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Session 12 - Binary Stars.
Display session, Monday, June 08
Atlas Ballroom,

[12.18] Determining the Mass-Luminosity Relation and Parallax for Faint (Mv<16) and Close (<50mas) Binary Systems with HST's Fine Guidance Sensors

E. Nelan, O. Lupie, L. Nagel, R. Makidon, J. Hershey (STScI), O. Franz, L. Wasserman (Lowell Obs.), G. Benedict, B. McArthur (U. Texas at Austin), T. Henry (CFA)

HST's Fine Guidance Sensors (FGS) are 2 dimensional white light shearing interferometers that can be operated in two different modes. In TRANSFER mode, the fringe pattern of an observed object is scanned multiple times to achieve the necessary SN, while in POSITION mode the fringe pattern is tracked to obtain an accurate position of the object in the detector's field of regard. The TRANSFER mode data of an observation of a binary system are deconvolved, using the reference fringe pattern from a point source, to determine the angular separation, position angle, and relative brightness of the binary's components. The POSITION mode data are used to determine the relative astrometry of the binary with respect to field stars (also observed in POSITION mode). By observing the binary several times over the course of its orbit, the FGS is well suited for determination of the binary's true relative orbit (from TRANSFER mode data) while simultaneously measuring its parallax and proper motion from the POSITION mode observations. Application of this observing technique can thereby solve for the binary's total mass and relative brightness without resorting to radial velocity studies. In cases where adequate positional accuracy is achieved, the barycentric position of the binary can be determined and the relative masses of the components can be measured. Alternatively, TRANSFER mode observations of a double lined spectroscopic binary system, which provide the orbit's inclination and angular extent of the semimajor axis, can be used in tandem with radial velocity data to derive the system's parallax, independently of any other astrometric (positional) measurements. This paper describes these techniques and discusses the performance of the instrument (FGS #3).

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