Figure 1.
Spectrum of a bright
region near the apex
of the HH 7 bowshock
(as published in Pike
et al., ApJ, 822: 82,
2016). All lines are
due to molecular
hydrogen and are
labeled according by
their vibrational and
rotational transitions.
The blue trace is the
same as the black trace,
but is multiplied by a
factor of 150 and offset
vertically to sho w the
weaker lines, which
had not been detected
previously in any
astronomical object.
two temperatures: 1,800 K and 5,000 K. Ap-
proximately 98.5% of the H 2 is at the lower
temperature, which corresponds closely to
the temperature expected for a C-shock. The
higher temperature component, which is
only 1.5% of the hot H 2 , accounts for virtually
all of the emission by the most highly excit-
ed H 2 . The origin of the 5,000 K component is
of intense interest. It seems most likely to be
due to H 2 that has reformed on dust grains
following destruction by the shock wave.
The formation of H 2 by the collision of two
H atoms in the gas phase is an extremely
unlikely process. However, hydrogen atoms
will stick to a dust particle and can easily hop
around on it, find each other, and make H 2 .
Their association produces a lot of energy,
some of which ejects the newly formed H 2
molecule from the dust particle and some
of which leaves the molecule in a highly ex-
cited state, from which it can emit spectral
lines as it cools. Qualitatively this explains
the observations, but many questions re-
January 2018 / 2017 Year in Review
main, especially regarding how well the rela-
tive line strengths match predictions of the
“formation spectrum.”
A Fundamental Question …
and the Answer
A basic question about this discovery was
whether the high temperature H 2 is unique
to HH 7 or is found in other clouds that have
been subjected to high velocity shocks. To
begin to answer this question, Burton, Pike,
and I observed the shocked H 2 in the loca-
tion where it was initially discovered in 1976,
and where it is brighter than anywhere else:
the Orion Molecular Cloud (OMC-1). Using
as a guide the exquisite images obtained by
John Bally (University of Colorado) and col-
laborators with the multi-conjugate adap-
tive optics System at Gemini South, we
positioned the long slit of the Gemini Near-
Infrared Spectrograph (GNIRS) on Gemini
North to traverse several regions of intense
GeminiFocus
31