2. Observations and data reduction
The IRC +10 420 speckle interferograms were obtained with the Russian 6 m telescope at the Special Astrophysical Observatory on June 13 and 14, 1998. The speckle data were recorded with our NICMOS-3 speckle camera (HgCdTe array, 2562 pixels, frame rate 2 frames/s) through an interference filter with a centre wavelength of 2.11 µm and a bandwidth of 0.19 µm. Speckle interferograms of the unresolved star HIP 95447 were taken for the compensation of the speckle interferometry transfer function. The observational parameters were as follows: exposure time/frame 50 ms; number of frames 8400 (5200 of IRC +10 420 and 3200 of HIP 95447); 2.11 µm seeing (FWHM) 1:000; field of view 7:0087:008; pixel size 30.5 mas. A diffraction-limited image of IRC +10 420 with 73 mas resolution was reconstructed from the speckle interferograms using the bispectrum speckle-interferometry method (Weigelt 1977, Lohmann et al. 1983, Hofmann & Weigelt 1986). The bispectrum of each frame consisted of 37 million elements. The modulus of the object Fourier transform (visibility) was determined with the speckle interferometry method (Labeyrie 1970).
It is noteworthy that 2.11 µm filters also serve to image hydrogen emission as, for instance, H2 (2.125 µm) or Br (2.166 µm) emission. Accordingly, it is possible that one might look at hydrogen emission rather than at the dust emission of a circumstellar shell. The low-resolution spectrum of IRC +10 420 published in the atlas of Hanson et al. (1996) shows a Br line in emission as the most prominent feature for the wavelength range considered here. Oudmaijer et al. (1994) carried out high-resolution infrared spectroscopy and found an equivalent width of 1.2 Å for the Br emission line. This is only 0.06% of the bandwidth of our interference filter and consequently negligible.
Fig. 1 shows the reconstructed 2.11 µm visibility function of IRC +10 420. There is only marginal evidence for an elliptical visibility shape (position angle of the long axis , axis ratio to 1.1). The visibility 0.6 at frequencies cycles/arcsec shows that the stellar contribution to the total flux is 60% and the dust shell contribution is 40%. In order to compare our results with speckle observations of other groups we determined the Gauß fit FWHM diameter of the dust shell to be mas. By comparison, Christou et al. (1990) found for 3.8 m telescope K-band data a dust-shell flux contribution of 50% and mas. However, as will be shown later, a ring-like intensity distribution appears to be much better suited than the assumption of a Gaussian distribution whose corresponding FWHM diameter fit may give misleading sizes (see Sect. 3.4.5). Fig. 2 displays the azimuthally averaged diffraction-limited images of IRC +10 420 and the unresolved star HIP 95447.
© European Southern Observatory (ESO) 1999
Online publication: August 13, 199