My research students and I work in
computational astrophysics. We use Smoothed Particle
Hydrodynamics (SPH) calculations to study stellar interactions, and,
in particular, stellar collisions and mergers. One environment where stellar interactions occur frequently is a
so-called globular cluster.
Globular clusters are collections of many hundreds of thousands of stars found in galactic
halos, the sparse region outside of the disk of a galaxy, and are
thought to have formed early in the evolution of the universe.
One exciting aspect of dense stellar systems such as globular clusters
is the simultaneous
importance of three principal areas of stellar astrophysics:
dynamics, evolution, and hydrodynamics. Many
simulation codes focus on one of these areas and have often
been lifelong works in progress. The first attempts at unifying
these treatments into a coherent model to describe clusters have
begun only recently (see the
MODEST home page).
Attempting to integrate stellar dynamics, evolution, and hydrodynamics codes
into one fully functional package will be challenging,
largely because each area
treats stellar properties that evolve on different time-scales.
However, by combining these areas, we will be able to better
model the origins, dynamics, evolution, and death of globular
clusters, galactic nuclei, and other dense stellar systems.
At Allegheny, our focus is on modelling hydrodynamic
interactions between stars.
There are many types of exotic stellar
objects that may be formed from stellar mergers and collisions, including
blue stragglers, binary neutron stars, ultracompact X-ray binaries,
catacylsmic variables, helium stars, and rapidly rotating horizontal branch stars.
One of our goals is to develop a
software module for quickly generating collision product models,
ultimately for any type of stellar collision, that could be incorporated into
simulations of dense star clusters.
Below is a sampling of visualizations of various stellar collisions
and mergers. To save any to your computer, right click on the link and choose "Save Target As...." Use freely, but please give appropriate credit.
Blue straggler formation
Given the the great age of clusters, it was predicted that stars
more massive than about 80% the mass of the Sun, the so-called
turnoff mass, should have evolved to later stages of stellar life,
ultimately "dying" off as a dim stellar remnant such as a white
dwarf, neutron star, or black hole. Nevertheless, when one observes
a globular cluster, a number of bluish main-sequence stars, more
massive than the turnoff mass, are seen scattered throughout. These
"blue stragglers" appear to 'straggle' behind in their evolution,
since, despite being more massive than the turnoff mass, they have
not yet left the main-sequence.
The calculations performed by Alexander Brown '09
and Lombardi help establish that these exotic blue stragglers
can indeed be formed through the collision of garden-variety
main-sequence stars in a cluster. Visualizations of one of their
simulations were featured in two recent episodes of
a History Channel series entitled "The Universe."
Here we provide visualizations and individual frames
of a collision between a
0.8 and a 0.7 solar mass star at a periastron separation of 0.89 solar
radii. The file size of these particular movies and frames exceed 100 MB, so be
patient!
Quicktime
movie,
jpeg's of frames (zipped): 2D cross section in orbital plane with colors representing
density. (Very large files!)
Quicktime
movie,
jpeg's of frames (zipped): Line of sight view with colors representing column density.
(Very large files!)
mov file: collision
between a 0.8 solar mass and 0.6 solar mass star. Clip is from the
"Cosmic Collision" skyshow now playing at the Hayden planetarium, the
National Air and Space Museum, the Museum of Nature and Science, and
the Shanghai Science and Technology Museum.
avi file: the different colors
represent different constant density surfaces. The top halves of the
outer two isodensity surfaces have been removed so that we can see
deep into the stars.
mpeg file: collision between a
0.8 and a 0.6 solar mass star at a periastron separation of 0.37 solar
radii. 2D cross section in orbital plane with colors representing density.
avi file: 3D particle plot showing the first
collision in a triple star merger.
mov file: 3D particle plot showing the second
collision in a triple star merger.
Binary neutron star formation
Mergers of binary red giants can ultimately lead to the formation of a
binary neutron star system.
This visualization presents the merger of two red giants in an
unstable contact binary, with the centers of the red giants separated
initially by 2.3 times the red giant radius.
mov file: the instability develops slowly
at first, but the merger is rapid near the end of the visualization.
Ultracompact X-ray binary formation
Collisions between neutron stars and subgiants can lead to the
formation of ultracompact x-ray binaries.
These visualizations present hydrodynamics calculations of a collision
between a 1.4 solar mass neutron star and an 0.9 solar mass subgiant
star, on initially parabolic trajectories with a periastron separation of 3.8 solar
radii. Colors correspond to the column density of
the fluid along the line of sight.
mov file: Periastron separation
of 1.9 solar radii.
mov file: Periastron separation
of 3.8 solar radii.
Intermediate mass black hole formation
Runaway collisions involving high mass main-sequence stars in young
star clusters may ultimately lead to the formation of intermediate
mass black holes.
mov file: 106 and 77 solar mass stars
colliding with a periastron separation of 24 solar radii and an
orbital eccentricity of 1.02 (from Valerie McVay's comp).
This next visualizations,
made in collaboration with Scott Fleming and Katherine Dooley,
present hydrodynamics calculations of a collision
between a 53 solar mass and an 18 solar mass star, with an
orbital eccentricity of 0.68 and periastron separation of 23 solar
radii (see the March 2004 Astronomy Magazine). Initial conditions were taken from the first stellar
collision in a runaway sequence in a cluster
dynamics simulation by Portegies Zwart et al., at a time of 3.0 Myr.
The stellar models were taken from stellar evolution calculations by D.
Arnett's Tycho code.
The visualization is given a three-dimensional
feel by assigning colors according to the column density of
the fluid along the line of sight:
512x384 mov
file: radiation and ideal gas pressure included.
1000x750 mov file: ideal gas only.
Learn More
Outreach links:
Popular short summaries, informative pictures and
animations, and press release type statements relevant to
MODEST (Modeling Dense Stellar Systems).