Vanadium 50 said:
What does one expect? I'd expect ejected bodies to be going a little faster (relative to the LSR) than the bodies they were ejected from. So maybe 30 km/s would be most probable. Now, 8 km/s is not an impossible number, but it is unusual - phase space considerations (an idealization - maybe even an oversimplification - to be sure) would suggest maybe 1 in 10 or so such objects would be going that fast or slower.
So 1km/s would be more puzzling. Not impossible, but slower than ~99.9% of the expected objects. 15 km/s would be less puzzling - maybe a quarter of the objects would be that slow or slower. 1000 km/s would be very surprising, as it is faster than the galaxy's escape velocity.
First, there seems to be a matter of language use: when you say puzzling, it implies that the cause is somehow unknown, perplexing, completely unexpected. As in, 'how in the world did it get to have that velocity?' kind of puzzling.
But from your post it looks like what you meant is that it is just an outlier in some distribution. I was objecting to the former (all of those velocities can be produced by known interactions, hence they're not puzzling), not to the latter.
But let's look at whether 8 km/s w/r to LSR is really an outlier. There are two things to take into consideration here:
1.
Ejection events are more likely to be caused by gradual (over many orbits) changes to orbital angular momentum of an asteroid whose orbit is coupled to some massive planet, rather than a one-off close encounter resulting in large delta V - simply because there will always be more asteroids affected at long range than in close encounters.
Since the most likely ejection is by incremental boosting of orbital momentum, the velocities of ejecta should be clustered around the escape velocity, i.e. one would expect rogue asteroids to have velocities close to the peculiar velocity of their parent star, with similar distribution.
2.
Peculiar velocities of stars w/r to LSR are, by definition, directed every which way. An asteroid ejected with some velocity in a random direction w/r to its parent system will then have its velocity w/r to LSR be a nett result of the two.
E.g. even in a fantasy scenario where all stars have peculiar velocities equal to 20 km/s, and all asteroids are ejected with 20 km/s over escape velocity, the expected LSR velocity of an asteroid would be anywhere in the range of [0, 40] km/s.
That is, one doesn't even need a slow-moving star, nor slow ejection, to get a slow-moving rogue asteroid.
This is the velocity distribution in the galactic plane of some 20 thousand stars in the solar neighbourhood:
Where the first dark contour contains approx 50% of stars. Vx is radial, Vy is in the direction or galactic rotation. The triangle is LSR.
(Taken from:
https://arxiv.org/abs/0912.3262, fig 1. This earlier paper contains contours for plane-normal direction as well:
https://arxiv.org/abs/0905.2980 - they're narrower, but similarly clustered around 0 km/s w/r to LSR)
So, taking into account both points mentioned above, as well as the velocity distribution of nearby stars, I think the 8 km/s is not only within the expected range (i.e. not-puzzling), but also in the most likely range.
snorkack said:
250 would NOT be puzzling, because that happens to be the peculiar speed of Kapteyn´s star, for example.
The lecture presentation gives velocity dispersion of 9 km/s for A stars, compared to 20 of Sun.
The dispersion you mentioned is for plane-normal velocities only. It is not the nett peculiar velocity which you compare it with. The plane-normal velocity of the Sun is approx. +7 km/s.
Same thing as with Vanadium's post, if by 'not puzzling' you mean 'likely', then 250 km/s would be unlikely, as velocities like that of Kapteyn's star are strong outliers.
