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We have recently initiated a speckle interferometric survey of bright Galactic O stars, in order to determine the frequency of binaries with angular separations in the range 0$''$\llap.035 $<$ $\rho$ $<$ 1$''$. Some 20\% of the survey has been completed thus far, and 4 new binaries have been discovered, suggesting that some 15 additional binaries may eventually be resolved. Our scientific goals are threefold:
\noindent (1)~~Determining new masses: Fundamental data on O-star masses are scarce, and masses determined by indirect means show large systematic discrepancies. Combined speckle and spectroscopic orbits, however, offer the potential for direct measurement of mass. For example, HD 47839 = 15 Mon (O7V) is a 0$''$\llap.05 speckle binary that has shown considerable motion. Combining these data with optical and IUE velocity data leads to a preliminary mass of 32 M$_{\odot}$ (assuming membership in the cluster NGC 2264), which is consistent with evolutionary tracks.
\noindent (2)~~Using the binary frequency as a test of cluster evolution theories: For example, massive O- and B- stars are often born in compact clusters and may be ejected through binary--binary collisions to produce OB runaway stars. We would expect that these ejected stars will be depleted in binaries. Frequency data are also important for estimating the incidence of unresolved O-star binaries in other galaxies.
\noindent(3)~~Interpretating composite spectrum objects: For example, HD 152623 is a 4-day SB, which has been shown (Wiemker, 1992, MS thesis, GSU) to have both a strong stationary component and a weaker velocity variable one. This suggested a low-mass companion, a finding at odds with the apparent spectral signatures of two O stars. Recent speckle data have confirmed Wiemker's suggestion that this is instead a triple system, in which the weaker component is a SB1 which is well separated from the single, strong--line star.
The CHARA speckle program is supported by NSF grant AST 9141927.
