Superwind phase in stellar evolution

[vertices]

why is there very rapid mass loss in the superwind phase. my notes simply say "because the envelope attains positive binding energy".

Firstly, how can binding energy be positive by definition?!

Secondly, I am not entirely sure what the lecturer is talking about? what makes the envelope "attain positive binding energy" in the first place? Can't find anything relavent in Carroll and Ostlie:(

thanks in advance:)

[vertices]

bump this one too:)

[astrorob]

I'm not sure this mechanism is fully known, however during the AGB phase we observe that some stars begin to pulsate (Mira variables). The systematic compression and expansion causes material to "bunch up" making the formation of molecules (and dust etc.) easier. These trap the outgoing flux and essentially drive a strong wind.

[astrorob]

why is there very rapid mass loss in the superwind phase. my notes simply say "because the envelope attains positive binding energy".
thanks in advance:)

I think it just means that the bound system becomes energetically unfavourable in favour of the unbound? That is to say, the envelope would prefer to be ejected..

[vertices]

so the mass loss is simply a consequence of the star pulsating.

thanks again astrorob.

[cadnr]

Mass loss is due to radiation pressure (outward) overcoming gravitation (inward).

In very massive stars, the gravity at the surface is weak and the radiation strong, so the matter gets blown away.

As astrorob says, in AGB stars molecules form as the star pulsates. This is due to the dredge-up of carbon and/or oxygen from inside the star. The molecules form into grains which are easily blown outwards by the stellar radiation.