Abstract:
Observing the vibrational/rotational lines in a comet’s optical spectrum requires high-resolution spectroscopy, as they are
otherwise seen as a blended feature. To achieve this, we have obtained medium and high-resolution (R (λ/λ) = 30 000
and 60 000) spectra of several comets, including C/2015 V2 (Johnson), 46P/Wirtanen, 41P/Tuttle–Giacobini–Kresak, ´ and
38P/Stephan–Oterma, using the Hanle Echelle Spectrograph (HESP) mounted on the 2-m Himalayan Chandra Telescope (HCT)
in India. The spectra effectively cover the wavelength range 3700–10 000 Å, allowing us to probe the various vibrational bands
and band sequences to identify the rotational lines in the cometary molecular emission. We were also able to separate the
cometary Oxygen lines from the telluric lines and analyse the green-to-red (G/R) forbidden oxygen [O I] ratios in a few comets.
For comets C/2015 V2, 46P, and 41P, the computed G/R ratios, 0.04 ± 0.01, 0.04 ± 0.01, and 0.08 ± 0.02, respectively, point
to H2O being a major source of Oxygen emissions. Notably, in the second fibre pointing at a location 1000 km away from the
photocentre of comet 46P, the G/R ratio reduced by more than half the value observed in the first fibre, indicating the effects
of quenching within the inner coma. We also measured the NH2 ortho-to-para ratio of comet 46P to be about 3.41 ± 0.05 and
derived an ammonia ratio of 1.21 ± 0.03 corresponding to a spin temperature of ∼26 K. With these, we present the results of the
study of four comets from different cometary reservoirs using medium and high-resolution optical spectroscopy, emphasizing
the capabilities of the instrument for future cometary studies.
