2 Observations and data reduction

2.1 Near-infrared observations

The near infrared observations of CB34 were carried out during the nights of December 8-9, 2000 using Omega Prime (Bizenberger et al. 1998) at the Calar Alto 3.5 m telescope. Omega Prime is equipped with a Rockwell $1024 \times 1024$ pixel HAWAII array detector. It provides a pixel scale of 0.4 arcsec/pixel scale and a total field of view of $6.8 \times 6.8$ arcmin² on the sky.

Images were obtained in the J, H and $K_{\rm s}$ broadband filters, with filter passbands centered at $\lambda$ = 1.275, 1.645, and 2.196 $\mu$m, respectively, as well as in the narrowband H₂ 1-0 S(1) filter centered at $\lambda$ = 2.125 $\mu$m and a width of 0.0206 $\mu$m. The detector integration time (DIT) was 1.677 s, and the total sum per "frame'' of 47 $\times$ 1.677s for all four filters was achieved by using the new Full MPIA read mode (Bizenberger et al. 1998). The total integration times were 1500s in J and H, 2000s in $K_{\rm s}$ and finally 3800s in the H₂ narrow band.

Images were obtained by positioning the globule in the center and in each of the four quadrants of the detector. The method avoids the explicit acquisition of sky frames and maximises the time spent on the object - see Cowie et al. (1990) and Stanke (2000) for a detailed description. Difference images obtained on a flat field screen with and without illumination by a halogen lamp were used to define the pixel-to-pixel response pattern of the detector. We identified hot and cold pixels using the flatfield frames taken each night. The bad pixels were flagged and excluded from processing during the following steps of data reduction.

Individual frames in each filter were passed through two stages of the DIMSUM package: First Pass and Mask Pass. During the First Pass sky subtraction was performed automatically by median filtering and object rejection using a 5$\sigma$ iterated rejection mechanism (Eisenhardt et al. 1996). This functions very well under the above observational strategy. Then, by applying a threshold algorithm to the ratio of the image and median filtered image, a cosmic ray mask was created and utilised. In the Mask Pass, the images were mosaiced and a wide field image obtained by choosing the reference star on the central frame and shifting and orientating the frames towards it. The thresholding for background detection and object masking were performed for the mosaiced picture.

Final mosaics of 8.8 $\times$ 8.8 arcmin² are obtained after the 2 runs. The results for J, H, $K_{\rm s}$ and H₂ are displayed in Figs. 14a-d, respectively.

The astrometric calibration was performed by use of stars from the Hubble Space Telescope guide star catalog (GSC) and the SIMBAD data base which were identified on our mosaics. We compare our images with those from 2MASS. There is a good positional accuracy of the order of 1 $\hbox {$^{\prime \prime }$ }$, which is similar to the positional errors in the SIMBAD and GSC data bases.

In the final stage of data reduction, the H₂ image was continuum subtracted. We took H₂(sub) = (H₂ $\times \,\,C$) - $K_{\rm s}$. Where the constant C was determined from the width of the $K_{\rm s}$ and H₂ filters and by eye examination of the subtracted image (in order to subtract the majority of field stars). From the subtracted image, H₂ excitation knots were revealed. Finally, an H₂ excitation mask was made (defined as 1 where H₂ excitation exist and 0 where not) which was multipled by the actual H₂ image in order to overlay it on the unsubtracted H₂ image. This distinguishes the H₂ excitation objects in Fig. 1.

Figure 1: The central part of CB34 in H2. Total integration of this frame is $\approx$3800 s. Crosses "+'' are SMM objects from Huard et al. (2000) and the "$\times$'' are HH objects and H2 knots discovered by Moreira & Yun (1995). CB34 VLA1 is marked by a "diamond'' (Yun et al. 1996). The YSOs NIR counterparts CB034YC1 and CB034YC1-I ("triangles'') are from Yun & Clemens (1994). Our discovered objects are indicated as chains of H and N letters as well as G and T. One more Q knot has been found - Q5.

2.2 H¹³CO⁺ and ¹²CO(2-1) observations

We obtained millimetre line maps in several molecular tracers of the central regions of CB34 using the IRAM 30 m telescope on Pico Veleta, Spain, on April 28th, 29th and 30th of 2001.

The observations were done using 4 receivers simultaneously, with the backends configured to observe the following lines: the ¹²CO(2-1) line (at 230.538GHz) with the A230 receiver and the autocorrelator as backend with a frequency resolution of 320kHz (velocity resolution of 0.42kms^-1); the ¹³CO(2-1) line (at 220.399GHz) with the B230 receiver and the autocorrelator (same resolution as for the ¹²CO(2-1) line); the ¹²CO(1-0) line (at 115.271GHz) with the B100 receiver and the autocorrelator (frequency resolution 320kHz corresponding to 0.83kms^-1); the SiO(2-1 v=0) line (at 86.847GHz) with the A100 receiver and the 1MHz backend (velocity resolution 3.5kms^-1); and finally the H¹³CO⁺(1-0) line (at 86.754GHz) also with the A100 receiver and the 100kHz backend (velocity resolution 0.35kms^-1).

The final maps consist of a number of smaller raster maps. Of those, the first map centred roughly on the north-eastern core containing SMM4 was taken on a 4 $\hbox {$^{\prime \prime }$ }$ grid covering 40 $\hbox {$^{\prime \prime }$ }$ $\times$ 40 $\hbox {$^{\prime \prime }$ }$ (i.e., fully sampled even for the high-frequency CO(2-1) lines; the beamsize at 230GHz is about 11 $\hbox {$^{\prime \prime }$ }$), the remaining maps were taken on 8 $\hbox {$^{\prime \prime }$ }$ or 10 $\hbox {$^{\prime \prime }$ }$ grids, i.e., fully sampled for the low-frequency data (the beamsize is about 27 $\hbox {$^{\prime \prime }$ }$ at 86GHz and 22 $\hbox {$^{\prime \prime }$ }$ at 115GHz), but undersampling the high-frequency data.

The data were flux calibrated using the standard hot/cold load calibration procedure at the 30 m. However, it should be noted that part of the data were taken in cloudy or foggy weather, thus the calibration (particularly for the high frequency CO(2-1) data) might be severely inaccurate for some of the data. We will therefore only use general morphological information rather than interpret the brightness of the features found in the data.

We present here only velocity channel maps of the ¹²CO(2-1) line and the H¹³CO⁺ line. No emission was detected in the SiO line (a shock tracer, e.g., Schilke et al. 1997); apparently the shocks in CB34 are not strong enough and too far away to produce detectable SiO emission.