This week on HST

HST Programs: July 28 - August 3, 2014

Selected highlights

GO 13314: Characterizing the Elusive Intragroup Medium and Its Role in Galaxy Evolution

Subaru image of the Hickson 40 compact group.

A high proportion of galaxies, perhaps as many as half, are found in relatively sparse, but gravitationally-bound groups of less than 50 members. These systems are generally less than 1-2 Mpcs in extent, with total mass between 1012 and 1013 MSun. The majority of the baryonic material in these systems is believed to lie in diffuse gas in the IntraGroup Medium. Direct detection of that gas is difficult; only the massive groups heat the IGrM sufficiently to excite soft X-ray emission. The present program aims to search for the gas in absorption: the Cosmic Origins Spectrograph will be used to obtain UV spectra of quasars that lie close to galaxy groups; hot gas in the IGrM absorbs the light from the background quasar. Those observations can be used to probe the composition and temperature of gas along the line of sight. The results can be compared against data from other HST programs that survey gas in the outer halos of galaxies, testing how the properties of the IGrM might be influenced by gas escaping from individual galaxies.

GO 13315: Pluto Satellite 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 fainter magnitudes to both better characterise the P4 and P5 orbits and search for even fainter moons.

GO 13453: The Elemental Compositions of Extrasolar Minor Planets

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typcail targets0, they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years.In subsequent years, a significant number of DA white dwarfs have been found to exhibit narrow metallic absorption lines in their spectra. Those lines are generally attributed to "pollution" of the white dwarf atmospheres. Given that the diffusion time for metals within the atmospheres is short (tens to hundreds of years), the only reasonable means of maintaining such lines in ~20% of the DA population is to envisage continuous accretion from a surrounding debris disk, probably generated by collisions within a population of asteroids. The present program aims to use these features to probe the composition of the parent asteroids through near- and far-UV spectroscopic observations of five white dwarfs, GD 408, GD 303, WD1425+540, GD 205 and WD2253+803. Given the presence of Ca II and Mg II lines in the optical spectra of these degenerates, the UV observations, with the Cosmic Origins Spectrograph, are expected to reveal absorpton lines due to neutral oxygen and ionised silicon and carbon, probing the relative abundances of these various elements within the accreted material.

GO 13454: The parallax and mass of the binary classical Cepheid V1334 Cyg

The radial velocity curve (left) and pulsational velocity curve (middle) of the primary component of V1334 Cyg (Gallene et al, A&A, 552, A21)

V1334 Cygni is a short-period (pulsation period 3.33 day) Cepheid variable lying at a distance of ~680 parsecs from the Sun. There have been suspicions for many years that this system harbours more than one star, with evidence for radial velocity variations and a possibly very close (0.1-0.2 arcsecond) companion. Recent observations with the CHARA interferometer have finally resolved the system; the companion, lying at a separation of 8 milliarcseconds is identified as a B5.5V (or earlier) companion of the F2 supergiant primary, with a flux ratio of ~1:33. The system has an orbital period of ~1938 days. These observations define most of the fundamental parameters of the system; there remains, however, an ambiguity in between the system mass ratio and the parallax/distance; the present set of HST observations aim to remove that ambiguity by obtaining FGS astrometry of the primary and mapping its apparent orbital trajectory. The program will also use STIS to obtain ultraviolet spectra; since the B5 companion dominates the flux at those wavelengths, those observation will map the radial velocity curve of the secondary, providing an unambiguous determination of the individual masses and the distance.