[Paper Review] High-mass star formation triggered by collision between CO filaments in N159 West in the Large Magellanic Cloud

Y. Fukui, Ryohei Harada+21 more|arXiv (Cornell University)|Mar 12, 2015

Astrophysics and Star Formation Studies4 references3 citations

TL;DR

This study presents the first detection of extragalactic protostellar molecular outflows in N159 West, LMC, using ALMA 13CO(2–1) observations. It proposes that a high-mass star (37 M☉) formed via collision-induced turbulence at the intersection of two CO filaments ~10⁵ years ago, with enhanced mass accretion (~6×10⁻⁴ M☉ yr⁻¹) driven by shock-induced turbulence overcoming stellar feedback.

ABSTRACT

We have carried out 13CO(J=2-1) observations of the active star-forming region N159 West in the LMC with ALMA. We have found that the CO distribution at a sub-pc scale is highly elongated with a small width. These elongated clouds called "filaments" show straight or curved distributions with a typical width of 0.5-1.0pc and a length of 5-10pc. All the known infrared YSOs are located toward the filaments. We have found broad CO wings of two molecular outflows toward young high-mass stars in N159W-N and N159W-S, whose dynamical timescale is ~10^4 yrs. This is the first discovery of protostellar outflow in external galaxies. For N159W-S which is located toward an intersection of two filaments we set up a hypothesis that the two filaments collided with each other ~10^5 yrs ago and triggered formation of the high-mass star having ~37 Mo. The colliding clouds show significant enhancement in linewidth in the intersection, suggesting excitation of turbulence in the shocked interface layer between them as is consistent with the magneto-hydro-dynamical numerical simulations (Inoue & Fukui 2013). This turbulence increases the mass accretion rate to ~6x10^-4 Mo yr^-1, which is required to overcome the stellar feedback to form the high-mass star.

Motivation & Objective

• Investigate the physical mechanism triggering high-mass star formation in the N159 West region of the Large Magellanic Cloud.
• Determine whether cloud-cloud collisions between CO filaments can initiate high-mass star formation in external galaxies.
• Assess the role of turbulence and mass accretion rates in enabling high-mass star formation despite strong stellar feedback.
• Provide observational evidence for a non-gravitational, collision-driven mechanism for core formation in high-mass star formation.

Proposed method

• Conducted high-resolution 13CO(J=2–1) observations of N159 West with ALMA to map molecular gas structure and kinematics at sub-pc scales.
• Identified elongated, filamentary CO structures with widths of 0.5–1.0 pc and lengths of 5–10 pc, aligned with known infrared YSOs.
• Analyzed line profiles to detect broad CO wings indicative of molecular outflows, with dynamical timescales ~10⁴ yrs.
• Modeled the collision scenario between two filaments intersecting at N159W-S, estimating collision timescale (~6×10⁴ yrs) from cloud velocity dispersion and separation.
• Calculated mass accretion rate (~6×10⁻⁴ M☉ yr⁻¹) from stellar mass (37 M☉) and inferred formation timescale (~10⁵ yrs), consistent with theoretical thresholds.
• Used ALMA continuum and CS(2–1) data to set upper limits on dust and dense gas line-mass densities, confirming lack of pre-existing dense cores.

Experimental results

Research questions

• RQ1 Can filament collisions in external galaxies trigger high-mass star formation, and what evidence supports this?
• RQ2 What is the role of turbulence generated by cloud-cloud collisions in enhancing mass accretion rates during high-mass star formation?
• RQ3 Is the observed high-mass star in N159W-S formed via gravitational collapse of a pre-existing dense core, or via collision-driven mass concentration?
• RQ4 How do the observed outflow dynamical timescales and mass accretion rates compare with theoretical requirements for high-mass star formation?

Key findings

• The first extragalactic protostellar molecular outflows were detected toward young high-mass stars in N159W-N and N159W-S, with dynamical timescales of ~10⁴ years.
• The high-mass star in N159W-S is located at the intersection of two CO filaments, suggesting a collision-triggered origin ~10⁵ years ago.
• The collision site shows enhanced linewidths, indicating shock-excited turbulence consistent with MHD simulations (Inoue & Fukui, 2013).
• The inferred mass accretion rate (~6×10⁻⁴ M☉ yr⁻¹) is sufficient to overcome stellar feedback and form a 37 M☉ star.
• No dense clumps or significant dust emission were detected in the filaments outside the intersection, indicating no pre-existing massive cores.
• The results support a scenario where collision-induced turbulence efficiently concentrates mass into a shock-compressed core without requiring initial self-gravity.