Moving-mesh Simulations of Star-forming Cores in Magneto-gravo-turbulence

Observations of star-forming regions by the current and upcoming generation of submillimeter polarimeters will shed new light on the evolution of magnetic fields over the cloud-to-core size scales involved in the early stages of the star formation process. Recent wide-area and high-sensitivity polarization observations have drawn attention to the challenges of modeling magnetic field structure of star forming regions, due to variations in dust polarization properties in the interstellar medium. However, these observations also for the first time provide sufficient information to begin to break the degeneracy between polarization efficiency variations and depolarization due to magnetic field sub-beam structure, and thus to accurately infer magnetic field properties in the star-forming interstellar medium. In this article we discuss submillimeter and far-infrared polarization observations of star-forming regions made with single-dish instruments. We summarize past, present and forthcoming single-dish instrumentation, and discuss techniques which have been developed or proposed to interpret polarization observations, both in order to infer the morphology and strength of the magnetic field, and in order to determine the environments in which dust polarization observations reliably trace the magnetic field. We review recent polarimetric observations of molecular clouds, filaments, and starless and protostellar cores, and discuss how the application of the full range of modern analysis techniques to recent observations will advance our understanding of the role played by the magnetic field in the early stages of star formation.

Section: Comparison With Simulationsmentioning

confidence: 99%

Submillimeter and Far-Infrared Polarimetric Observations of Magnetic Fields in Star-Forming Regions

Pattle

Fissel

2019

“…One of the predicted signposts of highly magnetized star formation is that at high enough densities ( 10 4 cm −3 ), the collapse of strongly magnetized gas should pinch the magnetic field into an "hourglass" shape with a symmetry axis perpendicular to the major axis of a flattened, ∼ 1000 au "pseudodisk" (Galli and Shu, 1993a,b). The hourglass is expected to persist down to scales < 1000 au (Fiedler and Mouschovias, 1993;Galli and Shu, 1993b;Allen et al, 2003;Gonçalves et al, 2008;Frau et al, 2011;Kataoka et al, 2012;Mocz et al, 2017); see Figure 1. And indeed, the predicted hourglass has now been seen in a number of interferometric observations of low-mass protostellar cores (Girart et al, 1999(Girart et al, , 2006Rao et al, 2009;Stephens 6 The two polarization results from OVRO are observations toward NGC 1333-IRAS 4A and IRAS 16293 (Akeson et al, 1996;Akeson and Carlstrom, 1997); however, OVRO was known to have issues with polarization calibration, which is the most likely explanation for the inconsistency of those results with later observations of the same sources (Girart et al, 1999(Girart et al, , 2006Rao et al, 2009).…”

Section: The Role Of the Magnetic Field In Protostellar Collapsementioning

confidence: 99%

Interferometric Observations of Magnetic Fields in Forming Stars

Hull

Zhang

2019

108

103

The magnetic field is a key ingredient in the recipe of star formation. However, the importance of the magnetic field in the early stages of the formation of low-and high-mass stars is still far from certain. Over the past two decades, the millimeter and submillimeter interferometers BIMA, OVRO, CARMA, SMA, and most recently ALMA have made major strides in unveiling the role of the magnetic field in star formation at progressively smaller spatial scales; ALMA observations have recently achieved spatial resolutions of up to ∼ 100 au and ∼ 1,000 au in nearby low-and high-mass star-forming regions, respectively. From the kiloparsec scale of molecular clouds down to the inner few hundred au immediately surrounding forming stars, the polarization at millimeter and submillimeter wavelengths is dominated by polarized thermal dust emission, where the dust grains are aligned relative to the magnetic field. Interferometric studies have focused on this dust polarization and occasionally on the polarization of spectral-line emission. We review the current state of the field of magnetized star formation, from the first BIMA results through the latest ALMA observations, in the context of several questions that continue to motivate the studies of highand low-mass star formation. By aggregating and analyzing the results from individual studies, we come to several conclusions: (1) Magnetic fields and outflows from low-mass protostellar cores are randomly aligned, suggesting that the magnetic field at ∼ 1000 au scales is not the dominant factor in setting the angular momentum of embedded disks and outflows. (2) Recent measurements of the thermal and dynamic properties in high-mass star-forming regions reveal small virial parameters, challenging the assumption of equilibrium star formation. However, we estimate that a magnetic field strength of a fraction of a mG to several mG in these objects could bring the dense gas close to a state of equilibrium. Finally, (3) We find that the small number of sources with hourglass-shaped magnetic field morphologies at 0.01-0.1 pc scales cannot be explained purely by projection effects, suggesting that while it does occur occasionally, magnetically dominated core collapse is not the predominant mode of low-or high-mass star formation. Magnetic fields in forming stars Polarized molecular-line emissionPolarization of molecular-line emission is another tracer of the magnetic field in star-forming regions. Molecular and atomic lines are sensitive to magnetic fields, which cause their spectral levels to split into magnetic sub-levels. For some molecules, linear polarization can arise when an anisotropy in the radiation and/or velocity field yields a population of magnetic sub-levels that are not in local thermodynamic equilibrium (LTE); this is known as the Goldreich-Kylafis (G-K) effect. Polarization from the G-K effect is most easily detected where the spectral line emission has an optical depth τ ≈ 1, when the ratio of the collision rate to the radiative transition rate (i.e., the spontaneous emi...

“…This narrowing will lead to less mass exceeding the threshold density for the onset of collapse. This effect has been studied by a number of authors (e.g., Cho and Lazarian, 2003;Kowal et al, 2007;Burkhart et al, 2009;Molina et al, 2012;Mocz et al, 2017), but its magnitude is still not entirely certain, because it depends crucially on the scaling of magnetic field strength with density. The density jump across an isothermal shock of sonic Mach number M with pre-shock ratio of thermal to magnetic pressure β 0 will depend on how the pre-and post-shock magnetic fields compare, which is determined by the relative orientation between the field and the shock plane.…”

Section: Star Formation Rates From Magnetized and Non-magnetized Turbmentioning

confidence: 99%

The Role of Magnetic Fields in Setting the Star Formation Rate and the Initial Mass Function

Krumholz

Federrath

2019

150

Star-forming gas clouds are strongly magnetized, and their ionization fractions are high enough to place them close to the regime of ideal magnetohydrodyamics on all but the smallest size scales. In this review we discuss the effects of magnetic fields on the star formation rate (SFR) in these clouds, and on the mass spectrum of the fragments that are the outcome of the star formation process, the stellar initial mass function (IMF). Current numerical results suggest that magnetic fields by themselves are minor players in setting either the SFR or the IMF, changing star formation rates and median stellar masses only by factors of ∼ 2−3 compared to non-magnetized flows. However, the indirect effects of magnetic fields, via their interaction with star formation feedback in the form of jets, photoionization, radiative heating, and supernovae, could have significantly larger effects. We explore evidence for this possibility in current simulations, and suggest avenues for future exploration, both in simulations and observations.