Diatomic carbon is a ubiquitous molecule in comets, with dense emission bands dominating 4600-5600 A in the optical. C2 has previously been shown in observations of a handful of comets to have a bimodal rotational temperature. The current explanations for this bimodality make predictions for how the rotational temperature(s) change as a function of heliocentric and cometocentric distance. Comet C/1995 O1 (Hale-Bopp), presents an ideal testbed for these predictions because it was targeted for an extensive observing campaign with the coude spectrograph of the 2.7m Harlan J Smith Telescope at McDonald Observatory. We collected at least one on and off optocenter spectrum for Hale-Bopp each observing night for heliocentric distances between 2.77 and 0.92 AU at high resolving power (R~60,000) and high signal-to-noise ratio. For some particularly good nights, we collected ~8 off optocenter data points, allowing exploration of the radial and angular variation in the temperature(s). We find evidence that a bimodal temperature is present in all observations analyzed. Furthermore, we see a clear evolution in the rotational temperatures as a function of cometocentric distance, with C2 observations beyond 30,000 km from the nucleus having upper temperatures exceeding the effective temperature of the Sun. Since the prior explanations for the bimodal temperature rely on the photosphere of the Sun as the driver of the heating, our observations at >30,000 km are incompatible with all previous explanations for the bimodality. We present an explanation for how this may occur.
This work was supported by NASA Grant NNX17AI86G and predecessor grants.