Diameter determinations from VINCI using global calibration solutions

Abstract

Although primarily built for testing and alignment purposes, the K band instrument VINCI at the VLTI has produced numerous scientific results on individual targets. The present work consists of a global analysis of the entire VINCI data archive. A visibility estimator using coherent integration has successfully obtained some 15500 raw visibilities on 293 objects. Those results are interpreted using a global solution algorithm which imultaneously solves for calibration (transfer function) fluctuations and for stellar diameters. Stars which do not follow the standard uniform disk visibility curve are automatically or manually flagged and are avoided in the solutions for nightly calibrations. The standard approach to calibration based on observations of "calibrator" stars having assumed diameters, is rejected in the present work. No a priori diameters are input to the algorithm, which relies on baseline diversity to simultaneously solve both for diameters of stars having well-behaved characteristics, and a quasistatic transfer function subject to various hardware fluctuations. The success of this approach depends on a diversified schedule of observing the same target on different nights in conjunction with a mixture of other targets, so that all visibilities can be "cross-calibrated." This is practical in the case of the VINCI data set where half of the 666 observing nights produced at least 22 successful visibilities on a median of 7 different targets. Half of the 293 objects were successfully observed at least 18 times and on at least 6 separate nights, with observations over a median baseline range of 4.8:1. Uniform disk diameters are a by-product of the cross-calibration procedure which uses this database to solve for an evolving transfer function on a nightly basis. © 2007 Springer.