Twinkling
stars are often compared with diamonds, sparkling and bright. Stars are made
almost entirely of hydrogen and helium with traces of other elements; diamonds
are made of carbon. However, there are real diamonds in the sky, and they
derive from the fiery furnaces that make stars shine. As stars fuse lighter
elements, releasing energy, carbon is produced along with other elements via
nuclear fusion. Eventually, a portion of these elements is released into the
interstellar medium when stars die. Astronomers find carbon compounds
throughout the interstellar medium including diamonds, the most compressed form
of carbon. How do we know that there are diamonds in the sky? By comparing the
astronomical spectra with laboratory spectra and calculated models of emission
and absorption spectra.
Interstellar
microdiamonds are found in meteoroids called carbonaceous chondrites. Think of these diamonds as "diamond dust," not jewels that could grace
an engagement ring. We can detect the fingerprints of diamonds by looking at
their near-infrared signature via spectral analysis. What we see is the
evidence of carbon atoms emitting or absorbing infrared energy as their bonds
stretch, twist and flex. While only a few astronomical objects, such as HD
97048 and Elias 1, show spectroscopic features in the 3.4 – 3.5
µm emission region that can be assigned to microdiamonds, as shown in
a) and b) respectively (see figure at right), many astronomical objects, such as dense clouds,
exhibit an interstellar absorption band (shown in red in c) which peaks at
about 3.47 µm and is superposed on the long wavelength wing of the
3µm H2O ice band. This feature has been tentatively
attributed to the tertiary C-H stretch of diamond-like carbon.
Part of the
difficulty associated with tracking interstellar microdiamonds stems from the
limited knowledge of their infrared and electronic properties.
Theoretical
calculations can help us understand the possible contribution diamondoids make
to the chemistry and physics of the interstellar medium.
Diamonds
are tightly bound chemically, and it takes considerable energy to cause them to
emit a characteristic spectral "fingerprint." Modeling helps us to
understand what we might see in the presence of a very energetic star. Our
computed spectroscopic properties imply that, if diamondoids are present in
these astronomical environments, the majority would be in the neutral form. The
combination of small absorption strengths (f-values) and large
excitation energies, which are close to the ionization energies, makes it
unlikely that, as a class, neutral diamondoids can become sufficiently excited
to emit strongly in the infrared in most astronomical environments. Thus, if
neutral diamondoids are abundant in the interstellar medium, their
spectroscopic properties favor detection by absorption in the 3 µm
region rather than emission. As shown in the figure, neutral diamondoids will
absorb most strongly near 3.47 µm. Our computed infrared spectra for
two neutral diamondoids (green and blue peaks shown in c) are centered around
3.47 µm and line up with the interstellar absorption band associated
with dense clouds. This result supports the assignment of the interstellar 3.47
µm absorption band to neutral diamondoids. Using the calculated
absorption/emission band for diamondoids, we can interpret astronomical
observations.
The very
small absorption strengths of both neutral and ionized diamondoids place tight
constraints on the astronomical environments in which they can become
sufficiently excited to emit strongly in the infrared. Clear-cut infrared
fluorescent emission from highly vibrationally excited diamondoids requires
radiation fields with both high energy and high flux. For diamondoids larger
than those shown in c), the C-H stretching band shifts into agreement with the
emission bands of HD 97048 and Elias 1. However, radiation fields strong enough
to excite the neutral diamondoids will also produce ionized diamondoids. Our
computed infrared spectra for small positively charged diamondoids, shown in d)
-f), line up with parts of the emission spectra for HD 97048 and Elias 1 and
suggest that the rare 3.5 µm interstellar emission feature originates
very close to the exciting star and strongly support its assignment in HD 97048
and Elias 1 to diamondoids species. We also show that neutral and cationic
diamondoids can contribute to the observed emission features.
There are
diamonds in the sky that reveal themselves when excited by the glow of brilliant
stars.
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