Self-Consistent Analysis of OH Zeeman Observations

Crutcher, Hakobian, and Troland (2009) used OH Zeeman observations of four nearby molecular dark clouds to show that the ratio of mass to magnetic flux was smaller in the ~0.1 pc cores than in the ~1 pc envelopes, in contradiction to the prediction of ambipolar diffusion driven core formation. A crucial assumption was that the magnetic field direction is nearly the same in the envelope and core regions of each cloud. Mouschovias and Tassis (2009) have argued that the data are not consistent with this assumption, and presented a new analysis that changes the conclusions of the study. Here we show that the data are in fact consistent with the nearly uniform field direction assumption; hence, the original study is internally self-consistent and the conclusions are valid under the assumptions that were made. We also show that the Mouschovias and Tassis model of magnetic fields in cloud envelopes is inconsistent with their own analysis of the data. However, the data do not rule out a more complex field configuration that future observations may discern.Comment: 3 pages, 1 figure, accepted for publication by MNRAS Letter

Structure and Composition of Molecular Clouds with CN Zeeman Detections I: W3OH

We have carried out a multi-species study of a region which has had previous measurements of strong magnetic fields through the CN Zeeman effect in order to to explore the relationship between CN and N$_2$H$^+$, both of which have evidence that they remain in the gas phase at densities of 10$^5$ - 10$^6$ cm$^{-3}$. To achieve this we map the 1 arcmin$^2$ region around the UCHII region of W3(OH) using the Combined Array for Millimeter-wave Astronomy (CARMA). Approximately 105 hours of data were collected in multiple array configurations to produce maps with an effective resolution of $\sim$ 2.5\arcsec at high signal-to-noise in CN, C$^{18}$O, HCN, HCO$^+$, N$_2$H$^+$, and two continuum bands (91.2 GHz and 112 GHz). These data allow us to compare tracer molecules associated with both low and high density regions to infer gas properties. We determine that CARMA resolves out approximately 35% of the CN emission around W3(OH) when compared with spectra obtained from the IRAM-30 meter telescope. The presence of strong absorption lines towards the continuum source in three of the molecular transitions infers the presence of a cold, dark, optically thick region in front of the continuum source. In addition, the presence of high-velocity emission lines near the continuum source shows the presence of hot clumpy emission behind the continuum source. These data determine that future high-resolution interferometric CN Zeeman measurements which cannot currently be performed (due to technical limitations of current telescopes) are feasible. We confirm that CN is indeed a good tracer for high density regions; with certain objects such as W3(OH) it appears to be a more accurate tracer than N$_2$H$^+$.Comment: 33 pages, 16 figures. Accepted by Ap

Testing Magnetic Star Formation Theory

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/Phi, between the cloud core and envelope. This relative M/Phi 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 R of the core to the envelope values of M/Phi are less than 1. Stated another way, the ratios R' of the core to the total cloud M/Phi 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/Phi to be approximately equal to the inverse of the original subcritical M/Phi, or R' > 1. The probability that all four of our clouds have R' > 1 is 3 x 10^{-7}; our results are therefore significantly in contradiction with the hypothesis that these four cores were formed by ambipolar diffuson. Highly super-Alfvenic turbulent simulations yield a wide range of relative M/Phi, but favor a ratio R < 1, 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 that have a regular magnetic field morphology. Nonetheless, our experimental results are not consistent with the "idealized" strong field, ambipolar diffusion theory of star formation.Comment: 30 pages, 6 figures; paper revised after journal review, now accepted by Ap

Misalignment of magnetic fields and outflows in protostellar cores

We present results of λ1.3 mm dust-polarization observations toward 16 nearby, low-mass protostars, mapped with ∼2.″5 resolution at CARMA. The results show that magnetic fields in protostellar cores on scales of ∼1000 AU are not tightly aligned with outflows from the protostars. Rather, the data are consistent with scenarios where outflows and magnetic fields are preferentially misaligned (perpendicular), or where they are randomly aligned. If one assumes that outflows emerge along the rotation axes of circumstellar disks, and that the outflows have not disrupted the fields in the surrounding material, then our results imply that the disks are not aligned with the fields in the cores from which they forme

TADPOL: A 1.3 mm Survey of Dust Polarization in Star-forming Cores and Regions

We present {\lambda}1.3 mm CARMA observations of dust polarization toward 30 star-forming cores and 8 star-forming regions from the TADPOL survey. We show maps of all sources, and compare the ~2.5" resolution TADPOL maps with ~20" resolution polarization maps from single-dish submillimeter telescopes. Here we do not attempt to interpret the detailed B-field morphology of each object. Rather, we use average B-field orientations to derive conclusions in a statistical sense from the ensemble of sources, bearing in mind that these average orientations can be quite uncertain. We discuss three main findings: (1) A subset of the sources have consistent magnetic field (B-field) orientations between large (~20") and small (~2.5") scales. Those same sources also tend to have higher fractional polarizations than the sources with inconsistent large-to-small-scale fields. We interpret this to mean that in at least some cases B-fields play a role in regulating the infall of material all the way down to the ~1000 AU scales of protostellar envelopes. (2) Outflows appear to be randomly aligned with B-fields; although, in sources with low polarization fractions there is a hint that outflows are preferentially perpendicular to small-scale B-fields, which suggests that in these sources the fields have been wrapped up by envelope rotation. (3) Finally, even at ~2.5" resolution we see the so-called "polarization hole" effect, where the fractional polarization drops significantly near the total intensity peak. All data are publicly available in the electronic edition of this article.Comment: 53 pages, 37 figures -- main body (13 pp., 3 figures), source maps (32 pp., 34 figures), source descriptions (8 pp.). Accepted by the Astrophysical Journal Supplemen

Observational study of the role of magnetic fields in star formation

This project is a multi-faceted approach to establish a link between proposed theories of star formation and direct observation. Some key factors explored include: if magnetic fields cause significant support against gravitational collapse, which physical parameters are sampled by different tracer molecules, and what these results tell us about the structure and development of the regions observed. Until recently, only ambipolar diffusion theory had numerical models that simulated possible physical results that could be compared to observational data. These theories interpret the models in terms of a physical parameter: the ratio of the mass to the magnetic flux (M/$\Phi$). Performing measurements of the magnetic field, to determine the magnetic flux ($\Phi$), is complicated. It is only possible to obtain direct measurements of the strength of the line-of-sight component of the magnetic field through the normal Zeeman effect. Only a few molecules have Zeeman splitting factors large enough to be successfully used to measure magnetic fields. Of these few, OH traces lower density molecular species, while CN is believed to trace higher density regions. Using the Green Bank Telescope (GBT) we mapped the magnetic fields of cores and envelopes of dark cloud cores using OH as a tracer molecule. From this, the ratio of M/$\Phi$ between the cores and envelopes were computed, and were consistently determined to be 1. This study can be extended to other types of objects by using different tracer molecules. CN can be used to probe hot dense regions; however, it requires high resolution mapping currently only obtainable with an interferometer. Using CARMA, we obtained maps of 6 high mass star formation regions with a spatial resolution of approximately 2" by combining data from the C, D, and E arrays. CARMA's correlator was used to sample several spectral lines simultaneously in order to compare the structure of the CN emission with emission of other tracer molecules. We determined that CN is a good high density tracer which correlates well with other tracers such as HCO+ and HCN. From this, we concluded that these regions can be observed at high resolution and long integration times with an instrument capable of performing CN Zeeman measurements, when such an interferometer array becomes available. A study was additionally performed to apply the Li & Houde method of estimating magnetic field strengths from linewidth differences of ion and neutral molecular species. We were unable to replicate the previously published results; however, there were several differences in the datasets that may contribute to the non-detection of this effect. Several possible reasons for this discrepancy were determined, and further investigation may be able to determine whether this analysis technique holds significant merit

Observational study of the role of magnetic fields in star formation

This project is a multi-faceted approach to establish a link between proposed theories of star formation and direct observation. Some key factors explored include: if magnetic fields cause significant support against gravitational collapse, which physical parameters are sampled by different tracer molecules, and what these results tell us about the structure and development of the regions observed. Until recently, only ambipolar diffusion theory had numerical models that simulated possible physical results that could be compared to observational data. These theories interpret the models in terms of a physical parameter: the ratio of the mass to the magnetic flux (M/$\Phi$). Performing measurements of the magnetic field, to determine the magnetic flux ($\Phi$), is complicated. It is only possible to obtain direct measurements of the strength of the line-of-sight component of the magnetic field through the normal Zeeman effect. Only a few molecules have Zeeman splitting factors large enough to be successfully used to measure magnetic fields. Of these few, OH traces lower density molecular species, while CN is believed to trace higher density regions. Using the Green Bank Telescope (GBT) we mapped the magnetic fields of cores and envelopes of dark cloud cores using OH as a tracer molecule. From this, the ratio of M/$\Phi$ between the cores and envelopes were computed, and were consistently determined to be 1. This study can be extended to other types of objects by using different tracer molecules. CN can be used to probe hot dense regions; however, it requires high resolution mapping currently only obtainable with an interferometer. Using CARMA, we obtained maps of 6 high mass star formation regions with a spatial resolution of approximately 2" by combining data from the C, D, and E arrays. CARMA's correlator was used to sample several spectral lines simultaneously in order to compare the structure of the CN emission with emission of other tracer molecules. We determined that CN is a good high density tracer which correlates well with other tracers such as HCO+ and HCN. From this, we concluded that these regions can be observed at high resolution and long integration times with an instrument capable of performing CN Zeeman measurements, when such an interferometer array becomes available. A study was additionally performed to apply the Li & Houde method of estimating magnetic field strengths from linewidth differences of ion and neutral molecular species. We were unable to replicate the previously published results; however, there were several differences in the datasets that may contribute to the non-detection of this effect. Several possible reasons for this discrepancy were determined, and further investigation may be able to determine whether this analysis technique holds significant merit