A two-wind interaction model for proplyds

Recent HST observations of the Orion Nebula show the presence of compact (similar to 2 '') emission-line objects (''proplyds'') with bowlike morphologies and tails pointing away from the theta(1)C Ori star. We model these objects as the result of the interaction between the fast wind from theta(1)C Ori 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 lambda = F(o)c(o)/n(w)v(w)(2), where c(o) is the sound speed of the ionized gas, F-o is the ionizing photon flux impinging on the surface of the accretion disks, and n(w)v(w)(2) is the specific momentum flux of the wind from theta(1)C Ori, both ''choked'' subsonic (low lambda) solutions and ''free'' supersonic (high lambda) solutions can be found. We argue that for the case of theta(1)C Ori, this second supersonic regime is relevant. For the supersonic regime, we find that both the properties of the exciting star (theta(1)C Ori) and the size of the accretion disk that ejects the photoevaporated wind enter the solution only as a direst 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 theta(1)C Ori. 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.