Diamonds in the Sky
Spectral analysis of diamonds reveals their their near-infrared signature.
Credit: SETI Institute

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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