Milliarcsecond angular resolution of reddened stellar sources in the vicinity of the Galactic center

Abstract

Context. We present lunar occultation (LO) observations obtained in August 2006 with the recently demonstrated burst mode of the ISAAC instrument at the ESO VLT. The results presented here follow the previously reported observations carried out in March 2006 on a similar but unrelated set of sources. Aims. Interstellar extinction in the inner regions of the galacti c bulge amounts to tens of magnitudes at visual wavelengths. As a consequence, the majority of sources in that area are poorly studied and large numbers of potentially interesting sources such as latetype giants with circumstellar shells, stellar masers, inf rared stars, remain excluded from the typical investigatio ns which are carried out in less problematic regions. Also undetected are a large numbers of binaries. By observing LO events in this region, we gain the means to investigate at least a selected number of sources with an unprecedented combination of sensitivity and angular resolution. Methods. The LO technique permits to achieve milliarcsecond resolution with a sensitivity of K≈12 mag at a very large telescope. We have used the opportunity of a favorable passage of the Moon over a crowded region in the general direction of the Galactic Center to observe 78 LO events of heavily reddened stellar sources. Results. We have detected six new binary and one triple star, with typical projected separation of≈ 0. 01. We have also detected the compact circumstellar emission around one maser and one central star of a planetary nebula. Additionally we have measured the diameter and/or circumstellar shell of two carbon stars and other IR sources. Conclusions. We have used the upper limits on the size of about 60 unresolved or marginally resolved sources to establish the performance of the method. In agreement with our previous result, we conclude that lunar occultations in fast read-out m ode on a detector subwindow at an 8 m-class telescope can achieve an angular resolution close to 0. ′′ 001 with a sensitivity K≈ 12 mag.