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I'm reading about planetary nebulae and how they are formed, but as is sometimes the case, I've gotten a little confused. So, I have a star, let's say 5 times the mass of the sun. At some point, when helium in the core has run out, the new core now consists primarily of carbon and oxygen, which cannot fuse in this medium mass star.

Now, In the meantime, a hydrogen and helium shell has been established outside the core. So when the helium shell starts to burn, it expands the distance between the two shells due to the contraction of the core. Then, at some point, the space between them is so huge, that the temperature at the hydrogen shell is not sufficient to keep the process going, and it "turns off". This makes the star expand greatly, because of only one burning shell, and that it makes a large convective zone where material from the core and such can be transported out into the surface.

So now the helium shell is burning out, and the hydrogen shell is getting closer to the core, i.e. higher temperature (due to the gravitational pull), and it re-ignites and start producing helium again, which then again can start burning at some point.

So, this results in the so-called thermal pulse as far as I understand.

My question is: What is it that "throws" the material out into space and makes planetary nebulae ? Is it the thermal pulse, that somehow bounces some of the material out every time it expands (And side question: Does it keep doing this until there is only the carbon-oxygen core left?), or is it radiation from the core that pushes some of it out, or something entirely different ? In short: I'm not sure what it is that exactly throws material out from the star !

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  • $\begingroup$ From what I have seen it's stellar wind caused by radiation pressure on dust condensing in the very thin and cool outer shell of the star. It's kind of hard to talk about a red giant star in terms of a dense object. At their largest they expand to hundreds of times their earlier diameter. This means that the density drops by six to eight orders of magnitude. The outer shell is therefor a gas with very low density. It seems that condensation of heavy elements to dust dominates the movement of the gas when these grains are driven strongly by radiation pressure due to their strong IR absorption. $\endgroup$ – CuriousOne Dec 22 '14 at 12:56
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    $\begingroup$ As far as I remember the radiation pressure is one of the terms in the equation of evolution of these nebulae. The equation(s) were developed and solved numerically by the group of A. Finzi. Do you have access to a library? Then, ask for the book of A. Finzi and R. Yahel, "Planetary Nebulae: Observations and Theory" books.google.co.il/books?isbn=9027708738 Y. Terzian - 1978 - ‎Science THEORETICAL ABUNDANCES IN PLANETARIES Arrigo Finzi and Raphael Yahel, Israel Institute of Technology, Haifa, Israel A satisfactory model of the ... $\endgroup$ – Sofia Dec 22 '14 at 13:18
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As the photosphere expands it eventually becomes cool enough to form dust. The dust formation is enhanced by shocks driven by pulsational instabilities. This dust is opaque to visible radiation and even though the radiation field from the star is weak, the surface gravity is so low that the radiation pressure is able to accelerate the material away. Typical terminal velocities are only $\sim 10$ km/s, but the mass loss rates can be very large ($\sim 10^{-5} M_{\odot}/yr$).

The process continues and appears to speed up - that is the mass loss increases as the star reaches the end of the AGB phase. This is termed the "AGB superwind". The exact mix of mechanisms responsible for this final phase be it pulsational instability or radiation pressure on copious dust, is still debated. What seems agreed is that it almost entirely removes the H-rich outer envelope.

AGB wind formation: Peter Woitke (c) 2006

See

http://iopscience.iop.org/0004-637X/698/2/1136

or a simpler version here; from which the image was taken (Peter Woitke (c)).

http://homepage.univie.ac.at/peter.woitke/AGB_popular.html

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