This week on HST

HST Programs: May 27 - June 2, 2013

Selected highlights

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 12897: Pluto system orbits in support of New Horizons

Hubble Space Telescope images of the Pluto system, including the recently discovered moons, P4 and P5

Pluto, one of the largest members of the Kuiper Belt and, until recently, the outermost planet in the solar system, has been in the news over the last year or two. Besides the extended "planet"/"dwarf planet" debate, Pluto is the primary target of the New Horizons Mission. In 1978, James Christy discovered from analyses of photographic plates that Pluto has a relatively large companion moon, Charon, with a diameter of ~1200 km, or almost half that of Pluto itself. In 2005, Hubble observations led to the discovery of two small moons, christened Nix and Hydra. These two new moons are 5,000 fainter than Pluto itself, implying diameters as small as ~30-50 km if the surface composition is similar to Pluto itself. Over the past two years, a series of observations were taken in support of the New Horizons mission, using WFC3 to search for faint rings due to dust particles that might jeopardise the space craft and require a course correction. While no rings were detected unequivocally, two small satellite, christened "P4" and "P5", have been discovered. Both are significantly fainter than Nix and Hydra, and may well be as small as 10-13 km in size. There is also some evidence that might point to the presence of a debris ring within Charon's orbit. The present observations, again in support of New Horizons, will use WFC3 to push to even fainter magnitudes to both better characterise the P4 and P5 orbits and search for even fainter moons.

GO 13018: Deciphering the Assembly History of Galactic Disks: The Resolved Record in the Outer Disk of M31

M31: the Andromeda spiral galaxy, superimposing GALEX UV imaging on the optical data

M31, the Andromeda galaxy, is the nearest large spiral system to the Milky Way (d ~ 700 kpc), and, with the Milky Way, dominates the Local Group. The two galaxies are relatively similar, with M31 likely the larger system; thus, Andromeda provides the best opportunity for a comparative assessment of the structural properties of the Milky Way. Moreover, while M31 is (obviously) more distant, our external vantage point can provide crucial global information that complements the detailed data that we can acquire on individual members of the stellar populations of the Milky Way. With the advent on the ACS and, within the last 2 years, WFC3 on HST, it has become possible to resolve main sequence late-F and G dwarfs, permitting observations that extend to sub-solar masses in M31's halo and disk. The galaxy has been targeted by many HST programs, including imaging the extended halo, probing the nuclear regions, surveying the disk for specific objects (LBVs, Miras, low-mass X-ray binaries), and, most recently, the PHAT Multi-Cycle Treasury program, a multi-waveband survey of approximately one third of disk and bulge, focusing on the north-east quadrant. The present program builds on past observations of a field in the outer disk, some 26 kpc from the M31 nucleus. Analysis of the stellar colour-magnitude diagram suggests a star formation history that was roughly constant until 5 Gyrs ago, after which the SFR generally declined but underwent a sharp burst some 2 Gyrs ago. The timescale of the latter burst coincides approximately with a potential dynamical interaction between M31 and M33. The present observations taget two additional fields with the goal of testing whether they show evidence for a similar star formation history.

HST-ACS image of the planetary-mass companion of Fomalhaut

Fomalhaut, or alpha Piscis Austrini, is one of the Sun's closest neighbours, an A-type star with a mass approximately twice that of the Sun and an age between 100 and 300 million years, lying at a distance of only ~7.7 parsecs. Observations with the IRAS satellite in the early 1980s revealed the presence of significant excess radiation at mid-infrared wavelengths, indicating the presence of substantial dust within a disk that is being irradiated by the luminous central star. Since then, observations of Fomalhaut and nearby stars of that ilk have led to a much more detailed characterisation of the debris disk phase. In particular, Spitzer has mapped warm dust in these systems, while HST imaging has provided exquisite resolution in reflected light. It is now recogised that debris disks are the evolutionary stage where planet formation has likely run to completion, the gas has fully dissipated but the disk remains well populated with dusty material spanning a wide range of sizes. Indeed, it is likely that this phase coincides with the heavy bombardment epoch within the Solar System. ACS imaging of Fomalhaut reveals extensive structure in the disk, notably a sharply-defined, eccentric inner edge to the disk, which led to the prediction of a ~Saturn-mass planet at that location. Subsequent ACS/HRC observations led to the identification of that planet, the first direct imaging of a "conventional" exoplanet. The detection was confirmed in 2010 by HST observations with STIS. Observations suggest that the exoplanet may be variable at optical wavelengths, suggesting that the observed flux is not only contributed by reflected light from the planetary "surface". Moreover, the orbit is clearly far from circular. The current program will use further STIS observations to detect the planet at the current epoch and hence map the orbit.