[Paper Review] The Formation of Massive Molecular Filaments and Massive Stars Triggered by a MHD Shock Wave

Tsuyoshi Inoue, P. Hennebelle+4 more|arXiv (Cornell University)|Jul 7, 2017

Astrophysics and Star Formation Studies15 references3 citations

TL;DR

This paper proposes that a strong magnetohydrodynamic (MHD) shock wave from a molecular cloud collision triggers the formation of massive molecular filaments and massive stars. Using high-resolution isothermal MHD simulations with adaptive mesh refinement and sink particles, it demonstrates that shock compression focuses gas into high-line-mass filaments perpendicular to the magnetic field, enabling global collapse and accretion rates exceeding $10^{-4}\,M_\odot\,\text{yr}^{-1}$, leading to sink masses over 50 $M_\odot$ within $\sim3\times10^5$ years—indicating viable pathways for O-type star formation.

ABSTRACT

Recent observations suggest that intensive molecular cloud collision can trigger massive star/cluster formation. The most important physical process caused by the collision is a shock compression. In this paper, the influence of a shock wave on the evolution of a molecular cloud is studied numerically by using isothermal magnetohydrodynamics (MHD) simulations with the effect of self-gravity. Adaptive-mesh-refinement and sink particle techniques are used to follow long-time evolution of the shocked cloud. We find that the shock compression of turbulent inhomogeneous molecular cloud creates massive filaments, which lie perpendicularly to the background magnetic field as we have pointed out in a previous paper. The massive filament shows global collapse along the filament, which feeds a sink particle located at the collapse center. We observe high accretion rate dot{M}_acc > 10^{-4} M_sun/yr that is high enough to allow the formation of even O-type stars. The most massive sink particle achieves M>50 M_sun in a few times 10^5 yr after the onset of the filament collapse.

Motivation & Objective

• To investigate how strong MHD shock waves from molecular cloud collisions drive the formation of massive molecular filaments and massive stars.
• To determine whether shock compression in magnetized, turbulent molecular clouds can produce conditions favorable for massive star formation.
• To assess the role of magnetic fields in altering shock compression dynamics and enabling high-mass accretion rates.
• To evaluate the robustness of massive filament and star formation under varying numerical resolution and initial conditions.
• To compare simulation outcomes with observational features in regions like S116 and NGC6334, where filamentary structures suggest collision-induced triggering.

Proposed method

• Numerical simulations using isothermal magnetohydrodynamics (MHD) with self-gravity to model shock compression in turbulent, magnetized molecular clouds.
• Adaptive-mesh refinement (AMR) is employed to resolve small-scale structures and high-density regions dynamically.
• Sink particle technique is implemented to model gravitational collapse and accretion, with formation threshold at $\sim3\times10^{-17}\,\text{g cm}^{-3}$.
• The simulations track long-term evolution of the shocked cloud, focusing on filament formation, line-mass evolution, and accretion history.
• The Alfvén Mach number and magnetic field strength are key parameters, with compression ratio $r \simeq \sqrt{2}\,M_{\rm A}$ for $M_{\rm A} > M_{\rm s}$.
• Sensitivity tests are performed by varying AMR levels and sink formation thresholds to assess numerical convergence.

Experimental results

Research questions

• RQ1 Can a strong MHD shock wave from a cloud collision produce massive molecular filaments with sufficient line-mass to form massive stars?
• RQ2 What is the role of magnetic fields in modifying shock compression and enabling high accretion rates in filaments?
• RQ3 Can the observed high accretion rates ($>10^{-4}\,M_\odot\,\text{yr}^{-1}$) in massive star-forming regions be reproduced by shock-induced filament collapse?
• RQ4 How does the mass and accretion rate of the most massive sink particle evolve over time in the presence of MHD shocks and self-gravity?
• RQ5 Is the formation of massive stars via shock-triggered filament collapse robust across different numerical resolutions and sink formation thresholds?

Key findings

• The shock compression of a turbulent, magnetized molecular cloud forms massive filaments with line-masses up to $100\,M_\odot\,\text{pc}^{-1}$, aligned perpendicularly to the magnetic field.
• These filaments undergo global longitudinal collapse, feeding a central sink particle with an accretion rate exceeding $10^{-4}\,M_\odot\,\text{yr}^{-1}$, sufficient for O-type star formation.
• The most massive sink particle reaches a mass of over $50\,M_\odot$ within $\sim3\times10^5$ years after the onset of filament collapse.
• The formation mechanism is driven by focusing flows due to curved MHD shocks, consistent with the theoretical framework proposed by Inoue & Fukui (2013).
• The simulations show that the massive star formation process is robust against resolution changes, as the sink mass and accretion rate remain stable across different AMR levels.
• The results are consistent with observations of massive filaments in regions like S116 and NGC6334, where shock-induced filament enhancement is inferred.