As I understand it it’s where these particles reach equilibrium with the stellar medium. The sun is like a comet at a large enough scale, with a long tail of particles as it moves through the galaxy.
You might want to re-read the Wikipedia page. It explicitly says that Voyager 1 saw the density of plasma around it increase by a factor of 40 as it crossed the heliopause. (For Voyager 2, it was a factor of 20, as I have posted elsewhere in this discussion.) It also explicitly says that the solar wind is stopped at the heliopause due to the pressure of the interstellar medium, which, last I checked, means the interstellar medium is interacting with the solar wind.
> conservation of momentum means the particles leaving the sun don’t stop rotating when they leave the sun - the sun is rotating
Sure, with a period of about 27 days. Go do the math and compare the tangential velocity that equates to with the tangential velocity required to orbit the Sun just above the Sun's surface.
Only if the space they were escaping into were vacuum. Which it isn't. What stops them is not the Sun's gravity but the plasma in the interstellar medium.
There is no threshold of low enough density at which there is suddenly "vacuum". If there are particles present, there are particles present, and they can have effects.
> Last I checked, it was literally billions of times lower density than the hardest vacuum we’ve been able to produce on earth.
[Edit--these numbers are off--see my post downthread]
And the solar wind is much, much less dense than that. Interstellar medium density is about a trillion particles per cubic meter. Solar wind density is about 5 thousand particles per cubic meter. So the interstellar medium is more than dense enough to stop the solar wind.
I suspect you got your numbers reversed.
You are correct that the numbers I cited were off, because I had neglected to check specifically for numbers at the heliopause. Here is a better set of numbers:
https://ui.adsabs.harvard.edu/abs/2019NatAs...3.1024G/abstra...
The plasma density in the outer heliosphere is typically about 0.002 cm-3. The first electron density measured by the Voyager 2 plasma wave instrument in the interstellar medium, 0.039 cm-3 ± 15%, was on 30 January 2019 at a heliocentric radial distance of 119.7 au. The density jump, about a factor of 20, confirms that Voyager 2 crossed the heliopause.
In other words, the density of the interstellar medium just outside the heliopause, as detected by Voyager 2, was about 20 times larger than the density of the plasma just inside the heliopause.
If they had greater than escape velocity, they’d be escaping and we’d not see the graph we see.