This week on HST

HST Programs: April 16 - April 22, 2007

Some selected highlights

GO 10524: Blue Stragglers: a key stellar population to probe internal cluster dynamics

Blue stragglers in NGC 6397 (Hubble heritage team)

Blue stragglers are found in old open clusters and globular cluster systems. Most appear to lie on the main sequence, but in a location `up and to the right' of the main-sequence turnoff; that is, these stars have higher mass than the turnoff stars, and, if coeval, ought to have evolved through the giant branches to become white dwarfs. The resolution of this paradox appears to lie in stellar mergers; blue stragglers represent the remnants of violent dynamical processes that can only occur within high density star clusters. Most recent models favour stellar collisions as the likely formation mmode for these stars. However, previous observations of M3, 47 Tuc and NGC 6752 suggest that the blue stragglers folla a bimodal radial distribution, a result that this team interprets in favour of formation by mass transfer in close binary systems, rather than by direct stellar collisions The present proposal aims to combine HST ACS observations with ground-based CCD imaging to trace the radial distribution of these stars in eight globular clusters, including M15, NGC 2808, NGC 5466 and M71.

GO 10862: Comprehensive Auroral Imaging of Jupiter and Saturn during the International Heliophysical Year

Hubble ultraviolet image of auroral activity near Jupiter's north magnetic pole

2007 has been designated the International Heliophysical Year, and HST will be playing a key part in the associated scientific activities by participating in a detailed investigation of auroral activity in jupiter and Saturn. Planetary aurorae are stimulated by the influx of charged particles from the Sun, which travel along magnetic field lines and funnel into the atmosphere near the magnetic poles. Aurorae therefore require that a planet has both a substantial atmosphere and a magnetic field. They are a common phenomenon on Earth, sometimes visible at magnetic latitudes more than 40 degrees from the pole, and have also been seen on Jupiter, Saturn, Uranus and Neptune. Jovian auroral activity is also affected by the Galilean satellites, which generate electric currents that can produce bright auroral spots (see figure), and, in some cases, have their own auroral storms. HST is using the ACS Solar Blind Channel to monitor activity on the two largest gas giants. The initial campaign, starting in early January, focused on Saturn while it was at opposition. In February, the New Horizons spacecraft flew by Jupiter, with closest approach on February 28th, and used the strong gravitational field to propel it on its way to Pluto. During the fly-by, New Horizons is carrying out a number of experiments ( see this link ) while HST monitors the large-scale behaviour. Finally, Jupiter will be surveyed while it is at opposition in June 2007.

GO 10889: The Nature of the Halos and Thick Disks of Spiral Galaxies

Ground-based imaging of the edge-on spiral, NGC 891

The stars in the Milky Way are generally grouped into stellar populations, building blocks that provide insight into the process of galaxy assembly. The traditional populations are the near-spherical, metal-poor Halo, representing the first significant burst of star formation; the Disk, whose constituents have higher metallicities, a flattened density distribution (which defines the Galactic Plane) and significant angular momentum, suggesting a formation history that includes collapse and dissipation; and the central Bulge, which, with a spheroidal distribution and broad metallicity range, may be something of an amalgam of disk and halo. The original models for the Disk envisaged a relatively simple population, with a continuous star formation history and an exponential density distribution, both radially and perpendicular to the Plane. However, in the mid-1980s, starcount analyses revealed more complexity in the vertical density distribution, with evidence for two components with scaleheights ~300 and 1000-1300 pc. Subsequent investigations of the more extended component, dubbed the thick disk, suggest that it probably formed as a result of a merger with a massive satellite early in the Milky Way's history (8-10 Gyrs ago). Ground-based observations suggest that some other spiral galaxies possess a similar component. Clearly, the frequency of such systems and their age distribution offer clues to the merging history of the average spiral galaxy. The present program will use ACS, WFPC2 and NICMOS to image seven edge-on spirals at several locations perpendicular to the Plane, with the aim of resolving the underlying stellar populations and tracing the metallicity distribution and overall morphology of the extended disk components.

GO 10931: Dynamical Masses and Radii of Four White Dwarf Stars

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 signifciant scatter in the observed initial-final mass relation. Binary white dwarfs are of particular interest, since mergers are predicted to lead to supernovae and/or gamma ray bursts, besides liberating significant energy as gravity waves. The present program is using the Fine Guidance Sensors on HST to carry out precise astrometry of a number of close WD binaries. Besides measuring the orbital motions, the data will also be used to determine parallaxes and distances.