Shocking Shock Waves
In the late 1980s and early 1990s, Burton
(first a graduate student at Edinburgh Uni-
versity and then a research fellow at NASA
Ames), Chrysostomou (also a grad student
at Edinburgh) and I (then employed at the
United Kingdom Infrared Telescope; UKIRT),
were part of a team led by Peter Brand at Ed-
inburgh that was attempting to understand
the physics of shock waves in star-forming
molecular clouds. In a pure hydrodynamic
shock, H 2 is dissociated into its constituent
hydrogen atoms when collisions involving
it and atoms or molecules in the wind from
the protostar occur at speeds exceeding 20
kilometers per second (km/s).
During 1978-1981, however, when I was a
Carnegie Fellow in Pasadena working with
Gary Neugebauer of Caltech and his gradu-
ate student Daniel Nadeau, we had found
that the H 2 lines in the Orion Molecular
Cloud have velocity widths of over 100 km/s.
Similar high velocity and high temperature
H 2 was later found in other clouds as well.
Molecule-molecule or atom-molecule col-
lisions occurring at even a small fraction of
that speed would have destroyed the H 2 ,
and the emission lines from H 2 thus would
not be observed.
Our finding helped to stimulate the develop-
ment by theorists of magneto-hydrodynam-
ic shock models in which the quiescent gas is
accelerated and heated more slowly and the
H 2 survives. Because these so-called continu-
ous shocks, or C-shocks, are naturally created
if the cloud contains a magnetic field, as is al-
ways the case, they appeared to be a natural
explanation for the observations.
Brand, Burton, Chrysostomou, and I, along
with a few other Brand grad students tested
the C-shock models by measuring the rela-
tive intensities of numerous lines of shocked
H 2 . To our surprise, the relative intensities did
not match the predictions for C-shocks. The
April 2017
highest excitation lines we could detect at
the time (with upper energy levels as high as
25,000 K above the ground state) were far too
strong; their strengths actually much more
closely matched the predictions for pure hy-
drodynamic shocks than for C-shocks. Yet at
the observed speeds, none of the H 2 could
have survived a hydrodynamic shock. Un-
able to find a satisfactory resolution to this
puzzle, we researchers eventually went our
separate ways and moved on to other unre-
lated projects.
On the Sky Again … at Gemini
My move from UKIRT to Gemini and its set
of powerful infrared spectrographs eventu-
ally led me to return to the problem, and I
reassembled part of my old Edinburgh team
(Burton and Chrysostomou) to do so. We
chose as our target the Herbig-Haro object
HH 7, a small patch of nebulosity associated
with a newly born star well known for its
strong H 2 line emission and its simple geom-
etry in the sky, that of a classic bow shock.
As our spectrograph, we selected Gemini’s
Near-Infrared Integral Field Spectrometer
(NIFS), which was capable of dicing the bow
shock into tiny regions that could be ana-
lyzed separately.
Gemini System Support Associate Rosemary
Pike (now a PhD astronomer) reduced the
complex NIFS spectral data on HH 7. In addi-
tion to the well-known high-excitation lines
of H 2 that were the intended target of the
program, the reduced data (see Figure 1) re-
vealed a large number of very faint emission
lines that were eventually identified as also
due to H 2 , but emitting from energy levels
far above the highest ones previously ob-
served (25,000 K). Some of these levels are
50,000 K above the ground state, very close
to the dissociation energy of H 2 .
Surprisingly, Burton successfully modeled
all the line emission as arising from H 2 at just
GeminiFocus
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