A catalog of near-IR sources found unresolved with milliarcsecond resolution

Abstract

Calibration is one of the long-standing problems in optical interferometric measurements, particularly with long baselines which demand stars with angular sizes on the milliarcsecond scale and no detectable companions. While systems of calibrators have been generally established for the near-infrared in the bright source regime (K$\la 3$\,mag), modern large interferometers are sensitive to significantly fainter magnitudes. We aim at providing a list of sources found unresolved from direct observations with high angular resolution and dynamic range, which can be used to choose interferometric calibrators. To this purpose, we have used a large number of lunar occultations recorded with the ISAAC instrument at the VLT to select sources found to be unresolved and without close companions. An algorithm has been used to determine the limiting angular resolution achieved for each source, taking into account a noise model built from occulted and unocculted portions of the light curves. We have obtained upper limits on the angular sizes of 556 sources, with magnitudes ranging from K$_{\rm s} \approx$4 to 10, with a median of 7.2\,mag. The upper limits on possible undetected companions (within $\approx 0\farcs5$) range from K$_{\rm s} \approx$8 to 13, with a median of 11.5\,mag. One-third of the sources have angular sizes $\le 1$, and two-thirds $\le 2$ milliarcseconds. This list of unresolved sources matches well the capabilities of current large interferometric facilities. We also provide available cross-identifications, magnitudes, spectral types, and other auxiliary information. A fraction of the sources are found to be potentially variable. The list covers parts of the Galactic Bulge and in particular the vicinity of the Galactic Center, where extinction is very significant and traditional lists of calibrators are often insufficient.