This week on HST

HST Programs: September 29 - October 5, 2008

HST suffered a failure of the side A electronics on the CU/SDF (Command Unit/Science Data Formatter). 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 11299: Calibrating the Mass-Luminosity Relation at the End of the Main Sequence

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 allow sub-milliarcsecond accuracy astrometry of 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 monitor six close binary systems. Observations are scheduled of GJ 1081 (also known as G 96-45). Lying at a distance of ~15.3 parsecs, this system consists of two M dwarfs. The primary has MV ~ 11.5, corresponding to spectral type M3), while the secondary is 1.5 magnitudes faint (probably spectral type ~M4).