This week on HST

HST Programs: August 25 - August 31, 2014

Selected highlights

GO 13301: Deep COS Spectra of the Two Brightest Quasars that Probe the He II Post-Reionization Era

an HST GHRS spectrum of the bright quasar, HE 2347-4342

In astronomy, 'reionisation" usually refers to the period in the early universe (6 < z < 10) when star formation and/or accreting supermassive black holes produced sufficient ultraviolet flux to ionise hydrogen and lift the veil of the cosmic dark ages. However, intergalactic helium remained neutral at that time. The reionisation epoch for intergalactic helium is thought to occur somewhere between redshifts 3 and 4. Tracking the onset of that epoch through analysis of the He II Lyman alpha absorption constrains the evolution of star formation in the univers at those epochs. Observations, however, are complicated by the continued presence of neutral hydrogen, which absorbs radiaton at those wavelengths. The present program is using the Cosmic Origins Spectrograph to obtain deep observations of two of the brightest known He II quasars, HE2347-4342 and HS1700+6416. Both have previous UV spectroscopic observations by HST, as witnessed by the GHRS spectrum, but the new COS observations will be orders of magnitude more sensitive and can resolve individual He II absorption lines for comparison with neutral hydrogen and metal lines (C IV, Si IV). Inded, these observations will set a new benchmark for QSO absorption line studies with COS.

GO 13344: A 1% Measurement of the Distance Scale with Perpendicular Spatial Scanning

HST WFPC2 image of NGC 4639, one of the Cepheid-rich spiral galaxies used to calibrate SNe Ia

The cosmic distance scale and dark energy are two key issues in modern astrophysics, and HST has played a vital role in probing both. On the one hand, HST has been involved in cosmic distance measurements since its inception, largely through the H0 Key Project, which used WFPC2 to identify and photometer Cepheids in 31 spiral galaxies at distances from 60 to 400 Mpc. On the other, HST is the prime instrument for investigating cosmic acceleration by searching for and following Type Ia supernovae at moderate and high redshift. These two cosmological parameters are directly related, and recent years have seen renewed interest in improving the accuracy of H0 with the realization that such measurements, when coupled with the improved constraints from the Cosmic Microwave Background, provide important constraints on cosmic acceleration and the nature of Dark Energy. Previous HST programs have focused on identifying and measuring light curves for cepheids in external galaxies (eg GO 10802 , GO 11570 ) or quantifying the effects of variations in intrinsic stellar parameters, such as metallicity (eg GO 10918 , GO 11297 ). The present program focuses on the Galactic Cepheids that form the foundation for the whole distance ladder, employing a revived version of an old technique to determine accurate astrometry, and hence trigonometric parallaxes and reliable distances. The technique is drift-scanning - tracking HST during the observation so that stars form trails on the detector. This mode of observations was available in the early years of HST's operations, and has been revived primarily as a means of obtaing high signal-to-noise grism spectroscolpic data of stars hosting transiting exoplanets. However, the same technique can be used in imaging mode, and the extended trails allow multiple measurements of position differences for stars in the field. The net result is a significant improvement in the relative precision of the final astrometry. The present program, a continuation of Cycle 20 program GO 12879, targets 11 Galactic cepheids and aims for astrometric accuracies of 20 micro-arcseconds.

GO 13467: Follow The Water: The Ultimate WFC3 Exoplanet Atmosphere Survey

Probing the atmosphere of a transiting exoplanet

The first exoplanet, 51 Peg b, was discovered in 1995 through high-precision radial velocity measurements. 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 400 in over 300 planetary systems, with the overwhelming majority identified by the Kepler satellite, which has also contributed close to 3,000 additional (very strong) candidates. As these observations have accumulated,the broad diversity of exoplanet systems has become increasingly apparent. Transiting systems are invaluable, since they provide not only unambiguous measurements of mass and diameter, but also an opportunity to probe the atmospheric structure by differencing spectra taken during and between primary secondary transit. Such observations are best done from space: indeed, while high-precision ground-based observations have succeeded in constraining atmospheric properties in a few systems, the only successful detections of atmospheric features to date have been with HST and Spitzer. HST capabilities have been enhanced in the last few years with addition of spatial scanning, moving the target star over the chip in a controlled fashion during an observation. This allows observers to accumulate images or spectra of substantially higher signal-to-noise, a crucial advantage if one is looking for flux differences of elss than 1 part in 104. Past programs have accumulated observations of over a dozen exoplanets, using STIS at optical wavelengths and WFC3 in the near-infrared. The present program targets eight exoplanet systems with a diverse range of properties: HD 209458b,GL 3470b, HAT-P-26b, WASP-12b, WASP-18b, WASP-43b, WASP-80b and WASP-19b. The WFC3-IR G141 grism will be used to search for the characteristic near-infrared spectral features due to water in the amospheres of these exoplanets.

GO 13479: The host and location of the candidate relativistic tidal disruption event Swift 2058+0516

An artistic rendition of the Swift satellite

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, with NASA's Swift mission being the main contributor to these discoveries since its launch in 2004. Analyses indicate that the majority can be divided into two classes with durations longer or shorter than 2 seconds. The short/hard bursts appear to arise from coalescing binary systems (probably pairs of neutron stars or black holes), but the long/soft bursts appear to originate in the collapse of very massive stars. The typical "long" GRB has a duration of around 100 seconds, with a tail extending to around 10,000 seconds. Recently, however, a new type of transient has been detected where the gamma-ray emission persists for several days. These events are suspected to have their origin in the tidal disruption of stars by central black holes in the nuclei of galaxies. The present program aims to test that hypothesis through deep imaging of the second such event detected, Swift J2058+0516. Hubble obtained images of that source when it was active, allowing an accurate positional determination. The new deep images should resolve the host galaxy, allowing a determination of where the source lay within that system. If the GRB was centred on the nucleus, then this bolsters the tidal disruption hypothesis.