I was tracking these runaway stars for a SF story i had in mind, but this is the closest one i have heard of yet, and the ArXiv paper describes one that also passed thru 2.5 mya.
Gliese 710 will pass the Sun even closer
Close approach parameters recalculated based on the first Gaia data release
http://www.aanda.org/articles/aa/abs/2016/11/aa29835-16/aa29835-16.html
Close encounters of the stellar kind
https://arxiv.org/abs/1412.3648
tl:dr article
http://www.businessinsider.com/star-hurting-towards-solar-system-2016-12\
"Gliese 710 is about half the size of our sun, and it is set to reach Earth in 1.35 million years, according to a paper published in the journal Astronomy & Astrophysics in November.
And when it arrives, the star could end up a mere 77 light-days away from Earth — one light-day being the equivalent of how far light travels in one day, which is about 26 billion kilometers, the researchers worked out.
As far as we know, Gliese 710 isn't set to collide directly with Earth, but it wil be passing through the Oort Cloud, a shell of trillions of icy objects at the furthest reaches of our solar system. "
Seems like a great opportunity to send out some interstellar probes. The star will be trailing lots of ISM, free gas that would help bring a ramjet up to speed, and track till you could curve towards another destination. Likewise, a solar sail probe launched out in front of it by laser could "hitchhike" , and get some deep space ISM , and EM measurements.
Can we think of some other opportunities that this might present ? If we are past the filter by then, then we will already prob have samples of the Oort objects, but looks like they will be delivering then...
You get infall at speeds like that by falling close to the star - it is the strongest gravitational field in the system and the closer an orbit takes you to it the faster you go. Something falling from the Oort cloud to the surface of the sun reaches a speed of 600 km/s at the moment of impact, and at four solar radii away it is moving 300 km/s. Speed goes down with the square root of distance. Sungrazing comets do this all the time in our solar system, but they are for the most part smallish. The data seeing cometary material moving across the face of another star at that speed was interpreted to mean that there were large numbers of large sungrazing comets falling very close to the star at that point in that star system's evolution. Keep in mind that at the distances you are looking at, the entire star system is basically a pixel for most instruments so you are seeing the whole thing superimposed over itself.
When two objects interact gravitationally, the maximum physically possible delta V provided to a small object by the big object by a near miss is equal to twice the orbital velocity of the large object. From far away it basically looks like an elastic collision in basic mechanics, and that maximum speed is provided if it comes in and 'bounces off' exactly along the orbital vector of the larger object with an initial velocity of nearly zero. Planet 9, if it exists, has an orbital velocity on the order of low single digits kilometers per second and if it sent things into the inner solar system that 'elastic collision' would be very much not along the line of its orbit and impart rather less. The biggest thing that could impart velocity changes would be the other star and it would be passing through at 15 kilometers per second, at right angles to the radial direction towards the inner solar system at close approach. Furthermore, near misses would be exceptionally rare. These things are notable for stirring the orbital parameters of very slow moving oort cloud objects by providing a small far-away force for a short time over only a piece of its orbit and allowing some to approach the inner system on orbits with very similar energies rather than adding appreciable energy to the system.
I understand this, and as a young system, you would potentially have a lot more rocks affected by the proposed gas giant, but as you also point out, any un-bounded material should have already been ejected from the system. It is difficult, but obviously not impossible to change parabolas into hyper parabolas to enable these kind of speeds, but they obviously got close enough to hit the roche limit, or simply dissolved like the Christmas Comet of 2014.
Planet 9 is also theorized to be near 90 d(edit:30d) to orbital plane also, so tossing things out where ... (read more)