The difficulty of observing faint emissions in the close environment of bright objects, like stars, increases with the contrast. Yet, many fields of astrophysical research depend on this ability. One important field is the study of the close environment of IRAS excess stars where the aim is to detect the origin of such infrared (IR) emissions as either disks, or red and faint objects. Alternatively, very faint structures around a number of Herbig stars could be imaged.
For instance, a circumstellar disk has been imaged around Pictoris (Smith & Terrile
1984), using a coronograph with a 7 diameter occulting mask
and a Lyot mask on a 2 meter class, classical (i.e. without adaptive
optics) telescope. This first detection has been followed by observations
of this disk closer to the star, using two different approaches. A first
one consisted of using image tip-tilt correction before the coronograph
(Golimovski et al. 1993), and allowed the detection of the disk
down to 2 from the star, and quantitative measurements to
2.5, to be compared to 6 obtained without correction. The
second method used an anti-blooming detector to achieve a very high dynamic
range instead of using a coronograph (Lecavelier et al. 1993) and
enabled a similar performance to be reached. The scientific interest in the
vicinity of the star is important: the physical nature of the medium is
supposed to change within 2 and this coincides also with the
expected location of planets and their area of influence. The thermal
emission of the dust has also been imaged at 10 m (Lagage & Pantin
1994). This provides complementary physical information.
A systematic search for similar disks around other main sequence (MS) stars has also been performed (Smith et al. 1992). No other disk was found among approximately 100 candidates. The reasons for these non detections are probably a combination of three facts (Lagrange 1994). The disks are possibly intrinsically fainter than that of Pictoris (according to IRAS indications), the disks may be unfavourably oriented, and they may also be closer to the star. In the context of such studies, scientific results and constraints would benefit greatly if one could observe faint (typically less than 15 magnitude per arcsec2) and extended structure much closer (typically closer than 2'') to the bright central object. Such performance requires the highest possible angular resolution as well as the use of techniques which provide a very high dynamic range.
Theoretical calculations indicate that the quality of coronographic images depends critically on the angular resolution. The improvement with the use of partial image correction has already been demonstrated by Malbet et al. (1992) on the COME-ON adaptive optics system and by Golimovski et al. (1993) with their Adaptive Optics Coronagraph. On the basis of the COME-ON experiment, we have built a coronograph to work with the COME-ON-PLUS adaptive optics system on the ESO 3.6 meter telescope at La Silla, which provides diffraction-limited images in K band (2.2 m), i.e. a resolution of 0.12. It was hoped to be able to use masks smaller in diameter than 1, which is significantly smaller than the ones used without or with partial image correction (respectively, typically 4 and 2). In 1994, six nights were devoted to testing the coronograph and measuring the performance of the AO + coronograph system in terms of dynamic range and the ability to observe faint emissions close to a central, bright object.
In Sect. 2 (click here), we review the theoretical reasons that support the use of a coronograph with adaptive optics. We describe the coronograph in Sect. 3 (click here) and we present in Sect. 4 (click here) a detailed description of the reduction procedure for this particular type of data. The resulting capabilities of the whole procedure are presented and the different sources of uncertainty are evaluated in Sect. 5 (click here). These are critically compared to theoretical calculations and the performance of other existing systems in Sect. 6 (click here). Conclusions and perspectives to this instrumental work are given in Sect. 7 (click here).