[Paper Review] A List of Bright Interferometric Calibrators measured at the ESO VLTI

A. Richichi, I. Percheron+1 more|Jun 22, 2009

Adaptive optics and wavefront sensing16 citations

TL;DR

This paper presents a homogeneous, high-accuracy list of 17 primary and 47 secondary interferometric calibrators observed with the ESO VLTI's VINCI instrument in the K-band, using a robust global fitting method on publicly available data. The primary calibrators achieve an average angular diameter accuracy of 0.35%, significantly improving upon prior catalogues and enabling more precise near-infrared interferometric measurements across baselines of 20–200 m.

ABSTRACT

In a previous publication (Richichi & Percheron 2005) we described a program of observations of candidate calibrator stars at the ESO Very Large Telescope Interferometer (VLTI), and presented the main results from a statistical point of view. In the present paper, we concentrate on establishing a new homogeneous group of bright interferometric calibrators, based entirely on publicly available K-band VLTI observations carried out with the VINCI instrument up to July 2004. For this, we have defined a number of selection criteria for the quality and volume of the observations, and we have accordingly selected a list of 17 primary and 47 secondary calibrators. We have developed an approach to a robust global fit for the angular diameters using the whole volume of quality-controlled data, largely independent of a priori assumptions. Our results have been compared with direct measurements, and indirect estimates based on spectrophotometric methods, and general agreement is found within the combined uncertainties. The stars in our list cover the range K=-2.9 to +3.0 mag in brightness, and 1.3 to 20.5 milliarcseconds in uniform-disk diameter. The relative accuracy of the angular diameter values is on average 0.4% and 2% for the primary and secondary calibrators respectively. Our calibrators are well suited for interferometric observations in the near-infrared on baselines between ~20m and ~200m, and their accuracy is superior, at least for the primary calibrators, to other similar catalogues. Therefore, the present list of calibrators has the potential to lead to significantly improved interferometric scientific results.

Motivation & Objective

• To establish a homogeneous, high-accuracy list of bright interferometric calibrators based solely on publicly available VLTI observations, avoiding reliance on pre-existing estimates.
• To improve the accuracy and robustness of angular diameter measurements for calibrator stars used in near-infrared interferometry.
• To provide a reference set of primary calibrators with high-precision data to support the derivation of secondary calibrators through a global numerical solution.
• To validate the reliability of the derived diameters by comparing them with direct measurements and spectrophotometric estimates.
• To support future interferometric observations and theoretical modeling by offering a benchmark dataset with superior accuracy for calibration.

Proposed method

• The authors selected 17 primary and 47 secondary calibrators based on strict criteria for data quality and observation volume from VINCI K-band observations up to July 2004.
• A robust global fitting method was applied to the entire volume of quality-controlled visibility data, minimizing dependence on a priori assumptions about stellar parameters.
• The angular diameters were derived from squared visibility measurements using the relation $ V^2 = |{ m TF}| imes V^2_{ m t} $, where TF accounts for instrumental and atmospheric degradation.
• Limb darkening corrections were applied using model atmosphere predictions for intensity variations across the stellar disk.
• The primary calibrators were used as reference points to numerically derive the diameters of secondary calibrators through a global solution framework.
• Systematic uncertainties were evaluated by comparing results with independent measurements and the Cohen et al. (1999) catalogue.

Experimental results

Research questions

• RQ1 Can a homogeneous list of high-accuracy interferometric calibrators be derived solely from publicly available VLTI observations, without relying on pre-existing estimates?
• RQ2 What level of angular diameter accuracy can be achieved using a global fitting approach on a consistent dataset from a single instrument?
• RQ3 How do the derived diameters compare with direct measurements and spectrophotometric estimates in terms of consistency and uncertainty?
• RQ4 To what extent do the primary calibrators serve as reliable reference points for deriving secondary calibrators?
• RQ5 What is the impact of assuming a constant transfer function per night on the accuracy of the final angular diameter measurements?

Key findings

• The primary calibrators achieve an average angular diameter accuracy of 0.35%, with a worst-case uncertainty of 0.9%, significantly outperforming the Cohen et al. (1999) catalogue.
• The secondary calibrators have an average accuracy of 2%, providing a reliable reference set for interferometric observations.
• The derived angular diameters range from 1.3 to 20.5 milliarcseconds, covering K-band magnitudes from −2.9 to +3.0.
• The sample is predominantly composed of early K-type stars, with about 70% of both primary and secondary calibrators having diameters between 2 and 5 milliarcseconds.
• Comparison with the Cohen et al. (1999) catalogue and direct measurements shows general agreement within combined uncertainties, validating the robustness of the method.
• The study demonstrates that the primary calibrators are ideal for testing theoretical predictions due to their high precision, offering an exacting benchmark for stellar atmosphere models.