Selection of Homework Questions
Topic 8: Theory II : Stellar Dynamics
(1) PotentialDensity Pairs
 Use Poisson's equation in spherical polar form to show that Jaffe's (1983) spherical density distribution:

(Q8.1) 
gives the potential:

(Q8.2) 
Where M and r_{J} are constants.
 Verify that the total mass is M.
 Show that the circular speed is roughly constant for r r_{J} and decreases as r^{½}
for r r_{J} (B&T1 Q 2.3)
(2) Potential Energy
 Show that the Gravitational potential energy of a spherical system
can be written:

(Q8.3) 
where M(r) is the mass interior to radius r (B&T Q2.2).
 Evaluate this for a uniform density sphere of radius R.
 Approximate a SN progentior
star as a small (R 1.4 M_{} core of radius
10^{4}km plus a 20 M_{} envelope of
radius 1AU, each of uniform density. Calculate the binding energy of
just the envelope.
 If the core collapses to form a neutron
star of uniform density and radius 10km, and 1% of the gravitational energy
released is dumped into the envelope (99% escapes as neutrinos), can
the core collapse jettison the envelope?
 If it can, what is the velocity of the ejected envelope material (assuming
it all moves radially at the same velocity) ?
(3) Power Law Cores, and the Jeans Equation :
The goal of this problem
is to explore the behaviour of the
velocity dispersion near the center of a spherical nonrotating galaxy.
At radii r < r_{o} assume that the density has the power law
form (r) = _{o}(r/r_{o})^{}, with 0 < < 3.
Assume that the velocity
dispersion is isotropic at all radii and equal to _{o} at r_{o}.
 Why is the constraint < 3 necessary ?
 Use the Jeans equation in spherical form to derive an expression for
the dispersion profile ^{2}(r) for r < r_{o}
 For what range of does
^{2}(r) 0 as r
0 ?
 For what range of does ^{2}(r) diverge as r 0 ?
 For what value(s) of is
^{2}(r) independent of r as r 0 ?
 For the latter situation, what value of _{o} (expressed in terms of
_{o}, r_{o}, G) makes independent of r at all r ? Evaluate this for the case in which _{o} = 100
M_{} pc^{3} and r_{o}=100 pc.
(4) Central Mass to Light Ratios :
Print out the pdf figure here (
link) which contains light
profiles for three elliptical galaxies (taken from Lauer et al). The units
for µ_{V} are
mag/ss in the V band. The central lineofsight velocity
dispersions in these galaxies are : (N1400) = 265 km/s;
(N2832) = 330 km/s;
(N3608) = 195 km/s. Assuming that
the galaxies are spherical and the velocity dispersion is isotropic and
the core is approximately isothermal, use "King's Method" to find the
core masstolight ratio of each galaxy in solar units. (Note that the
physical scale is plotted along the TOP axis; and think how core radius
is defined in terms of central surface brightness).
(5) Relaxation Times :
Estimate the 2body relaxation time in the following systems :
 The galactic bulge, which we approximate as a singular isothermal
sphere with circular speed V_{c} = 200 km/s containing stars of mass
0.6 M_{}. The relaxation time should be given
as a function of radius. At what radius is the relaxation time equal to
10^{10} years?
 A typical open cluster, with median radius 2 pc, mass 250
M_{}, and stellar mass 1
M_{}.
 The core of the
clobular cluster M4, with core radius 0.5 pc and central surface
brightness 17.88 mag/ss in V. You may assume that the typical
stellar mass is 0.6 M_{} and the masstolight ratio is 1.6
M_{}/L_{}.
(6) Conceptual Question on DFs :
 Systems of stars can be described by a 7dimensional distribution function, DF or just f. What are those 7 dimensions and what, exactly, does the DF describe? What, in qualitative terms, is the
form of the velocity portion of the DF for (i) stars at the galaxy center; (ii) stars in the solar neighborhood?
 Write down the collisionless Boltzmann equation (CBE) for f, and briefly discuss each term.
Why must physically plausible DFs also be solutions to the CBE? In other words, what does the CBE
describe about a system of stars and the nature of the DF?
 Imagine you are living "on" a star which is caught in a galaxy merger. Although your trajectory
in physical space may hurl you through dense bulges or sparse halos, your trajectory through the 6D
positionvelocity phase space keeps you moving along a path of constant stellar density.
Why is this?
 For a static potential, why is a distribution function with simple form f(E) automatically a solution
of the CBE, where E is the energy at a particular point in positionvelocity phase space? What kind of
potentials would support DFs of the form f(E,L) and f(E,L,L_{z})?
 Describe, in conceptual terms, how the CBE is "processed" to yield an observationally more
accessible equation: the Jeans equation? What properties of a stellar system does the Jeans equation
describe?
 Write down the Jeans equation for a spherical galaxy or star cluster. How do astronomers use
the Jeans equation to derive the mass distribution in a spherical nonrotating elliptical galaxy.
What basic observations and assumptions must be made, and how can higher quality observations help
inform those assumptions?