2.1. ARNICA observations
NGC 2346 was observed during two observing runs in January 1996 using ARNICA (ARcetri Near Infrared CAmera) mounted on the 1.5m TIRGO 2 telescope. ARNICA is equipped with a 256x256 NICMOS3 array, the pixel size with the optics used at TIRGO is ; for a complete description of instrument performances, see Lisi et al. (1996) and Hunt et al. (1996). Images were obtained in the K broad-band filter (centered at 2.2 µm) and in a narrow-band filter centered on the 2.12 H2 1-0S(1) line (, Vanzi et al. 1998). The seeing was approximately and the observed field was , covering all the nebula.
Data reduction was carried out using the IRAF 3 and ARNICA (Hunt et al. 1994) software packages. Photometric calibration in the K band was performed by observing the photometric standards of the FS14 group from the list of Hunt et al. (1997). The quality of the night was rather poor, and the calibration accuracy is estimated to be .
The image in the H2 1-0S(1) line has been calibrated using the 5 brightest (unsaturated) stars in the ARNICA images, under the assumption that for each star the flux density measured in the line filter was equal to the flux density measured in the K band. Integrated line fluxes on the nebula were then obtained multiplying the flux density by the bandwith of the narrowband filter (Vanzi et al. 1998). The accuracy is 15%.
2.2. LONGSP observations
K (2.2 µm) band spectra of NGC 2346 were obtained using the LonGSp (Longslit Gornergrat Spectrometer) spectrometer mounted at the Cassegrain focus on the TIRGO telescope. The spectrometer is equipped with cooled reflective optics and grating in Littrow configuration. The detector is a 256256 engineering grade NICMOS3 array (for detector performances see Vanzi et al. 1995). The pixel sizes are 11.5 Å (first order) 0073 in the dispersion and slit directions, respectively. LONGSP operates in the and 1: range 0.9-2.5 µm achieving a spectral resolution at first order of 950 in K. For a more comprehensive description of the instrument, refer to Vanzi et al. (1997).
Observations were conducted in March 1998 under non-photometric conditions. The slit had dimensions 3:005 and was oriented N-S. The seeing during the observations was in the range . NGC 2346 was observed at three slit positions labeled as E, S, W and shown in Fig. 1 superimposed on the image in the H2 1-0S(1) line. Position E and W are centered on the peaks of the line emission located east and west of the central star, respectively (see Zuckerman and Gatley 1988). Position S is centered on the star. At each grating position we performed 5 ABBA cycles (A=on source, B=on sky) with an on-chip integration time of 60 sec, for a total of 10 min integration on source.
Data reduction was performed with the ESO package MIDAS, within the context IRSPEC, modified to take into account LonGSp instrumental characteristics. The frames were corrected for bad pixels, flat-fielded, sky subtracted and wavelength calibrated using the OH sky lines present in all the frames (Oliva & Origlia 1992). After direct subtraction, sky removal was optimized by minimizing the standard deviation in selected areas where the OH sky lines were poorly subtracted but no object emission was present. The wavelength calibration was performed to better than 1/5 of a pixel (2 Å). The spectra were then corrected for telluric absorption by dividing the spectra by the spectrum of the A star BS 2714 after removing its photospheric features (mainly Br). For more details on LonGSp data reduction, see Vanzi et al. 1997.
Flux calibration of the spectra was achieved by rescaling the observed flux distribution along the slit to match that obtained from the ARNICA image in the 1-0S(1) line at the positions of the slits.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999