979 research outputs found
Jeans Instability of Palomar 5's Tidal Tail
Tidal tails composed of stars should be unstable to the Jeans instability and
this can cause them to look like beads on a string. The Jeans wavelength and
tail diameter determine the wavelength and growth rate of the fastest growing
unstable mode. Consequently the distance along the tail to the first clump and
spacing between clumps can be used to estimate the mass density in the tail and
its longitudinal velocity dispersion. Clumps in the tidal tails of the globular
cluster Palomar 5 could be due to Jeans instability. We find that their spacing
is consistent with the fastest growing mode if the velocity dispersion in the
tail is similar to that in the cluster itself. While all tidal tails should
exhibit gravitational instability, we find that clusters or galaxies with low
concentration parameters are most likely to exhibit short wavelength rapidly
growing Jeans modes in their tidal tails.Comment: sumbmitted to MNRA
Disk heating by more than one spiral density wave
We consider a differentially rotating, 2D stellar disk perturbed by two
steady state spiral density waves moving at different patterns speeds. Our
investigation is based on direct numerical integration of initially circular
test-particle orbits. We examine a range of spiral strengths and spiral speeds
and show that stars in this time dependent gravitational field can be heated
(their random motions increased).This is particularly noticeable in the
simultaneous propagation of a 2-armed spiral density wave near the corotation
resonance (CR), and a weak 4-armed one near the inner and outer 4:1 Lindblad
resonances. In simulations with 2 spiral waves moving at different pattern
speeds we find: (1) the variance of the radial velocity, sigma_R^2, exceeds the
sum of the variances measured from simulations with each individual pattern;
(2) sigma_R^2 can grow with time throughout the entire simulation; (3)
sigma_R^2 is increased over a wider range of radii compared to that seen with
one spiral pattern; (4) particles diffuse radially in real space whereas they
don't when only one spiral density wave is present. Near the CR with the
stronger, 2-armed pattern, test particles are observed to migrate radially.
These effects take place at or near resonances of both spirals so we interpret
them as the result of stochastic motions. This provides a possible new
mechanism for increasing the stellar velocity dispersion in galactic disks. If
multiple spiral patterns are present in the Galaxy we predict that there should
be large variations in the stellar velocity dispersion as a function of radius.Comment: 20 pages, 13 figures. Submitted to MNRA
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