Magnetized interstellar molecular clouds – I. Comparison between simulations and Zeeman observations

Li, Pak Shing; McKee, Christopher F.; Klein, Richard

doi:10.1093/mnras/stv1437

Cited by 80 publications

(110 citation statements)

References 55 publications

(181 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This increase can be interpreted as being due to flux freezing in the collapse of spherical, weakly-magnetized clouds (Mestel 1966). Such a dependence of B-field strength with density has also been seen in the simulations of self-gravitating turbulence of, e.g., Li et al (2015) and Mocz et al (2017). In more strongly magnetized clouds, i.e., with Alfvén Mach numbers 1, a shallower relation, with κ 0.5 is expected (Tritsis et al 2015;Mocz et al 2017) and Tritsis et al (2015) have also argued that such a value of κ is a better match to the observational data.…”

Section: Introductionsupporting

confidence: 57%

“…An increase in the mass-to-flux ratio from the envelope to the core is expected to be a sign of ambipolar diffusion acting in subcritical clouds, a result not favored by the Crutcher et al (2009) analysis. However, Mouschovias & Tassis (2009 have called into question the validity of the conclusions on statistical grounds (but, see also Li et al 2015). Li & Houde (2008) proposed that the action of ambipolar diffusion could be inferred through the differing velocity dispersions observed for spatially coincident ions and neutrals.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

GMC Collisions as Triggers of Star Formation. IV. The Role of Ambipolar Diffusion

Christie

Tan

2017

ApJ

View full text Add to dashboard Cite

We investigate the role of ambipolar diffusion (AD) in collisions between magnetized giant molecular clouds (GMCs), which may be an important mechanism for triggering star cluster formation. Three dimensional simulations of GMC collisions are performed using a version of the Enzo magnetohydrodynamics code that has been extended to include AD. The resistivities are calculated using the 31-species chemical model of Wu et al. (2015). We find that in the weak-field, 10 µG case, AD has only a modest effect on the dynamical evolution during the collision. However, for the stronger-field, 30 µG case involving near-critical clouds, AD results in formation of dense cores in regions where collapse is otherwise inhibited. The overall efficiency of formation of cores with n H ≥ 10 6 cm −3 in these simulations is increases from about 0.2% to 2% once AD is included, comparable to observed values in star-forming GMCs. The gas around these cores typically has relatively slow infall at speeds that are a modest fraction of the free-fall speed.

show abstract

Section: Introductionsupporting

confidence: 57%

Section: Introductionmentioning

confidence: 99%

GMC Collisions as Triggers of Star Formation. IV. The Role of Ambipolar Diffusion

Christie

Tan

2017

ApJ

View full text Add to dashboard Cite

show abstract

“…As the discovery of ordered cloud B-fields has just started to constrain theories and numerical simulations of star formation (e.g. reference 10,17,18,26,27), being able to explain the bimodal cloud-field alignment 5 and SFR ( Figure 2) should also be included among the criteria of a successful cloud/star-formation theory. Figures 2), to define the regions for which we calculate cloud mean fields.…”

mentioning

confidence: 99%

The link between magnetic field orientations and star formation rates

Li¹,

Jiang²,

Fan³

et al. 2017

Nat Astron

View full text Add to dashboard Cite

Understanding star formation rates (SFR) is a central goal of modern star-formation models, which mainly involve gravity, turbulence and, in some cases, magnetic fields (Bfields) 1,2 . However, a connection between B-fields and SFR has never been observed. Here, a comparison between the surveys of SFR 3,4 and a study of cloud-field alignment 5 -which revealed a bimodal (parallel or perpendicular) alignment-shows consistently lower SFR per solar mass for clouds almost perpendicular to the B-fields. This is evidence of B-fields being a primary regulator of SFR. The perpendicular alignment possesses a significantly higher magnetic flux than the parallel alignment and thus a stronger support of the gas against selfgravity. This results in overall lower masses of the fragmented components, which are in agreement with the lower SFR.It is not difficult to imagine that the SFR of a molecular cloud should be related to the mass of gas it contains, due to self-gravity. On the whole, this trend has indeed been observed 3,4 . On the other hand, star formation efficiencies of molecular clouds are usually just a few percent 6 while cloud ages are at least comparable to their free-fall time (~Myr) 7 . This requires other forces to slow down gravitational contraction 8 . In addition, clouds with a similar mass and age can have different SFR; for example, it is well known that the Ophiuchus cloud has a significantly higher SFR than its neighbour, the Pipe Nebula 3,4 . In Figure 1, we can see a significant difference between the two dark clouds: Ophiuchus is accompanied by the colourful nebulae, which is a signature of active stellar feedback, while Pipe looks very quiescent in comparison. No one knows the reason for these differences in SFR. Another good example is the pair of Rosette and G216-2.5 molecular clouds 9 , where turbulence had been suspected as the cause of their very different SFRs. But later their turbulent velocity spectra were found almost identical 9 . Recently, a good agreement has been discovered between the empirical column density threshold for cloud contraction and the magnetic critical column density of the Galactic field (~10 µG) 5,10 . For densities lower than this threshold, the field strength is independent of densities 11 ; i.e., gas must accumulate along field lines. Also, a bimodal cloud-field alignment, which is another signature of field-regulated (sub-Alfvenic turbulence) cloud formation, was recently observed in the Gould Belt 5 , where, interestingly, Pipe and Ophiuchus (Figure 1) are aligned differently from their local fields. More intriguingly, it is the one aligned with the B-field that holds the higher SFR. Together with another observed piece of evidence that Galactic B-field direction anchors deeply into cloud cores 12 and thus plays a role in cloud fragmentation 13 , we were prompted to survey whether the cloud-field alignment has a connection with SFR.Luckily, both the cloud-field alignment 5 and SFR 3,4 of the Gould Belt clouds have been very well studied (Table 1). Both Heiderman e...

show abstract

“…It depends on the collapse state of the gas, the magnetic pressure, and the 6 This is referred to as the critical density in Li et al (2015).…”

Section: Physical Interpretation Of S Tmentioning

confidence: 99%

The Razor’s Edge of Collapse: The Transition Point From Lognormal to Power-Law Distributions in Molecular Clouds

Burkhart

Stalpes

Collins

2016

ApJ

View full text Add to dashboard Cite

We derive an analytic expression for the transitional column density value (s t ) between the lognormal and power-law form of the probability distribution function (PDF) in star-forming molecular clouds. Our expression for s t depends on the mean column density, the variance of the lognormal portion of the PDF, and the slope of the power-law portion of the PDF. We show that s t can be related to physical quantities such as the sonic Mach number of the flow and the power-law index for a self-gravitating isothermal sphere. This implies that the transition point between the lognormal and power-law density/column density PDF represents the critical density where turbulent and thermal pressure balance, the so-called "post-shock density." We test our analytic prediction for the transition column density using dust PDF observations reported in the literature as well as numerical MHD simulations of self-gravitating supersonic turbulence with the Enzo code. We find excellent agreement between the analytic s t and the measured values from the numerical simulations and observations (to within 1.5 A V ). We discuss the utility of our expression for determining the properties of the PDF from unresolved low density material in dust observations, for estimating the post-shock density, and for determining the HI-H 2 transition in clouds.

show abstract

Magnetized interstellar molecular clouds – I. Comparison between simulations and Zeeman observations

Cited by 80 publications

References 55 publications

GMC Collisions as Triggers of Star Formation. IV. The Role of Ambipolar Diffusion

GMC Collisions as Triggers of Star Formation. IV. The Role of Ambipolar Diffusion

The link between magnetic field orientations and star formation rates

The Razor’s Edge of Collapse: The Transition Point From Lognormal to Power-Law Distributions in Molecular Clouds

Contact Info

Product

Resources

About