APOD: A Spiral Nebula Surrounding Star R... (2012 Oct 16)

[dougettinger]

Let me guess what you are thinking. The ejecta is expelled rather slowly and steadily. The heliomagnetosphere of the smaller star attracts and/or drags the expelled material causing it to spiral. As this ejecta passes the companion star in its outward migration it maintains a spiralling trajectory. As the material moves farther away it loses its spiral and initiates a circular orbit. Am I close?

Trying to think spirally,
Doug

[Ann]

Well, I don't think the ejecta are expelled slowly and steadily. At a very late stage of evolution, stars with a mass comparable to the Sun or greater become very unstable. They turn on their helium burning at or near their cores, which then expand and cool, which in turn shuts the helium burning off. But then the core contracts again, making the temperature rise, turning on the helium burning again. The onset of the helium burning often causes a very strong "pulse" working its way all the way through to the outer layers of the star, giving the outer layers an outward push. The way I understand it, it is these repeated "pulses" that make the red giant star expel more and more of its mass, eventually turning them into planetary nebulae surrounding hot exposed cores.

But you should really ask someone like Chris about the exact mechanisms here.

Ann

[neufer]

Let me guess what you are thinking. The ejecta is expelled rather slowly and steadily. The heliomagnetosphere of the smaller star attracts and/or drags the expelled material causing it to spiral. As this ejecta passes the companion star in its outward migration it maintains a spiralling trajectory. As the material moves farther away it loses its spiral and initiates a circular orbit. Am I close?

You are probably sort of close to what I was originally thinking; however, I am now modifying that somewhat:

<<The system modeled here consists of a primary AGB star going through a thermal pulse and a small companion star. The separation between the stars used in the simulation is 60 astronomical units with a total mass of the system of two solar masses. The orbital period is 350 years.>>

Thus we have two (near solar mass) stars orbiting each other at a distance of 60 AU with one of them bloated out as a red giant with a radius of ~1 AU. I think that it is safe to say that the strong but slow (~17 km/s) solar wind of the red giant will preferably funnel through the center of gravity point at 30 AU and then be more or less swept up by the leading edge of the non-red giant's magneto-tail in the fashion of a giant wisk broom.

[FLPhotoCatcher]

Let me guess what you are thinking. The ejecta is expelled rather slowly and steadily. The heliomagnetosphere of the smaller star attracts and/or drags the expelled material causing it to spiral. As this ejecta passes the companion star in its outward migration it maintains a spiralling trajectory. As the material moves farther away it loses its spiral and initiates a circular orbit. Am I close?

I don't think the heliomagnetosphere of the smaller star attracts the expelled material at all. Nor do I think it is as neufer says, "the... solar wind of the red giant will preferably funnel through the center of gravity point at 30 AU..." I think the spiral is created where the solar winds from the two stars collide, and the collision point rotates around the red giant as the smaller star orbits, then the piled-up gas is pushed away by the stronger solar wind from the red giant. The spiral turns into a circle (really a sphere) as the spiral bands crush together at the edge of the heliosphere called the heliopause.

[neufer]

I don't think the heliomagnetosphere of the smaller star attracts the expelled material at all. Nor do I think it is as neufer says, "the... solar wind of the red giant will preferably funnel through the center of gravity point at 30 AU..." I think the spiral is created where the solar winds from the two stars collide, and the collision point rotates around the red giant as the smaller star orbits, then the piled-up gas is pushed away by the stronger solar wind from the red giant. The spiral turns into a circle (really a sphere) as the spiral bands crush together at the edge of the heliosphere called the heliopause.

Perhaps, but remember that a steady solar wind comes from the high velocity component of an extended hot solar corona and, consequently, it should be very sensitive to small "gravitational" effects well above the solar surface.