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Register a new userSlingshot Mechanism for Clusters: Gas Density Regulates Star Density in the Orion Nebula Cluster (M42)
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Amelia M. Stutz
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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.
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The spatial morphology and dynamical status of a young, still-forming stellar cluster provide valuable clues on the conditions during the star formation event and the processes that regulated it. We analyze the Orion Nebula Cluster (ONC), utilizing the latest censuses of its stellar content and membership estimates over a large wavelength range. We determine the center of mass of the ONC, and study the radial dependence of angular substructure. The core appears rounder and smoother than the outskirts, consistent with a higher degree of dynamical processing. At larger distances the departure from circular symmetry is mostly driven by the elongation of the system, with very little additional substructure, indicating a somewhat evolved spatial morphology or an expanding halo. We determine the mass density profile of the cluster, which is well fitted by a power law that is slightly steeper than a singular isothermal sphere. Together with the ISM density, estimated from average stellar extinction, the mass content of the ONC is insufficient by a factor $sim 1.8$ to reproduce the observed velocity dispersion from virialized motions, in agreement with previous assessments that the ONC is moderately supervirial. This may indicate recent gas dispersal. Based on the latest estimates for the age spread in the system and our density profiles, we find that, at the half-mass radius, 90% of the stellar population formed within $sim 5$-$8$ free-fall times ($t_{rm ff}$). This implies a star formation efficiency per $t_{rm ff}$ of $epsilon_{rm ff}sim 0.04$-$0.07$, i.e., relatively slow and inefficient star formation rates during star cluster formation.
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