2. Observations and results
We searched for glycine line emission from IRAS16293, a well studied solar type protostar in the Ophiuchus complex (distance 120 pc: Knude & Hog 1998). The observations were performed on Mar 18, 2000 with the IRAM 30m telescope near Pico Veleta (Spain).
The two conformations of glycine with the lowest energies are called conformer I and II respectively, where conformer II is cm-1 higher in energy than conformer I (Lovas et al. 1995). Both conformers have numerous rotational transitions () in the radio to millimeter wavelength range (Pickett et al. 1998). Although higher in energy, conformer II has a dipole moment times larger than that of conformer I in a-type transitions (Brown et al. 1978 and Suenram & Lovas 1978). Finally, a-type transitions of conformer I have a slightly higher dipole moment than b-type transitions.
In our search for line emission from glycine we selected two observable frequency ranges: 216.305-216.815 GHz at 1.3 mm and 101.150-101.665 GHz at 3 mm. These bands include glycine lines expected to be among the strongest and whose upper level energies are relatively low ( cm-1), i.e. a-type transitions of conformer I. Besides, lines in the same band have similar level energies and line strengths, which makes it possible to lower the upper limit of the line intensity, averaging them together (see Discussion). Finally, in the selected bands, a few conformer II a-type lines are also present. The list of the glycine a-type transitions in the observed frequency ranges are reported in Table 1. In total, we have 9 a-type transitions from conformer I and 5 transitions from conformer II.
Table 1. List of glycine a-type transitions in the two observed frequency ranges. The last two columns report the frequencies and low level energy of each transition.
Two dual 1.3mm/3mm receivers were used simultaneously to observe the two frequency ranges. Each receiver was connected to a filterbank of 256 channels, each 1 MHz wide. The receivers were tuned in single side band and the image side band rejection was always higher than 10 dB. The system temperature throughout the observations was 120 K at 3 mm and 240 K at 1.3 mm.
We searched for glycine line emission from two positions: the center of IRAS16293, at (1950) = and (1950) = -, and a position 20" south, where there is a peak in the deuteration of formaldehyde (see below). The observations were performed in position switching mode with the OFF position located at = -180" and = 0 from the center of IRAS16293. The total usable ON+OFF integration times at each position were 50 minutes at 1.3 mm and 115 minutes at 3 mm, yielding rms's of about 10 mK per MHz channel at 1.3 mm and 4 mK at 3 mm. The beam sizes are 21" and 11" at 3 mm and 1.3 mm respectively.
Fig. 1 shows the observed spectra towards the center and 20" south of IRAS16293 at 1.3 mm and 3 mm respectively. No lines are seen where glycine transitions are expected, implying a 1 upper limit to their signal of Tv K km s-1 in the 1.3 mm band and K km s-1 in the 3 mm band respectively, assuming that the line is unresolved down to the resolution of the background spectrometer (1.4 km s-1 and 3.0 km s-1 at 1.3 mm and 3 mm respectively) 1.
It is worth noting the richness of the spectra in the central position (Fig. 1), which disappears in the 20" position. In the figure, we report tentative identifications of the detected lines, based on the coincidence of a transition with the line frequency. Obviously, mere coincidence does not constitute a definitive identification for the large molecules in the figure, as these molecules have multiple transitions in the millimeter range: more lines in other frequency bands are necessary to definitively identify them. Nevertheless, the many lines detected from the central position almost certainly reflect the presence of large molecules. Those molecules are not seen 20" south, in the cold and dense envelope surrounding IRAS16293 (Ceccarelli et al. 2000a,b). The 20" south position in particular was chosen because the doubly deuterated formaldehyde, shown to be % of the formaldehyde of this source (Ceccarelli et al. 1998; Loinard et al. 2000), has a peak emission in this position (Ceccarelli et al. in preparation). The absence of line emission from large molecules in the envelope may suggest that the large molecules are indeed formed in the interior of the envelope, probably in the 2" hot core probed by the H2O, SiO and H2CO line emission (Ceccarelli et al. 2000a,b). In principle, it may be due to different excitation conditions rather than to a different chemical composition: however, in LTE conditions, a change of temperature from 20 K to 100 K would imply a gain of a factor 3 only in the population of levels at cm-1, a factor smaller than the dilution factor given by the 2" hot core size (the dilution factor is in the 1.3 mm band and higher in the 3 mm band).
© European Southern Observatory (ESO) 2000
Online publication: October 30, 2000