douglas heggie university of edinburgh, ukross/nbody-2015/slides/heggie.pdf · douglas heggie...
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16 September 2015 Lund 1
12 things they don't tell you about the dynamics of star clusters
Douglas HeggieUniversity of Edinburgh, UK
With apologies to Ha-Joon Chang
16 September 2015 Lund 2
The things (minus the small print)1. Black holes don't escape
2. Low-mass stars don't escape preferentially
3. Primordial binaries don't matter
4. Neutron stars matter
5. There is no equipartition between stellar masses
6.. High-concentration clusters are nowhere near core collapse
7. Star clusters don't fill their tidal radius
8. Escapers don't escape
9. There is no such thing as tidal heating
10. Stars don't escape on the relaxation time scale
11. Lagrange points don't exist
12.. The size of a cluster isn't set at perigalacticon
16 September 2015 Lund 3
Thing 1.Stellar-mass Black holes don't all
escape in all clusters
What they say
1.Black holes segregate to
the centre
2.They then behave like an
isolated cluster
3.They have a small
relaxation time
4. 2+3 ⇒ the black holes escape very fast
16 September 2015 Lund 4
What they don't say
1. Isolated clusters also expand on the relaxation time scale
2. Expansion counteracts tendency to segregate further
3. Black holes sit inside a much deeper potential well than an isolated cluster.
4. Would-be escapers cannot escape so easily
5. The would-be escapers donate their escape energy to the other stars
6. The black holes drive the expansion of the whole star cluster
7. They escape on the long relaxation time scale of the whole cluster
See Breen & H (2013)
16 September 2015 Lund 5
Numerical illustrations
Mackey, Wilkinson, Davies, Gilmore (2008)
0 10Gyr
Giersz &H (2014)
Numbers of BH and BH binaries
M4, NGC 6397, 47 Tuc
M22Additional remarks
1. Disappearance of BH coincides with “second core collapse”
2. Stellar-mass BH expected in uncollapsedclusters, and not in post-collapse clusters
16 September 2015 Lund 6
Thing 2. Low-mass stars don't always
escape preferentiallyWhat they say
1. Stars escape in two-body encounters
2. Heavy stars tend to lose energy, low-mass stars tend to gain energy in encounters (tendency to equipartition of energy)
3. It is easier for low-mass stars to gain energy above the escape energy
16 September 2015 Lund 7
What they don't say
1. Heavy stars tend to sink to the centre
2. At high central densities, binary stars form from the heavy stars by three-body encounters
3. One of the stars in a three-body encounter gains high energy
4. Heavy stars escape preferentially in such circumstances
See Kruijssen (2009)
When the upper stellar mass is large enough,the low-mass stars are least rapidly removed
16 September 2015 Lund 8
Thing 3. Primordial binaries don't matter much, at least for uncollapsed clusters
What they say
1. In binaries with P ⪅ 10 year, binary components have more energy than single stars
2. Time scale for changing energy of stars in binary-single interactions ~ relaxation time/binary fraction
3. “Heating” of single stars causes expansion on time scale of a few relaxation times
4. “Heating” by binaries sets the core radius in post-collapse evolution
16 September 2015 Lund 9
What they don't say
1. It's a second-order effect
2. Expansion of the cluster is powered mostly by central stellar-mass black holes
3. Binaries gradually take over as black holesescape, approaching (second) core collapse
4. Questionable if primordial binaries set the post-collapse core radius
5. They affect the time to core collapse a lot 47 Tuc (fb = 0.018)
Evolution of core and half-mass radiiGiersz & H (2011)
Note: interactions affect the binaries a lot
16 September 2015 Lund 10
Thing 4. Neutron stars matter for some cluster models
What they say
1. Neutron stars are about 2% of cluster mass
2. Few-percent effect on relaxation time, escape time scale, etc.
16 September 2015 Lund 11
What they don't tell you
1. Presence or near-absence of NS depends on natal kicks (typically ≫ escape speed from cluster)
2. Presence or absence can change lifetime by factor ~4 (Contenta, Varri, H 2015)
3. Clusters dissolve by
two processes
a) Two- and three-body
encounters (long lifetime)
b) Mass-loss of stellar
evolution (short lifetime)
4. The models (from Baumgardt
& Makino 2003) sit on a separatrix
where these processes are finely
balanced
16 September 2015 Lund 12
Thing 5. There is no equipartition between stellar masses among visible stars in
globular clusters
What they say
1. Two-body encounters lead to equipartition in a few relaxation times
16 September 2015 Lund 13
What they don't say
1. In equipartition low-mass stars would evaporate too quickly
2. Multi-mass King models do not give equipartition (Miocchi 2006)
3. N-body models do not give equipartition (Trenti & van der Marel 2013)
4. Fokker-Planck models do not give equipartition (Inagaki & Saslaw 1985), except at highest masses
16 September 2015 Lund 15
Thing 6. High-concentration clusters are nowhere near core collapse
What they say
1. Clusters are evolving from low concentration (large cores) to high concentration (small cores, c ≈ 2.5)
2. 47 Tuc has c = 2.07, a small
dense core, and is undergoing
rapid evolution towards core
collapse
Harris catalogue
16 September 2015 Lund 16
What they don't say
1. Models imply that core collapse will take at
least another 20 Gyr
16 September 2015 Lund 17
Thing 7. Star clusters don't fill their tidal radius
What they say
1. The Galactic tide strips off stars beyond a “tidal radius”
2. King models have a finite “tidal radius” to incorporate this effect
3. Globular clusters fit King models quite well
4. Therefore the radius of globular clusters is determined by the Galactic tide
5. We can use radii of globular clusters to estimate strength of tidal field
16 September 2015 Lund 18
What they don't say
1. Edge radius may be set by initial conditions
2. If a cluster starts smaller than its tidal radius
i. It first expands so that its relaxation time is of order its age, until its radius
equals the tidal radius
ii.After that it contracts,
and its relaxation time
is of order its remaining
lifetime (Henon 1961;
Gieles, H & Zhao 2011)
3. GHZ say 2/3 are (i), 1/3
are (ii)
4. Gives a reinterpretation
of the “survival triangle”
Gnedin & Ostriker 1997
Thing 8. Some Escapers don't escape
What they say
1. For a cluster on a circular
Galactic orbit, switch to rotating
frame centred at the cluster
2.Combined potential
(cluster, tidal field,
centrifugal acceleration)
has last closed
equipotential Vcrit
3. Stars with higher energy escape
What they don’t say
1. The condition E > Vcrit is necessary, not sufficient
2. There’s also a Coriolis acceleration, which can
keep high-energy stars inside the cluster
3. There is a population of
“potential escapers”
4. Can reach up to 10% of
cluster members (Baumgardt
2001)
5. Ignored in all snapshot
modelling of globular clusters
Henon 1970
Thing 9. There is no such thing as tidal heating on a circular Galactic orbit
What they say
1. Clusters exhibit non-Keplerian
velocity dispersion profile
2. Tidal field is time-dependent
(“bulge shocking”, “disk
shocking”) and strongest
at large radii
3. Velocity dispersion elevated by
“tidal heating”, or even something “non-Newtonian”
Drukier et al 1998 (M15)
What they don't say
1. For a cluster on a circular
Galactic orbit, in the
rotating frame the
potential is static.
2. Energy is conserved (Jacobi
integral), therefore there is
no tidal heating.
3. Velocity dispersion is
elevated by potential
escapers (see Thing 8)
Kuepper et al 2010
Thing 10. Stars don't escape on the relaxation time scale except for very large N
What they say
1. Stars escape by two-body encounters, which may elevate the energy of one star above the escape energy.
2. Two-body encounters change stellar energies on the relaxation time scale
3. Stars escape on the relaxation time scale
What they don't say1. Stars with E > Vcrit remain
as potential escapers for a
time
2. During this time E changes
on relaxation time scale, and
escape becomes easier as E
increases
3. The balance of these
processes gives an escape
time scale (i.e. time scale for
loss of mass) ∝ tr/N1/4
(Baumgardt 2001)
4. Eventually this must turn over
to ∝ tr
Thing 11. Lagrange points don't exist for clusters on elliptical orbits
What they say
1. There is a point where
the attraction of the
cluster and Galaxy are
in balance.
2. It is an equilibrium point
3. It is a critical point of the “potential”
4. Stars beyond this point are escaping
What they don't say
1. For a cluster on an elliptical Galactic orbit,
there is a point where the forces balance, but
it's a moving point,
not an equilibrium
2. There is no potential,
and no critical point
3. The nearest analogue
to the Lagrange point of
the circular problem is
a periodic orbit
“Lagrange” periodic orbit for variouspower-law Galactic potentials
Thing 12. The size of a cluster isn't set at perigalacticon
What they say
1. The tidal radius is smallest at perigalacticon
2. Stars beyond the tidal radius escape
3. Cluster members are non-escapers, and must lie inside the tidal radius at perigalacticon
What they don't say
1. It's not (Kuepper+ 2010), at least up to e = 0.5.Fit a King profile to an N-body simulation, and compare edge radius (red) with tidal radius (dots) for e = 0 (left), 0.5 (right).
Edge radius agrees with mean tidal radius to about 10%.
Summary
What they say
“...underlying it all is a basic dynamical structure that is very simple”
What they don't say
“Nothing is rich but the inexhaustible wealth of nature. She shows us only surfaces, but she is a million fathoms deep.” - Emerson