A Two-Wind Interaction Model for Proplyds

Astronomy and Astrophysics – Astronomy

Scientific paper

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Ism: H Ii Regions, Hydrodynamics, Ism: Individual Name: Orion Nebula, Ism: Jets And Outflows, Stars: Individual Constellation Name: Theta 1 Orionis C, Stars: Mass Loss

Scientific paper

Recent HST observations of the Orion Nebula show the presence of compact (˜2") emission-line objects ("proplyds") with bowlike morphologies and tails pointing away from the θ1C Ori star. We model these objects as the result of the interaction between the fast wind from θ1C and slow dense winds from accretion disks around young, low-mass stars, which are photoevaporated by the ionizing radiation coming from this massive star.
We develop a fully analytic model for this two-wind interaction, which shows that depending on the value of the dimensionless parameter λ = F0c0/nwVw2 where c0 is the sound speed of the ionized gas, F0 is the ionizing photon flux impinging on the surface of the accretion disks, and nwVw2 is the specific momentum flux of the wind from θ1C, both "choked" subsonic (low λ) solutions and "free" supersonic (high λ) solutions can be found. We argue that for the case of θ1C, this second supersonic regime is relevant.
For the supersonic regime, we find that both the properties of the exciting star (θ1C) and the size of the accretion disk that ejects the photoevaporated wind enter the solution only as a direct scaling of the size of the proplyd. The only physical parameter with a more complex effect on the problem is the orientation between the axis of the accretion disk and the direction to θ1C.
We finally use this analytic model to produce predicted emission measure maps (which are directly compared to the HST images of O'Dell & Wen 1994). A good qualitative agreement is found at least for some of the proplyds observed in the Orion Nebula.

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