Hubble Space Telescope Observations of Population II Binaries
 

 Above: A few scans made with FGS 1 for the project. Data obtained July, 2001.

HST Fine Guidance Sensors

The Hubble Space Telescope (HST) has three Fine Guidance Sensors (FGSs) that, for normal observations, do exactly as the name indicates: they help the telescope point at the target throughout the observation. However, the FGSs also have the ability to carry out very precise astrometry. The data come to the scientist as shown above, as a series of scans across the target in two orthogonal directions (shown above are several scans in one of the two axes only).
If a source is binary, than two copies of the shape of the above transfer function will be seen per scan. Thus, like speckle imaging, the FGSs provide a way to do high-precision astrometry of binary star systems, for example.

The Masses and Luminosities of Population II Stars

Population I stars: metal rich stars that typically live in the disk of the galaxy, such as the sun.
Population II stars: metal poor stars that are typically part of the galactic halo, thought to have been formed at an early epoch of the history of the Milky Way.

Very little is currently known concerning the mass-luminosity relation (MLR) of Population II stars. However, with the advent of the Hipparcos Catalogue, improved distances to many spectroscopic binaries known to be Pop II systems are now available. After surveying the literature, we found 13 systems whose minimum separation should be larger than the resolution limit on FGS. Because of the expected magnitude differences and separations, it is not possible to resolve these systems from the ground. In combination with Hipparcos distances and the known spectroscopic orbits, FGS observations will yield up to 26 precise stellar mass determinations of metal-poor stars, if all systems are resolved and the relative orbits are determined. A combination of FGS data nd ground based spectroscopic observations will lead to component luminosities and effective temperatures. This program will allow for a significanly better understanding of the Pop II main sequence, which in turn will lead to better ages and distances of the galactic globular clusters, and a Pop II MLR will be constructed for the first time.

Who's Involved?

Elliott Horch is the PI for the project, but the real FGS experts are Otto Franz and Larry Wasserman at Lowell Observatory. Jim Heasley, of the Institute for Astronomy, University of Hawaii is also a Co-I, who will be very involved in the ground-based spectroscopic observations of our target systems.

For More Information:

Contact Elliott Horch.