopy, using the two grisms available on the IR channel of Wide-Field Camera 3. Those observations provide an important avenue for complementing the various dep imaging surveys undertaken by HST. The 3D-HST program used relatively shallow observations to observe a significant fraction of the area covered by the CANDELS Multi-Cycle Treasury program. The present program, FIGS, targets only 4 fields, split between GOODS South and GOODS North, but with integrations totalling 40 orbits for each field. As a consequence, the observations will have significantly greater sensitivity, with the potential of measuring Lyman-alpha emission from galaxies at redshifts 5.5 < z < 8.5. The spectroscopic data will provide important additional information on the galaxy redshift distribution, on the formation of early-type galaxies at 1 < z < 2 and the evolution of star formation for moderate luminosity galaxies at z > 1.

GO 14125: A Remarkable Recurrent Nova in M31: The Leading Single Degenerate Supernova Ia Progenitor Candidate(?)

The recurrent nova, M31 2008 - 12a (from www.rochesterastronomy.com)

Novae are generally believed to originate in binary systems with a white dwarf paired with a non-degenerate companion. The orbital separation of the components is sufficiently small that the non-degenerate star fills its Roche lobe, leading to accretion onto the surface of the white dwarf. That material accumulates until a runaway nuclear reaction occurs, leading to the nova outburst. Typcially, the system survives to undergo subsequent outbursts. In some cases, the mass loss is sufficiently high that the time between outbursts is a measured in years rather centuries. These systems are known as recurrent novae; the companion is generally a subgiant or a red giant star, rather than the main sequence star in classical novae. One of the most remarkable such systems is M31 2008-12a, originally discovered in Deecmber 2008. Since that date, this system has undergone no less than eight outbursts. This indicates a very high accretion rate. Moreover, models suggest that typically 30% of the accreted material is retained after each outburst. Thus, the mass of the white dwarf is likely growing significantly with each outburst. This system is therefore a strong candidate as a progenitor of a supernova, triggered when the white dwarf mass exceeds the Chandrasekhar limit. The present HST observations are part of a target-of-opportunity program, submitted in anticipation of the system udnergoing another outburst in the present cycle. The program was triggered by an outburst detected on August 28th. The aim is to obtained ultraviolet spectra, combined with UV and optixcal photometric data through the outburst.