This week on HST

HST Programs: February 22, 2010 - February 28, 2010

Selected highlights

GO 11589: Hypervelocity Stars as Unique Probes of the Galactic Center and Outer Halo

Artistic view of a hypervelocity star

Hypervelocity stars are stars that have velocities that exceed the escape velocity of the Milky Way by a very substantial margin. Stars can only achieve such velocities through violent gravitational interactions with other bodies. Indeed, to achieve velocities of 1,000 km/sec or more, the proposed mechanism involves binary stars interacting with the black hole at the Galactic Centre: one of the binary components is lost to the black hole, and the other ejected at from the core. The existence of such objects was hypothesised over 20 years ago, but the first candidate was only identified in 2005: SDSS J090745.0+024507, an apparently non-descript sunlike star, lying at a distance of 71 kpc from the Sun in the Galactic, and moving at a velocity of 850 km/sec, or more than twice the escape velocity. Since then, a number of other candidates have been identified, and the present HST program aims to measure precise proper motions that will quantify their space motions.

GO 11613: GHOSTS: Stellar Outskirts of Massive Spiral Galaxies

The (relatively) nearby barred spiral galaxy, NGC 253

The subdwarf stars that populate the Galactic halo are generally recognised as fossil remnants of the first episode of substantial star formation to afflict the Milky Way galaxy. The structure and density distribution of our own halo has been inferred partly from deep starcounts, partly from globular cluster systems and partly from the kinematics of local subdwarfs; most analyses favour a near-spherical system with density r-3.5. The aim of this proposal is to extend these studies to a number of other nearby spiral systems. ACS and WFC3/UVIS are being used to obtain SNAP observations in the F606W and F814W filters in regions within the galactic halos. The observations are capable of obtaining photometry extending 2-3 magnitudes below the tip of the red giant branch in those galaxies. The data should permit separation of the contributions from disk, thick disk and bulge, and isolation of the halo population. The observations will allow measurement of the halo metallicity distribution and an estimate of the shape of the halo in these systems.

GO 11650: Orbits, Masses, and Densities of Three Cold Classical Transneptunian Binaries

Preliminary orbital determination for the KBO WW31, based on C. Veillet's analysis of CFHT observations; the linked image shows the improved orbital derivation, following the addition of HST imaging

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs (or trans-Neptunian objects, TNOs) are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known KBOs are binary (including Pluto, one of the largest known KBOs, regardless of whether one considers it a planet or not). This is a surprisingly high fraction, given the difficulties involved in forming such systems and the relative ease with which they can be disrupted. It remains unclear whether these systems formed from single KBOs (through collisions or 3-body interactions) as the Kuiper Belt and the Solar System have evolved, or whether they represent the final tail of an initial (much larger) population of primordial binaries. These issues can be addressed, at least in part, through deriving a better understanding of the composition of KBOs - and those properties can be deduced by measuring the orbital parameters for binary systems. The present proposal aims to use HST WFC3 observations to map the orbits of three binary systems. Those observations will be ued to determine the orbital period and semi-major axis and the total system mass, while the mid-infrared properties (measured by Spitzer) allow an assessment of the surface area/diameters; combining these measurements gives an estimate of the mean density.

GO 12011: Magnetic Heating of the Outer Atmospheres of Very Low Mass Dwarfs

Artist's impression of a flaring low-mass star.

One of the major surprises delivered by the Einstein Observatory was the high level of chromospheric activity present in cool, late-type dwarfs. Proportionately,. low-mass dwarfs are significantly more active than (and often as luminous as) solar-type stars, and are frequently subject to high energy flares and outbursts. This was surprising since these cool dwarfs are fully convective, and are therefore incapable of supporting the rotational dynamo that is believed to power magnetic activity in sun-like stars. In the succeeding 30 years, subsequent observatories, notably Chandra and HST, with increasingly sensitive X-ray and UV detectors have pushed observations to fainter flux limits and lower luminosity stars. Those observations demonstrate that activity persists through spectral class M and is even present among ultracool L dwarfs, probably powered through shear dynamos maintrained by convective motions. The present program targets two late-type M dwarf: LHS 3003 (M7) and LHS 2065 (M9) and will use COS observations of the far-UV spectra to probe the conditions in the transition region between the chromosphere and corona, and search for coronal structures.