This week on HST

HST Programs: January 12, 2008 - January 18, 2009

Selected highlights

GO 11113: Binaries in the Kuiper Belt: Probes of Solar System Formation and Evolution

A composite of HST images of the Kuiper Belt binary, WW31

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 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. This proposal will use WFPC2 imaging of known KBOs to identify new binary systems.

GO 11564: Optical and Ultraviolet Photometry of Isolated Neutron Stars

Artist's impression of a neutron star

Neutron stars are extremely compact (~10 km diameter) massive (~1.4 to 2 solar mass) remnants of high mass (> 7 solar mass) stars. Formed during the gravitational core-collapse of Type Ib, Ic and II supernovae, they have initial temperatures exceeding 1011 Kelvon, but cool very rapidly with time. Their existence was originally proposed by Zwicky and Baade in the 1930s, but they remained unobserved until the late-1960s, when Hewish and Bell identified a pulsing radio source in the Crab nebula as a neutron star. (Hewish and Okoye had previously identified "an unusual source of high radio brightness temperature" within the crab.) Neutron stars have strong magnetic fields and rapid rotation, leading to radiation beaming along the magnetic poles; the observed pulses result from the beams sweeping across the terrestrial line of sight. Numerous pulsars are currently known, but direct observations of the neutron star's "photosphere" are much rarer. The present observations will used WFPC2 to image five previously-detected neutron stars in the U and B bands, mapping the blue/ultraviolet flux distribution and enabling the potential measurement of proper motions.

GO 11966:

Spitzer IRAC image of the one-armed spiral, NGC 4725; the 5.8 and 8 micron data (red) highlight warm dust clouds

SINGS is the Spitzer Infrared Nearby Galaxy Survey, a comprehensive imaging and spectroscopic study of 75 nearby galaxies (D < 30 Mpc). The program combines Spitzer mid-infrared observations (With IRAC, MIPS and the IRS) with optical data from ground-based telescope and HST, radio observations and UV imaging with GALEX. The overall goalm of the program is to map the distribution of gas and dust in these nearby systems, and hence probe the nature and distribution of physical processes within the ISM. Many of the SINGS targets lack high-resolution ultraviolet imaging, and the present communtiy program was allocated Director's Discretionary Time of HST for WFPC2 F336W (U-band) and (in a few cases) F656N (H-alpha) imaging of a subset of the sample. The data will be of particular use in resolving and providing reliable age estimates for individual star clusters.