This week on HST

HST Programs: September 16 - September 22, 2013

Selected highlights

GO 12880: The Hubble Constant: Completing HST's Legacy with WFC3

A GALEX image of Messier 106 (NGC 4258), one of the galaxies targeted in this program

The Hubble constant remains a key parameter in understanding cosmology and the evolution of the Universe. Refining measurements of H0 therefore still represents a vital means of probing the nature of dark energy. The present program aims to tackle this question by laying a firmer foundation to the SNe Ia distance scale. The WFC3 IR camera will be used to identify and characterise Cepheid variables in eight relatively nearby galaxies that have hosted Type Ia SNe. Cepheids have signficantly lower amplitude at near-infrared wavelengths, and the measured magnitudes are less subject to uncertainties due to foreground reddening and variations in metallicity. As a consequence, determining the mean apparent magnitude, and hence the period/apparent magnitude relation, is substantially more straightforward than at optical wavelengths. WFC3 has revolutionised this field by providing substantial greater areal coverage and higher precision photometry than NICMOS. Past observational program have targeted the Cepheids in the maser galaxy, NGC 4258, as well as Galactic Cepheids. The present program targets eight galaxies that have hosted Type Ia supernovae, offering the prospect of tying the SNe Ia scale directly to the Galaxy and to NGC 4258, avoiding the many intermediate steps of previous analyses. The aim is to reduce the level of systematics in determinations of H0 to the 1-2 percent level, setting signficantly stronger constraints on dark energy.

GO 12955: Comparing Planet Formation Signatures in two Systems

Artist's impression of the Keple 11 planetary system

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. Following those discoveries, a number of ground-based surveys were estbalished that used small, wide-field telescopes to search specifically for such systems. Those surveys bore fruit, adding some tens of systems to theexopanet catalogues. Transit searches, however, were revolutionaised by the launch of the Kepler spacecraft in March 2009. Kepler monitored some ~200,000 stars within a ~115 square degree field in the constellation of Cygnus, i.e. in the Galactic Plane. Kepler's reaction wheels failed in May 2013, but by that time the mission had resulted in the discovery of 137 confirmed planets around 76 stars together with around 3250 additional candidates. As the statistics suggest, multiple planet systems are comonn, and the present HST program focuses on follow-up observations of one such system, Kepler 11, which hosts six planets. The WFC3 near-IR grism will be used to measure the stellar spectrum while three members of this system, Kepler 11d, e and f, are in and out of eclipse, allowing a determination of the atmospheric spectrum. These observations will be compared with observations of two planets in the Kepler 9 system, observed in Cycle 19 as part of HST program GO 12482.

GO 13178: Spitzer Trigonometric Parallaxes of the Solar Neighborhood's Coldest Brown Dwarfs

The stellar menagerie: Sun to Jupiter, via brown dwarfs

Brown dwarfs are objects that form in the same manner as stars, by gravitational collapse within molecular clouds, but which do not accrete sufficient mass to raise the central temperature above ~2 million Kelvin and ignite hydrogen fusion. As a result, these objects, which have masses less than 0.075 MSun or ~75 M<\sub>Jup, lack a sustained source of energy, and they fade and cool on relatively short astronomical (albeit, long anthropological) timescales. Following their discovery over a decade ago, considerable observational and theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes. At their formation, most brown dwarfs have temperatures of ~3,000 to 3,500K, comparable with early-type M dwarfs, but they rapidly cool, with the rate of cooling increasing with decreasing mass. As temperatures drop below ~2,000K, dust condenses within the atmosphere, molecular bands of titanium oxide and vanadium oxide disappear from the spectrum to be replaced by metal hydrides, and the objects are characterised as spectral type L. Below 1,300K, strong methane bands appear in the near-infrared, characteristics of spectral type T. At present, the coolest T dwarfs known have temperatures of ~650 to 700K. At lower temperatures, other species, notably ammonia, are expected to become prominent, and a number of efforts have been undertaken recently to find examples of these "Y" dwarfs. The search is complicated by the fact that such objects are extremely faint instrinsically, so only the nearest will be detectable. Identifying such ultra-ultracool dwarfs was a goal of the WISE satellite mission, which recently completed its all-sky survey. WISE has succeeded in identifying close to a dozen very red, and therefore very cool, stellar sources. Initial follow-up observations with both Spitzer and HST suggest that at least may have effective temperatures less than 400K, although subsequent analyses suggests that those temperatures may be udnerestimated. These are the first examples of the new spectral class of Y dwarfs. The present program follows up on the initial discoveries by combining astrometry from Spitzer imaging, deriving proper motions and parallaxes, with low-resolution spectroscopy with the G102 and G141 grisms on WFC3-IR. The overall goal is to establish reliable distances, luminosities and temperatures, defining these sources as benchmarks in the lowest reaches of the brown dwarf regime.

GO 13400:The Surprising Ejecta Geometry of Recurrent Nova T Pyx

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 2011. HST obtained a series of multi-wavelength narrowband images of the illuminated ejecta over the past two years, tracking the evolution of the expanding ejecta. The present program builds on those observations by using WFC3 and STIS to map the spatial distribution and kinematics of the evolving remnent.