Latest on white dwarfs in globulars from Brad, John and Stefan.
White Dwarfs in Globular Clusters - Hansen - video and podcast
White Dwarfs in Globular Clusters - Fregeau and Rosswog - video and podcast
As we learned this morning, white dwarf cooling gives an absolute age indicator, essentially from ab initio physics calculations.
So you can independently measure cluster ages if you have very good photometry of very faint stars in very crowded regions. And we do.
cf NGC6397 - had a bit too much of M4 for now...
NB: white dwarf cooling age for NGC6397 is 11.5 Gyrs +/- 0.5
which is 1.2 Gy younger than M4. +/- maybe 0.5 Gy systematics.
Omega Cen: (Calamida et al '08) see anomalous colour distribution of white dwarfs in &Omega: Cen. Upper part of white dwarf cooling track is much too broad.
Due to lots of He white dwarfs? With some pp H burning???
Single star evolution? Do minor CNO/NaAlMg abundance anomalies lead to bypassing the helium flash??? - Requires tens of % He white dwarfs, compared to ~ 2% in the field.
Remember Omega Cen is a multiple stellar pop system...
Metal rich old open cluster NGC6791 also has weird multiple peak white dwarf luminosity function - due to He core white dwarfs???
Large binary fraction??? (cf Bedin et al '08 at 40+% binaries)
Binary white dwarfs seen in superposition.
Recent results suggest the white dwarfs are not relaxed - their spatial distribution is over extended radially, and the "puffing out" seems to occur late on the AGB. (Davis et al 2008)
How?
Crossing times are short, so any kinematic kicks on WDs which can cause them to spread out have to occur fast - faster than 100,000 years.
Which implies on second ascent of AGB, during thermal pulsing, but there is very little envelope mass left then, so you need very asymmetric mass loss and a fast outflow.
How?
Why don't we see this?
On the other hand the "deficit" of white dwarfs in open clusters, relative to main sequence stars - would be elegently explained by impulsive kicks of modest size at white dwarf formation.... few km/s will suffice
So, what is implied - if you assume ~ 1% of a solar mass is available late on the AGB, then you need a strongly asymmetric outflow with speeds of ~ 300-500 km/sec dumping the whole mass in
This might be observable if you look at few hundred tip AGB stars in globulars, and do multiband imaging - looking for IR features, shock, snakes or jets
At this speed we'd be looking at linear outflows with sizes about the size of the whole cluster. Could also do slower outflows, but then need more mass.
Fishing expedition, but possible. Would make a nice paper.
Anyone have any ideas on a physical mechanism?
It almost has to happen late on (second) AGB ascent, thermal pulsation during late stage shell burning and low envelope mass.
But not too low, because momentum flow is already a a problem
(unless you do it with asymmetric neutrino emission as Brad speculatively suggested!)
So for a fast flow you're not ejecting from the surface of the envelope, speeds are too slow, so this has to come from the base of the envelope.
Maybe some sort of asymmetric cooling instability? One side of envelope has catastrophic cooling and then puffs back out fast, so about half the envelope takes off with asymptotic speeds of few hundred km/sec - needs strong thermal pulsing - are the energetics adequate?
Tricky.
Ok, now Stefan is onto tidal disruption of white dwarfs by intermediate mass black holes.
A subject near and dear to my heart.
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