The role of initial magnetic field structure in the launching of protostellar jets

Monthly Notices of the Royal Astronomical Society

Körtgen

2020

Self Cite

Complex turbulent motions of magnetized gas are ubiquitous in the interstellar medium. The source of this turbulence, however, is still poorly understood. Previous work suggests that compression caused by supernova shockwaves, gravity, or cloud collisions, may drive the turbulence to some extent. In this work, we present threedimensional (3D) magnetohydrodynamic (MHD) simulations of contraction in turbulent, magnetized clouds from the warm neutral medium (WNM) of the ISM to the formation of cold dense molecular clouds, including radiative heating and cooling. We test different contraction rates and find that observed molecular cloud properties, such as the temperature, density, Mach number, and magnetic field strength, and their respective scaling relations, are best reproduced when the contraction rate equals the turbulent turnover rate. In contrast, if the contraction rate is significantly larger (smaller) than the turnover rate, the compression drives too much (too little) turbulence, producing unrealistic cloud properties. The relation σ 2 s = ln(1 + b 2 M 2 ) between logarithmic density fluctuations (σ s ) and turbulent Mach number (M) is found to be consistent with previous theoretical models that were based on artificially-driven isothermal turbulence. Here we find that the effective turbulence driving parameter of contraction-driven MHD turbulence subject to heating and cooling grows from solenoidal (b ∼ 1/3) to compressive (b ∼ 1) during the contraction. Overall, the physical properties of the simulated clouds that contract at a rate equal to the turbulent turnover rate, indicate that large-scale contraction induced by processes such as supernova shockwaves, gravity, spiral-arm compression, or cloud collisions, may explain the origin and evolution of turbulence in the ISM.

Section: Construction Of Initial Turbulent Magnetic Fieldmentioning

confidence: 99%

Molecular cloud formation by compression of magnetized turbulent gas subjected to radiative cooling

Mandal

Monthly Notices of the Royal Astronomical Society

Körtgen

2020

Self Cite

“…The ejection of a companion may enhance or initiate these processes (Moe & Kratter 2018). Turbulence and magnetic fields also play a significant role in the structure and evolution of the disc (Seifried et al 2015;Kuffmeier et al 2017;Gerrard et al 2019).…”

Section: Introductionmentioning

confidence: 99%

The dependence of episodic accretion on eccentricity during the formation of binary stars

Kuruwita

Haugbølle

2020

A&A

Self Cite

Context. Episodic accretion has been observed in short-period binaries, where bursts of accretion occur at periastron. The binary trigger hypothesis has also been suggested as a driver for accretion during protostellar stages. Aims. Our goal is to investigate how the strength of episodic accretion bursts depends on eccentricity. Methods. We investigate the binary trigger hypothesis in longer-period (> 20 yr) binaries by carrying out three-dimensional magnetohydrodynamical simulations of the formation of low-mass binary stars down to final separations of ∼10 AU, including the effects of gas turbulence and magnetic fields. We ran two simulations with an initial turbulent gas core of one solar mass each and two different initial turbulent Mach numbers, ℳ = σv/cs = 0.1 and ℳ = 0.2, for 6500 yr after protostar formation. Results. We observe bursts of accretion at periastron during the early stages when the eccentricity of the binary system is still high. We find that this correlation between bursts of accretion and passing periastron breaks down at later stages because of the gradual circularisation of the orbits. For eccentricities greater than e = 0.2, we observe episodic accretion triggered near periastron. However, we do not find any strong correlation between the strength of episodic accretion and eccentricity. The strength of accretion is defined as the ratio of the burst accretion rate to the quiescent accretion rate. We determine that accretion events are likely triggered by torques between the rotation of the circumstellar disc and the approaching binary stars. We compare our results with observational data of episodic accretion in short-period binaries and find good agreement between our simulations and the observations. Conclusions. We conclude that episodic accretion is a universal mechanism operating in eccentric young binary-star systems, independent of separation, and it should be observable in long-period binaries as well as in short-period binaries. Nevertheless, the strength depends on the torques and hence the separation at periastron.

“…The complex interplay of these physical processes is still not fully understood (Mouschovias et al 2006;Lazarian et al 2012;Hennebelle & Inutsuka 2019). Magnetic fields also may play a role in feedback, for example, by driving protostellar jets that inject mass, momentum and energy into the medium, possibly stirring turbulent motions (Frank et al 2014;Gerrard et al 2019;Krumholz & Federrath 2019;. They also seem to play a role in the orientation of filamentary structures and striations in the MCs which may not directly influence the star-forming potential of the cloud, but certainly influences the global structure of the cloud through the density PDF (Planck Collaboration et al 2016a,b;Cox et al 2016;Malinen et al 2016;Soler et al 2017;Tritsis & Tassis 2016;Soler 2019).…”

Section: Introductionmentioning

confidence: 99%

Filaments and striations: anisotropies in observed, supersonic, highly magnetized turbulent clouds

Beattie

Monthly Notices of the Royal Astronomical Society

2019

Self Cite

Stars form in highly-magnetised, supersonic turbulent molecular clouds. Many of the tools and models that we use to carry out star formation studies rely upon the assumption of cloud isotropy. However, structures like high-density filaments in the presence of magnetic fields, and magnetosonic striations introduce anisotropies into the cloud. In this study we use the two-dimensional (2D) power spectrum to perform a systematic analysis of the anisotropies in the column density for a range of Alfvén Mach numbers (M A = 0.1-10) and turbulent Mach numbers (M = 2-20), with 20 highresolution, three-dimensional (3D) turbulent magnetohydrodynamic simulations. We find that for cases with a strong magnetic guide field, corresponding to M A < 1, and M 4, the anisotropy in the column density is dominated by thin striations aligned with the magnetic field, while for M 4 the anisotropy is significantly changed by high-density filaments that form perpendicular to the magnetic guide field. Indeed, the strength of the magnetic field controls the degree of anisotropy and whether or not any anisotropy is present, but it is the turbulent motions controlled by M that determine which kind of anisotropy dominates the morphology of a cloud.