T. H. Troland

The Millennium Arecibo 21 Centimeter Absorption-Line Survey. IV. Statistics of Magnetic Field, Column Density, and Turbulence

We discuss observations of the magnetic field, column density, and turbulence in the cold neutral medium (CNM). The observed quantities are only indirectly related to the intrinsic astronomical ones. We relate the observed and intrinsic quantities by relating their univariate and bivariate probability distribution functions (pdfs). We find that observations of the line-of-sight component of a magnetic field do not constrain the pdf of the total field Btot very well but do constrain the median value of Btot. In the CNM, we find a well-defined median magnetic field 6.0 ± 1.8 μG. The CNM magnetic field dominates thermal motions. Turbulence and magnetism are in approximate equipartition. We find that the probability distribution of column density N⊥(H ) in the sheets closely follows N⊥(H )-1 over a range of 2 orders of magnitude, 0.026 N⊥(H ) 2.6 × 1020 cm-2. The bivariate distributions are not well enough determined to constrain structural models of CNM sheets.

MAGNETIC FIELDS IN DARK CLOUD CORES: ARECIBO OH ZEEMAN OBSERVATIONS

We have carried out an extensive survey of magnetic field strengths toward dark cloud cores in order to test ambipolar-diffusion-drivenandturbulence-drivenmodelsof starformation.Thesurveyinvolved � 500hrof observing with the Arecibo telescope in order to make sensitive OH Zeeman observations toward 34 dark cloud cores. Nine new probable detections were achieved at the2.5 � level;the certainty of the detectionsvaries from solid to marginal, so we discuss each probable detection separately. However, our analysis includes all the measurements and does not depend on whether each position has a detection or just a sensitive measurement. Rather, the analysis establishes mean (or median) values over the set of observed cores for relevant astrophysical quantities. The results are that the mass-to-flux ratio is supercritical by � 2, and that the ratio of turbulent to magnetic energies is also � 2. These results are compatible with both models of star formation. However, these OH Zeeman observations do establish for thefirst time on astatistically soundbasis the energetic importance of magneticfieldsin dark cloudcores at densities of order 10 3 Y10 4 cm � 3 , and they lay the foundation for further observations that could provide a more definitive test.

CN Zeeman measurements in star formation regions

Aims. Magnetic fields play a primordial role in the star formation process. The Zeeman effect on the CN radical lines is one of the few methods of measuring magnetic fields in the dense gas of star formation regions. Methods. We report new observations of the Zeeman effect on seven hyperfine CN N = 1−0 lines in the direction of 14 regions of star formation. Results. We have improved the sensitivity of previous detections, and obtained five new detections. Good upper limits are also achieved. The probability distribution of the line-of-sight field intensity, including non-detections, provides a median value of the total field Btot = 0.56 mG while the average density of the medium sampled is n(H2) = 4.5 × 10 5 cm −3 . We show that the CN line probably samples regions similar to those traced by CS and that the magnetic field observed mostly pervades the dense cores. The dense cores are found to be critical to slightly supercritical with a mean mass-to-flux ratio M/Φ ∼ 1 to 4 with respect to critical. Their turbulent and magnetic energies are in approximate equipartition.

OH Zeeman observations of dark clouds

We have made measurements with the Green Bank 43 m telescope of the Zeeman effect in the 1665 and 1667 MHz lines of OH toward dark clouds. The typical 1 σ sensitivity was 3 μG. The only certain detection of a magnetic field was toward B1, for which we measured a line-of-sight component |B|cos θ=−19.1±3.9 μG. Comparison with our earlier measurement of the field toward B1 with the Arecibo telescope provided evidence for a 40% enhancement in field strength between the molecular envelope and core of the B1 cloud, which is consistent with quasi-static contraction of the cloud driven by ambipolar diffusion. Because the Zeeman effect is only sensitive to the line-of-sight component of the magnetic field, a statistical analysis of the detection and upper limits was necessary

TESTING MAGNETIC STAR FORMATION THEORY

Zeeman observations of molecular clouds yield the line-of-sight component B LOS of the magnetic vector B, which makes it possible to test the two major extreme-case theories of what drives star formation?ambipolar diffusion or turbulence. However, only one of the three components of B is measurable, so tests have been statistical rather than direct, and they have not been definitive. We report here observations of the Zeeman effect in the 18 cm lines of OH in the envelope regions surrounding four molecular cloud cores toward which detections of B LOS have been achieved in the same lines, and evaluate the ratio of mass-to-magnetic flux, M/?, between the cloud core and envelope. This relative M/? measurement reduces uncertainties in previous studies, such as the angle between B and the line of sight and the value of [OH/H]. Our result is that for all four clouds, the ratios of the core to the envelope values of M/? are less than 1. Stated another way, the ratios of the core to the total cloud M/? are less than 1. The extreme case or idealized (no turbulence) ambipolar diffusion theory of core formation requires the ratio of the central to total M/? to be approximately equal to the inverse of the original subcritical M/?, or 1

Detection of the CN Zeeman Effect in Molecular Clouds

SRC=http://ej.iop.org/images/0004-637X/692/1/844/apj296388ieqn3.gif/>. The probability that all four of our clouds have 1

A Magnetically Supported Photodissociation Region in M17

SRC=http://ej.iop.org/images/0004-637X/692/1/844/apj296388ieqn4.gif/> is 3 ? 10?7; our results are therefore significantly in contradiction with the hypothesis that these four cores were formed by ambipolar diffusion. Highly super-Alfv?nic turbulent simulations yield a wide range of relative M/?, but favor a ratio , as we observe. Our experiment is limited to four clouds, and we can only directly test the predictions of the extreme-case idealized models of ambipolar-diffusion driven star formation, which have a regular magnetic field morphology. Nonetheless, our experimental results are not consistent with the idealized strong field, ambipolar diffusion theory of star formation. Comparisons of our results with more realistic models and simulations that include both ambipolar diffusion and turbulence may help to refine our understanding of the relative importance of magnetic fields and turbulence in the star formation process.

OH Zeeman Magnetic Field Detections toward Five Supernova Remnants Using the VLA

Observations of the Zeeman effect in the 3 mm lines of CN have been carried out with the 30 m IRAM telescope toward seven dense molecular clouds. Detections were achieved toward the Orion Molecular Cloud 1 (OMC1), toward two cores in the DR21OH molecular cloud, and probably toward the M17SW molecular cloud. The line-of-sight magnetic field strengths inferred are Blos(OMC1)=-0.36±0.08, Blos(DR21OH1)=-0.36±0.10, Blos(DR21OH2)=-0.71±0.12, and Blos(M17SW)=-0.33±0.14 mG. The theoretical implications of these results are discussed.

The Millennium Arecibo 21 Centimeter Absorption-Line Survey. I. Techniques and Gaussian Fits

The southwestern (SW) part of the Galactic H II region M17 contains an obscured ionization front that is most easily seen at infrared and radio wavelengths. It is nearly edge-on, thus offering an excellent opportunity to study the way in which the gas changes from fully ionized to molecular as radiation from the ionizing stars penetrates into the gas. M17 is also one of the very few H II regions for which the magnetic field strength can be measured in the photodissociation region ( PDR) that forms the interface between the ionized and molecular gas. Here we model an observed line of sight through the gas cloud, including the H+, H0 (PDR), and molecular layers, in a fully self-consistent single calculation. An interesting aspect of the M17 SW bar is that the PDR is very extended. We show that the strong magnetic field that is observed to be present inevitably leads to a very deep PDR, because the structure of the neutral and molecular gas is dominated by magnetic pressure, rather than by gas pressure, as previously had been supposed. We also show that a wide variety of observed facts can be explained if a hydrostatic geometry prevails, in which the gas pressure from an inner X-ray hot bubble and the outward momentum of the stellar radiation field compress the gas and its associated magnetic field in the PDR, as has already been shown to occur in the Orion Nebula. The magnetic field compression may also amplify the local cosmic-ray density. The pressure in the observed magnetic field balances the outward forces, suggesting that the observed geometry is a natural consequence of the formation of a star cluster within a molecular cloud.

Physical Conditions in Orion's Veil

We have observed the OH (1720 MHz) line in five galactic SNRs with the VLA to measure their magnetic field strengths using the Zeeman effect. We detected all 12 of the bright (Sν > 200 mJy) OH (1720 MHz) masers previously detected by Frail et al. and Green et al. and measured significant magnetic fields (i.e., >3 σ) in 10 of them. Assuming that the thermal Zeeman equation can be used to estimate for OH masers, our estimated fields range from 0.2 to 2 mG. These magnetic field strengths are consistent with the hypothesis that ambient molecular cloud magnetic fields are compressed via the SNR shock to the observed values. Magnetic fields of this magnitude exert a considerable influence on the properties of the cloud with the magnetic pressures (10-7-10-9 ergs cm-3) exceeding the pressure in the ISM or even the thermal pressure of the hot gas interior to the remnant. This study brings the number of galactic SNRs with OH (1720 MHz) Zeeman detections to 10.