Unidentified Molecular Bands in the Plasma Tail of Comet Hyakutake (C/1996 B2)

Abstract

Unidentified molecular emission bands first observed in optical spectra of the plasma tail of comet Halley (1P/1982 U1) have been discovered in pure ion tail spectra ( Å , ) of comet Dl ∼ 4700-6300 l/Dl ∼ 1000-4400 Hyakutake (C/1996 B2) and three other recent comets (Nakamura-Nishimura-Machholz P/1994 N1, Austin C/1989 X1, and Brorsen-Metcalf 32P/1989 N1), but not in comet Hale-Bopp (C/1995 O1). The strengths of the bands near 4940, 5310, and 6000 Å are correlated among the comets observed, indicative that the unidentified bands arise from a single molecular species. The bands have an asymmetric spatial distribution that extends in the antisolar direction, resembling molecular ions observed in comet plasma tails. Thus, the source of the unidentified bands is probably a single species of molecular ions. Of known laboratory molecular ion spectra, none can be attributed to the unidentified cometary emission bands. The absence of the unidentified ions in the tail of Hale-Bopp may be explained by a parent species of the unidentified ions that mimics the coma dynamics and photochemistry of H 2 O more closely than CO. This is inferred from the large tail CO ϩ /H 2 O ϩ ratio observed in Hale-Bopp, which arises from the comet's large production rate that extends the ionopause and coma collision zone. Our observations provide evidence that the source region for the ions in plasma tails of comets lies external to the magnetic field-free ionopause.

A High Spectral Resolution Atlas of Comet 122P/de Vico

“…We compared our tail spectrum of de Vico with the list of identified and unidentified species in the spectrum of the tail of comet C/1996 B2 (Hyakutake) (Wyckoff et al 1999). Some of Wyckoff et al's H 2 O + lines are seen in our spectra, while others are not, and we detect many H 2 O + lines that they did not see (however, our data are at much higher spectral resolution and so exact matches in wavelength with their identifications are difficult).…”

Vibronic transition moments and line intensities for H2O+

“…In 1999, Wyckoff et al [1] reported spectroscopic observations of the comets Halley, Hyakutake, and Hale-Bopp. They detected several unassigned molecular emission bands in the plasma tails of the comets.…”

On the possible luminescence nature of unidentified cometary emissions

“…However, in some cases, it became possible to establish the fact that such unidentified emissions belong to the radiation of comet ions. This is evidenced by the profiles of unidentified emissions obtained by several authors, including (Arpigny 1995;Wyckoff et al 1999). At the same time, until recently, a great number of unidentified emissions has been considered as the radiation from unknown sources.…”