This week on HST

HST Programs: October 6 - October 12, 2008

HST suffered a failure of the side A electronics on the CU/SDF (Command Unit/Science Data Formatter) on 27 September 2008. This units handles telemetry from the science instruments. Work is currently under way to determine when the CU/SDF can be switched to side B operations. In the meantime, astrometric observations continue to be made using the Fine Guidance Sensors.

GO 11212: Filling the Period Gap for Massive Binaries

Artist's impression of the O star binary LH54-425, an LMC system

The relative number of single and binary (or multiple) star systems as a function of stellar mass is an important constraint on star formation theories. Current indications are that the overall binary frequency decreases with decreasing mass, running at around 70% for solar-type stars and falling to less than 30% for late-type M dwarfs. Massive stars, particularly highly luminous O stars, pose a particular challenge for binary investigations, since even relatively massive companions are overwhelmed by the flux from the primary star. The present program aims to use the Fine Guidance Sensors on HST to probe binarity among these stars. The FGS is a white-light interferometer, which is not only capable of achieving sub-milliarcsecond accuracy astrometry, but also allows the binary detection via fringe analysis. The effective limits are separations of ~15 milliarcseconds for binaries with magnitude differences less than 2 magnitudes, and combined apparent magnitudes brighter than ~15th magnitude. Observations of more than 40 OB stars are scheduled for the coming week.