This week on HST

HST Programs: April 22 - April 28, 2013

Selected highlights

GO 12789: Through a Lens, Darkly - New Constraints on the Fundamental Components of the Cosmos

The cluster MACS J1206.2-0.47, imaged by HST as part of the CLASH program

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 aims to capitalise fully on HST's imaging capabilities, utilising the refurbished Advanced Camera for Surveys and the newly-installed Wide-Field Camera 3 to obtain 17-colour imaging of 25 rich clusters. The data will be use to map the mass profiles of the clusters and probe the characteristics of the high-redshift lensed galaxies. Since ACS and WFC3 can be operated in parallel, the program will also use parallel imaging in offset fields to search for high-redshift supernovae. The present observations target the cluster MACS 1311-031 at z=0.492.

GO 12870: The mass and temperature distribution of accreting white dwarfs

An accreting white dwarf starn in a close binary system

Supernovae are the most spectacular form of stellar obituary. Since B2FH, the physical processes underlying their eruptive deaths have been known to play a key role in populating the ISM with metals beyond the iron peak. More recently, these celestial explosions have acquired even greater 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 significant uncertainties remain concerning the physical details of the disruption, and, potentially, the overall uniformity of these events. Consequently, there is potential for systematic bias in the distance estimates. The present program aims to set constraints on the various mechanisms associated with white dwarf stars by investigating the rotational properties of over 40 degenerate companions in catclysimic variable systems. All of these systems are in the process of accreting material from the companion star, as the latter voerflows its Roche lobes. The program aims to obtain ultraviolet spectra with the Cosmic origins Spectrograph, probing both the spin rates and the orbital parameters. Over the next decade, these data may lead to the determinaton of reliable masses for both stars once accurate parallax measurements become available from Gaia.

GO 12944: A High-Resolution Survey of the Very Youngest Brown Dwarfs

Multi-colour image of the Rho Ophiuchus region

Rho Ophiuchus is a star-forming region that lies within a molecular cloud complex at a distance of ~130 parsecs from the Sun. There are two major stellar concentrations, the young clusters Lynds 1688 and 1689. Both have been the subject of extensive ground-based observations, which have succeeded in identifying over several members, spread over an area of ~0.5 square degrees, with masses ranging from 2-3 solar masses to below the hydrogen burning limit. The clusters are much less massive than the Orion Nebula Cluster (ONC), and appears to break into two sub-units. Matched against theoretical isochrones, the colour-magnitude data suggest that the cluster is significantly younger than the ONC, with age estimates around ~0.5 Myrs These clusters therefore presents an interesting opportunity to investigate brown dwarfs at extremely young ages, and probe the extent to which environment plays a role in determining the properties of stars and brown dwarfs. The present program aims to focus on the lower mass members, using WFC3 to obtain multiwaveband, high resolution images that can search for extremely low-mass planetary companions.

GO 13051: Long term observations of Saturn's northern auroras

Planetary aurorae are stimulated by the influx of charged particles from the Sun, which travel along magnetic field lines and funnel into the atmosphere near the magnetic poles. Aurorae therefore require that a planet has both a substantial atmosphere and a magnetic field. They are a common phenomenon on Earth, sometimes visible at magnetic latitudes more than 40 degrees from the pole, and have also been seen on Jupiter, Saturn, Uranus and Neptune. Saturn pasased through its equinox four years ago, in August 2009, with the Sun moving into the northern Saturnian hemisphere. The present program has obtained HST has used the ACS/SBC to monitor auroral activity in the northern regions over the past three cycles, enabling a compariosn with previous observations of activity around Saturn's south pole. In particular, the southern aurorae tend to occur in an oval whose centre is displaced from the south (rotational) pole. These observations are correlated with in situ measurements by the Cassini spacecraft.