Astron. Astrophys. 347, 194-202 (1999)

3. Results

3.1. Line shapes

The discussion in this section, except in Sect. 3.5, is based on the spectra obtained towards the map centre position. The ¹²CO(21) line profile has the highest S/N-ratio in our data (see Fig. 1), and a significantly higher S/N-ratio than the line profiles presented by Loup et al. (1990), Trams et al. (1990), and van der Veen et al. (1993). The line profile is well described by a weakly double-peaked central part (the Intermediate Velocity Feature, IVF, covering the central 100) with extended line wings (the High Velocity Wings, HVWs), and two narrow (in a relative sense) features at the extremes of the line wings (the High Velocity Features, HVFs, occasionally referred to as bHVF and rHVF, where b and r indicate blue- and redshifted emission with respect to the systemic velocity, respectively). The line profile is close to symmetric around its centre, but the line wings become somewhat asymmetric at velocities beyond 80 of the centre velocity. The bHVF is also narrower (FWHM15) than the rHVF (FWHM20). The ¹²CO(10) line profile is very similar to the ¹²CO(21) line profile except that the central part may be more flat-topped. The S/N-ratio of our ¹²CO(10) spectrum is comparable to that obtained by van der Veen et al. (1993). The ¹³CO line profiles, in particular the high S/N-ratio ¹³CO(21) spectrum, clearly reveal an additional feature; a narrow component at the centre of the line profile (the Low Velocity Feature, LVF). Furthermore, the ¹³CO spectra have, in a relative sense, weaker extended line wings than the ¹²CO spectra, and the two horns seen in the central part of the ¹²CO(21) spectrum are not seen in the ¹³CO(21) line. When comparing with the spectra of van der Veen et al. (1993) we confirm the existence of their extreme blue- and redshifted features (features d and e in their nomenclature) and the inner peaks (features b and c ), but not the intermediate high-velocity features f and g , which we believe can be attributed to the limited S/N-ratio in their spectra. The central feature a is not prominent enough to be safely identified in our ¹²CO spectra, but it has the same characteristics as the LVF, which is clearly seen in the ¹³CO spectra.

3.2. Centre velocities

The horns of the IVF part of the ¹²CO(21) line profile appear at -42 and 882, i.e., they are symmetrically placed around a velocity of 423. The bHVF and rHVF have centre velocities of -921 and 1731, respectively, as determined by fits of Gaussians profiles, i.e., they are symmetrically placed around a velocity of 412. The LVF of the ¹³CO(21) line lies at 411 (as estimated by a fit of a Gaussian). Thus, we estimate a systemic velocity of 412 from the CO data (corresponding to a heliocentric velocity of 502). Based on this we find that the blue- and redshifted velocity-integrated intensities of the ¹²CO(21) line profile differ by 1% and those of the ¹³CO(21) line profile by 2%. That is, the emission is very symmetric with respect to the systemic velocity, which is also indicated by the close agreement between the intensity-weighted centre velocities and the estimated systemic velocity, see Table 2.

3.3. Line widths

Most likely none of the line components in the CO spectra originate in a symmetrically expanding circumstellar envelope, thus all inferred expansion velocities are projected velocities. The widths of the LVF in the ¹³CO lines correspond to an expansion velocity of 7. The full width at half intensity of the ¹³CO(21) IVF is 100, and the two horns in the IVF of the ¹²CO(21) profile are separated by 92 (i.e., an expansion velocity of 50 can be inferred). The HVWs probably extend to about 145 on each side of the systemic velocity, and the HVFs both expand with a (projected) velocity of 1322.

3.4. Line intensities and optical depths

In order to analyze the emission in more detail we have calculated the integrated intensity over seven velocity intervals: the LVF, the IVF regions on both sides of the LVF, the HVWs, and the HVFs, see Table 3. A measure of the uncertainties in the integrated intensities are obtained as 30, where is the peak-to-peak noise in spectra with a velocity resolution reduced to 30. Note that the integrated intensities in Table 3 do not necessarily come from the individual features, since there is considerable overlap in velocity space between these features, and we have made no attempt to separate them.

Table 3. Line intensities and line intensity ratios in seven velocity ranges

The CO emission is essentially unresolved at a resolution of 20". Thus, the high and nearly constant 21/10 ¹²CO intensity ratios, 3-4, over the entire velocity range is most likely attributed to beam dilution, assuming that the 10 and 21 line-emitting gas coincide (a not unreasonable assumption considering the similarity of the line profiles), rather than being an effect of low optical depths, in which case differences in the excitation conditions in the different regions should show up. The ¹³CO lines also have a fairly constant intensity ratio, only marginally higher than the ¹²CO ratio, 3-5, indicating relatively high optical depths also in these lines. There is a clear trend in the ¹²CO/¹³CO line intensity ratio in the sense that the emission in the LVF and IVF has a significantly lower ratio than the HVW emission; this applies especially to the LVF. The HVFs may also have a lower ratio (in particular, if one subtracts the line wing emission in this velocity range). This suggests lower optical depths in the high velocity emission.

3.5. The ¹²CO(21) brightness distribution

We will here concentrate on the ¹²CO(21) map data since they have a much higher quality than the ¹²CO(10) map data. We have produced brightness distribution maps in 20-intervals centred at -90, -70, ..., 170, Fig. 2. In each velocity interval the emission is unresolved with a HPBW=23". The maxima of the brightness distributions in the central part of the emission, -20 to 100, lie within a radius of 1" of each other, Fig. 3 [this is not an effect of the way we centred the maps with respect to each other; we will in the following discussions assume that the central star coincides in position with the (0,0)-position)]. However, for the higher-velocity emission maxima there is a clear trend: they are displaced with respect to the centre along a PA90^o, with the blueshifted emission to the W and the redshifted emission to the E in a very symmetric way. The separation between the HVFs is 9". Furthermore, the velocities are clearly (essentially linearly) increasing (in an absolute sense with respect to the centre velocity) with increasing offset, Fig. 4.

Fig. 2. 12CO(J=) brightness distributions in 20-intervals centred at -90, -70, ..., 170. The contours represent 10%, 20%, ..., 90% of the maximum intensity in each panel

Fig. 3. Positions of the 12CO(J=) brightness maxima, in 20-intervals centred at -90, -70, ..., 170, with respect to the position of the total brightness maximum

Fig. 4. Offsets of the 12CO(J=) brightness maxima, in 20-intervals centred at -90, -70, ..., 170, with respect to the position of the total brightness maximum [positive (negative) offset for a point lying on the east (west) side of a line with PA=0o]

© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999
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