Magnetic Fields in the dust cloud M17-SW from Extreme-Infrared (800 micron) polarimetry

Figure 1 shows the magnetic field results. The high angular resolution of the JCMT makes it more suitable for detecting the magnetic fields in the higher density gas (10(5) cm(-3)) nearer the central ridge line of the cloud, eg in denser cores. The proposed magnetic field lines across the M17-SW area are shown (long dashes). The six dust peaks P1 to P6 along a slightly curve ridge are shown within a light area (densest thermal gas density), and this ridge is surrounded by a darker area (medium gas density) and then by the cloud halo in the darkest area (lowest gas density). In the densest portion of M17-SW, the magnetic field directions are shown (thick bars with open circles at the six dust peaks P1 to P6). The lower angular resolution of the Kuiper Airborne Observatory (KAO) at 100 micron makes it more suitable for detecting the magnetic field in the lower density gas (10(3) cm(-3)) in more extended but less dense halo-type areas ~2' or 1.3 pc). Polarimetry of M17 has recently been obtained with the KAO by Dotson (1994, PhD Thesis, Univ. Chicago). In the less dense portion of M17-SW in Figure 1, the halo magnetic field directions are shown as thick bars with filled circles (adapted from the KAO Far-IR data). JCMT measurements at peaks P4, P3, P2, and P1 show a changing magnetic field orientation over a short distance, consistent with the nearly zero polarization detected there at KAO with their larger beam.

Figure 1: Magnetic field map of M17-SW - the proposed magnetic field lines across the M17-SW area are shown as long dashes (--). In the higher gas density areas, the thick bars show the magnetic field directions from our JCMT 800 micron data.

Theories. The role of magnetic fields in the evolution of molecular clouds and in regions of star formation is poorly understood. Many predicted strengths for the magnetic fields in molecular clouds have been published (eg, whether you assume magnetic equipartition or not). The magnetic field amplitude B cannot be deduced directly from the amplitude of the linear polarization, due to our incomplete knowledge of grains. We can use the total continuum emission I coming from dust particles to obtain the thermal gas density n, and the well-known statistical relation betweeen B and n, ie:

B(stat) = 0.4 x 10(-6) Gauss (n/cm(-3))**0.55

to yield an estimate of B within a factor of 5. Many predicted shapes for the magnetic fields in molecular clouds have been published (see below). Many predicted scales for the magnetic field have been made, ranging from 10 kpc down to 0.1 pc. We grouped the theoretical models for the magnetic fields in molecular clouds into eleven 'magnetic classes', according to 2 parameters: the shape and the scale of the magnetic field involved.

Figure 2 shows our cartoon-style drawings with the main features for each magnetic class, as described further below. Class A: Cloud B-vectors are parallel to the galactic plane. Class B: B-vectors follow a U-shaped bowl, coming from the galactic halo and being parallel to the galactic plane at the bottom of the bowl. Class C: Cloud B-vectors follow the cloud elongation, except near the edges of the cloud where B-vectors enter and leave in a Y-shape fashion (taken at ±40 degrees from the cloud elongation). Class D: Cloud B-vectors are perpendicular to the regional magnetic field (over 100 pc). Class E: Cloud B-vectors are parallel to the regional magnetic field (over 100 pc). Class F: B-vectors in clump centres parallel to cloud elongation, with pinching effects of magnetic field lines in clump edges (X-shaped). Class G: Cloud B-vectors are locally perpendicular to cloud elongation. Class H: B-vectors in clumps are perpendicular to cloud elongation, and B-vectors in-between clumps are aligned along cloud elongation. Class I: B-vectors in clumps are parallel to cloud elongation, and B-vectors in-between clumps are aligned perpendicular to cloud elongation. Class J: Cloud's B-vectors are skewed -20 degrees from the direction of cloud elongation. Class K: B-vectors randomly oriented in clumps.

Comparisons. We defined two 'Acceptance Criteria', to compare model predictions to polarization observations, one on the differences between the observed B-vector PA (ie OB) and the predicted B-vector PA (ie PB), ie (OB - PB) should be ~<13 degrees, and one on the standard deviation of the mean, sdm, should be ~<20 degrees. Results: While four of the 11 magnetic classes of models satisfy the first criterion (means), and six of the 11 magnetic classes of models satisfy the second criterion (sdm), only two of the 11 magnetic classes of models can satisfy both criteria simultaneously (Classes E and G). The two successful classes share in common the idea of an environmental magnetic field being perpendicular to the cloud elongation. This orientation has implications on star forming activities, favoring the formation of a large elongated disk (> 2000 AU) or cocoon, with a disk elongation perpendicular to the cloud magnetic field (and thus a disk elongation parallel to the cloud elongation).

Figure 2: Cartoon-style drawings showing the main features of each 'magnetic class', described further in the text.

Jacques P. Vallée, Institut Herzberg d'Astrophysique, Conseil National de Recherches du Canada, 5071 West Saanich Rd., Victoria, BC, Canada V8X 4M6

jacques.vallee@hia.nrc.ca

& Pierre Bastien, Dépt. de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, Canada H3C 3J7

bastien@physcn.umontreal.ca