Optical and submillimetre observations of Bok globules – tracing the magnetic field from low to high density

We present optical and submillimetre polarimetry data of the Bok globule CB3. We also present optical polarimetry and submillimetre continuum data of the Bok globule CB246. We use each set of polarimetry data to infer the magnetic field orientation in each of the clouds. The optical data can only trace the field orientation in the low density edge regions of clouds, because if the extinction is too high then no optical emission is transmitted. The submillimetre data can only trace the field orientation in the high density central regions of the clouds, because current submillimetre polarimeters are only sensitive to high column densities. It has previously been found that near-infrared polarisation mapping of background stars does not accurately trace the magnetic field in dense cloud regions, and hence that the grains responsible for near-infrared polarisation are under-represented in those regions. This may be due to a lack of aligned grains in dense regions. We test this by comparing the field orientations measured by our two independent methods of optical and submillimetre polarimetry. We find that the field orientation deduced from the optical data matches up well with the orientation estimated from the submillimetre data. We therefore claim that both methods are accurately tracing the same magnetic field in CB3. Hence, in this case, there must be significant numbers of aligned dust grains in the high density region, and they do indeed trace the magnetic field in the submillimetre. We find an offset of 40±14 degrees between the magnetic field orientation and the short axis of the globule. This is consistent with the mean value of 31±3 degrees found in our previous work on prestellar cores, even though CB3 is a protostellar core. CB246 is a prestellar core, and in this case the offset between the magnetic field orientation inferred from the optical polarisation data and the short axis of the core inferred from the submillimetre continuum data is 28±20 degrees. Taken together, the six prestellar cores that we have now studied in this way show a mean offset between magnetic field orientation and core short axis of ∼30±3 degrees, in apparent contradiction with some models of magnetically dominated star formation.