Josep M. Girart

Magnetic Fields in the Formation of Sun-Like Stars

We report high-angular-resolution measurements of polarized dust emission toward the low-mass protostellar system NGC 1333 IRAS 4A. We show that in this system the observed magnetic field morphology is in agreement with the standard theoretical models of the formation of Sun-like stars in magnetized molecular clouds at scales of a few hundred astronomical units; gravity has overcome magnetic support, and the magnetic field traces a clear hourglass shape. The magnetic field is substantially more important than turbulence in the evolution of the system, and the initial misalignment of the magnetic and spin axes may have been important in the formation of the binary system.

Magnetic Fields in the Formation of Massive Stars

Stellar Hourglass Figure Star-forming clouds are thought to be supported against gravity by ordered interstellar magnetic fields, which are strong enough to slow gravitation collapse but too weak to prevent it. Girart et al. (p. 1408) measured polarized radio waves from dust particles around a forming massive star, which reveal an hourglass shape. The data imply that a magnetic field strength dominates over turbulence—the telltale signs of magnetically controlled star formation. These conditions mimic those found in low-mass star-forming regions, suggesting that the magnetic field plays an important role in star formation, irrespective of differences in mass. Observations of polarized dust emission show that the magnetic field controls the dynamical evolution of a massive star-forming region. Massive stars play a crucial role in the production of heavy elements and in the evolution of the interstellar medium, yet how they form is still a matter of debate. We report high-angular-resolution submillimeter observations toward the massive hot molecular core (HMC) in the high-mass star-forming region G31.41+0.31. We find that the evolution of the gravitational collapse of the HMC is controlled by the magnetic field. The HMC is simultaneously contracting and rotating, and the magnetic field lines threading the HMC are deformed along its major axis, acquiring an hourglass shape. The magnetic energy dominates over the centrifugal and turbulence energies, and there is evidence of magnetic braking in the contracting core.

Optical polarimetry toward the Pipe nebula: Revealing the importance of the magnetic field ⋆

Context. Magnetic fields are proposed to play an important role in the formation and support of self-gravitating clouds and the formation and evolution of protostars in such clouds. Aims. We attempt to understand more precisely how the Pipe nebula is affected by the magnetic field. Methods. We use R-band linear polarimetry collected for about 12 000 stars in 46 fields with lines of sight toward the Pipe nebula to investigate the properties of the polarization across this dark cloud complex. Results. Mean polarization vectors show that the magnetic field is locally perpendicular to the large filamentary structure of the Pipe nebula (the “stem”), indicating that the global collapse may have been driven by ambipolar diffusion. The polarization properties clearly change along the Pipe nebula. The northwestern end of the nebula (B59 region) is found to have a low degree of polarization and high dispersion in polarization position angle, while at the other extreme of the cloud (the “bowl”) we found mean degrees of polarization as high as ≈15% and a low dispersion in polarization position angle. The plane of the sky magnetic field strength was estimated to vary from about 17 μG in the B59 region to about 65 μG in the bowl. Conclusions. We propose that three distinct regions exist, which may be related to different evolutionary stages of the cloud; this idea is supported by both the polarization properties across the Pipe and the estimated mass-to-flux ratio that varies between approximately super-critical toward the B59 region and sub-critical inside the bowl. The three regions that we identify are: the B59 region, which is currently forming stars; the stem, which appears to be at an earlier stage of star formation where material has been through a collapsing phase but not yet given birth to stars; and the bowl, which represents the earliest stage of the cloud in which the collapsing phase and cloud fragmentation has already started.

Formic Acid in Orion KL from 1 Millimeter Observations with the Berkeley-Illinois-Maryland Association Array

We present Berkeley-Maryland-Illlinois Association array observations of formic acid (HCOOH) at 1 mm toward the Orion KL region. Near the compact ridge, HCOOH emission is spatially resolved; its partial shell morphology is different from that of other complex O-bearing molecules such as methyl formate and dimethyl ether. This unique distribution suggests that HCOOH is located in a layer that delineates the interaction region between the outflow and the ambient quiescent gas. HCOOH is also detected toward the hot core. For both cases, ejection of grain mantles is likely to be responsible for the observed HCOOH.

THE OUTBURST DECAY OF THE LOW MAGNETIC FIELD MAGNETAR SGR 0418+5729

N.R. is supported by a Ramon y Cajal Research Fellowship, and by grants AYA2009-07391, AYA2012-39303, SGR2009-811, TW2010005, and iLINK 2011-0303. J.A.P. and D.V. acknowledge support from the grants AYA 2010-21097-C03-02 and Prometeo/2009/103. R.T. and S.M. are partially funded through an INAF 2011 PRIN grant. A.P. is supported by a JAE-Doc CSIC fellowship co-funded with the European Social Fund under the program “Junta para la Ampliacion de Estudios,” by the Spanish MICINN grant AYA2011-30228-C03-02 (co-funded with FEDER funds), and by the AGAUR grant 2009SGR1172 (Catalonia).

DETAILED INTERSTELLAR POLARIMETRIC PROPERTIES OF THE PIPE NEBULA AT CORE SCALES

We use R-band CCD linear polarimetry collected for about 12,000 background field stars in 46 fields of view toward the Pipe nebula to investigate the properties of the polarization across this dark cloud. Based on archival Two Micron All Sky Survey data, we estimate that the surveyed areas present total visual extinctions in the range 0.6 mag ? AV ? 4.6 mag. While the observed polarizations show a well-ordered large-scale pattern, with polarization vectors almost perpendicularly aligned to the clouds long axis, at core scales one sees details that are characteristics of each core. Although many observed stars present degrees of polarization that are unusual for the common interstellar medium (ISM), our analysis suggests that the dust grains constituting the diffuse parts of the Pipe nebula seem to have the same properties as the normal Galactic ISM. Estimates of the second-order structure function of the polarization angles suggest that most of the Pipe nebula is magnetically dominated and that turbulence is sub-Alv?nic. The Pipe nebula is certainly an interesting region to investigate the processes that prevailed during the initial phases of low-mass stellar formation.

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.