This week on HST

HST Programs: October 20 - October 26, 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. Operations were switched to the side B electronics on 16 October, but two anomalies occurred later that day, affecting the ACS/SBC and the NSSC-1. Work is currently under way to determining whether operations can be resumed using side B, or whether operations are possible in a hybrid mode. 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.

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).

GO 11789: An Astrometric Calibration of Population II Distance Indicators

Measuring trigonometric parallax

Trigonometric parallax measurement remains the fundamental method of determining distances to astronomical objects. The best ground-based parallax measurements can achieve accuracies of ~1 milliarcsecond, comparable with the typical accuracies achieved by the ESA Hipparcos astrometric satellite. This level of accuracy allows us to obtain reliable distances and luminosities for main sequence stars, subgiants, red giants and even a number of metal poor subdwarfs. However, with an effective distance limit of 100-150 parsecs, the sampling volume includes less than a handful of rarer, shorter-lived celestial objects. In particular, there are no RR Lyraes or Cepheids, two of the principal calibrators in the extragalactic distance scale. There is only one instrument currently available that can achieve astrometry of higher accuracy - the Fine Guidance Sensors (FGS) on HST. The present team used the FGS to measure a parallax of 3.82+/10.2 milliarseconds for RR Lyrae, the nearest star of its type. this corresponds to a distance of 262 parsecs. The present program (an extension of GO 11211) aims to improve the calibration by extending observations to four more relatively nearby RR Lyraes (XZ Cyg, UV Oct, RZ Cep and SU Dra) and two Pop II Cepheids (Kappa Pav and VY Pyx).