CO Observations of the -Orionis Ring

Interpretation

Total Mass

We can make a direct estimate of the total molecular mass of the Ring via the relation M XdS, where d is the distance and S is W summed over the angular extent of the cloud. X here is the standard conversion factor N/W, and we use the value of X=(1.060.14) cm(K km s) derived for the Orion region by Digel et al. (1995). This is considerably lower than the 2.6 cm(K km s) used by MM. Therefore, even though we have detected a substantial amount more CO-emission than MM, our mass estimate is slightly lower at  1.110 M.

Kinematics

We consider a simple expanding shell model to fit to the clump peaks. Using code originally designed to model circumstellar masers (Yates and Cohen 1994), we obtain a reasonably good numerical fit to a model in which the shell expands at 16.5 km s from datacube position (v,l,b)=(3.8, 195.8, -12.1), giving a present day radius of 30pc and an expansion age of 1.8 Myrs, younger than the 2.3 Myrs derived by MM. This model shell closely follows the IRAS dust ring, unlike the MM shell, which is strongly emission biased towards their western clouds MM 2,3,6 and 7, which may not be involved in the HII region expansion.

The assumption of spherical expansion is far from adequate, however, and these preliminary results should be treated with caution. Velocity maps suggest that we are dealing with two flattened cloud complexes. Further work must be undertaken using expanding ring models, for example. We must also be careful which clumps to include in model fits. Only those directly interacting with the HII region should be used. IRAS and H images can help determine which parts of the region fall into this category. Diffuse H images (Isobe 1973, Gaustad 1997) clearly show that the southern B223 complex, in contrast to the rest of the Ring, has its optical bright rims facing away from the OB-association. The bulk of the cloud is slightly in front of the HII region, and care should be taken in drawing conclusions from its kinematics, which could well be dominated by other influences, the expansion of the Ori-Eri Bubble for example.

© Copyright Astronomical Society of Australia 1997