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While magnetic fields likely play an important role in driving the evolution of protoplanetary disks through angular momentum transport, observational evidence of magnetic fields has only been found in a small number of disks. Although dust continuum linear polarization has been detected in an increasing number of disks, its pattern is more consistent with that from dust scattering than from magnetically aligned grains in the vast majority of cases. Continuum linear polarization from dust grains aligned to a magnetic field can reveal information about the magnetic field's direction, but not its strength. On the other hand, observations of circular polarization in molecular lines produced by Zeeman splitting offer a direct measure of the line-of-sight magnetic field strength in disks. We present upper limits on the net toroidal and vertical magnetic field strengths in the protoplanetary disk AS 209 derived from Zeeman splitting observations of the CN 2-1 line. The 3$\sigma$ upper limit on the net line-of-sight magnetic field strength in AS 209 is 5.0 mG on the redshifted side of the disk and 4.2 mG on the blueshifted side of the disk. Given the disk's inclination angle, we set a 3$\sigma$ upper limit on the net toroidal magnetic field strength of 8.7 and 7.3 mG for the red and blue sides of the disk, respectively, and 6.2 and 5.2 mG on the net vertical magnetic field on the red and blue sides of the disk. If magnetic disk winds are a significant mechanism of angular momentum transport in the disk, magnetic fields of a strength close to the upper limits would be sufficient to drive accretion at the rate previously inferred for regions near the protostar., Comment: June 22, 2021: Corrected disk size scale bar in Figures 1, 2 and 5. Corrected percent polarization scale bar label in Figure 5

(Edited for length) The Zeeman effect is the only observational technique available to measure directly the strength of magnetic fields in regions of star formation. We review the physics of the Zeeman effect and its practical use in both extended gas and in masers. We discuss observational results for the five species for which the Zeeman effect has been detected in the ISM -- H~I, OH, and CN in extended gas and OH, CH$_3$OH, and H$_2$O in masers. These species span densities from $\sim10$ cm$^{-3}$ to $\sim10^{10}$ cm $^{-3}$, which allows magnetic fields to be measured over the full range of cloud densities. However, there are significant limitations, including that only the line-of-sight component of the magnetic field strength can usually be measured and that there are often significant uncertainties about the physical conditions being sampled, particularly for masers. We discuss statistical methods to partially overcome these limitations. The results of Zeeman observations are that the mass to magnetic flux ratio is subcritical (gravity dominates magnetic support) at lower densities but supercritical for $N_H \gtrsim 10^{22}$ cm$^{-2}$. Above $n_H\sim 300$ cm$^{-3}$, which is roughly the density at which clouds typically become self-gravitating, the strength of magnetic fields increases approximately as $B \propto n^{2/3}$, which suggest that magnetic fields do not provide significant support at high densities. This is consistent with high-density clouds being supercritical. However, magnetic fields have a large range in strengths at any given density, so the role of magnetic fields should differ significantly from one cloud to another. And for maser regions the dependence of field strength on density may have a slightly lower slope. Turbulent reconnection theory seems to best match the Zeeman observational results., 33 pages, 6 figures

Here we present new ALMA observations of polarized dust emission from six of the most massive clumps in W43-Main. The clumps MM2, MM3, MM4, MM6, MM7, and MM8, have been resolved into two populations of fragmented filaments. From these two populations we extracted 81 cores (96 with the MM1 cores) with masses between 0.9 \Msun\ to 425 \Msun\ and a mass sensitivity of 0.08 M$_{\odot}$. The MM6, MM7, and MM8 clumps show significant fragmentation, but the polarized intensity appears to be sparse and compact. The MM2, MM3, and MM4 population shows less fragmentation, but with a single proto-stellar core dominating the emission at each clump. Also, the polarized intensity is more extended and significantly stronger in this population. From the polarized emission, we derived detailed magnetic field patterns throughout the filaments which we used to estimate field strengths for 4 out of the 6 clumps. The average field strengths estimations were found between 500 $\mu$G to 1.8 mG. Additionally, we detected and modeled infalling motions towards MM2 and MM3 from single dish HCO$^{+}(J=4 \rightarrow 3)$ and HCN$(J=4 \rightarrow 3)$ data resulting in mass infall rates of $\dot{\mathrm{M}}_{\mathrm{MM2}} = 1.2 \times 10^{-2}$ \Msun\ yr$^{-1}$ and $\dot{\mathrm{M}}_{\mathrm{MM3}} = 6.3 \times 10^{-3}$ \Msun\ yr$^{-1}$. By using our estimations, we evaluated the dynamical equilibrium of our cores by computing the total virial parameter $\alpha_{\mathrm{total}}$. For the cores with reliable field estimations, we found that 71\% of them appear to be gravitationally bound while the remaining 29\% are not. We concluded that these unbound cores, also less massive, are still accreting and have not yet reached a critical mass. This also implies different evolutionary time-scales, which essentially suggests that star-formation in high mass filaments is not uniform., Comment: Accepted for Publication in the Astrophysical Journal

We report new polarimetric and photometric maps of the massive star-forming region OMC-1 using the HAWC+ instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA). We present continuum polarimetric and photometric measurements of this region at 53, 89, 154, and 214 microns at angular resolutions of 5.1, 7.9, 14.0, and 18.7 arcseconds for the four bands, respectively. The photometric maps enable the computation of improved SEDs for the region. We find that at the longer wavelengths, the inferred magnetic field configuration matches the `hourglass' configuration seen in previous studies, indicating magnetically-regulated star formation. The field morphology differs at the shorter wavelengths. The magnetic field inferred at these wavelengths traces the bipolar structure of the explosive Becklin-Neugebauer (BN)/Kleinman-Low (KL) outflow emerging from OMC-1 behind the Orion Nebula. Using statistical methods to estimate the field strength in the region, we find that the explosion dominates the magnetic field near the center of the feature. Farther out, the magnetic field is close to energetic equilibrium with the ejecta and may be providing confinement to the explosion. The correlation between polarization fraction and the local polarization angle dispersion indicates that the depolarization as a function of unpolarized intensity is a result of intrinsic field geometry as opposed to decreases in grain alignment efficiency in denser regions., Comment: 28 pages, 14 figures, ApJ, accepted

Observations of magnetic fields in molecular clouds are essential for understanding their role in the evolution of dense clouds and in the star formation process. The two extreme-case models of star formation are the weak magnetic field scenario, in which turbulence drives molecular cloud evolution and therefore star formation, and the strong magnetic field scenario, with magnetic fields governing molecular cloud formation and ambipolar diffusion ultimately driving star formation. Continuum and spectral line polarization observations make it possible to infer magnetic field strengths and morphologies, and therefore to test which of the two star formation scenarios dominates. The observational techniques available for such observations – the Zeeman effect, mapping of polarized dust emission, and mapping of linearly polarized line emission – are briefly described, together with how these observations can test star formation theory. Examples of the existing observations and the current state of the testing of magnetically controlled star formation are presented. Finally, the prospects for the future of study of magnetic fields in molecular clouds and their role in the star formation process is discussed.

Polarimetry at far-infrared wavelengths is a key tool for studying physical processes on size scales ranging from interstellar dust grains to entire galaxies. A multi-wavelength continuum polarimeter at these wavelengths will allow studies of thermal dust polarization in an effort to constrain the grains’ physical properties and test grain alignment theory. High spatial resolution (5–30 arcsec) and sensitive observations will measure the influence of magnetic fields on infrared cirrus clouds, the envelopes and disks of YSOs, outflows from both low- and high-mass star forming regions, and the relative strength of magnetic, gravitational, and turbulent effects in star- and cloud-formation.

The only direct measurements of interstellar magnetic field strengths depend on the Zeeman effect, which samples the line-of-sight component B{sub z} of the magnetic vector. In this paper, we use a Bayesian approach to analyze the observed probability density function (PDF) of B{sub z} from Zeeman surveys of H I, OH, and CN spectral lines in order to infer a density-dependent stochastic model of the total field strength B in diffuse and molecular clouds. We find that at n 300 cm{sup -3}, with an uncertainty at the 50% level in the power-law exponent of about {+-}0.05. This break-point density could be interpreted as the average density at which parsec-scale clouds become self-gravitating. Both the uniform PDF of total field strengths and the scalingmore » with density suggest that magnetic fields in molecular clouds are often too weak to dominate the star formation process. The stochasticity of the total field strength B implies that many fields are so weak that the mass/flux ratio in many clouds must be significantly supercritical. A two-thirds power law comes from isotropic contraction of gas too weakly magnetized for the magnetic field to affect the morphology of the collapse. On the other hand, our study does not rule out some clouds having strong magnetic fields with critical mass/flux ratios.« less

We have used the Berkeley-Illinois-Maryland Array (BIMA) to make deep N2H+ 1-0 maps of the pre-stellar core L183, in order to study the spatial and kinematic substructure within the densest part of the core. Three spatially and kinematically distinct clumps are detected, which we label L183-N1, L183-N2 and L183-N3. L183-N2 is approximately coincident with the submillimetre dust peak and lies at the systemic velocity of L183. Thus we conclude that L183-N2 is the central dense core of L183. L183-N1 and 3 are newly-discovered fragments of L183, which are marked by velocity gradients that are parallel to, but far stronger than, the velocity gradient of L183 as a whole, as detected in previous single-dish data. Furthermore, the ratio of the large-scale and small-scale velocity gradients, and the ratio of their respective size-scales, are consistent with the conservation of angular momentum for a rotating, collapsing core undergoing spin-up. The inferred axis of rotation is parallel to the magnetic field direction, which is offset from its long axis, as we have seen in other pre-stellar cores. Therefore, we propose that we have detected a fragmenting, collapsing, filamentary, pre-stellar core, rotating about its B-field, which is spinning up as it collapses. It will presumably go on to form a multiple protostellar system., 7 figures, 1 table, 21 pages, accepted for publication in ApJ

We have carried out OH Zeeman observations toward a position at the edge of a Taurus molecular cloud where a continuum polarization minimum and excessive Faraday rotation measure were reported by Wolleben & Reich in 2004. They had interpreted this result as requiring a line-of-sight magnetic field strength in excess of 20 μG. Our Zeeman measurement failed to detect a field consistent with this result.

We have carried out an extensive survey of magnetic field strengths toward dark cloud cores in order to test models of star formation: ambipolar-diffusion driven or turbulence driven. The survey involved $\sim500$ hours of 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 the 2.5-sigma level; the certainty of the detections varies 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 $\sim 2$, and that the ratio of turbulent to magnetic energies is also $\sim 2$. These results are compatible with both models of star formation. However, these OH Zeeman observations do establish for the first time on a statistically sound basis the energetic importance of magnetic fields in dark cloud cores at densities of order $10^{3-4}$ cm$^{-3}$, and they lay the foundation for further observations that could provide a more definitive test., Comment: 22 pages, 2 figures, 2 tables

We present a 3 mm spectral line and continuum survey of L1451 in the Perseus Molecular Cloud. These observations are from the CARMA Large Area Star Formation Survey (CLASSy), which also imaged Barnard 1, NGC 1333, Serpens Main and Serpens South. L1451 is the survey region with the lowest level of star formation activity---it contains no confirmed protostars. HCO+, HCN, and N2H+ (J=1-0) are all detected throughout the region, with HCO+ the most spatially widespread, and molecular emission seen toward 90% of the area above N(H_2) column densities of 1.9x10^21 cm^-2. HCO+ has the broadest velocity dispersion, near 0.3 km/s on average, compared to ~0.15 km/s for the other molecules, thus representing a range from supersonic to subsonic gas motions. Our non-binary dendrogram analysis reveals that the dense gas traced by each molecule has similar hierarchical structure, and that gas surrounding the candidate first hydrostatic core (FHSC), L1451-mm, and other previously detected single-dish continuum clumps have similar hierarchical structure; this suggests that different sub-regions of L1451 are fragmenting on the pathway to forming young stars. We determined the three-dimensional morphology of the largest detectable dense gas structures to be relatively ellipsoidal compared to other CLASSy regions, which appeared more flattened at largest scales. A virial analysis shows the most centrally condensed dust structures are likely unstable against collapse. Additionally, we identify a new spherical, centrally condensed N2H+ feature that could be a new FHSC candidate. The overall results suggest L1451 is a young region starting to form its generation of stars within turbulent, hierarchical structures., Comment: Accepted to The Astrophysical Journal (ApJ), 45 pages, 24 figures (some with reduced resolution in this preprint); Project website is at http://carma.astro.umd.edu/classy

Here we present the first results from ALMA observations of 1 mm polarized dust emission towards the W43-MM1 high mass star forming clump. We have detected a highly fragmented filament with source masses ranging from 14Msun to 312Msun, where the largest fragment, source A, is believed to be one of the most massive in our Galaxy. We found a smooth, ordered, and detailed polarization pattern throughout the filament which we used to derived magnetic field morphologies and strengths for 12 out of the 15 fragments detected ranging from 0.2 to 9 mG. The dynamical equilibrium of each fragment was evaluated finding that all the fragments are in a super-critical state which is consistent with previously detected infalling motions towards W43-MM1. Moreover, there are indications suggesting that the field is being dragged by gravity as the whole filament is collapsing., Comment: Accepted for publication is Astrophysical Journal Letters

We present high-resolution observations of G5.89-0.39 in CO(1 → 0), 13CO(1 → 0), C18O(1 → 0), and HCO+(1 → 0). We characterize the G5.89-0.39 outflow using the 13CO emission. The outflow is found to be young, massive, and powerful. We conclude that (1) the outflow is nearly along the line of sight, (2) there is dynamical evidence for entrainment of ambient interstellar material into the outflow, (3) the mass entrainment rate is ~4 × 10-3 M☉ yr-1, (4) in the blue lobe, only ~27% of the outflow mass is due to entrainment, and (5) expansion of the outflow lobes perpendicular to the flow axis is occurring at ~1-10 vsound. A neutral and ionized outflow tracer are compared. Watson and coworkers predicted that if entrainment through the Kelvin-Helmholtz shear instability adds significantly to the outflow mass, a difference in the turbulent velocity widths of neutral and ionized outflow tracers could be measurable. We cannot conclude that the Kelvin-Helmholtz shear instability is the physical process causing this entrainment, but it may operate at a level below our detection limit.

Observations of the Zeeman effect in OH and H2O masers provide valuable information about magnetic field strength and direction, but only for the very high density gas in which such masers are found. In order to understand the role of magnetic fields in the evolution of the interstellar medium and in the star formation process, it is essential to consider the maser results in the broader context of magnetic fields in lower density gas. This contribution will (very briefly) summarize the state of observational knowledge of magnetic fields in the non-masing gas. Magnetic fields in H I and molecular clouds may be observed via the Zeeman effect, linear polarization of dust emission, and linear polarization of spectral-line emission. Useful parameters that can be inferred from observations are the mass-to-flux ratio and the scaling of field strength with density. The former tells us whether magnetic fields exert sufficient pressure to provide support against gravitational contraction; the latter tells whether or not magnetic fields are sufficiently strong to determine the nature (spherical or disk geometry) of the contraction. Existing observations will be reviewed. Results are that the strength of interstellar magnetic fields remains roughly invariant at 5-10 microgauss between densities of 0.1 cm−3 < n(H) < 1,000 cm−3 but increases proportional to approximately the square root of density at higher densities. Moreover, the mass-to-flux ratio is significantly subcritical (strong magnetic support with respect to gravity) in diffuse H I clouds that are not self-gravitating, but becomes approximately critical in high-density molecular cloud cores. This suggests that MCs and GMCs form primarily by accumulation of matter along magnetic field lines, a process that will increase density but not magnetic field strength. How clumps in GMCs evolve will then depend crucially on the mass-to-flux ratio in each clump. Present data suggest that magnetic fields play a very significant role in the evolution of molecular clouds and in the star formation process.

We present polarization maps of NGC2071IR from thermal dust emission at 1.3 mm and from CO J=$2 \to 1$ line emission. The observations were obtained using the Berkeley-Illinois-Maryland Association array in the period 2002-2004. We detected dust and line polarized emission from NGC2071IR that we used to constrain the morphology of the magnetic field. From CO J=$2 \to 1$ polarized emission we found evidence for a magnetic field in the powerful bipolar outflow present in this region. We calculated a visual extinction $A_{\rm{v}} \approx 26$ mag from our dust observations. This result, when compared with early single dish work, seems to show that dust grains emit polarized radiation efficiently at higher densities than previously thought. Mechanical alignment by the outflow is proposed to explain the polarization pattern observed in NGC2071IR, which is consistent with the observed flattening in this source., Comment: 17 pages, 4 Figures, Accepted for publication in ApJ

Although the subject of this meeting is triggered star formation in a turbulent interstellar medium, it remains unsettled what role magnetic fields play in the star formation process. This paper briefly reviews star formation model predictions for the ratio of mass to magnetic flux, describes how Zeeman observations can test these predictions, describes new results – an extensive OH Zeeman survey of dark cloud cores with the Arecibo telescope, and discusses the implications. Conclusions are that the new data support and extend the conclusions based on the older observational results – that observational data on magnetic fields in molecular clouds are consistent with the strong magnetic field model of star formation. In addition, the observational data on magnetic field strengths in the interstellar medium strongly suggest that molecular clouds must form primarily by accumulation of matter along field lines. Finally, a future observational project is described that could definitively test the ambipolar diffusion model for the formation of cores and hence of stars.

We present polarization maps of G30.79 FIR 10 (in W43) from thermal dust emission at 1.3 mm and from CO J=$2 \to 1$ line emission. The observations were obtained using the Berkeley-Illinois-Maryland Association array in the period 2002-2004. The G30.79 FIR 10 region shows an ordered polarization pattern in dust emission, which suggests an hourglass shape for the magnetic field. Only marginal detections for line polarization were made from this region. Application of the Chandrashkar-Fermi method yielded $B_{pos} \approx 1.7$ mG and a statistically corrected mass to magnetic flux ratio $\lambda_{C} \approx 0.9$, or essentially critical., Comment: 11 pages, 2 Figures, Published in ApJ

We present Very Large Array (VLA) observations of 21 cm H I absorption in the high-velocity (HV) system toward the external galaxy NGC 1275 (Per A). The primary motivation for our observations is to examine whether the only existing (single-dish) detection of the magnetic field in an external galaxy will stand up to independent scrutiny with an interferometer. The H I absorption profile in the HV system toward NGC 1275 has a number of velocity components. In this paper, we focus on the prominent narrow component at 8114 km s-1, in which we have detected the Zeeman effect. The measured line-of-sight magnetic field is Blos = -21.5 ± 6.5 μG. Within the errors, this is consistent with the single-dish detection. A weighted average of the single-dish and VLA detections gives a value for the line-of-sight magnetic field of Blos = -10.7 ± 3.2 μG. The derived ratio of total internal kinetic energy to gravitational energy suggests that the clouds responsible for H I absorption are likely not gravitationally bound. The ratio of magnetic energy to total internal kinetic energy indicates that magnetic effects are likely dominant in these clouds. Finally, the measured ratio of mass to magnetic flux is found to be much lower than the critical value, indicating that the observed H I absorbing clouds are significantly subcritical. This agrees with a recent study of H I absorbing clouds in the cold neutral medium of our Galaxy, thereby extending an important result for Galactic clouds to an extragalactic case.

We present results of JCMT and BIMA CO J = 2 − 1 polarization observations towards the Orion KL/IRc2 high mass star forming region. The linear polarization fraction of the JCMT CO J = 2 − 1 spectra presents a clear decrease towards the center of the line, as expected, due to the increase of the optical depth. The position angle remains constant along the spectral line, except at the line center, where the highest optical depth and lower fractional polarization are measured. The combined BIMA and JCMT maps of the redshifted and blueshifted CO emission show a uniform polarization pattern that does not coincide with previous dust continuum observations at similar angular resolution. This suggests that the CO and dust are tracing different spatial components along the line of sight.

Prestellar cores represent the stage of star formation immediately prior to protostellar collapse. We present polarisation maps of three prestellar cores, L183, L1544 and L43. In each case the magnetic field lines are uniform but not parallel to the semi-minor axis of the core. This suggests that magnetic and thermal pressure support alone are inconsistent with the data. We also calculate the magnetic field strength using the Chandrasekhar-Fermi technique and find that all three cores are magnetically supercritical by a factor of ∼ 2. This is consistent with the observation that the magnetic field is not dominating the evolution of these cores.

We present recent JCMT and BIMA array polarimetry data of nearby star-forming regions in order to compare the core and cloud-scale magnetic field geometries in two regions of Orion. The similarity of the magnetic field geometry in these cores to that of their ambient clouds is contrasted with JCMT data toward the Barnard 1 dark cloud in Perseus, which reveal a different magnetic field orientation between the majority of the cores and the surrounding cloud; each of the cores exhibits a different mean polarization position angle. We conclude that the preservation of the magnetic field geometry is better in cores formed within clouds with ordered large scale structures. In Barnard 1, the cores may quickly exhibit a different polarization pattern if they have, for example, rotation which differs from the large scale cloud motions, or a weaker component of ordered fields. This could also explain why the cores exhibit such different geometries from each other in Barnard 1.

We present the polarization detections in DR21(OH) from both the thermal dust emission at 1.3 mm and the CO J=2-1 line obtained with the Berkeley-Illinois-Maryland Association (BIMA) array. Our results are consistent with the prediction of the Goldreich-Kylafis effect that the CO polarization is either parallel or perpendicular to the magnetic field direction. The detection of the polarized CO emission is over a more extended region than the dust polarization, while the dust polarization provides an aide in resolving the ambiguity of the CO polarization. The combined results suggest that the magnetic field direction in DR21(OH) is parallel to the CO polarization and therefore parallel to the major axis of DR21(OH). The strong correlation between the CO and dust polarization suggests that magnetic fields are remarkably uniform throughout the envelope and the cores. The dispersion in polarization position angles implies a magnetic field strength in the plane of the sky of about 1 mG, compared with about 0.5 mG inferred for the line-of-sight field from previous CN Zeeman observations. Our CO data also show that both MM1 and MM2 power high-velocity outflows with $v\gtrsim$25 km/s relative to the systematic velocity., 20 pages, 7 figures, accepted for publication in ApJ

We present VLA observations of the Zeeman effect in 22 GHz H2O masers in several high-mass star-forming regions. These masers are believed to arise from collisional pumping in postshock environments. Therefore, the Zeeman effect data provide the most direct measurements of magnetic field strengths in high-density (n 108 cm-3) postshock gas, where the field energy determines other physical conditions. Our observations yield significant magnetic field detections in W3 IRS 5, W3(OH), W49 N, and OH 43.8-0.1. In these sources, we detect line-of-sight field strengths ranging from 13 to 49 mG. For some regions, the detected fields provide a 2-3 point sampling of the magnetic field, indicating the nature of field variations on arcsecond scales. These field strengths are consistent with a shock-driven maser model having relatively low speed (20 km s-1), C-type shocks. We examine the balance between magnetic field energy and turbulent kinetic energy in the masing regions. These energies appear close to equilibrium.

The mechanism for producing polarized emission from protostellar disks at (sub)millimeter wavelengths is currently uncertain. Classically, polarization is expected from non-spherical grains aligned with the magnetic field. Recently, two alternatives have been suggested. One polarization mechanism is caused by self-scattering from dust grains of sizes comparable to the wavelength while the other mechanism is due to grains aligned with their short axes along the direction of radiation anisotropy. The latter has recently been shown as a likely mechanism for causing the dust polarization detected in HL Tau at 3.1 mm. In this paper, we present ALMA polarization observations of HL Tau for two more wavelengths: 870 $\mu$m and 1.3 mm. The morphology at 870 $\mu$m matches the expectation for self-scattering, while that at 1.3 mm shows a mix between self-scattering and grains aligned with the radiation anisotropy. The observations cast doubt on the ability of (sub)millimeter continuum polarization to probe disk magnetic fields for at least HL Tau. By showing two distinct polarization morphologies at 870 $\mu$m and 3.1 mm and a transition between the two at 1.3 mm, this paper provides definitive evidence that the dominant (sub)millimeter polarization mechanism transitions with wavelength. In addition, if the polarization at 870 $\mu$m is due to scattering, the lack of polarization asymmetry along the minor axis of the inclined disk implies that the large grains responsible for the scattering have already settled into a geometrically thin layer, and the presence of asymmetry along the major axis indicates that the HL Tau disk is not completely axisymmetric., Comment: Accepted to ApJ, typos corrected from proof

For the Class 0 protostar, L1527, we compare 131 polarization vectors from SCUPOL/JCMT, SHARP/CSO and TADPOL/CARMA observations with the corresponding model polarization vectors of four ideal-MHD, non-turbulent, cloud core collapse models. These four models differ by their initial magnetic fields before collapse; two initially have aligned fields (strong and weak) and two initially have orthogonal fields (strong and weak) with respect to the rotation axis of the L1527 core. Only the initial weak orthogonal field model produces the observed circumstellar disk within L1527. This is a characteristic of nearly all ideal-MHD, non-turbulent, core collapse models. In this paper we test whether this weak orthogonal model also has the best agreement between its magnetic field structure and that inferred from the polarimetry observations of L1527. We found that this is not the case; based on the polarimetry observations the most favored model of the four is the weak aligned model. However, this model does not produce a circumstellar disk, so our result implies that a non-turbulent, ideal-MHD global collapse model probably does not represent the core collapse that has occurred in L1527. Our study also illustrates the importance of using polarization vectors covering a large area of a cloud core to determine the initial magnetic field orientation before collapse; the inner core magnetic field structure can be highly altered by a collapse and so measurements from this region alone can give unreliable estimates of the initial field configuration before collapse., Comment: 43 pages, 9 figures, 4 tables. Accepted by the Astrophysical Journal

This chapter reviews Zeeman observations of magnetic fields in molecular clouds and their use in testing star formation theory. The result is that while ambipolar diffusion driven star formation appears to be inconsistent with observations, reconnection diffusion seems to be consistent.

We present details of the CARMA Large Area Star Formation Survey (CLASSy), while focusing on observations of Barnard 1. CLASSy is a CARMA Key Project that spectrally imaged N2H+, HCO+, and HCN (J=1-0 transitions) across over 800 square arcminutes of the Perseus and Serpens Molecular Clouds. The observations have angular resolution near 7" and spectral resolution near 0.16 km/s. We imaged ~150 square arcminutes of Barnard 1, focusing on the main core, and the B1 Ridge and clumps to its southwest. N2H+ shows the strongest emission, with morphology similar to cool dust in the region, while HCO+ and HCN trace several molecular outflows from a collection of protostars in the main core. We identify a range of kinematic complexity, with N2H+ velocity dispersions ranging from ~0.05-0.50 km/s across the field. Simultaneous continuum mapping at 3 mm reveals six compact object detections, three of which are new detections. A new non-binary dendrogram algorithm is used to analyze dense gas structures in the N2H+ position-position-velocity (PPV) cube. The projected sizes of dendrogram-identified structures range from about 0.01-0.34 pc. Size-linewidth relations using those structures show that non-thermal line-of-sight velocity dispersion varies weakly with projected size, while rms variation in the centroid velocity rises steeply with projected size. Comparing these relations, we propose that all dense gas structures in Barnard 1 have comparable depths into the sky, around 0.1-0.2 pc; this suggests that over-dense, parsec-scale regions within molecular clouds are better described as flattened structures rather than spherical collections of gas. Science-ready PPV cubes for Barnard 1 molecular emission are available for download., Accepted to The Astrophysical Journal (ApJ), 51 pages, 27 figures (some with reduced resolution in this preprint); Project website is at http://carma.astro.umd.edu/classy

Magnetic fields in accretion disks play a dominant role during the star formation process but have hitherto been observationally poorly constrained. Field strengths have been inferred on T Tauri stars themselves and possibly in the innermost part of the accretion disk, but the strength and morphology of the field in the bulk of the disk have not been observed. Unresolved measurements of polarized emission (arising from elongated dust grains aligned perpendicular to the field) imply average fields aligned with the disks. Theoretically, the fields are expected to be largely toroidal, poloidal, or a mixture of the two, which imply different mechanisms for transporting angular momentum in the disks of actively accreting young stars such as HL Tau. Here we report resolved measurements of the polarized 1.25 mm continuum emission from HL Tau's disk. The magnetic field on a scale of 80 AU is coincident with the major axis (~210 AU diameter) of the disk. From this we conclude that the magnetic field inside the disk at this scale cannot be dominated by a vertical component, though a purely toroidal field does not fit the data well either. The unexpected morphology suggests that the magnetic field's role for the accretion of a T Tauri star is more complex than the current theoretical understanding., Accepted for publication in Nature

We have measured the Zeeman splitting of the CCS JN = 32-21 line at 33 GHz toward L1498, a dense preprotostellar core, in an effort to measure the line-of-sight component of its magnetic field. With approximately 35 hr of data on source (70 hr total) in good weather, the data suggest a line-of-sight component of the magnetic field in L1498 of 48 ± 31 μG, yielding an upper limit of Blos < 100 μG at the 95% confidence level. This upper limit provides some constraints on models. Our results show that the technique we have adopted to measure CCS Zeeman splitting holds great promise for determining magnetic field strengths in cloud cores using lower-frequency transitions, in particular the CCS JN = 10-00 line at 11 GHz. At this transition, the frequency splitting per gauss is 3 times that at 33 GHz, the brightness temperature is comparable to the 32-21 line, and receiver systems can be made more sensitive.

We report observations with the Arecibo telescope of the Zeeman effect in the 1665 and 1667 MHz lines of OH toward eight dark cloud cores. For L1544, the inferred line-of-sight magnetic field is BLOS = +11 ± 2 μG. The L1544 starless core has been observed to have infall motions; it may be very close to forming a star. BLOS ≈ 11 μG is consistent with the prediction of an ambipolar diffusion model computed specifically for this core before the Zeeman measurements were made; however, in order to obtain agreement with the data this model has inclined by only 16° to the plane of the sky. Virial arguments show that unless the magnetic field is mainly in the plane of the sky, it is not important for support of the L1544 core. For L1457S our result is BLOS = -24 ± 7 μG, a possible but not definite detection. The other six cores were observed only with short integration times, and the Zeeman effect was not detected in these cores.

We have conducted a survey of the CCS $J_N = 3_2-2_1$ line toward 11 dark clouds and star-forming regions at 30 arcsec spatial resolution and 0.054 km/s velocity resolution. CCS was only detected in quiescent clouds, not in active star-forming regions. The CCS distribution shows remarkable clumpy structure, and 25 clumps are identified in 7 clouds. Seven clumps with extremely narrow nonthermal linewidths < 0.1 km/s are among the most quiescent clumps ever found. The CCS clumps tend to exist around the higher density regions traced by NH_3 emission or submillimeter continuum sources, and the distribution is not spherically symmetric. Variation of the CCS abundance was suggested as an indicator of the evolutionary status of star formation. However, we can only find a weak correlation between N(CCS) and $n_{H_2,vir}$. The velocity distributions of CCS clouds reveal that a systematic velocity pattern generally exists. The most striking feature in our data is a ring structure in the position-velocity diagram of L1544 with an well-resolved inner hole of 0.04 pc x 0.13 km/s and an outer boundary of 0.16 pc x 0.55 km/s. This position-velocity structure clearly indicates an edge-on disk or ring geometry, and it can be interpreted as a collapsing disk with an infall velocity $\gtrsim$ 0.1 km/s and a rotational velocity less than our velocity resolution. Nonthermal linewidth distribution is generally coherent in CCS clouds, which could be evidence for the termination of Larson's Law at small scales, $\sim$ 0.1 pc., 21 pages, 25 ostscript figures, accepted for publication in the Supplement Series of the Astrophysical Journal (May 2000)

We report the first interferometric detection and mapping of linearly polarized spectral line emission due to the Goldreich-Kylafis effect. Our polarization maps of the CO J = 2 → 1 line in the molecular outflow powered by the very young stellar system NGC 1333 IRAS 4A make it possible to define the direction of the magnetic field in the outflow. Comparison with theoretical predictions implies that the magnetic field is parallel to the polarization. Our data suggest that the deflection of the outflow may be the result of the interaction between the outflow and the magnetic field. We also detect and map the linearly polarized dust continuum emission at 1.3 mm. The polarization map of the dust continuum is roughly consistent with an hourglass magnetic field morphology; i.e., it is in agreement with theoretical models of interstellar cloud contraction with a frozen-in magnetic field. The two techniques for mapping magnetic field morphologies agree. In general, the two techniques sample different column densities, and together they allow the study of magnetic field morphology over wider areas than either technique by itself would permit.

This paper presents a summary of all 27 available sensitive Zeeman measurements of magnetic field strengths in molecular clouds together with other relevant physical parameters. From these data input parameters to magnetic star formation theory are calculated, and predictions of theory are compared with observations. Results for this cloud sample are the following: (1) Internal motions are supersonic but approximately equal to the Alfv?n speed, which suggests that supersonic motions are likely MHD waves. (2) The ratio of thermal to magnetic pressures ?p ? 0.04, implying that magnetic fields are important in the physics of molecular clouds. (3) The mass-to-magnetic flux ratio is about twice critical, which suggests but does not require that static magnetic fields alone are insufficient to support clouds against gravity. (4) Kinetic and magnetic energies are approximately equal, which suggests that static magnetic fields and MHD waves are roughly equally important in cloud energetics. (5) Magnetic field strengths scale with gas densities as |B| ?? with ? ? 0.47; this agrees with the prediction of ambipolar diffusion driven star formation, but this scaling may also be predicted simply by Alfv?nic motions. The measurements of magnetic field strengths in molecular clouds make it clear that magnetic fields are a crucial component of the physics governing cloud evolution and star formation.

We have carried out VLA Zeeman observations of HI absorption lines toward the HII region in the M17 giant molecular cloud complex. The HI absorption lines toward M17 show between 5 and 8 distinct velocity components which vary spatially in a complex manner across the source. We explore possible physical connections between these components and the M17 region based on calculations of HI column densities, line of sight magnetic field strengths, as well as comparisons with a wide array of previous optical, infrared, and radio observations. In particular, an HI component at the same velocity as the southwestern molecular cloud (M17 SW) ~20 km/s seems to originate from the edge-on interface between the HII region and M17 SW, in un-shocked PDR gas. We have detected a steep enhancement in the 20 km/s HI column density and line of sight magnetic field strengths (Blos) toward this boundary. A lower limit for the peak 20 km/s HI column density is N_{HI}/T_s > 5.6 x 10^{19} cm^{-2}/K while the peak Blos is ~ -450 muG. In addition, blended components at velocities of 11-17 km/s appear to originate from shocked gas in the PDR between the HII region and an extension of M17 SW, which partially obscures the southern bar of the HII region. The peak N_{HI}/T_s and Blos for this component are > 4.4 x 10^{19} cm^{-2}/K and +550 muG, respectively. Comparison of the peak magnetic fields detected toward M17 with virial equilibrium calculations suggest that ~1/3 of M17 SW's total support comes from its static magnetic field and the other 2/3 from its turbulent kinetic energy which includes support from Alfven waves., Contains 29 pages, 14 figures (Latex). Text and figures can be downloaded separately at http://www.pa.uky.edu/~brogan, submitted to ApJ

We have carried out Very Large Array (VLA) Zeeman observations of absorption lines of H I and OH toward the molecular cloud associated with the NGC 2024 (Orion B) H II region. The synthesized beam diameters are 68'' × 52'', p.a.=38°, and 81'' × 65'', p.a.=-6°, for OH and H I, respectively. The absorption lines could be mapped over the NGC 2024 continuum source, which has an extent (at the 1 Jy beam−1 level) of Δα≈7' by Δδ≈5'. The maps of the magnetic field, together with comparisons with additional data from the published literature, lead to the following conclusions: (1) The magnetic field comes from a line subcomponent at vLSR≈10.2 km s-1, which corresponds in velocity and in spatial morphology with the northern dense molecular ridge in NGC 2024. (2) Blos varies from 0 to the northeast of the northern molecular ridge to almost 100 μG to the southwest. The variation in Blos may be due to the field being mainly in the plane of the sky to the northeast but having a significant line-of-sight component to the southwest. (3) Velocities in the cloud are supersonic but approximately equal to the Alfven velocity, which is consistent with motions being dominated by magnetohydrodynamical waves rather than thermal motion or hydrodynamical turbulence. (4) The mass-to-magnetic flux ratio is supercritical, which suggests that the static magnetic field does not support the cloud against collapse. Simple virial estimates of the relative importance of gravitational, kinetic, and magnetic energies show that the ratio of kinetic/gravitational energy is about 0.5, while the magnetic/gravitational energy ratio is less than 0.1. At face value, these results imply that the cloud is supported mainly by nonthermal motions rather than by the static magnetic field. However, since we only measure directly the line-of-sight component of B, this result is not conclusive.

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.

Recent studies of the gas-phase abundances of carbon and oxygen in diffuse clouds have suggested that the depletions of both elements are invariant and independent of extinction curve properties. We show, however, that in the line of sight toward HD 24534 (X Persei), the depletion of carbon is at least a factor of 3 greater than in the diffuse clouds previously surveyed. For HD 24534, the interstellar gas-phase C/H ratio is lower than the mean of diffuse sight lines by a factor of about 3.5. We conclude that the carbon depletion may be enhanced in translucent molecular clouds and also that carbon begins to show an enhanced level of depletion at lower extinctions than oxygen, which is also discussed. We argue that it is imperative to measure abundances and depletions in translucent and denser clouds in order to understand the interaction between gas and dust as the thicknesses and densities of clouds increase.

The W3 molecular cloud core has been observed with the BIMA and NRAO 12 m telescopes in the 13CO (J = 1-0) and C18O (J = 1-0) lines. Two BIMA fields were imaged, centered on the infrared sources IRS 5 and IRS 4 (80'' west of IRS 5). In order to observe large-scale structure, a full-synthesis cube was made with a resolution of 95 × 80. Additionally, a high-resolution cube was made with only the IRS 5 pointing of BIMA to investigate the distribution of gas near IRS 5 at high angular resolution. The total H2 masses of the molecular cores associated with IRS 4 and IRS 5 are 1700 and 1100 M☉, respectively. Six clumps are identified. Derived H2 masses of the clumps are between 120 and 480 M☉, and under simple geometrical assumptions, n(H2) is between 1.8 and 4.5 × 105 cm-3. The virial masses of the clumps are within 30% of the derived H2 masses. For the clumps located at IRS 5 and 20'' south of IRS 4, the 13CO opacities derived were 0.3 and 0.7, respectively. At the positions of IRS 4 and IRS 5, individual clumps are observed; the IRS 4 clump in particular has a n(H2) a factor of 2 larger than the other clumps. At the position of IRS 5, the high-resolution data show a velocity structure suggestive of two clumps, one north and another south of IRS 5. Integrating the emission from these clumps gives an H2 mass of 110 M☉. Using the derived H2 column density toward IRS 5 and a previous H I Zeeman result, we can derive the ratio | B |/NH (which is proportional to the magnetic flux-to-mass ratio ΦB/M). The determined ratio is near the magnetically critical value, which is consistent with the standard theory of high-mass star formation.

We present the first detection of polarization around the Class 0 low-mass protostar L1157-mm at two different wavelengths. We show polarimetric maps at large scales (10 '' resolution at 350 mu m) from the SHARC-II Polarimeter and at smaller scales (1.'' 2-4.'' 5 at 1.3 mm) from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). The observations are consistent with each other and show inferred magnetic field lines aligned with the outflow. The CARMA observations suggest a full hourglass magnetic field morphology centered about the core; this is only the second well-defined hourglass detected around a low-mass protostar to date. We apply two different methods to CARMA polarimetric observations to estimate the plane-of-sky magnetic field magnitude, finding values of 1.4 and 3.4 mG.

We present results of 1.3 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 formed., Comment: 6 pages, 2 figures, 1 table

We discuss the significance of previously published OH Zeeman effect measurements of the magnetic field in the ρ Oph cloud (L1688) and also a new OH Zeeman effect measurement for the dark cloud core TMC-1C, the latter obtained with the Arecibo telescope. Results for both of these clouds, like previously published OH Zeeman results for other dark clouds, show that the line-of-sight field strength |B||| ≤ 10 μG. The ρ Oph cloud is significantly higher in column density than other clouds studied, and recent H I Zeeman effect observations have revealed B|| ≈ +10 μG in the H I self-absorption region in front of the cloud. The low values of B derived from the OH Zeeman effect may indicate that OH poorly samples the dense gas in the ρ Oph cloud, or else that the cloud is in a state of supercritical collapse. The TMC-1C cloud core may have formed via ambipolar diffusion in a manner similar to recent modeling of dark cloud Barnard 1.