Observations of magnetic fields in diffuse clouds
P. C. Myers, A. A. Goodman, R. Gusten, C. Heiles;
ApJ, 1995, 442, 177
ABSTRACT:We report 32 statistically significant measurements of the line-of-sight component of the magnetic field strength, B(sub z), in four diffuse
clouds, via the Zeeman effect in the 21 cm line of H I.
The region near Magnani,
Blitz, and Mundy 1985 (MBM) 27-30 in the Ursa Major complex has B(sub z) greater
than 4 micro-G throughout a filamentary region 15 pc long, with significant
structure on scales as small as 1.6 pc.
The greatest field strength measured in
this cloud is 19 +/- 2 micro-G, greater than in most diffuse clouds by a factor
approximately 2.
Comparison of measurements with different telescopes suggests
that the field strength at the map peak may be significantly greater than 19
micro-G on scales smaller than 1.6 pc.
The magnetic and kinetic energy
densities M and K in this cloud are comparable, within a factor 2 of 2 x 10(exp
-11)ergs/cu cm, and greater than the gravitational energy density by a factor
approximately 500.
Among the four clouds surveyed, six positions where CO emission
is a local maximim have essentially the same mean line-of-sight field
strength, B(sub z) approximately = 8 micro-G, as do four positions where CO
emission is too weak to be detected.
The similarity of M and K in the diffuse
clouds discussed here, as well as in denser, self-gravitating clouds,
suggests strong coupling between magnetic fields and gas motions in some
interstellar clouds, independent of their self-gravity.
This coupling probably
arises from ion-neutral collisions, which allow propagation of MHD waves.
Cartographie du cirrus dans Ursa Major observ\'e \`a Penticton.
37 mesures Zeeman ont \'et\'e effectu\'ees. Le champ magn\'etique du
cirrus est $\sim 2$ fois plus \'ele\'ve que la normale. L'\'energie
cin\'etique et l'\'energie magn\'etique sont comparables. Le beam
permet de voir des structures jusqu'\`a 1.6 pc. On
affirme qu'il existe des structures a encore plus petite \'echelle.
On modelise le filament d'Ursa comme etant form\'e de 2 parties:
1) la portion HI-H2 constitu\'ee d'un coeur mol\'eculaire et
d'une enveloppe HI et 2) la portion HI pure. On compare les \'energies
cin\'etique et magn\'etique. Encore une fois ces
2 \'energies sont comparables malgr\'e le fait que le champ
magn\'etique soit particuli\`erement \'elev\'e. En effet, la
dispersion de vitesse et la densit\'e dans Ursa Major sont
particuli\`erement \'elev\'ees pour ce type de nuage.
KEYWORDS: h i regions, interstellar gas, interstellar magnetic fields, line of sight, molecular clouds, radio observation, turbulence, flux density, magnetic flux, magnetohydrodynamic waves, radio spectra, spectrum analysis, zeeman effect
PERSOKEY:ursa major, general ism, magnetic field, cirrus, h_i, 21 cm, ,
CODE: myers95