4. Morphology of dust emission
Fig. 6 shows the 1300 µm contours of the remaining sources. Most of them display a structure consisting of a rather compact but resolved source embedded in a more diffuse environment. We have included in Table 1 the 1300 µm flux density within an 11 beam () centered on the peak of emission (column 8) as well as the total flux from the compact source () as obtained from the Gaussian fit (column 9). The ratio of both quantities reflects the central concentration of the emission, ranging from rather flat distributions with % (1548C27) to highly peaked sources with % (HL Tau). Neglecting the influence of a radial temperature gradient, C measures the degree of condensation of the circumstellar cloud into a more compact configuration, i.e. most probably into a disk.
The diffuse environment often attains a complicated structure, that does not resemble the shape of the compact component. The integrated 1300 µm flux within the lowest contour () is given with its uncertainty in Table 1(columns 10 and 11). Defining a new quantity , which ultimately measures the fraction of material left in the diffuse parental cloud, we find that D ranges from 0% to 85%. In a few sources like HL Tau or SR 24 the entire emission is confined to the compact component whereas in sources like HH 106-107 up to 85% of the flux originates from the extended region. Comparing the degree of concentration C of the inner component to the remaining diffuse cloud D of the source environment one finds that all combinations occur in the sample: There are 5 objects with a highly concentrated inner source ( %) embedded in a massive surrounding cloud ( %). There are several objects, which have a rather flat central source distribution ( %) with a relatively weak diffuse environment ( %). Four objects are without a diffuse environment at all.
Apart from HH 7-11, which appears to be a triple source, the compact components can be approximated by Gaussians, whose deconvolved major and minor axes a and b have been determined with a formal accuracy between 0.4 and 1.5 . This, and a possible elevation dependent change in the beam shape of the IRAM 30 m, which, however, should be always better than , leads us to introduce a safe limit of below which we consider the components to be of spherical shape. Ratios indicate an elongated structure. Whether this is due to additional fainter sources or to a real flattened dust distribution can not be distinguished with the present spatial resolution. It seems plausible, however, that contours which are asymmetric with respect to the minor axes are likely to indicate multiple embedded sources whereas symmetric contours are more in favour of a non-spherical dust configuration. With these remarks we note that there are 17 spherical and 10 elongated sources; the corresponding ratios are contained in Table 1 (column 7).
We have compared the orientation of the dust major axes for the 10 elongated sources with that of the corresponding optical HH flow. Even allowing for large errors in the determination of the orientation of the dust major axis we find no correlation at all, i.e. the relative angles of the flattened dust configurations and the HH flows are distributed between 0 and 90 degrees. This suggests that the collimation of the flows must occur on much smaller scales than what can be achieved with an 11 , beam.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998