Angular diameter estimation of interferometric calibrators Example of λ Gruis, calibrator for VLTI-AMBER

Context. Accurate long-baseline interferometric measurements require careful calibration with reference stars. Small calibrators with high angular diameter accuracy ensure the true visibility uncertainty to be dominated by the measurement errors. Aims. We review some indirect methods for estimating angular diameter, using various types of input data. Each diameter estimate, obtained for the test-case calibrator star lambda Gru, is compared with the value 2.71 mas found in the Borde calibrator catalogue published in 2002. Methods. Angular size estimations from spectral type, spectral index, in-band magnitude, broadband photometry, and spectrophotometry give close estimates of the angular diameter, with slightly variable uncertainties. Fits on photometry and spectrophotometry need physical atmosphere models with plausible stellar parameters. Angular diameter uncertainties were estimated by means of residual bootstrapping confidence intervals. All numerical results and graphical outputs presented in this paper were obtained using the routines developed under PV-WAVE (R), which compose the modular software suite SPIDAST, created to calibrate and interprete spectroscopic and interferometric measurements, particularly those obtained with VLTI-AMBER. Results. The final angular diameter estimate 2.70 mas of lambda Gru, with 68% confidence interval 2.65-2.81 mas, is obtained by fit of the MARCS model on the ISO-SWS 2.38-27.5 mu m spectrum, with the stellar parameters T-e = 4250 K, log g = 2.0, z = 0.0 dex, M = 1.0 M-circle dot, and xi(t) = 2.0 km s(-1).