Astronomy and Astrophysics – Astronomy
Scientific paper
May 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995adil...dr...04r&link_type=abstract
NCSA Astronomy Digital Image Library
Astronomy and Astrophysics
Astronomy
Absorption Spectra, Abundance, H Ii Regions, Hydrogen Atoms, Hydroxyl Radicals, Interstellar Magnetic Fields, Magnetic Field Configurations, Magnetic Flux, Molecular Clouds, Neutral Gases, Radio Observation, Shock Fronts, Velocity Distribution, Zeeman Effect
Scientific paper
VLA Zeeman observations of absorption lines of neutral hydrogen (H I) and the hydroxyl radical (OH) have been carried out toward the bipolar outflow H II region S106 at a resolution of about 20" with an rms noise of about 3 mJy/beam. The results of the observations are interpreted in terms of a model for the absorption line gas in which some of the OH and H I gas has been accelerated by the passage of a shock front and some of the gas lies outside the shock front at the undisturbed velocity of the molecular cloud. Two distinct OH absorption features are due to gas associated with S106. One component, which is identified with the unshocked gas, is narrow (delta-v = 1.2 to 2.9 km/s) and is observed at small negative velocities (v(LSR) = -2.4 to -1.0 km/s). This component shows a large peak in opacity at the eastern edge of the H II region near a previously observed molecular cloud core; the H2 mass of this component is estimated to be about 13 solar masses. Additionally, there is a wide (delta-v = 3.7 to 8.2 km/s) OH velocity component at a higher negative velocity (v(LSR) = -4.8 to -3.3 km/s) that is identified as shocked gas; the H2 mass in front of S106 from this component is about 4 solar masses. Our observations suggest an enhancement of the OH abundance in both velocity components by a factor of about 5 relative to dark clouds; the ratio N(OH)/N(H2) approximately 4 x 10(-7) and N(OH)/A(v) approximately 5 x 10(14). This increased abundance is likely due to the penetration of UV photons into the molecular cloud resulting in enhanced OH formation. A line-of-sight magnetic field (B(los)) has been determined for the two OH velocity components between 150 and 400 microGauss over the inner arcminute (0.18 pc) of the H II region. The most significant detection of B(los) in OH was 400 +/- 23 microGauss about 0.5 arcminutes (0.09 pc) northeast of the exciting source, IRS 4. Given simple assumptions about the geometry of the magnetic field and the mass distribution, the observed magnetic field appears to be near the critical value for magnetic support. Together with far IR and submm measurements of the direction of magnetic field in the plane of the sky, our Zeeman map suggests that the field may lie along the long axis of S106 at large scales and be twisted into a toroidal morphology near the central star, IRS 4. One of the H I velocity components appears to originate in shocked gas near the ionization front. This H I gas has a high negative radial velocity (v(LSR) = -9.8 to -5.8 km/s), has a large line width (delta-v = 3.9 to 11.6 km/s), and is believed to be from shocked gas associated with the outflow. B(los) approximately 70 +/- 12 microGauss for this velocity component near the center of the H II region. The small value of B(los) determined for H I relative to OH may be due to a tangling of a strong field near the ionization front on scales smaller than our synthesized beamsize. A weak, narrow component with a velocity about -2 km/s is observed in the eastern portion of the source; this is probably unshocked H I in the molecular cloud. B(los) in this unshocked component is estimated to be between 200 and 300 microGauss. The ratio of N(H I)/N(H2) approximately 2 x 10(-2) and 4 x 10(-2) for the unshocked and shocked components, respectively. The increased strength of the shocked H I component is probably due to an increased relative abundance of H I from the photodissociation of H2.
Crutcher Richard M.
Roberts Daniel A.
Troland Thomas H.
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