Technical Comments

Observations of Emission Bands in Comet Hale-Bopp

Science  05 Sep 1997:
Vol. 277, Issue 5331, pp. 1526-1527
DOI: 10.1126/science.277.5331.1526

Heike Rauer et al. observed anomalous H2O+ and NH2 emission bands in comet Hale-Bopp (C/1995 01) at large heliocentric distances before perihelion (1). The subject bands are from bending vibrational transitions, (0,υ2′,0)-(0,0,0), of theÃ2A1 −X̃2B1 system of the two isoelectronic molecular species. Except at the shortest heliocentric distances studied, only emissions from even bending vibrational levels were observed, a phenomenon for which existing fluorescence excitation models provide no explanation, according to Rauer et al.

An equivalent observation, however, was made in comet Kohoutek by Wehinger et al. (2), who identified H2O+ and attributed the phenomenon to fluorescence excitation of molecules at temperatures below 50 K. The reason for the missing vibrational bands can be found in the electronic structure of H2O+ and NH2 (3,4) that gives rise to optical transitions involving a lower state bent asymmetric rotor with quantum numbers J", N", Ka" , and Kc" and an upper state linear symmetric rotor with quantum numbers J′, N′, and K′, the latter quantum number being equivalent to Ka′. While there is no vibrational level-dependent constraint on Ka" values in the ground electronic state, odd K′ vibronic sublevels of the linear excited state are restricted to even bending vibrational states, while even K′ sublevels are associated with odd bender states.

Given the ΔKa = ±1 selection rule of the H2O+ and NH2Ã2A1 −X̃2B1 transitions, absorption to the unobserved odd bender states can only occur from odd Ka" levels of the lower state. Population of Ka" = 1 states requires a minimum rotational excitation of 37 cm−1 for H2O+ (4) [32 cm−1 for NH2 (3)], corresponding to a temperature of 53 K. We calculated (Fig. 1) the temperature dependence of H2O+ fluorescence excitation spectra for the same spectral range in which H2O+ emissions were observed by Rauer et al. (1). The calculations use term values, frequencies, line strengths given by Lew (4), and the methodology described by Dressler et al. (5, 6). The H2O+ Ã− state bending vibrational assignment adopted by Rauer et al. has recently been revised (7, 8). The (0,9,0)-(0,0,0) band becomes apparent between 15 and ∼25 K, suggesting that odd bands should be visible closer to perihelion. This was indeed the case for Kohoutek (2) and was also observed by Rauer et al. in Hale-Bopp for NH2 at r h < 3 AU (1).

Figure 1

Fluorescence excitation spectra calculated for the H2O+ Ã 2 A 1 2 B 1system at four temperatures of the ground-state molecule. Calculation assumes 1-nm resolution and a uniform spectral sensitivity.


Response: We thank Dressler for his interesting comment to our observation of the excitation of NH2 and H2O+ in comet Hale-Bopp. His calculation shows that odd vibrational bands should become visible at temperatures larger than 25 K and might therefore be visible closer to perihelion as the temperature in the coma increases. However, we would like to point out in more detail than we could in our short report (1) that a straightforward interpretation of the observations is not possible and a more detailed model of the excitation in a cometary coma is required.

The calculation by Dressler assumes thermal populations of the lower rotational levels and determines relative line intensities under fluorescence excited by an unspecified source. In such a case, the input kinetic temperature, Tk, is equivalent to the rotational excitation temperature, Texc, in the computed spectrum. However, in nonthermal equilibrium conditions as they are found throughout most of the cometary coma, Texc, determined by the relative population of the rotational or vibrational levels, is not generally equivalent to Tk. While the absence of the odd bands in a comet at large heliocentric distance is indicative of a low rotational Texc as we pointed out [page 1911, column 3, paragraph 3 in our report (1)], it does not allow any conclusion to be drawn as to a thermal excitation at a corresponding Tk.

In comets, a temperature-dependent excitation should play a role only in the innermost, collisionally dominated coma. In comet Hale-Bopp, this collisionally dominated region is larger than that in other comets at the same heliocentric distance range as a result of its higher gas production rate. However, throughout most of the coma and in the ion tail, the populations of all the levels—and therefore the NH2 and H2O+ emissions—are governed by purely radiative processes; they depend on the incoming solar radiation as well as on the molecular characteristics, notably the strength of pure rotational transitions within the ground state (2). Existing resonance fluorescence models (3) are unable to reproduce the observed visibility of even and odd bands, mainly because they do not account for the rotational structure of the molecule or ion.

A realistic excitation model of NH2 and H2O+ in a cometary coma and ion tail must take into account that thermal rotational equilibrium does not hold in such low density environments. The temporal evolution of the population of rotational levels with increasing nucleocentric distance must be taken into account, in addition to the pumping by solar flux as a result of resonance fluorescence processes and the rotational de-excitation processes mentioned above. The latter are important in the species in hand because they are in the hydride radicals OH, NH, and CH. The spectra modeled by Dressler show the effect of temperature-dependent excitation in thermal equilibrium conditions for NH2 and H2O+. However, only a detailed investigation including all significant excitation processes, considering the spatial distribution of the emissions, and covering a range of heliocentric distances will provide a full explanation of the observations.

A correct treatment of the populations of even and odd levels for NH2 and H2O+ is important. Neglecting to account for the appropriate selection rules leads to production rates that are underestimated by a factor of about 2 near 1 AU from the sun (4). For H2O+ it was shown (5) that the large discrepancy of the ion production rates in comparison to its parent, water, is most likely caused by incorrect g-factors for the emission bands observed. Furthermore, the observations of comet Kohoutek (6), quoted by Dressler, showed that odd bands of H2O+ were weak in a comet still at heliocentric distances of about 1.4 AU. Some other spectra of the same comet and of comet West at the same rh, however, showed these bands (also those of NH2) with comparable intensities to the even bands (7). A similar remark can be made regarding comet West near 1.6 AU (8).

Finally, we would like to thank Dressler for making us aware of the changed assignments of vibrational levels for H2O+.


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