This week on HST

HST Programs: November 21, 2011 - November 27, 2011

Selected highlights

GO 11617: HAT-P-1: A Direct Glimpse into the Atmosphere of a Hot Jupiter

Artist's impression of a planetary transit & a partial light curve

Transiting extrasolar planets offer particularly valuable insight into the structure of these non-Solar System gas giants. Besides providing direct measures of mass (with no complications for v sin(i)) and radius (from accurate time-series photometry), spectroscopic observations obtained during either transit or planetary eclipse can probe the atmospheric structure and chemical composition. The first such systems, including HD 209658b and HD 189733b, were originally discovered as radial velocity variables, and only subsequently identified as transits based on follow-up photometry. However, the last five-six years have seen the instigation of almost a dozen wide-field, ground-based photometric surveys that are searching specifically for such systems - and those surveys are starting to bear fruit. even with the recent advent of the initial set of Kepler discoveries, these ground-based programs remain important since their targetes are generally brighter, nearer stars that can be targeted for more detailed follow-up observations. The present HST program is using STIS to obtain spectroscopic observations of HAT-P-1, the first transiting system discovered by the Hungarian Automated Telescope Network (HATNet). The target star is a ~9th magnitude G0 dwarf lying at a distance of ~120 parsecs in the constellation of Lacerta. The planet is a hot jupiter, with a period of ~4.47 days and a mass of ~0.53 MJup. The present program aims to use STIS to search for the signature of reflected light by carefully differencing observations taken during and outside secondary eclipse (planet behind star). Spitzer has carried out similar observations, and has succeeded in detecting thermal emission due to the planet. If successful, this will be the first detection of reflected light from a planet.

GO 12177: 3D-HST: A Spectroscopic Galaxy Evolution Treasury

Part of the GOODS/Chandra Deep Field South field, as imaged by HST

One of the exciting new capabilities offered by the post-SM4 Hubble Telescope is multi-object, low-resolution, near-infrared spectroscopy, using the two grisms available on the IR channel of Wide-Field Camera 3. These observations provide an important avenue for complementing wide-field imaging surveys. In particular, the present program aims to build on the extensive database currently being accumulated as part of the CANDELS Multi-Cycle Treasury program. CANDELS, itself, rests on past HST Treasury programs, and will provide multi-tiered imaging of five fields. 3D-HST will supplement portions of four fields (GOODS-south, AEGIS, the UDS and COSMOS fields) with WFC3/G141 and ACS/G800L grism data. The spectroscopic data will provide important additional information on the galaxy redshift distribution, and on the star formation characteristics in the redshift range 1 < z < 3.5. The data should also be useful in identifying quasars at high redshifts, potentially extending beyond z~6.

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 designed to obtain multi-wavelength narrowband images of the illuminated ejecta. Further observations may be obtained at future dates to trace the further evolution of this interesting system.

All single stars, and most binary stars, with masses less than ~7 solar masses are expected to end their lives as white dwarfs - extremely compact objects made of degenerate material, compressing ~0.3 to 1.4 solar masses of material into a sphere little larger in radius than the Earth. Theoretical evolutionary models predict a broad correlation between the mass of the main-sequence star and the mass of the remnant, although there is significant scatter in the observed initial-final mass relation. The models also predict that white dwarfs should follow mass-radius relations that depend on the composition, temperature and internal structure. The present program aims to test the predictions of those models by determining accurate masses and radii for a sample of white dwarfs in resolved binary systems. STIS spectra will be used to measure accurate Balmer line profiles for these hot degenerates, and those pofiles can be analysed to yield effective temperatuers and surface gravities. Moreover, the H-beta line profile has a sharp core that allows accurate measurement of the apparent radial velocity of the system. This measured velocity has two main components: the star's peculiar velocity relative to the Sun; and the gravitational redshift induced by the high field on a degenerate white dwarf. Sicne these stars are members of wide binary systems, observations of the main sequence companion can be used to determine the former quantity and hence allow emasurement of the latter, and set constraints on the white dwarf mass.