This week on HST

HST Programs: May 4 - May 10, 2009

Selected highlights

GO 11178: Probing Solar System History with Orbits, Masses, and Colors of 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 will use the Planetary camera on WFPC2 to determine the relative orbits for several known KBO binaries. Just as with binary stars, the orbital period and semi-major axis give 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 11944: Binaries at the Extremes of the H-R Diagram

The MV-mass relation for low-mass stars (from T. Henry)

The mass-luminosity relation remains one of the key underpinnings of stellar astrophysics, notably in probing the grey area that separates hydrogen-burning stars from cooling-powered brown dwarfs. The calibration of thsi relation rests on observations of binary systems, primarily eclipsing binaries at masses above 1 MSun, and primarily astrometric binaries at sub-solar masses. In the latter case, reliable mass determinations require orbital measurements of extremely high accuracy, which, in turn, requires high precision astrometry over at least one orbital period. The Fine Guidance Sensors on HST have proven invaluable in this regard, since they permit both the detection of closely-separated binary systems, and sub-milliarcsecond accuracy astrometry of resolved binary systems with sub-arcsecond separations; in other words, HST allows measurement of nearby, low-mass binaries with periods short enough to allow completion of the observations in significantly less than an astronomer's lifetime. The current program is using the FGS to search for new binary systems among the more extreme constituents of the HR diagram: specifically, massive, luminous O stars; nearby lower main-sequence K and M dwarfs and subdwarfs; and cool white dwarfs. Suitable binary systems detected through these initial observations will be targeted for detailed monitoring in subsequent observational programs

GO 11975: UV light from old stellar populations: a census of UV sources in Galactic Globular Clusters

Hubble Heritage image of the globular cluster, M15

Globular clusters are the oldest structures within the Milky Way that are directly accessible to observation. They are relatively simple systems, with relatively simple colour-magnitude diagrams (albeit with some complexities adduced from recent HST observations, see GO 11233 ). Matching those CMDs against theoretical models not only allows us to set constraints on the age of the oldest stars in the Galaxy, and hence on the age of the Milky Way and the epoch of galaxy formation, but also probes the range of properties of stellar populations at these ages. In the latter respect, a long-standing issue centres on the morphology of the the horizontal branch - specifically, the relative number of UV-bright, extreme horizontal branch stars. Such stars are believed likely to be the source of UV light in (otherwise red and dead) ellipticla galaxies, but the exact origin (or origins), and frequency, of EHB stars remains unclear. The present HST program will use WFPC2 and the ACS/SBC to map the central regions of 46 globular clusters, and carry out a census of UV-bright stars.

GO 11980: Deep FUV Imaging of Cooling Flow Clusters

Chandra X-ray image of a cooling flow in the galaxy cluster, Abell 1795

Galaxy clusters are embedded within an extensive halo of high temperature gas, typically estimated as having a total mass that significantly exceeds the stellar mass in the component galaxies. The gas in this intracluster medium can undergo cooling, largely through X-ray emission, leading to its coalescence into filametary structures that were originally postulated as likely to produce significant star formation. Over the last two decades, detailed observations indicate that, while such cooling flows are present in many clusters, the level of star formation is substantially lower than the original projections. Nonetheless, this phenomena is of considerable interest, and the present observing program is part of a multi-telescope, multi-wavelength survey of 19 nearby clusters. Large ground-based telescopes have already been used to obtain H-alpha imaging (with the Magellan 6.5-metre tel4scope) and radio observations (with the VLA), while Chandra and Galex have provided space-based X-ray and low-=resolution UV imaging. HST will now add high-resolution far-UV imaging with the ACS Solar Blind Channel, probing the details of filamentary structure evident in the H-alpha images.