Magnetic field alignment in low-mass molecular clouds: the role of turbulence and density of the clouds (2025)

Anchoring Magnetic Field In Turbulent Molecular Clouds

The Astrophysical …, 2009

The Relative Orientation between Local Magnetic Field and Galactic Plane in Low Latitude Dark Clouds

Research in Astronomy and Astrophysics

In this work, we study the magnetic field morphology of selected star-forming clouds spread over the galactic latitude (b) range −10° to 10°. The polarimetric observations of clouds CB24, CB27 and CB188 are conducted to study the magnetic field geometry of those clouds using the 104 cm Sampurnanand Telescope (ST) located at ARIES, Manora Peak, Nainital, India. These observations are combined with those of 14 further low latitude clouds available in the literature. Most of these clouds are located within a distance range 140–500 pc except for CB3 (∼2500 pc), CB34 (∼1500 pc), CB39 (∼1500 pc) and CB60 (∼1500 pc). Analyzing the polarimetric data of 17 clouds, we find that the alignment between the envelope magnetic field ( θ B env ) and galactic plane (GP) (θ GP) of the low-latitude clouds varies with their galactic longitudes (l). We observe a strong correlation between the longitude (l) and the offset ( θ off = ∣ θ B env − θ GP ∣ ) which shows that θ B env is parallel to the GP when t...

The alignment of molecular cloud magnetic fields with the spiral arms in M33

The formation of molecular clouds, which serve as stellar nurseries in galaxies, is poorly understood. A class of cloud formation models suggests that a large-scale galactic magnetic field is irrelevant at the scale of individual clouds, because the turbulence and rotation of a cloud may randomize the orientation of its magnetic field. Alternatively, galactic fields could be strong enough to impose their direction upon individual clouds, thereby regulating cloud accumulation and fragmentation, and affecting the rate and efficiency of star formation. Our location in the disk of the Galaxy makes an assessment of the situation difficult. Here we report observations of the magnetic field orientation of six giant molecular cloud complexes in the nearby, almost face-on, galaxy M33. The fields are aligned with the spiral arms, suggesting that the large-scale field in M33 anchors the clouds.

On the Evidence for Dynamically Important Magnetic Fields In Molecular Clouds

Arxiv preprint arXiv: …, 2010

Magnetic Fields in Molecular Clouds

Annual Review of Astronomy and Astrophysics, 2012

This review examines observations of magnetic fields in molecular clouds and what those observations tell us about the theory of molecular cloud evolution and star formation. First, the review briefly summarizes classes of theoretical models of molecular clouds and specific predictions of the models that can be tested by observation. Then, the review describes the techniques for observing and mapping magnetic fields in molecular clouds, followed by discussion of important examples of observational studies using each technique. A synthesis of results from all observational techniques summarizes the current state, which is that though magnetic fields generally dominate turbulence, there is no definitive evidence for magnetic fields dominating gravity in molecular clouds or for ambipolar-diffusion-driven star formation. Finally, the review discusses prospects for advances in our observational capabilities with telescopes and instruments now beginning operation or under construction.

The Average Magnetic Field Strength in Molecular Clouds: New Evidence of Super-Alfvnic Turbulence

The Astrophysical Journal, 2004

The magnetic field strength in molecular clouds is a fundamental quantity for theories of star formation. It is estimated by Zeeman splitting measurements in a few dense molecular cores, but its volume-averaged value within large molecular clouds (over several parsecs) is still uncertain. In this work we provide a new method to constrain the average magnetic field strength in molecular clouds. We compare the power spectrum of gas density of molecular clouds with that of two 350 3 numerical simulations of supersonic MHD turbulence. The numerical simulation with approximate equipartition of kinetic and magnetic energies (model A) yields the column density power spectrum P (k) ∝ k −2.25±0.01 , the super-Alfvénic simulation (model B) P (k) ∝ k −2.71±0.01 . The column density power spectrum of the Perseus, Taurus and Rosetta molecular cloud complexes is found to be well approximated by a power law, P o (k) ∝ k −a , with a = 2.74 ± 0.07, 2.74 ± 0.08 and 2.76 ± 0.08 respectively. We conclude that the observations are consistent with the presence of super-Alfvénic turbulence in molecular clouds (model B) while model A is inconsistent (more than 99% confidence) with the observations.

Magnetic field orientation in self-gravitating turbulent molecular clouds

Monthly Notices of the Royal Astronomical Society, 2021

Stars form inside molecular cloud filaments from the competition of gravitational forces with turbulence and magnetic fields. The exact orientation of these filaments with the magnetic fields depends on the strength of these fields, the gravitational potential, and the line of sight (LOS) relative to the mean field. To disentangle these effects we employ three-dimensional magnetohydrodynamical numerical simulations that explore a wide range of initial turbulent and magnetic states, i.e. sub-Alfvénic to super-Alfvénic turbulence, with and without gravity. We use histogram of relative orientation (HRO) and the associated projected Rayleigh statistics (PRS) to study the orientation of density and, in order to compare with observations, the integrated density relative to the magnetic field. We find that in sub-Alfvénic systems the initial coherence of the magnetic is maintained inside the cloud and filaments form perpendicular to the field. This trend is not observed in super-Alfvénic m...

The orientations of molecular clouds in the outer Galaxy: evidence for the scale of the turbulence driver?

Monthly Notices of the Royal Astronomical Society, 2009

Supernova (SN) explosions inject a considerable amount of energy into the interstellar medium (ISM) in regions with high-to-moderate star formation rates. In order to assess whether the driving of turbulence by supernovae is also important in the outer Galactic disc, where the star formation rates are lower, we study the spatial distribution of molecular cloud (MC) inclinations with respect to the Galactic plane. The latter contains important information on the nature of the mechanism of energy injection into the ISM. We analyse the spatial correlations between the position angles (PAs) of a selected sample of MCs (the largest clouds in the catalogue of the outer Galaxy published by Heyer et al). Our results show that when the PAs of the clouds are all mapped to values into the [0 • , 90 • ] interval, there is a significant degree of spatial correlation between the PAs on spatial scales in the range of 100-800 pc. These scales are of the order of the sizes of individual SN shells in low-density environments such as those prevailing in the outer Galaxy and where the metallicity of the ambient gas is of the order of the solar value or smaller. These findings suggest that individual SN explosions, occurring in the outer regions of the Galaxy and in likewise spiral galaxies, albeit at lower rates, continue to play an important role in shaping the structure and dynamics of the ISM in those regions. The SN explosions we postulate here are likely associated with the existence of young stellar clusters in the far outer regions of the Galaxy and the ultraviolet emission and low levels of star formation observed with the Galaxy Evolution Explorer (GALEX) satellite in the outer regions of local galaxies.

Probing the Turbulence Dissipation Range and Magnetic Field Strengths In Molecular Clouds

The Astrophysical Journal, 2008

We study the turbulent velocity dispersion spectra of the coexistent HCN and HCO+ molecular species as a function of length scale in the M17 star-forming molecular cloud. We show that the observed downward shift of the ion's spectrum relative to that of the neutral is readily explained by the existence of an ambipolar diffusion range within which ion and neutral turbulent energies dissipate differently. We use these observations to evaluate this decoupling scale and show how to estimate the strength of the plane-of-the-sky component of the embedded magnetic field in a completely novel way.

The Structure of the Galactic Magnetic Field toward the High‐Latitude Clouds

The Astrophysical Journal, 1997

We present the results of an optical polarization survey toward the galactic anticenter, in the area 5h º a º 2h and 6¡ ¹ d ¹ 12¡. This region is characterized by the presence of a stream of high-velocity H I as well as high galactic latitude molecular clouds. We used our polarization data together with 100 km IRAS maps of the region to study the relation between the dust distribution and the geometry of the magnetic Ðeld. We Ðnd that there is a correlation between the percent polarization and the 100 km Ñux such that When the IRAS Ñux is converted into H I column densities this

The Link Between Magnetic Fields and Cloud/Star Formation

Protostars and Planets VI, 2014

The question whether magnetic fields play an important role in the processes of molecular cloud and star formation has been debated for decades. Recent observations have revealed a simple picture that may help illuminate these questions: magnetic fields have a tendency to preserve their orientation at all scales that have been probed -from 100-pc scale inter-cloud media down to sub-pc scale cloud cores. This ordered morphology has implications for the way in which self-gravity and turbulence interact with magnetic fields: both gravitational contraction and turbulent velocities should be anisotropic, due to the influence of dynamically important magnetic fields. Such anisotropy is now observed. Here we review these recent observations and discuss how they can improve our understanding of cloud/star formation.

DISPERSION OF MAGNETIC FIELDS IN MOLECULAR CLOUDS. II

The Astrophysical Journal, 2009

We apply our technique on the dispersion of magnetic fields in molecular clouds to high spatial resolution Submillimeter Array polarization data obtained for Orion KL in OMC-1, IRAS 16293, and NGC 1333 IRAS 4A. We show how one can take advantage of such high resolution data to characterize the magnetized turbulence power spectrum in the inertial and dissipation ranges. For Orion KL we determine that in the inertial range the spectrum can be approximately fitted with a power law k −(2.9±0.9) and we report a value of 9.9 mpc for λ AD , the high spatial frequency cutoff presumably due to turbulent ambipolar diffusion. For the same parameters we have ∼ k −(1.4±0.4) and a tentative value of λ AD ≃ 2.2 mpc for NGC 1333 IRAS 4A, and ∼ k −(1.8±0.3) with an upper limit of λ AD 1.8 mpc for IRAS 16293. We also discuss the application of the technique to interferometry measurements and the effects of the inherent spatial filtering process on the interpretation of the results.

The link between molecular cloud structure and turbulence

Astronomy & Astrophysics, 2011

Aims. We aim to better understand how the spatial structure of molecular clouds is governed by turbulence. For that, we study the large-scale spatial distribution of low-density molecular gas and search for characteristic length scales. Methods. We employ a 35 square degrees 13 CO 1→0 molecular line survey of Cygnus X and visual extinction (A V) maps of 17 Galactic clouds to analyze the spatial structure with the Δ-variance method. This sample contains a large variety of different molecular cloud types with different star-forming activity. Results. The Δ-variance spectra obtained from the A V maps show differences between low-mass star-forming (SF) clouds and massive giant molecular clouds (GMC) in terms of shape of the spectrum and its power-law exponent β. Low-mass SF clouds have a double-peak structure with characteristic size scales around 1 pc (though with a large scatter around this value) and 4 pc. The GMCs show no characteristic scale in the A V-maps, which can partly be ascribed to a distance effect owing to a larger line-of-sight (LOS) confusion. The Δ-variance for Cygnus, determined from the 13 CO survey, shows characteristic scales at 4 pc and 40 pc, either reflecting the filament structure and large-scale turbulence forcing or-for the 4 pc scale-the scale below which the 13 CO 1→0 line becomes optically thick. Though there are different processes that can introduce characteristic scales, such as geometry, decaying turbulence, the transition scale from supersonic to subsonic turbulence (the sonic scale), line-of-sight effects and energy injection caused by expanding supernova shells, outflows, HII-regions, and although the relative contribution of these effects strongly varies from cloud to cloud, it is remarkable that the resulting turbulent structure of molecular clouds shows similar characteristics.

Observational Aspects of Magnetic Fields in Molecular Clouds

Galactic and Intergalactic Magnetic Fields, 1990

A small but growing body of observational information now exists regarding magnetic field strengths in molecular regions. Most of these data come from study of the Zeeman effect in 18 cm OH lines. The field is strong enough in many such regions to be dynamically important.

Dispersion of Magnetic Fields in Molecular Clouds. III

The Astrophysical Journal, 2011

We apply our technique on the dispersion of magnetic fields in molecular clouds to high spatial resolution Submillimeter Array polarization data obtained for Orion KL in OMC-1, IRAS 16293, and NGC 1333 IRAS 4A. We show how one can take advantage of such high resolution data to characterize the magnetized turbulence power spectrum in the inertial and dissipation ranges. For Orion KL we determine that in the inertial range the spectrum can be approximately fitted with a power law k −(2.9±0.9) and we report a value of 9.9 mpc for λ AD , the high spatial frequency cutoff presumably due to turbulent ambipolar diffusion. For the same parameters we have ∼ k −(1.4±0.4) and a tentative value of λ AD ≃ 2.2 mpc for NGC 1333 IRAS 4A, and ∼ k −(1.8±0.3) with an upper limit of λ AD 1.8 mpc for IRAS 16293. We also discuss the application of the technique to interferometry measurements and the effects of the inherent spatial filtering process on the interpretation of the results.

The Magnetic Field versus Density Relation in Star-forming Molecular Clouds

The Astrophysical Journal Letters, 2022

We study the magnetic field to density (B–ρ) relation in turbulent molecular clouds with dynamically important magnetic fields using nonideal three-dimensional magnetohydrodynamic simulations. Our simulations show that there is a distinguishable break density ρ T between the relatively flat low-density regime and a power-law regime at higher densities. We present an analytic theory for ρ T based on the interplay of the magnetic field, turbulence, and gravity. The break density ρ T scales with the strength of the initial Alfvén Mach number  A 0 for sub-Alfvénic (  A 0 < 1 ) and trans-Alfvénic (  A 0 ∼ 1 ) clouds. We fit the variation of ρ T for model clouds as a function of  A 0 , set by different values of initial sonic Mach number  0 and the initial ratio of gas pressure to magnetic pressure β 0. This implies that ρ T, which denotes the transition in mass-to-flux ratio from the subcritical to the supercritical regime, is set by the initial turbulent compression of the molec...

The Role of Magnetic Field in Molecular Cloud Formation and Evolution

Frontiers in Astronomy and Space Sciences, 2019

We review the role that magnetic field may have on the formation and evolution of molecular clouds. After a brief presentation and main assumptions leading to ideal MHD equations, their most important correction, namely the ion-neutral drift is described. The nature of the multi-phase interstellar medium (ISM) and the thermal processes that allows this gas to become denser are presented. Then we discuss our current knowledge of compressible magnetized turbulence, thought to play a fundamental role in the ISM. We also describe what is known regarding the correlation between the magnetic and the density fields. Then the influence that magnetic field may have on the interstellar filaments and the molecular clouds is discussed, notably the role it may have on the prestellar dense cores as well as regarding the formation of stellar clusters. Finally we briefly review its possible effects on the formation of molecular clouds themselves. We argue that given the magnetic intensities that have been measured, it is likely that magnetic field is i) responsible of reducing the star formation rate in dense molecular cloud gas by a factor of a few, ii) strongly shaping the interstellar gas by generating a lot of filaments and reducing the numbers of clumps, cores and stars, although its exact influence remains to be better understood. Moreover at small scales, magnetic braking is likely a dominant process that strongly modifies the outcome of the star formation process. Finally, we stress that by inducing the formation of more massive stars, magnetic field could possibly enhance the impact of stellar feedback.

Turbulence in high latitude molecular clouds

Proceedings of the International Astronomical Union

We summarize a continuing investigation of turbulence in high-latitude translucent molecular clouds. These low mass (~50–100 M), nearby (~100 pc), non-star forming clouds appear to be condensing out of the atomic cirrus. Unlike star-forming clouds the velocity fields in the clouds must be driven by external processes. Our detailed mapping of the clouds MBM 3,16 and 40 indicates that the dynamics in these clouds result from the combination of shear-flow and thermal instabilities, not shocks. These clouds also show coherent structures, non-Gaussian PDFs but no clear velocity-size relation. Lastly, the energetics of these clouds indicate that radiative loss may terminate the cascade before local heating takes place.

Magnetic Field Structures and Turbulent Components in the Star-Forming Molecular Clouds OMC-2 and OMC-3

The Astrophysical Journal, 2010

The SCUBA polarized 850 µm thermal emission data of the region OMC-2 in Orion A are added to and homogeneously reduced with data already available in the region OMC-3. The data set shows that OMC-2 is a region generally less polarized than OMC-3. Where coincident, most of the 850 µm polarization pattern is similar to that measured in 350 µm polarization data. Only 850 µm polarimetry data have been obtained in and around MMS7, FIR1 & FIR2, and in the region south of FIR6. A realignment of the polarization vectors with the filament can be seen near FIR1 in the region south of OMC-3. An analysis shows that the energy injected by CO outflows and H 2 jets associated to OMC-2 and OMC-3 does not appear to alter the polarization patterns at a scale of the

The physical mechanism behind magnetic field alignment in interstellar clouds

arXiv (Cornell University), 2024

Context. A tight correlation between interstellar clouds contours and their local magnetic field orientation has been widely observed. However, the physical mechanisms responsible for this correlation remain unclear. Aims. We investigate the alignment mechanism between the magnetic field and interstellar clouds. Methods. We perform three-and two-dimensional MHD simulations of warm gas streams in the thermally-bistable atomic interstellar medium (ISM) colliding with velocities of the order of the velocity dispersion in the ISM. In these simulations, we follow the evolution of magnetic field lines, identify and elucidate the physical processes causing their evolution. Results. The collision produces a fast MHD shock, and a condensation front roughly one cooling length behind it, on each side of the collision front. A cold dense layer forms behind the condensation front, onto which the gas settles, decelerating smoothly. We find that the magnetic field lines, initially oriented parallel to the flow direction, are perturbed by the fast MHD shock, across which the magnetic field fluctuations parallel to the shock front are amplified. The downstream perturbations of the magnetic field lines are further amplified by the compressive downstream velocity gradient between the shock and the condensation front caused by the settlement of the gas onto the dense layer. This mechanism causes the magnetic field to become increasingly parallel to the dense layer, and the development of a shear flow around the latter. Furthermore, the bending-mode perturbations on the dense layer are amplified by the non-linear thin-shell instability (NTSI), stretching the density structures formed by the thermal instability, and rendering them parallel to the bent field lines. By extension, we suggest that a tidal stretching velocity gradient such as that produced in gas infalling into a self-gravitating structure must straighten the field lines along the accretion flow, orienting them perpendicular to the density structures. We also find that the upstream superalfvénic regime transitions to a transalfvénic regime between the shock and the condensation front, and then to a subalfvénic regime inside the condensations. Finally, in two-dimensional simulations with a curved collision front, the presence of the magnetic field inhibits the generation of turbulence by the shear around the dense layer. Conclusions. Our results provide a feasible physical mechanism for the observed transition from parallel to perpendicular relative orientation of the magnetic field and the density structures as the density structures become increasingly dominated by self-gravity.