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First Magnetic Field Measurements in Bok GlobulesThomas Henning - Astrophysikalisches Institut und Universitäts-Sternwarte Jena, Germany, Ralf Launhardt - Division of Physics, Mathematics and Astronomy, California Institute of Technology, USA Rens Waters - Astronomical Institute, University of Amsterdam, The Netherlands
AbstractIn order to study the influence and structure of the magnetic field in the early phases of low-mass star formation, we obtained spatially resolved polarization maps of three Bok globules as part of a larger programme at a wavelength of 850 micron, using the Submillimeter Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT). We observed the following sources: CB 26 - a globule with a nearly dispersed dense core containing a source with a circumstellar disk, CB 54 - a deeply embedded young stellar cluster, and DC 253-1.6 (CG 30) - a protostellar double core.IntroductionMagnetic fields are an important factor in the star formation process (see, e.g., Greaves et al. 1999, McKee 1999, Mouschovias & Ciolek 1999, Shu et al. 1999). They can influence the contraction timescale, the gas-dust coupling, and the shape of cloud fragments. In the dusty envelopes around young stellar objects, polarization due to dichroic extinction and thermal emission by spinning dust grains is the most important signature of magnetic fields (see, e.g., Weintraub et al. 2000, Clemens & Kraemer 1999, Greaves et al. 1994). The dust grains become partially aligned with the magnetic field, generally with their long axes perpendicular to the field (see, e.g., Lazarian et al. 1997, Draine & Weingartner 1997). Thus, the thermal emission from grains at far infrared and millimeter wavelengths is partially linearly polarized, with a polarization direction perpendicular to the magnetic field as projected onto the plane of sky.Due to their relatively isolated location, Bok globules are well suited
to study the direct interplay between protostellar collapse, fragmentation,
and magnetic fields since they are less affected by strong turbulence and
other nearby star-forming events. The submillimeter continuum maps trace
mainly the dense cores which often consist of central condensations
unresolved in single-dish observations and their envelopes. The central
condensation can just represent the central dense and warm part of
the protostellar core, or an embedded unresolved circumstellar disk. The
observations are not sensitive to the low-density material along the path
length between the observer and the Bok globule. Thus, one
should be able to test the geometrical predictions of theoretical
models for star-forming clouds.
Magnetic field strengthsThe polarization maps of the Bok globules CB 26, CB 54, and DC 253-1.6 at 850 micron are shown in Figs. 1, 2, and 3. The mean percentage polarization degrees for CB 26, CB 54, and DC 253-1.6 are 7.3%, 5.1%, and 5.0%.
Following Chandrasekhar & Fermi (1953), the magnetic field strength
B [Gauss] can be derived from the gas density (
The magnetic field strengths we derived are comparable to those found
in molecular clouds (see, e.g., Bhatt & Jain 1992), pre-protostellar
cores (Levin et al. 2001), and other star-forming regions (see, e.g., Davis
et al. 2000, Glenn et al. 1999, Itoh et al. 1999, Minchin & Murray
1994, Chrysostomou et al. 1994, Crutcher 1999).
Further Results and ConclusionsFor the first time, we obtained spatially resolved submillimetre polarization maps of dense envelopes around the very high-density protostellar condensations in Bok globules. We observed the three objects CB 26, CB 54, and DC 253-1.6 and obtained polarization maps at 850 micron. Despite the fact that these Bok globules harbour a different number of embedded sources (CB 26: single source, DC 253-1.6: double core, CB 54: young stellar cluster and unresolved massive core) and show qualitatively different polarization patterns (CB 26, DC 253-1.6: aligned polarization vectors; CB 54: polarization vectors not aligned), we found the following similarities:
In the particular case of DC 253-1.6, we found for the first time that this source harbours a double core with a projected distance of about 4000 AU. The fact that the projected orientation of this possible binary system is oriented nearly perpendicular to the magnetic field direction projected onto the plane of the sky supports the hypothesis that the fragmentation process of a collapsing molecular core occurs perpendicular to the magnetic field lines. A preprint of the article (Henning et al. 2001)summarizing the results
is available here.
AcknowledgementsThis research was supported by the DFG grant Ste 605/10 within the program ``Physics of star formation'', the travel grant He 1935/19-1 of the DFG, and INTAS (Open Call 99/625). R. Launhardt acknowledges financial support through NFS grant AST 9981546. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation.References
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(P) are
shown.
[g/cm³]), the rms turbulence velocity (v [cm/s]) and the standard
deviation to the mean orientation angle of the polarization vectors (
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