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Register a new userSlingshot Mechanism in Orion: Kinematic Evidence For Ejection of Protostars by Filaments
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By comparing 3 constituents of Orion A (gas, protostars, and pre-main-sequence stars), both morhologically and kinematically, we derive the following. The gas surface density near the integral-shaped filament (ISF) is well represented by a power law, Sigma(b)=72 Msun/pc^2(b/pc)^{-5/8} for our entire range, 0.05<b/pc<8.5, of distance from the filament ridge. Essentially all protostars lie on the ISF or other filament ridges, while almost all pre-main-sequence stars do not. Combined with the fact that protostars move <1 kms relative to the filaments while stars move several times faster, this implies that protostellar accretion is terminated by a slingshot ejection from the filaments. The ISF is the 3rd in a series of star bursts that are progressively moving south, with separations of a few Myr in time and 3 pc in space. This, combined with the filaments observed undulations (spatial and velocity), suggests that repeated propagation of transverse waves thru the filament is progressively digesting the material that formerly connected Orion A and B into stars in discrete episodes. We construct an axially symmetric gas density profile rho(r)=16 Msun/pc^3(r/pc)^{-13/8}. The model implies that the observed magnetic fields are supercritical on scales of the observed undulations, suggesting that the filaments transverse waves are magnetically induced. Because the magnetic fields are subcritical on scales of the filament on larger scales, the system as a whole is relatively stable and long lived. Protostellar ejection 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 other lines of evidence. The southern filament has a broken power law, which matches the ISF profile for 2.5<b/pc<8.5, but is shallower closer in. It is also kinematically older than the ISF.
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We characterize the stellar and gas volume density, potential, and gravitational field profiles in the central $sim$ 0.5 pc of the Orion Nebula Cluster (ONC), the nearest embedded star cluster (or rather, proto-cluster) hosting massive star formation available for detailed observational scrutiny. We find that the stellar volume density is well characterized by a Plummer profile $rho_{stars}(r) = 5755,{rm M}_{odot},{rm pc}^{-3},(1+(r/a)^2)^{-5/2}$, where $a = 0.36$ pc. The gas density follows a cylindrical power law $rho_{gas}(R) = 25.9,{rm M}_{odot},{rm pc}^{-3},(R/{rm pc})^{-1.775}$. The stellar density profile dominates over the gas density profile inside $r,sim,1$ pc. The gravitational field is gas-dominated at all radii, but the contribution to the total field by the stars is nearly equal to that of the gas at $r,sim,a$. This fact alone demonstrates that the proto-cluster cannot be considered a gas-free system or a virialized system dominated by its own gravity. The stellar proto-cluster core is dynamically young, with an age of $sim$ 2-3 Myr, a 1D velocity dispersion of $sigma_{rm obs} = 2.6$ km s$^{-1}$, and a crossing time of $sim$ 0.55 Myr. This timescale is almost identical to the gas filament oscillation timescale estimated recently by Stutz & Gould (2016). This provides strong evidence that the proto-cluster structure is regulated by the gas filament. The proto-cluster structure may be set by tidal forces due to the oscillating filamentary gas potential. Such forces could naturally suppress low density stellar structures on scales $gtrsim,a$. The analysis presented here leads to a new suggestion that clusters form by an analog of the slingshot mechanism previously proposed for stars.
We have measured astrometry for members of the Orion Nebula Cluster with images obtained in 2015 with the Wide Field Camera 3 on board the Hubble Space Telescope. By comparing those data to previous measurements with NICMOS on Hubble in 1998, we have discovered that a star in the Kleinmann-Low Nebula, source x from Lonsdale et al. (1982), is moving with an unusually high proper motion of 29 mas/yr, which corresponds to 55 km/s at the distance of Orion. Previous radio observations have found that three other stars in the Kleinmann-Low Nebula (BN and sources I and n) have high proper motions (5-14 mas/yr) and were near a single location ~540 years ago, and thus may have been members of a multiple system that dynamically decayed. The proper motion of source x is consistent with ejection from that same location 540 years ago, which provides strong evidence that the dynamical decay did occur and that the runaway star BN originated in the Kleinmann-Low Nebula rather than the nearby Trapezium cluster. However, our constraint on the motion of source n is significantly smaller than the most recent radio measurement, which indicates that it did not participate in the event that ejected the other three stars.
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