it is raining, might as well liveblog the morning session...
runaway mergers of colliding stars - the quick and dirty intro...
stars are, in fact extended bodies.
This is mostly irrelevant to astronomers, since typical stellar separations are very large.
But, in dense stellar clusters, the density of stars can be a billion times higher than in the field - and hence separations thousand times smaller - and then the finite size can be really important.
Especially if stars also happen to be in binaries, but that is a different story...
So... to make a long story short, sometimes stars go "splat".
They physically collide, typically at parabolic speeds of a few hundred km/sec, and such collisions tend to be "inelastic".
ie stars are "sticky"
well, they are, if they are not too "puffy" - thin envelopes just get cannonballed and maybe even stripped, without much sticking...
Stellar Collision (mov - large)
Now, things can become interesting, because two stars stuck together are more massive, and bigger, than a single star; and, all things being equal (which they are not), if a star is going to hit anything, it will hit the largest other star around.
splat - from Freitag's set of simulations
Which implies in high density systems stars could undergo "runaway mergers", where they collide, stick, and the resultant merged system is bigger and more likely to undergo subsequent collisions.
But, we're not done yet...
a more massive star burns its nuclear fuel more rapidly, and this lives a shorter time; also, if massive enough, such stars can drive strong winds of their surface, and therefore lose mass.
This is particularly the case for young, massive stars; such as might be found in very dense recently formed clusters.
High mass stellar collision (mov) - from a "real cluster simulation" - Jamie again
So, which wins? Do stars collide more and more frequently, until a hypermassive star forms (with mass >> 100 solar masses)? Or, is there diminishing return and stars blow off their envelopes or blow up as supernovae before they can reach very high masses?
We don't know, but it is very exciting if they can runaway to high masses, since normal star formation probably can not make star with masses greather than somewhere around 80-120 solar masses.
Having hypermassive stars, even a few, is very fun, because they can make nice things like intermediate mass black holes (black holes with masses ~> 100 solar masses) and once you have some of these you can do fun physics.
Right now, the theoretical inclination is that we can in fact make hypermassive stars, given a normal spread of stellar masses in a high enough a density star forming cluster, as "mass segregation" (most massive stars "sink" to the center of the cluster, quite rapidly) increases the spatial density of high mass stars, which can then collide frequently enough that interval between collisions becomes as short as 100,000 years or shorter.
Of course if the collision time scale becomes very short, then the star can't cool between collisions and may get very "puffy" - it will then be extended all the time, not in thermal equilibrium, and collision rate will remain high, but the collisions become less sticky because the outer region of the star is more tenuous. Tricky.
And if the initial density of stars is too high, this starts to happen during the protostellar phase, with even high mass stars having extended structure and massive disks - but then it becomes convolved with the basic issue of star formation and role of mergers in forming the high end of the initial mass function, and that is far too hard a problem...
There are two basic issues: one is whether the stars can hold together long enough to actually grow in mass, this is probably a composition dependent issue - high metallicity stars ought to have stronger winds, low metallicity stars would have a harder time shedding mass; secondly, do high mass stellar clusters actually form compact enough for the collision rate ever to be high enough - it seems clear, theoretically, that if clusters are massive enough and dense enough, then the high mass stellar collision rate will diverge and many of the more massive stars will pile in and form one (or few) hypermassive stars.
Maybe.
More data is needed.
Actually any data at all would be nice.
More simulation movies:
and Jun
also at UKAFF
- y'all really need to get some of these onto YouTube!
Get some viral stellar collisions going on the intertoobs.
ah, the Sun is out.
and John is about to violate the 3' rule...
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How fitting that the sun came out from behind the clouds when the discussion was ending...
Interesting discussion. I just hope I'm still around when the question is finally settled by high-res, feedback-including simulations of star formation in clusters. Even then the answer may not be clear...
Do clusters of sufficient density occur very often in the modern (cosmological) age? Or was this sort of thing common during the glory days of the cosmos (first two or three billion years)?
Other issues, what happens during soon after an individual merger.
I would presume the new transient stage star has very high angular momentum. How long does it take to lose it? How does this time compare to either the collision time, or the time for significant chemical evolution to occur? Could the extra mass or pressure added during the merger process be sufficient to set of a gravitational core collapse? If so how might a high angular momentum SN be different from the usual (high mass) case?
clusters like this certainly were there at high redshift
in the local universe, superclusters in starbursts galaxies would do, and possibly compact young clusters like the Arches might do it
the mergers might be high spin, though if they form from many successive mergers the angular momentum is suppressed by ~ 1/SQRT(N_mergers)
the shedding of angular momentum depends on envelope thermal time, winds, and magnetic fields... so you tell me!
all things being equal, I'd BoE that high angular momentum stars might live longer (lower effective core pressure at fixed mass)
supernovae from high mass rapid rotators may well be hypernovae/long Gamma Ray Bursts.
Maybe.
Is the eruption of V838 Monocerotis thought to be the result of a stellar collision? Last I heard it was one of (if not the) leading hypothesis.
In my opinion the largest threat for California are cataclysms and ecological catastrophes. Not important is how many money we have because one tragedy can us take all.