Image sources: H. Bond, NASA, Frank Gregorio, and Yours Truly
Back in the early 1990s, Professor Volker Weideman of the University of Kiel noticed that the Hyades (a nearby star cluster that makes the "V" in the head of Taurus the Bull) don't have as many white dwarf stars as they should. We think we can make a good estimate of how many stars lived and died in a star cluster, but we didn't see nearly enough white dwarfs. So, Professor Weidemann proposed that, when a star loses its mass as a planetary nebula, the material may come off a bit lopsided. If enough matter comes off in this way, the star may get a little bit of a push, and slowly drift away from the star cluster.
When I did my doctoral thesis, I noticed that white dwarfs seemed to be missing from other star clusters as well. I also estimated how many white dwarfs astronomers might miss because they are next to a bright star that hides the white dwarf, and it wasn't enough. So, I put this forward as more evidence that white dwarfs were somehow getting a push when they were formed. Michael Fellhauer, an astronomer working with some of my colleagues, did a computer simulation that showed that the white dwarfs would only need a push of about 3 to 5 kilometers per second (about 7000-10,000 miles per hour, which sounds like a lot, but by astronomy standards, that is pretty slow). But it is hard to prove what is happening when we couldn't even find the white dwarfs, and many colleagues who worked on how stars lose mass said that there was not much evidence for a lopsided loss of material.
Yesterday, a press release was announced involving research by Saul Davis at the University of British Columbia involving a paper Davis and his collaborators had published on white dwarfs in the globular cluster NGC 6397. You can read the full story here, but in short, the newest white dwarfs in the cluster are moving around about 3 to 5 kilometers per second faster than older white dwarfs. If young white dwarfs get a push when they are born, this is what we would expect -- because of gravity, over time, will slow down any faster white dwarfs to match the average speeds of stars in the star cluster.
So, Davis's work gives some new and different evidence that white dwarfs go get a push when they end their lives. And it is nice to see someone find that evidence!
But I don't think the case is sealed yet. Theory still doesn't explain how these white dwarfs can get a push when they die, and we haven't seen strong evidence that dying stars are losing mass in the lopsided manner needed to give the white dwarfs a push. I have no other reasonable ideas as to how the white dwarfs could be getting a push (maybe bigger stars are bullying the white dwarfs, shoving them around, and putting "kick me" signs on their backs?), but until we actually see stars losing mass in a lopsided manner, the mystery remains.