Slingshot mechanism in Orion: Kinematic evidence for ejection of protostars by filaments

Abstract

By comparing three constituents of Orion A (gas, protostars, and pre-main-sequence stars), both morphologically and kinematically, we derive the following conclusions. The gas surface density near the integral-shaped filament (ISF) is very well represented by a power law, Σ(b) = 37 M pc −2 (b/pc) −5/8 , for the entire range to which we are sensitive, 0.05 pc < b < 8.5 pc, of projected separation from the filament ridge. Essentially all Class 0 and Class I protostars lie superposed on the ISF or on identifiable filament ridges farther south, while almost all pre-main-sequence (Class II) stars do not. Combined with the fact that protostars are moving < ∼ 1 km s −1 relative to the filaments, while stars are moving several times faster, this implies that protostellar accretion is terminated by a slingshot-like "ejection" from the filaments. The ISF is the third in a series of identifiable star bursts that are progressively moving south, with separations of several Myr in time and 2-3 pc in space. This, combined with the observed undulations in the filament (both spatial and velocity), suggest that repeated propagation of transverse waves through the filament is progressively digesting the material that formerly connected Orion A and B into stars in discrete episodes. We construct a simple, circularly symmetric gas density profile ρ(r) = 17 M pc −3 (r/pc) −13/8 consistent with the two-dimensional data. The model implies that the observed magnetic fields in this region are subcritical on spatial scales of the observed undulations, suggesting that the transverse waves propagating through the filament are magnetically induced. Because the magnetic fields are supercritical on scales of the filament as a whole (as traced by the power law), the system as a whole is relatively stable and long lived. Protostellar "ejection" (i.e., the slingshot) occurs because the gas accelerates away from the protostars, not the other way around. The model also implies that the ISF is kinematically young, which is consistent with several other lines of evidence. In contrast to the ISF, the southern filament (L1641) has a broken power law, which matches the ISF profile for 2.5 pc < b < 8.5 pc, but is shallower closer in. L1641 is kinematically older than the ISF.

Resolving the fragmentation of high line-mass filaments with ALMA: the integral shaped filament in Orion A

“…However, the different grouping of stars with disks shows that it is erased over the life-time of stars with disks (∼2 Myr; e.g., Evans et al 2009;Dunham et al 2014). This provides support for the dynamical slingshot model of Stutz & Gould (2016), in which the gravitational potential well of the surrounding gas reservoir in the ISF holds the protostars close to their maternal filament section. However, the (proto-)stars decouple from the gas reservoir due to the slingshot, at which point they can loose their initial grouping properties.…”

“…The hierarchical, scale-dependent fragmentation we observe in the ISF is not self-consistently predicted by any existing gravitational fragmentation models. We use the density profile along the filament derived by Stutz & Gould (2016) and show that the predictions for the Jeans' fragmentation scale is ∼10 000 AU and for the filamentary gravitational fragmentation scale is ∼40 000 AU. These scales are similar to the scales at which the cores show grouping, suggesting that gravitational fragmentation is an important process, even though our understanding of how exactly it proceeds is incomplete.…”

“…Specifically, filaments that have line-masses greatly in excess to the critical value of the self-gravitating, thermally supported, non-magnetised, infinitely long equilibrium model, i.e., ≫16 M ⊙ pc −1 (Ostriker 1964), contain large enough mass reservoirs to give birth to high-mass stars and star clusters (e.g., Pillai et al 2006;Beuther et al 2010Beuther et al , 2015aHenning et al 2010;Schneider et al 2012;Kainulainen et al 2013;Stutz & Gould 2016;Contreras et al 2016). This makes understanding their fragmentation and gravitational collapse important for Galactic-scale star formation.…”

“…At the distance of 420 pc (e.g., Schlafly et al 2014), the ISF is the nearest highly supercritical filament, and especially the nearest such filament in a giant molecular cloud that exhibits high-mass star formation. The ISF, located in the northern part of Orion A, is an ∼8 pc long structure with the line-mass of 385 × (d/pc) 3/8 M ⊙ pc −1 from 0.05 pc ¡ d ¡ 8.5 pc of projected separation from the filament ridge (Stutz & Gould 2016). Importantly, the young stellar population of Orion A is well characterised with Spitzer (Megeath et al 2012) and Herschel (the Herschel Orion Protostar Survey, HOPS; Furlan et al 2016).…”

Episodic accretion in binary protostars emerging from self-gravitating solar mass cores

“…The integral shaped filament in the Orion A cloud shows strong indications for a dynamical heating of the stars, which can possibly be explained by an oscillation of the filament (Stutz and Gould 2016). The latter…”