Astron. Astrophys. 353, L5-L8 (2000)

3. CO high-velocity bullets

Our CO observations of BD+30^o3639 are shown in the right hand panels of Fig. 1. The CO distribution is striking, with two discrete condensations on opposite sides of the central star. Note that the CO map contains essentially all (%) the flux seen in our single dish observations (Bachiller et al. 1991), so there is no significant emission from an extended component that is filtered out by the interferometer.

The spectra of the two condensations are also striking. The emission to the NE is centered at -60 km s^-1, whereas that to the SW is at +40 km s^-1. These velocities are roughly symmetric with respect to the systemic velocity of the ionized nebula (-13 km s^-1, Schneider et al. 1993), and imply expansion velocities of 50 km s^-1, about twice that of the bulk of the ionized gas and the H I envelope. In agreement with the single-dish spectra of Bachiller et al. (1991), no emission is detected in the velocity range from about -40 km s^-1 to +20 km s^-1.

On account of the discreteness and high expansion velocity of the CO condensations, we will refer to them as "bullets". Their double symmetry, in both position and velocity, shows that they are aligned in a narrowly collimated, bipolar distribution with respect to the central star. The bipolar axis lies at a P.A. of 22^o. This does not lie along the major axis of the nebula seen in the radio continuum at 2.6 mm (Fig. 1) or at cm wavelengths (where the P.A. is -80^o), so the bullet axis does not appear to be a principal axis of the nebula.

The detailed structure of the bullets is shown in the velocity channel maps of Fig. 2. The left panels show the blue shifted (approaching) emission, and the right panels show the red shifted (receding) emission; there is no CO in the channels within  km s^-1 of the systemic velocity, so these are not shown. Both CO bullets are seen to exhibit substructure. A certain degree of symmetry is also seen in the individual velocity channels, since the lower velocity gas (both blue and red shifted) lies closer to the star than that at higher velocity.

Fig. 2. Channel maps of the CO 2-1 emission in BD+30o3639. The channels are 3 km s-1 wide, with the central velocity of each one, in km s-1 with respect to the systemic velocity, indicated in each panel. They are arranged with the blue and red shifted channels in the left and right columns, respectively. No CO emission detected between 33 km s-1, so the corresponding channels are not shown. First contour and step are 5.25 K km s-1. The cross and coordinates are as in Fig. 1.

The angular distance of the bullets from the central star is which gives a projected distance of 0.025 pc at the adopted distance of 1.5 kpc (Kawamura & Masson 1996). Using the average expansion velocity, this corresponds to an expansion timescale for the bullets of 500 yr, assuming an inclination angle of 45^o. In fact the inclination angle cannot be derived from the present observations, so there is some uncertainty in the kinematic parameters.

The ratio of the intensities of the 2-1 to the 1-0 CO lines is 2, which implies that the emission is optically thin. We therefore estimate the mass of the bullets using the optically thin formula from Huggins et al. (1996). The mass of each bullet is found to be  , where the lower limit corresponds to full association of CO with an abundance of . This could be a small fraction ( 4%) of the mass of the ionized nebula (0.02 ), but if the CO abundance is lower than assumed (which is likely given the unusual conditions) the mass is correspondingly higher. The mass and size of the bullets imply a gas density in H₂ of  cm^-3.

© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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