On the Interaction of a Bonnor–Ebert Sphere with a Stellar Wind

Abstract

The structure of protostellar cores can often be approximated by isothermal Bonnor–Ebert spheres (BES), which are stabilized by an external pressure. For the typical pressure of 10 4 k B K cm −3 to 10 5 k B K cm −3 found in molecular clouds, cores with masses below 1.5 M ⊙ are stable against gravitational collapse. In this paper, we analyze the efficiency of triggering gravitational collapse with a nearby stellar wind, which represents an interesting scenario for triggering low-mass star formation. We analytically derive a new stability criterion for a BES compressed by a stellar wind, which depends on its initial nondimensional radius ξ max . If the stability limit is violated the wind triggers a core collapse. Otherwise, the core is destroyed by the wind. We estimate its validity range to 2.5 < ξ max < 4.2 and confirm this in simulations with the SPH-Code GADGET-3. The efficiency of triggering a gravitational collapse strongly decreases for ξ max < 2.5 since in this case destruction and acceleration of the whole sphere begin to dominate. We were unable to trigger a collapse for ξ max < 2 , which leads to the conclusion that a stellar wind can move the smallest unstable stellar mass to 0.5 M ⊙ and that destabilizing even smaller cores would require external pressure larger than 10 5 k B K cm −3 . For ξ max > 4.2 the expected wind strength according to our criterion is small enough that the compression is slower than the sound speed of the BES and sound waves can be triggered. In this case our criterion somewhat underestimates the onset of collapse and detailed numerical analyses are required.