This week on HST

HST Programs: July 14 - July 20, 2008

Some 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 11172: Defining Classes of Long Period Variable Stars in M31

M31, The Andromeda galaxy

Most long period variable stars (LPVs) are red giants on the asymptotic giant branch (AGB). These are intermediate mass stars, between ~1.5 and ~7 solar masses, that are powered by hydrogen and helium shell-burning. The interactions between the two energy sources lead to instabilities that can generate substantial pulsations, with periods from ~50 to ~500 days. Mira, or omicron Ceti, is the Galactic prototype for this type of variable, and numerous LPVs have been identified throughout the Milky Way and in the neighbouring Small and Large Magellanic Clouds. The LMC and SMC variables show a clear period-luminosity relation, particularly at near-infrared wavelengths, with longer period stars having higher intrinsic luminosities. This P-L relation, while not as well established for classical cepheids, allows LPVs to contribute to measurements of the extragalactic distance scale. The present program aims to expand LPV surveys to M31, using the NIC1 and NIC2 cameras within NICMOS to obtain follow-up near-infrared imaging of regions of the Andromeda galaxy that have been observed with ACS and WFPC2 in previous cycles. These new observations will both provide detailed colour-magnitude diagrams for the selected areas and allow the identification of LPVs within the Andromeda disk population.

GO 11235: HST NICMOS Survey of the Nuclear Regions of Luminous Infrared Galaxies in the Local Universe

NICMOS image of the interacting LIRG, NGC 6090

Luminous infrared galaxies (LIRGs) have total luminosities that exceed 1011.4 LSun, with most of the energy emitted at wavelengths longward of 10 microns. Many (perhaps most) of these galaxies are interacting or merging disk galaxies, with the excess infrared luminosity generated by warm dust associated with the extensive star formation regions. Many systems also have an active nucleus, and may be in the process of evolving towards an S0 or elliptical merger remnant. The present program surveys a total of 88 such systems from the IRAS Revised Bright Galaxy Sample, imaging each system with the H-band F160W filter on the NICMOS NIC2 camera. The observations will be used to probe the luminosity and distribution of star formation activity, particularly embedded star clusters; the presence of AGNs and the relationship between star formation and AGN activity; the correlations between near-infrared emission and the mid-IR structures identified from Spitzer IRAC imaging; the presence of coherent structure, bars or bridges, that might funnel gas into the nuclei; and the overall structural properties of the LIRGS as a function of luminosity and environment Observations of several interacting systems are scheduled for this week.