The high angular resolution images presented in this paper have been obtained at the 3.60-m telescope of ESO-La Silla with the COME-ON+ adaptive optics system on 1994 September 21. The combination of the high angular resolution and the high sensitivity offered in the near infrared by this instrument was essential for the project (Rigaut et al. 1991). On one hand, the expected size of the region responsible for the near-infrared circumstellar emission needs the angular resolution allowed by the full correction ( at J). On the other hand, the brightness ratio between the unresolved central star and the resolved circumstellar shell needs a high dynamic range (about 1000:1) allowed by the instrument. In order to increase the contrast in the resulting image without saturating the NICMOS array when observing the brightest stars () of our sample, a large number of short exposure frames (0.1 to 3.0 s) have been co-added. The non-saturation of the array was ensured during the acquisition process. This has also been controlled a posteriori by measuring the maximum level of intensity of each frame.
The visible magnitudes of all our targets are also well in agreement with the sensitivity of the wavefront sensor which uses the central parts of our objects themselves as reference stars. Source calibrators (non-resolved stars) have also been observed in view of a posteriori deconvolution.
Fig. 1 is the long exposure map of the calibration star Psc. Three symmetrical parts of the first Airy ring can be seen near the center. The lowest contour is about 3 times the standard deviation of the noise and is equal to 0.1% of the maximum of intensity (corresponding to a dynamic range of 1000:1). The noise has been measured in 10 by 10 pixel boxes situated at each corner of the long exposure image of the calibrator.
Table 1 (at the end of the paper) lists the program stars together with their spectral types, broad band magnitudes (V magnitudes are at the maximum of luminosity), and angular diameter of their inner dust shell, measured with the ISI interferometer (Danchi et al. 1994) or estimated (Rowan-Robinson & Harris 1983a, 1983b). The observations are summarized in Table 2 and were obtained during the night from 1994 September 21 to 22.
Table 1. Program stars
Table 2. Logbook of the observations
© European Southern Observatory (ESO) 1998
Online publication: September 8, 1998