Astron. Astrophys. 320, 594-604 (1997)

2. Observations

2.1. J H K broad band images

The near infrared observations were carried out on February 12, 1994 with the 1.5 m TIRGO ¹ telescope at the Gornergrat Observatory and with the Arcetri near-infrared camera ARNICA (Lisi et al. 1993; Hunt et al. 1996a). The camera is based on a NICMOS3 256 256 pixel HgCdTe array detector developed by Rockwell International. The scale on the detector is , hence the field of a single frame covers about . For each filter thirteen offset frames were taken, all of which contain the S 235 A-B region. The standard star AS 10 from the ARNICA list (Hunt et al. 1996b) was observed for photometric calibration.

All the observed images were reduced by using the ARNICA (Hunt et al. 1994) and IRAF software packages. Sky subtraction and flat fielding were performed using the median average image of the thirteen frames. We combined all the images and produced a mosaic image. Aperture photometry was performed using the APPHOT routines of IRAF. The limiting magnitudes in our mosaic image are 17.3, 16.2 and 16.0 mag ( in aperture) in J, H, K bands, respectively.

One of the sources (S 235 B , in the following we shall indicate with the stars within the nebulosities, to distinguish them from the diffuse emission) was so bright at K that it was saturated in our images; for this reason a shorter integration image was taken at K on the February 16, 1994. This image had a much lower sensitivity and was used only to determine the K magnitude of S 235 B .

Careful astrometry of the near infrared images was made with the Digitized Sky Survey of the Space Telescope Science Institute (Testi 1993). The astrometric calibration error is less than one arcsecond.

In Fig. 1 the full mosaic image in K-band centered on the maser position is presented. Three diffuse nebulosities can be seen: S 235 A and S 235 B in the center and S 235 C in the southern part. From the figure we can also distinctly see that there is a stellar cluster in the center of the region, which is highly obscured optically.

Fig. 1. The full mosaic image in K-band centered on the S 235 A-B region. Axes are Right Ascension and Declination at 1950.0 equinox.

2.2. Narrow band images

Narrow band images were obtained at the TIRGO telescope with ARNICA on the November 8, 1995, at the H₂  S and at the Br wavelengths. The plate scale of the instrument is the same as for the broad band observations, but the useful field of view is (Gennari & Vanzi 1994). The data reduction has been performed in the same way as for the broad band data, but, due to the smaller field of view, the final mosaics cover only a region around the water maser position.

To calibrate the images and to subtract the continuum emission, the K band image was used. The narrow band images were first convolved with a gaussian of proper width in order to match the K band PSF, then flux calibration was obtained assuming that a set of stars does not have detectable line emission, hence the flux density at the wavelength of the narrow band images should be almost the same as that at K. After calibration, the K band image was subtracted from the narrow band images. In Table 1 the central rest frequencies, the bandwidth of the filters, and the noise level of the final images are reported.

Table 1. Central rest wavelength, bandwidth and noise in the final images for the narrow band observations.

Due to non perfect PSF matching and saturation of S 235 B , the continuum subtraction performed as explained above was not satisfactory in removing strong point sources from the images. For this reason we tried also to subtract the two calibrated narrow band images from each other. Comparing this result with that obtained subracting the K band image, we found that this method was more efficient in removing the point sources. In Fig. 9 the narrow band images obtained with this method are presented.

2.3. Molecular line observations

With a resolution of NY found that the position of the maximum intensity of CS(J=1-0) is located northeast of S 235 B, while the emission has a more elongated shape, extending approximately in the northeast/southwest direction from the H₂ O maser to S 235 B. They also reported the detection of a molecular outflow centered on S 235 B.

The region between S 235 A and S 235 B was observed with a better resolution and sensitivity by Cesaroni, Felli and Walmsely (1996) as part of a larger survey in several molecular lines of a selected list of H₂ O masers without nearby radio continuum emission. Here we report the results that are relevant to the present discussion.

The observations were made with 30 m IRAM radiotelescope of Pico Veleta. The parameters of the observed lines are reported in Table 2. The pointing was checked every hour and it was found to be better than .

Table 2. List of observed molecules, rest frequencies, forward efficiency, main beam efficiency and half power beamwidth

The three C³⁴ S transitions were observed simultaneoulsy, while the ¹³ CO(J=2-1) line was observed in a separate setup. The alignment between the different receivers was checked through continuum cross scans on Jupiter, at the three frequencies of observation, and found to be accurate to within .

The data were calibrated by using the standard chopper wheel technique (Kutner and Ulich 1981). The observed line intensities are expressed as main beam brightness temperature (), corresponding to a temperature scale corrected for forward scattering and spillover losses, but not for the coupling of the source to the antenna beam. The main beam brightness temperature is given in terms of the effective beam efficiency, , and the antenna forward efficiency, , using the formula , where is the antenna tempearture outside the atmosphere corrected for rear spillover and resistive scattering.

The front-end receivers employ SIS mixers having system temperature, after correction for atmosphere and telescope efficiency, of 340 K at 96 GHz, 470 K at 144 GHz and 1500-2200 K between 220-240 GHz. Our spectrometer was a filter bank consisting of 256 channels with 25 MHz bandwidth for ¹³ CO(2-1) and two filter banks of 512 channels (one of this split into two parts of 256 channels each) with 512 MHz bandwith for the C³⁴ S lines. The corresponding velocity resolutions are 0.14 km s^-1 for ¹³ CO(2-1) and 0.12, 0.08 and 1.2 km s^-1 for the three C³⁴ S lines, respectively.

The integration time was 2 minutes in the total power mode. The molecular cloud was mapped at offsets of with respect to the H₂ O maser position, over an incomplete grid of 5 5 positions.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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