This week on HST

HST Programs: October 21 - October 27, 2013

Selected highlights

GO 13046: RAISIN: Tracers of cosmic expansion with SN IA in the IR

The first supernova discoevered by the Pan-STARRs survey

Supernovae are the most spectacular form of stellar obituary. In recent years, these celestial explosions have acquired even more significance through the use of Type Ia supernovae as distance indicators in mapping the `dark energy' acceleration term of cosmic expansion. However, while there are well-established models for the two main types of supernovae (runaway fusion on the surface of a white dwarf in a binary system for Type Ia, or detonation of the core in Type II), some uncertainties remain as to the uniformity of the events. Moreover, as the sample of known supernova has grown, so has the range of photometric systems and the methods used to fit the light curve and account for the ever-present uncertainites inroduced by dust absorption. Consequently, the potential remains for systematic bias in distance estimates due both to intrinsic differences and to measurement errors. The persent program aims to minimise these systematics by compiling standard sequences of observations, primarily in the Y, J, and H filters, of supernovae at redshifts between z~0.3 and 0.5. Focusing on those wavelengths minises the effects, and hence the uncertainties, due to dust absorption. The supernovae themselves are drawn from the Pan-STARRS survey, with the WFC3-IR camera on HST employed to obtain the photometry.

GO 13047: The D/H Ratio and Escape of Water from Venus

Pioneer V image of Venus

Ground-based amateur astronomers are always warned never to look directly at the Sun with a telescope (or, come to that, with the naked eye). The same consideration holds for HST; indeed, Hubble is normally unable to observe any targets that lie within 50 degrees of the Sun (the solar avoidance zone). This has obviously restricted HST's ability to provide information on objects in the inner Solar System, but there are a few exceptional occasions when it is possible to (legally) work around the observing constraints. One case arises when Venus is at its maximum elongation from the Sun. At that time, the angular separation is ~45 degrees, so the planet is still within the solar avoidance zone. However, it is possible to obtain observations by taking advantage of Hubble's location in low-Earth orbit. During each 90-minute orbit, the Earth occults the Sun and HST enters "night". If Venus is in the appropriate elongation, then Venus will appear from behing the Earth before the next HST "sunrise". If HST is placed in the appropriate orientation, it can obtain ~5 minutes observations before slewing away to avoid the Sun. HST used this technique to observe Venus in 1997 and in late 2010. The The same techniques are being used in the current cycle to obtain STIS ultraviolet spectra to determine accurate densities for deueterium and hydrogen, and hence the D/H ratio, in the upper atmosphere.

GO 13334: The Longest Period Cepheids, a bridge to the Hubble Constant

ESO image of the material surrounding the long-period Cepheid, RS Puppis

Cepheid variable stars have been the prime extragalactic distance indicator since Henrietta Leavitt's discovery of the period-luminosity relation described by Cepheids in the Small Magellanic Cloud. It was Hubble's identification of Cepheids in NGC 6822 that finally established that at least some nebulae were island universes. Cepheids and the extragalactic distance scale figure largely in HST's history, notably through the Hubble Constant Program, one of the initial Key Projects. HST has since observed Cepheids in more than 30 galaxies. Establishing a Galactic sample with reliable distance determinations is obviously crucial to this process. Long period Cepheids, with pulsation periods in excess of 25 days, play a key role, since they are more luminous, easier to detect and can be observed with higher photometric accuracy in distant galaxies. The present program aims to add to the sample of well-observed Galactic Cepheids by using spatial scanning on WFC3 to determine accurate parallaxes for nine Cepheids at distances up to 4 kpc from the Sun. Spatial scanning enables astrometry to an acuracy of ~40 microarcseconds, offering the prospect of distances accurate to 4% for individual Cepheids, and an ovall distance scal calibration accurate to ~1%.

GO 13492: Hubble Spectroscopy of Sungrazing Comet ISON

GO 13474: Imaging Polarimetry of the 2013 Comet ISON with ACS: A Study of the Heterogeneous Coma

HST image of Comet ISON, late April 2013

At least once every decade, the astronomical community gets excited by the discovery of a comet that offers the prospect of becoming a "comet of the century", developing a spectacular tail speading across the sky at sunset or sunrise. Unfortunately, while Comet McNaught put on a spectacular shows for the southern hemisphere in 2007, most of the predicted rivals of the Great Comets of 1680/1811/1882 have fizzled. The latest in line is Comet ISON. Discovered in late 2012 when it lay beyond jupiter's orbit, the comet is scheduled to pass within 700,000 miles of the Sun on November 28th. Despite its distance, at that time the comet already had a substahtial coma and tail, leading to hopes that it might be highly volatile rich. However, subsequent observations have shown that the brightness has increased at a slower rate than hoped for, suggesting that Comet ISON may prove to be another disappointment - at least for the public. The comet passed interior to Mars' orbit in late September, and will cross the Earth's orbit on its inward journey around November 2nd. As a sun-grazer, there is a significant chance that the comet might break up close to perihelion, potentially offering an even more spectacular show and more volatiles are exposed to solar radiation. Hubble caught observations of Comet ISON in April, before it entered the solar avoidance zone, withi 50 degrees of the Sun.The comet has become accessible again for most of October, and is being targeted by multiple programs with HST. The present programs will use STIS and COS to obtain ultraviolet spectra of the pre-perihelion gases, and will use ACS polarimketric imaging to probe the alignment of dust within the coma.

GO 13495: HST Frontier Fields - Observations of Abell 2744

Pandora's Cluster, Abell 2744: the Chandra X-ray image, tracking hot gas, is plotted in ed; the infered dark matter distribution in blue

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. The present program builds on the highly successful CLASH program,which used 17-colour ACS/WFC3 images to map 25 galaxy clusters, tracing the mas profile and the dark matter distribution. in addition, the observations identified several lensed galaxies at redshifts that enter the JWST domaine, with the most distant object lying at a redshift z~11, within a few hundred million years of the Big Bang. The Frontier Fields program is a large-scale Director's Discretionary program that capitalises on the latter characteristic by targeting 4-6 strong-lensing galaxy clusters for very deep optical and near-infrared imaging. WFC3 and ACS will be used to observe the clusters, with simultaneous imaging obtained in parallel of a nearby "blank" field. Since the observations need to made at a specific orientation, they are being taken in two sets, ~6 months apart, alternating between detectors. Abell 2744, Pandora's Cluster, is the first target, with the cluster being imaged with WFC3-IR and the nearby blank field with ACS on the cluster field and WFC3-IR on the parallel field.