NAVY PROTOTYPE OPTICAL INTERFEROMETER IMAGING OF LINE EMISSION REGIONS OF β LYRAE USING DIFFERENTIAL PHASE REFERENCING

We present the results of an experiment to image the interacting binary star beta Lyrae with data from the Navy Prototype Optical Interferometer using a differential phase technique to correct for the effects of the instrument and atmosphere on the interferometer phases. We take advantage of the fact that the visual primary of beta Lyrae and the visibility calibrator we used are both nearly unresolved and nearly centrally symmetric, and consequently have interferometric phases near zero. We used this property to correct for the effects of the instrument and atmosphere on the phases of beta Lyrae and to obtain differential phases in the channel containing the H alpha emission line. Combining the H alpha-channel phases with information about the line strength, we recovered complex visibilities and imaged the H alpha emission using standard radio interferometrymethods. Our images show the position of the H alpha-emitting regions relative to the continuum photocenter as a function of orbital phase, indicating a major axis line of nodes along Omega = 249 degrees +/- 4 degrees. The orbit is smaller than previously predicted, a discrepancy that can be alleviated if we assume that the system is at a larger distance, or if the stellar continuum contribution to the H alpha channel was underestimated. We do not detect a jet in the H alpha images, which may be due to the limited resolution of the observations along the direction perpendicular to the orbital plane. We find that the differential phase results are consistent with those obtained from a more standard analysis using squared visibilities (V(2)s) and closure phases, which also indicate an H alpha disk radius of 0.6 +/- 0.1 mas, and Delta V 1.30 +/- 0.1 and Delta R = 1.20 +/- 0.1 mag for the magnitude difference between the stars.