Magnetic Fields in Bok Globules: CCD Polarimetry of CB4 and CB17

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Session 16 -- Molecular Clouds
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[16.13] Magnetic Fields in Bok Globules: CCD Polarimetry of CB4 and CB17

Brian D. Kane (Boston University), Dan P. Clemens (Boston University)

We present detailed $V$-band polarimetric maps of CB4 and CB17, two small Bok globules showing no current signs of star formation. Sub-percent polarimetric accuracy down to 17th mag was achieved with a CCD-based imaging polarimeter employing bidirectional charge shifting and synchronous polaroid rotation. The narrowly peaked distributions of polarization position angles (for almost 100 stars in each globule) around the peripheries of these globules indicate ambient magnetic fields dominated by large-scale uniform components. Second, we investigate the field scale length(s) by employing a two-point correlation function of angular separation and polarization position angle difference for each pair of stars in our polarization samples. Third, we model and test the mean-field-subtracted residual position angles for pinching and twisting of the field directions in the near environs of these globules. For CB4, the correlation of differences of polarization position angle with different angular scales has a distinct peak at 0.4 arcmin separation. This favored scale is much smaller than any trend seen in radial distributions of the polarization position angle or percentage and, at an assumed CB4 distance of 600 pc, corresponds to a physical size of about 0.1 pc. This length, which can be interpreted as the peak wavelength in a spectrum of Alfv\'en waves, may also be the characteristic size of dust and molecular gas clumps in the globule, as is the case in CB63 (LBN11). Best model fits to the residual pattern of polarization position angles indicate that CB4 has, within errors, no pinching or twisting effect on the magnetic field passing through it.

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