Abstract
Abstract Wind-driven outflows are observed around a broad range of accreting objects throughout the universe, ranging from forming low-mass stars to supermassive black holes. We study the interaction between a central isotropic wind and an infalling, rotating envelope, which determines the steady-state cavity shape formed at their interface under the assumption of weak mixing. The shape of the resulting wind-blown cavity is elongated and self-similar, with a physical size determined by the ratio between wind ram pressure and envelope thermal pressure. We compute the growth of a warm turbulent mixing layer between the shocked wind and the deflected envelope, and calculate the resultant broad-line profile, under the assumption of a linear (Couette-type) velocity profile across the layer. We then test our model against the warm broad velocity component observed in CO J = 16–15 by Herschel/HIFI in the protostar Serpens-Main SMM1. Given independent observational constraints on the temperature and density of the dust envelope around SMM1, we find an excellent match to all its observed properties (line profile, momentum, temperature) and to the SMM1 outflow cavity width for a physically reasonable set of parameters: a ratio of wind to infall mass flux of ≃4%, a wind speed of v w ≃ 30 km s−1, an interstellar abundance of CO and H2, and a turbulent entrainment efficiency consistent with laboratory experiments. The inferred ratio of ejection to disk accretion rate, ≃6%–20%, is in agreement with current disk wind theories. Thus, the model provides a new framework to reconcile the modest outflow cavity widths in protostars with large observed flow velocities. Being self-similar, it is applicable over a broader range of astrophysical contexts as well.
Highlights
Massive outflows are observed everywhere in the universe, ranging from individual forming stars through galactic-scale events
Turbulent dissipation within the mixing layer provides a heating mechanism to make this material warmer than the shocked wind layer and brighter in high-J CO lines. We investigate whether such a model could explain at the same time the line profile, momentum, and temperature of the broad component observed in CO by Herschel toward the Serpens-Main SMM1 protostar, as well as the observed outflow cavity size, for reasonable envelope and wind parameters
With this value of cs, we find that the observed size of the outflow cavity in SMM1 can be reproduced with a wide-angle wind mass flux on the order of 4% of the envelope infall rate, which is quite modest
Summary
Massive outflows are observed everywhere in the universe, ranging from individual forming stars through galactic-scale events. Smith (1986) showed that elongated “flame-like” cavities could be obtained for isotropic envelopes with a purely radial pressure profile p(r) ∝ r− n, provided that n < 2 and the wind is obliquely deflected at its closest point of impact (e.g., by a small-scale thin disk) Both of these early calculations show that the addition of a dense, disk-like component along the horizontal axis can provide the required equatorial pinch to create steady, elongated outflow shapes similar to those observed around protostars. Despite an identical ambient density distribution, our cavity morphologies will strongly differ from the calculations of Wilkin & Stahler (2003), Mendoza et al (2004), and López-Vázquez et al (2019) in that we assume weak mixing, instead of full mixing, between the shocked wind and the shocked envelope, and we include the effect of thermal pressure in the envelope These two ingredients allow the existence of stable stationary solutions on large scales, with pointed shapes at the pole.
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