This week on HST

HST Programs: January 28 - February 3, 2013

Selected highlights

GO 12448: Towards a Detailed Understanding of T Pyx, Its Outbursts and Shell

Artist's impression of the recurrent nova, RS Oph (by David Hardy)

Recurrent novae are generally agreed to be close binary systems, comprising a white dwarf and a companion main sequence star that is overflowing its Roche lobe, leading to period transfers of mass onto the white dwarf surface. The mass transfer episode triggers nuclear ractions, which lead the star increasing significantly in it luminosity. T Pyxidis is one such system, and it exhibited fairly regular outbursts every 20 years between its discovery, in 1890, and 1966. Since then, however, it has been dormant, a prolonged period of quiescence that led to suggestions, earlier this year, that it might either be headed for hibernation, or in the process of accumulating sufficient mass to trigger a type Ia supernova explosion (in about 1 million years). Perhaps prompted by these suggestions (a la Monty Python Mary Queen of Scots radio sketch), T Pyxidis erupted into activity on or around April 15th 2012. The present HST observations are part of a time series designed to obtain multi-wavelength narrowband images of the illuminated ejecta, extending over a period of almost a year following the eruption.

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 12918: Origin of UCDs in the Coma Cluster

Wide-field iamge of the cluster (KPNO 4-metre)

The Coma cluster is the nearest rich galaxy cluster, lying at a distance of ~100 Mpc from the Milky Way. The cluster includes well over 1000 major galaxies, centred on two giant ellipticals, NGC 4874 and NGC 4889. Chandra observations show that the galaxies are embedded in very hot intracluster gas (see this site ). Cluster galaxies have also been surveyed at mid-infrared wavelengths by Spitzer, and in the ultraviolet by GALEX. Individual galaxies have been studied in the past using HST, and a Cycle 15 Treasury program aimed to obtain systematic imaging with ACS (and parallel observations with NICMOS) of the cluster core ( a 7x6 mosaic, covering approximately 400 sq. arcmin.), together with 40 fields at larger radii, sampling infalling galaxies in the outer cluster. That program was cut short by the failure of ACS in January, 2007. However, before that unfortunate event ACS had succeeded in capturing images of a subset of the central regions, allowing the detection of significant numbers of ultra-compact dwarf (UCD) galaxies. The present program aims to use the refurbished ACS to complete coverage of the core regions, providing a detailed census of the low luminosity tail of the galaxian mass function. The observations will be used to probe colour gradients and internal chemical evolution in these systems, and match the UCD luminosity function against the luminosity of globular cluster systems in these galaxies.

GO 12928: Gaseous outflows from low mass galaxies: Understanding local laboratories for high redshift star formation

The nearby starburst galaxy, NGC 1313

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution of galaxy assembly therefore is tied very to how gas is accreted, recycled, circulated through the halo and disk, and, perhaps, ejected back into the intergalactic medium. In particular, starburst-driven outflows may represent an important means of removing gas from and galaxy and quenching star formation, producing the systems that populate the "red and dead" sequence. Tracing the overall history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. The present program is using the Cosmic Origins Spectrograph to probe gas in nine compact, active star-forming galaxies at redshifts z~0.2. The targets are drawn from the Sloan Digital Sky Survey, and were selected for observation based on the presence of strong, high equivalent width nebular emission lines. These relatively low-mass systems are relatively metal-poor, and therefore may provide good analogues for starbursts at earlier epochs. COS will be used to obtain far-UV spectra, measuring the interstellar absorption from metal lines and the hydrogen Lyman series.