This week on HST

HST Programs: November 26 - December 2, 2012

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. The present HST observations are part of a time series designed to obtain multi-wavelength narrowband images of the illuminated ejecta.

HST-NICMOS imaging of the three outer planets in the HR 8799 system

HR 8799 is bright A-type star, lying ~39 parsecs from the Sun within the constellation Pegasus. It is one of the youngest stars in the Solar Neighbourhood, with an age of roughly 30 million years, and is likely a member of the loose group known as the Columba Association. The star itself is a member of the Lambda Bootis class, a type of star that exhibits unusually weak metallic features, suggesting that the surface atmospheric layers are depleted in abundance. Possible explanations include accretion of metal-poor gas from a surrounding circumstellar disk. HR 8799's main claim to fame is as the first star to have direct imaging observations of an associated planetary system. In 2008, a Canadian team led by Christian Marois announced the discovery of three giant planet companions, denoted HR 8799b, c and d, at orbital separations from 24 to 68 AU. The discoveries were based on near-infrared adaptive-optics assisted observations with the Gemini North and Keck telescopes. The planets likely have masses between 5 and 10 times that of Jupiter. Subsequent observations revealed a fourth planet, HR8799e, lying at a projected distance of ~14.5 AU from the parent star. Moreover, an examination of data within the HST archives showed that the outer three planets were detected on NICMOS observations taken in 1998, and careful re-analysis of those data, together with subsequent observations, by a team led by Remi Soummer has resulted in strong constraints on the orbital parameters. Mid-infrared observations with Spitzer have revealed complex debris disk structure. Program GO 12511 targets HR 8799 with the IR channel of Wide-Field Camera 3. The prime aim of this program is to obtain photometry of the planetary companions at a range of wavelengths between 8,000 Angstroms and 1.6 microns, and use the resultant data to constrain the spectral properties,and hence the likely composition of the planetary atmospheres.

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.

GO 13025: Unveiling the progenitor of the most luminous supernova

Chandra X-ray image of G292.0+1.9, a ~3000-year old supernoa remnant

Supernovae are generally believed to originate through two mechanisms: accretion onto a white dwarf in a close binary system, driving the white dwarf above the Chandrasekhar limit; and the implosion of the core of very massive (> 7 solar masses) stars. The resultant supernovae have distinctive spectral characteristics: in the former case, type I supernovae, the spectra lack any hydrogen lines and the majority have strong absorption due to ionised silicon and are classified as type Ia; in the latter case, Type II, hydrogen features are present in the spectra, which show very broad emission lines. Both processes result in explosive nucleosythesis that enriches the interstellar medium, with the ejecta forming a rapidly expanding shell. In general, the luminosities of these explosive events fall within well-defined limits. However, recent years have seen the identification of several highly luminous Type II supernovae in nearby galaxies. This program aims to set limits on the likely progenitors of these unusual stars by using the UVIS channel on Wide-Field Camera 3 to obtain high-resolution images of the appropriate regions within the galaxies. Those data will be used to derive colour-magnitude diagrams for the local stellar populations, and hence constrain the likely age distribution and progenitor mass.