BibSonomy
CiteUlike
Connotea
Del.icio.us
Digg
Facebook
A&A 486, L13-L16 (2008)
DOI: 10.1051/0004-6361:200810091
Letter
Optical polarimetry toward the Pipe nebula: revealing the importance of the magnetic field
F. O. Alves1, G. A. P. Franco2, and J. M. Girart11 Institut de Ciències de l'Espai (CSIC-IEEC), Campus UAB, Facultat de Ciències, C5 par 2, 08193, Bellaterra, Catalunya, Spain
e-mail: [oliveira;girart]@ieec.uab.es
2 Departamento de Física - ICEx - UFMG, Caixa Postal 702, 30.123-970 Belo Horizonte, Brazil
e-mail: franco@fisica.ufmg.br
Received 29 April 2008 / Accepted 3 June 2008
Abstract
Context. Magnetic fields are proposed to play an important role in the formation
and support of self-gravitating clouds and the formation and evolution of
protostars in such clouds.
Aims. We attempt to understand more precisely how the Pipe nebula is affected
by the magnetic field.
Methods. We use R-band linear polarimetry collected for about 12 000 stars in
46 fields with lines of sight toward the Pipe nebula to investigate the
properties of the polarization across this dark cloud complex.
Results. Mean polarization vectors show that the magnetic field is locally
perpendicular to the large filamentary structure of the Pipe nebula (the
"stem"), indicating that the global collapse may have been driven by ambipolar
diffusion. The polarization properties clearly change along the Pipe nebula.
The northwestern end of the nebula (B59 region) is found to have a low
degree of polarization and high dispersion in polarization position angle,
while at the other extreme of the cloud (the "bowl") we found mean degrees of
polarization as high as 
15% and a low dispersion in polarization
position angle. The plane of the sky magnetic field strength was
estimated to vary from about 17 
G in the B59 region to about 65 G
in the bowl.
Conclusions. We propose that three distinct regions exist, which may be related to
different evolutionary stages of the cloud; this idea is supported by both
the polarization properties across the Pipe and the estimated mass-to-flux
ratio that varies between approximately super-critical toward the B59 region
and sub-critical inside the bowl. The three regions that we identify are: the
B59 region, which is currently forming stars; the stem, which appears to be
at an earlier stage of star formation where material has been through a
collapsing phase but not yet given birth to stars; and the bowl, which
represents the earliest stage of the cloud in which the collapsing phase and
cloud fragmentation has already started.
Key words: ISM: clouds -- ISM: individual objects: Pipe nebula -- ISM: magnetic fields -- techniques: polarimetric
© ESO 2008