Astron. Astrophys. 327, 1185-1193 (1997)

4. Masses and kinematics of the globules

We have made a ¹³ CO J=1-0 map with a FWHM beam of 44 arcsec towards the largest globule Thackeray 1 centered at 11:35:55.8, = -63:04:24, and at a number of positions find a double lined profile, with v_lsr of -20 and -25 km s . Fig. 6a shows the map of these two lines, and it is evident that we see two globules superposed, not a single large one. In the following we refer to Thackeray 1A for the larger globule at v_lsr of -20 km s , and Thackeray 1B for the smaller globule located towards south west at v_lsr of -25 km s . It is interesting to note that closer inspection of the optical image of Thackeray 1 in Fig. 3 indeed reveals what appears as two large globules superposed along the line-of-sight. The averaged ¹³ CO line profile from the central 9 pointings is shown in Fig. 6b, and it is evident that Thackeray 1A has a much larger velocity dispersion than Thackeray 1B.

Fig. 3. A detailed view of Thackeray 1 and surrounding globules. Note the bright rims of Thackeray 1, and how the globule appears to consist of two nearly overlapping components, to the N-E and to the S-W. The field is 2.1 x 3.0 arcmin, which at 1800 pc corresponds to 1.1 x 1.6 pc. North is up and east is left

Fig. 4. A detailed view of the smaller globules Thackeray 4, 5, 6 and 7. The field is 2.1 x 3.0 arcmin, which at 1800 pc corresponds to 1.1 x 1.6 pc. North is up and east is left

Fig. 5. A detailed view of the globules Thackeray 8, 9 and 10. The field is 2.1 x 3.0 arcmin, which at 1800 pc corresponds to 1.1 x 1.6 pc. North is up and east is left

We have also observed Thackeray 1 in the ¹² CO J=1-0 transition and the lowest spectrum in Fig. 6c shows the profile towards the center of the globule at (0,0). Again we see the same double lined profile. The peak antenna temperature of the CO emission is 5.2 K, which corresponds to an excitation temperature of 11 K, assuming the transition is optically thick and the globule fills the main beam.

Applying the same excitation temperature for the ¹³ CO emission, we can convert the integrated intensities into estimates of total ¹³ CO column densities according to

where is the temperature of the cosmic microwave background, is the Planck function and is the main beam efficiency given in Sect. 2 to convert from the antenna temperature scale to main beam brightness temperatures.

Summed over the whole map as shown in Fig. 6 the integrated antenna temperatures are 36 K km s and 13 K km s for the line components at -20 km s and -25 km s , respectively. For a distance of 1800 pc to the globule and an assumed [¹³ CO]/[H₂ ] abundance ratio of the resulting column densities correspond to total molecular masses of 11 M and 4 M , for Thackeray 1A and 1B, respectively.

Fig. 6. a A 13 CO map of Thackeray 1. The fully drawn contours are from the -20 km s line and the dotted line is emission at -25 km s . Levels start at 1 K km s in steps of 1 K km s of integrated antenna temperature. The dots mark observed positions. b  The average 13 CO antenna temperature line profile averaged from the central 9 points of the map. c  12 CO line profiles towards seven globules. The scale in units of antenna temperature is indicated.

We have also obtained ¹² CO spectra towards six other globules, as shown in Fig. 6c and at the positions listed in Table 1. All spectra show a faint emission at v_lsr = -16 km s , which is not detected in the ¹³ CO spectra towards Thackeray 1, and we assume it is from a low density fore- or background region unrelated to the globules. The surprising result is that the globules show a wide range in velocity, from -8 km s to -29 km s . This strongly suggests that the globules are not high-density condensations that originally existed inside a more tenuous cloud and is now exposed by the strong uv radiation field. Rather, the chaotic distribution of globules suggests that violent and highly dynamic processes are at play, and this is borne out by the kinematics of the complex.

Table 1. Positions of selected globules

If we make the assumption that, to first order, the 3.9 pc extent of the globule field is also representative of the depth of the complex, then the maximum radial velocity difference of 21 km s between the globules suggests a dynamic timescale of 180000 yr. Since the uv radiation and the expansion of the HII region, which are the main forces on the globules, is along the line-of-sight, the complex may actually stretch out so that it is deeper than it is wide. Also, most velocity differences are smaller than the observed maximum of 21 km s , plus these velocities were presumably much smaller earlier. Altogether, we are probably dealing with a timescale closer to 1 million years. This is comparable or slightly less than the age of the most massive stars in the newborn OB association, and thus suggests that the formation of the massive stars heralded the beginning of the demise of the globules.

The only mechanism that could realistically be invoked to create the observed large velocity differences is a Rayleigh-Taylor instability in an expanding dense shell pushed by the hot HII region. Seen in the restframe of the accelerating shell, the whole body of a Rayleigh-Taylor instability is in free fall towards the OB cluster, with a velocity gradient along the body and the tip having the largest velocity. Seen from the OB stars the tip of the elephant trunk is at rest or slowly moving away, while more distant parts of the trunk move away with gradually higher velocities (e.g. Spitzer 1954).

We have evidence that just such a kinematic behaviour is present in another elephant trunk like structure in IC 2944. To the northwest of the OB stars, at a projected distance of roughly 10 pc, there is a large dense region, seen in the upper right corner of Fig. 1. We have mapped this structure in ¹³ CO, and the resulting map is shown in Fig. 7. We additionally show the accumulated radial velocity differences for each map point relative to the front of the globule. We have chosen to display this radial velocity map with vectors pointing away from HD 101205. It is evident that we are seeing precisely the kinematic behavior expected for a Rayleigh-Taylor unstable cloud, and we believe that, given enough time, the dense structure will develop into a fullfledged elephant trunk, before eventually disintegrating. An observer behind the remnants of this future globule complex should see a structure very similar to the Thackeray's globules of today.

Fig. 7. A 13 CO map of the cometary structure seen on Fig. 1 just west of ESO H  302, which is marked "2" in the figure. Tickmarks are in arcminutes. Levels start at 2 K km s in steps of 2 K km s of integrated antenna temperature. There is a clear increase in radial velocity as one moves in the direction away from the O star HD 101205

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
helpdesk.link@springer.de