New high-sensitivity, milliarcsecond resolution results from routine observations of lunar occultations at the ESO VLT

Abstract

Context. Lunar occultations (LO) are a very efficient and powerful technique that achieves the best combination of high angular resolution and sensitivity possible today at near-infrared wavelengths. Given that the events are fixed in time, that the sources are occulted randomly, and that the telescope use is minimal, the technique is very well suited for service mode observations.Aims: We have established a program of routine LO observations at the VLT observatory, especially designed to take advantage of short breaks available in-between other programs. We have used the ISAAC instrument in burst mode, capable of producing continuous read-outs at millisecond rates on a suitable subwindow. Given the random nature of the source selection, our aim has been primarily the investigation of a large number of stellar sources at the highest angular resolution in order to detect new binaries. Serendipitous results such as resolved sources and detection of circumstellar components were also anticipated.Methods: We have recorded the signal from background stars for a few seconds, around the predicted time of occultation by the Moon's dark limb. At millisecond time resolution, a characteristic diffraction pattern can be observed. Patterns for two or more sources superimpose linearly, and this property is used for the detection of binary stars. The detailed analysis of the diffraction fringes can be used to measure specific properties such as the stellar angular size and the presence of extended light sources such as a circumstellar shell.Results: We present a list of 191 stars for which LO data could be recorded and analyzed. Results include the detection of 16 binary and 2 triple stars, all but one of which were previously unknown. The projected angular separations are as small as 4 milliarcsec and magnitude differences as high as Δ K = 5.8 mag. Additionally we derive accurate angular diameters for 2 stars and resolve circumstellar emission around another one, also all for the first time. We have established upper limits on the angular size of 177 stars, mostly in the 1 to 5 mas range, and we plan to include them in a future list of sources well suited for the calibration of interferometers.Conclusions: We confirm the performance of the technique already established in our previous work. LO at an 8 m-class telescope can achieve an angular resolution close to 0farcs001 with a sensitivity K ≈ 12 mag.