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

Monthly Notices of the Royal Astronomical Society

Kuiper

2020

Self Cite

We investigate the turbulence driving mode of ionizing radiation from massive stars on the surrounding interstellar medium (ISM). We run hydrodynamical simulations of a turbulent cloud impinged by a plane-parallel ionization front. We find that the ionizing radiation forms pillars of neutral gas reminiscent of those seen in observations. We quantify the driving mode of the turbulence in the neutral gas by calculating the driving parameter b, which is characterised by the relation σ 2 s = ln(1 + b 2 M 2 ) between the variance of the logarithmic density contrast σ 2 s (where s = ln(ρ/ρ 0 ) with the gas density ρ and its average ρ 0 ), and the turbulent Mach number M. Previous works have shown that b ∼ 1/3 indicates solenoidal (divergence-free) driving and b ∼ 1 indicates compressive (curl-free) driving, with b ∼ 1 producing up to ten times higher star formation rates than b ∼ 1/3. The time variation of b in our study allows us to infer that ionizing radiation is inherently a compressive turbulence driving source, with a time-averaged b ∼ 0.76 ± 0.08. We also investigate the value of b of the pillars, where star formation is expected to occur, and find that the pillars are characterised by a natural mixture of both solenoidal and compressive turbulent modes (b ∼ 0.4) when they form, and later evolve into a more compressive turbulent state with b ∼ 0.5-0.6. A virial parameter analysis of the pillar regions supports this conclusion. This indicates that ionizing radiation from massive stars may be able to trigger star formation by producing predominately compressive turbulent gas in the pillars.

Section: Introductionmentioning

confidence: 99%

On the turbulence driving mode of expanding H ii regions

Menon

Monthly Notices of the Royal Astronomical Society

Kuiper

2020

Self Cite

“…Certainly the presence of magnetic fields suppresses the density fluctuations caused by the turbulent motions (e.g. Molina et al 2012) and reduces the star-formation rate by a factor of a few (Padoan & Nordlund 2011;Federrath & Klessen E-mail: beattijr@mso.anu.edu.au † E-mail: christoph.federrath@anu.edu.au 2012; Federrath 2015; Krumholz & Federrath 2019). However, magnetic fields may also be responsible for overcoming resistance to star formation, for example, by magnetic reconnection, ambipolar diffusion or magnetic breaking.…”

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.

“…In both the SFK15 SF model and this work's Equation 26, this will result in a reduction in the amount of Σ SFR predicted to be in any system when compared to assuming complete lack of magnetic fields. Numerical simulations have also supported the presence of star formation suppression as an effect of introducing magnetic fields (Padoan & Nordlund 2011;Federrath 2015;Krumholz & Federrath 2019).…”

Section: Limitations and Caveatsmentioning

confidence: 86%

“…This supports many previous studies that have suggested that SF is not only a function of the molecular gas density. Other factors that have found to control SF rate include turbulence (Krumholz & McKee 2005;Federrath 2010;Padoan & Nordlund 2011;Hennebelle & Chabrier 2011;Federrath 2013b;Salim et al 2015), magnetic fields (Federrath 2015;Krumholz & Federrath 2019) and free-fall time, which is the time scale required for a medium with negligible pressure support to collapse ).…”

Section: Spatial Distribution Of Depletion Timesmentioning

confidence: 99%

Spinning Bar and a Star-formation Inefficient Repertoire: Turbulence in Hickson Compact Group NGC 7674

Salim

Alatalo

et al. 2020

ApJ

Self Cite

The physics regulating star formation (SF) in Hickson Compact Groups (HCG) has thus far been difficult to describe, due to their unique kinematic properties. In this study we expand upon previous works to devise a more physically meaningful SF relation able to better encompass the physics of these unique systems. We combine CO(1-0) data from the Combined Array from Research in Millimeter Astronomy (CARMA) to trace the column density of molecular gas Σ gas and deep Hα imaging taken on the Southern Astrophysical Research (SOAR) Telescope tracing Σ SFR to investigate star formation efficiency across face-on HCG, NGC7674. We find a lack of universality in star formation, with two distinct sequences present in the Σ gas − Σ SFR plane; one for inside and one for outside the nucleus. We devise a SF relation based on the multi-freefall nature of gas and the critical density, which itself is dependent on the virial parameter α vir , the ratio of turbulent to gravitational energy. We find that our modified SF relation fits the data and describes the physics of this system well with the introduction of a virial parameter of about 5-10 across the galaxy. This α vir leads to an order-of-magnitude reduction in SFR compared to α vir ≈ 1 systems.