Astron. Astrophys. 335, 1040-1048 (1998)

2. Observations

Imaging data on AFGL 961 using a narrow band (2%) molecular hydrogen (H₂) filter centred on the 2.122µm v=1-0 S(1) line were acquired on UT Feb 9, 1996 at the United Kingdom Infrared Telescope, Mauna Kea, Hawaii using the facility 1-5µm camera, IRCAM3. This instrument, based on a 256256 Santa Barbara Research Center (SBRC) InSb detector, is described in detail in Puxley et al. (1994). Using a pixel scale 0.286"/pixel, we obtained a small mosaic of images (3x3) with an exposure time per tile of 5 minutes. The final mosaic image, shown in Fig. 1, covers a spatial area of 3.25´ square. A low-contrast version of the central region is overlaid on the image to indicate the location of the AFGL 961 binary stars. A contour map of the central region of this mosaic image showing the area immediately surrounding the binary is presented in Fig. 2. The images were dark subtracted, flat-fielded using a median-filtered flat-field calculated from the object images, and mosaiced in the usual manner. We refer the reader to Aspin, Sandell & Russell (1994) for specific details of the data reduction techniques used. No observations of a standard star were obtained due to the presence of thin variable cirrus during the observations. Astronomical seeing during the observations was 0.75".

Fig. 1. A mosaic image of the AFGL 961 region using the narrow band 2.122µm v=1-0 S(1) H2 filter covering 3.253.25´. The major H2 nebulous objects are indicated and designated with AS numbers 1 through 9. To allow the two stellar sources to be seen, a low-contrast image of the region immediately surrounding these sources has been superimposed on the high-contrast mosaic.

Fig. 2. Contour map of the central region of the mosaic S(1) image of AFGL 961. AFGL 961 W is the region from -20" West to the western edge of the image. The image has been sky subtracted and the contours are in ADU/sec in steps of 2 ADU/sec from a base of 0.5 ADU/sec.

Spectroscopy of AFGL 961a and AFGL 961b was obtained several years before the imaging data, specifically on UT Dec 28, 1993. These data were also acquired at the UKIRT and utilized the facility cooled-grating spectrometer, CGS4, also based on a 256256 SBRC InSb array and which is described in detail in Mountain, et al. (1990). The CGS4 slit was positioned North-South and two spectra were acquired centred on AFGL 961a and AFGL 961b, respectively. The slit was 90" in extent and therefore we obtained simultaneous spectra of the two stellar source and of the regions 45" to both north and south of each. The spectra were acquired using the 75 l/mm grating resulting on a spectral resolution of 1000 over the wavelength range 2.05-2.45µm. The spectra were sky-subtracted after preliminary processing (bias subtraction, flat-fielding) using blank sky observations taken interspersed between multiple target observations. The target spectra were extracted from the spectroscopic images and ratioed with observations of a standard star taken at a similar airmass. This effectively removed atmospheric effects/bands. These standard star observations also allow us to flux-calibrate the target spectra. For these data, observations of the bright standard star BS2845 (B8Ve, K=3.04) were used. The Br present in the spectrum of this star was removed by linear interpolation prior to ratioing. The resulting spectra of AFGL 961a, AFGL 961b and nebulosity directly south of AFGL 961b are shown in Fig. 3.

Fig. 3. 2µm spectroscopy of AFGL 961a and AFGL 961b. Also shown is the spectrum of nebulosity 15" south of AFGL 961b. The range of numbers associated with the AFGL 961a and AFGL 961b spectra are the fluxes at the extremely of the spectra in Watts m-2 µm-1.

© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998
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