A 2-WIND INTERACTION-MODEL FOR PROPLYDS

Recent HST observations of the Orion Nebula show the presence of compa ct (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 arou nd young, low-mass stars, which are photoevaporated by the ionizing ra diation coming from this massive star. We develop a fully analytic mod el for this two-wind interaction, which shows that depending on the va lue of the dimensionless parameter lambda = F(o)c(o)/n(w)v(w)(2), wher e c(o) is the sound speed of the ionized gas, F-o is the ionizing phot on 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 (hi gh lambda) solutions can be found. We argue that for the case of theta (1)C Ori, this second supersonic regime is relevant. For the supersoni c 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 photoevap orated wind enter the solution only as a direst scaling of the size of the proplyd. The only physical parameter with a more complex effect o n the problem is the orientation between the axis of the accretion dis k and the direction to theta(1)C Ori. We finally use this analytic mod el to produce predicted emission measure maps (which are directly comp ared to the HST images of O'Dell & Wen 1994). A good qualitative agree ment is found at least for some of the proplyds observed in the Orion Nebula.