Pau Frau

DR 21(OH): A HIGHLY FRAGMENTED, MAGNETIZED, TURBULENT DENSE CORE

We present high angular resolution observations of the massive star-forming core DR21(OH) at 880 μ mu sing the Submillimeter Array (SMA). The dense core exhibits an overall velocity gradient in a Keplerian-like pattern, which breaks at the center of the core where SMA 6 and SMA 7 are located. The dust polarization shows a complex magnetic field, compatible with a toroidal configuration. This is in contrast with the large, parsec-scale filament that surrounds the core, where there is a smooth magnetic field. The total magnetic field strengths in the filament and in the core are 0.9 and 2.1 mG, respectively. We found evidence of magnetic field diffusion at the core scales, far beyond the expected value for ambipolar diffusion. It is possible that the diffusion arises from fast magnetic reconnection in the presence of turbulence. The dynamics of the DR 21(OH) core appear to be controlled energetically in equal parts by the magnetic field, magnetohydrodynamic turbulence, and the angular momentum. The effect of the angular momentum (this is a fast rotating core) is probably causing the observed toroidal field configuration. Yet, gravitation overwhelms all the forces, making this a clear supercritical core with a mass-to-flux ratio of � 6 times the critical value. However, simulations show that this is not enough for the high level of fragmentation observed at 1000 AU scales. Thus, rotation and outflow feedback are probably the main causes of the observed fragmentation.

On the radiation driven alignment of dust grains: Detection of the polarization hole in a starless core

Aims. We aim to investigate the polarization properties of a starl ess core in a very early evolutionary stage. Linear polariza tion data reveal the properties of the dust grains in the distinct phas es of the interstellar medium. Our goal is to investigate how the polarization degree and angle correlate with the cloud and core gas. Methods. We use optical, near infrared and submillimeter polarization observations toward the starless object Pipe-109 in the Pipe nebula. Our data cover a physical scale range of 0.08 to 0.4 pc, comprising the dense gas, envelope and the surrounding cloud. Results. The cloud polarization is well traced by the optical data. The near infrared polarization is produced by a mixed population of grains from the core border and the cloud gas. The optical and near infrared polarization toward the cloud reach the maximum possible value and saturate with respect to the visual extinction. Th e core polarization is predominantly traced by the submillimeter data and have a steep decrease with respect to the visual extinction. Modeling of the submillimeter polarization indicates a magnetic field main direction projected onto the plane-of-sky and loss of grain alignment for densities higher than 6× 10 4 cm −3 (or AV> 30 mag). Conclusions. Pipe-109 is immersed in a magnetized medium, with a very ordered magnetic field. The absence of internal source of radiation significantly a ffects the polarization effi ciencies in the core, creating a polarization hole at the cen ter of the starless core. This result supports the theory of dust grain alignment via r adiative torques

Comparing star formation models with interferometric observations of the protostar NGC 1333 IRAS 4A - I. Magnetohydrodynamic collapse models

Observations of dust polarized emission toward star forming regions trace the magnetic field component in the plane of the sky and provide constraints to theoretical models of cloud collapse. We compare high-angular resolution observations of the submillimeter polarized emission of the low-mass protostellar source NGC 1333 IRAS 4A with the predictions of three different models of collapse of magnetized molecular cloud cores. We compute the Stokes parameters for the dust emission for the three models. We then convolve the results with the instrumental response of the Submillimeter Array observation toward IRAS 4A. Finally, we compare the synthetic maps with the data, varying the model parameters and orientation, and we assess the quality of the fit by a \chi^2 analysis. High-angular resolution observations of polarized dust emission can constraint the physical properties of protostars. In the case of IRAS 4A, the best agreements with the data is obtained for models of collapse of clouds with mass-to-flux ratio >2 times the critical value, initial uniform magnetic field of strength ~0.5 mG, and age of the order of a few 10^4 yr since the onset of collapse. Magnetic dissipation, if present, is found to occur below the resolution level of the observations. Including a previously measured temperature profile of IRAS 4A leads to a more realistic morphology and intensity distribution. We also show that ALMA has the capability of distinguishing among the three different models adopted in this work. Our results are consistent with the standard theoretical scenario for the formation of low-mass stars, where clouds initially threaded by large-scale magnetic fields become unstable and collapse, trapping the field in the nascent protostar and the surrounding circumstellar disk. In the collapsing cloud, the dynamics is dominated by gravitational and magnetic forces.

YOUNG STARLESS CORES EMBEDDED IN THE MAGNETICALLY DOMINATED PIPE NEBULA

The Pipe Nebula is a massive, nearby dark molecular cloud with a low star formation efficiency which makes it a good laboratory in which to study the very early stages of the star formation process. The Pipe Nebula is largely filamentary and appears to be threaded by a uniform magnetic field at scales of a few parsecs, perpendicular to its main axis. The field is only locally perturbed in a few regions, such as the only active cluster-forming core B59. The aim of this study is to investigate primordial conditions in low-mass pre-stellar cores and how they relate to the local magnetic field in the cloud. We used the IRAM 30 m telescope to carry out a continuum and molecular survey at 3 and 1 mm of early- and late-time molecules toward four selected starless cores inside the Pipe Nebula. We found that the dust continuum emission maps trace the densest regions better than previous Two Micron All Sky Survey (2MASS) extinction maps, while 2MASS extinction maps trace the diffuse gas better. The properties of the cores derived from dust emission show average radii of ~0.09 pc, densities of ~1.3×105 cm–3, and core masses of ~2.5 M ☉. Our results confirm that the Pipe Nebula starless cores studied are in a very early evolutionary stage and present a very young chemistry with different properties that allow us to propose an evolutionary sequence. All of the cores present early-time molecular emission with CS detections in the whole sample. Two of them, cores 40 and 109, present strong late-time molecular emission. There seems to be a correlation between the chemical evolutionary stage of the cores and the local magnetic properties that suggests that the evolution of the cores is ruled by a local competition between the magnetic energy and other mechanisms, such as turbulence.

Adaptable radiative transfer innovations for submillimetre telescopes (ARTIST) - Dust polarisation module (DustPol)

We present a new publicly available tool (DustPol) aimed to model the polarised thermal dust emission. The module DustPol, which is publicly available, is part of the ARTIST (Adaptable Radiative Transfer Innovations for Submillimetre Telescopes) package, which also offers tools for modelling the polarisation of line emission together with a model library and a Python-based user interface. DustPol can easily manage analytical as well as pre-gridded models to generate synthetic maps of the Stokes I, Q, and U parameters. These maps are stored in FITS format which is straightforwardly read by the data reduction software used, e. g., by the Atacama Large Millimeter Array (ALMA). This turns DustPol into a powerful engine for the prediction of the expected polarisation features of a source observed with ALMA or the Planck satellite as well as for the interpretation of existing submillimetre observations obtained with other telescopes. DustPol allows the parameterisation of the maximum degree of polarisation and we find that, in a prestellar core, if there is depolarisation, this effect should happen at densities of 10(6) cm(-3) or larger. We compare a model generated by DustPol with the observational polarisation data of the low-mass Class 0 object NGC 1333 IRAS 4A, finding that the total and the polarised emission are consistent.

THE IMPORTANCE OF THE MAGNETIC FIELD FROM AN SMA-CSO-COMBINED SAMPLE OF STAR-FORMING REGIONS

Submillimeter dust polarization measurements of a sample of 50 star-forming regions, observed with the SMA and the CSO covering pc-scale clouds to mpc-scale cores, are analyzed in order to quantify the magnetic field importance. The magnetic field misalignment

Chemical differentiation toward the Pipe nebula starless cores

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BARNARD 59: NO EVIDENCE FOR FURTHER FRAGMENTATION

-- the local angle between magnetic field and dust emission gradient -- is found to be a prime observable, revealing distinct distributions for sources where the magnetic field is preferentially aligned with or perpendicular to the source minor axis. Source-averaged misalignment angles

Formation of dense structures induced by filament collisions - Correlation of density, kinematics, and magnetic field in the Pipe nebula

\langle|\delta|\rangle

A correlation between chemistry, polarization, and dust properties in the Pipe nebula starless core FeSt 1-457

fall into systematically different ranges, reflecting the different source-magnetic field configurations. Possible bimodal