Spectroscopic investigation of Comet 46P Wirtanen at UV wavelengths

Introduction

Comets are small solar system bodies that formed in the outer solar system and are believed to preserve the primordial composition of the solar nebula [1]. Hence, the study of cometary cores provides information regarding the chemical composition of the protoplanetary cloud from which the solar system was formed. Organic materials are anticipated in comet with molecules that are observed in the interstellar medium [2].  Polycyclic aromatic hydrocarbons (PAHs), composed of fused benzene rings that are observed widely in the universe are observed in cometary materials at infrared wavelengths [3].

The UV spectroscopy of comets contains resonance lines of atoms, molecules, and ions to study the chemical composition of comets [5]. The near-UV spectral features of comet 1P/Halley observed at 362, 367.5, 373 and 382.5 nm observed using Vega-TKS spectrometer showed the presence of anthracene (C₁₄H₁₀) [6]. The PAHs acenaphthylene (C₁₂H₈) show broad emission at wavelength ~ 200 nm while pentacene (C₂₂H₁₄) display emissions at ~ 200 nm and 280 nm [7].

Observation and data analysis

The spectroscopic data of comet 46P observed at near UV (NUV) and far UV (FUV) wavelengths on 18th and 20th January 2019 respectively using the Space Telescope Imaging Spectrograph (STIS) onboard Hubble Space Telescope (HST) was used for the present study. The observed spectra were extracted using suitable routines in Python.

Results and discussion

The extracted spectra showed emissions due to the PAH molecule, in this case, could be pentacene (C₂₂H₁₄), display emissions at ~ 200 nm and 280 nm. In addition, the FUV spectra of the comet showed emissions due to Lyman alpha, S I, and C I. The comet was at a distance of 0.08 AU from the Earth during the time of observation. The detection of PAH may be due to its close encounter with the Earth. Figure 1 shows the extracted spectra. The detection of PAH emissions at UV wavelengths corroborates with our earlier detection of PAH from the mid-IR spectra [8]. The emission at 309 nm may be a blended feature due to emissions from PAH and OH molecules. Perhaps, refractory residues, such as benzene derivatives, from irradiated aromatic molecules might also contribute to the observed band around 200 nm [9].

Fig 1: HST spectra of 46P Wirtanen with emissions due to HI, CI, SI, and PAH. 

Conclusion

The NUV spectra of the comet 46P Wirtanen showed emissions due to the PAH molecule and/or benzene derivative. The FUV spectra showed emissions due to Lyman alpha, C I, and S I.

References

[1] M.C. Festou, H. U. Keller, H.A. Weaver, Eds., Comets II (Univ. of Arizona Press, Tucson, AZ, 2004)

[2] A. H. Delsemme, Adv. Space Res. 12, 5 (1992)

[3] M. Combes, J. Crovisier, T Encrenaz, V.I Moroz, J.P. Bibring, Icarus, 76, 404, 1988

[4] T. Ootsubo, H. Kawakita, Y. Shinnaka, J.I. Watanabe, M. Honda, Icarus, 338, 113450, 2020

[5] P. Feldman, H. Weaver, M. Festou, et al, Nature, 286, 132, 1980

[6] J. Clairemidi, G. Moreels, O.Mousis, P. Brechignac, A&A, 492, 245, 2008

[7] G. Malloci, G. Mulas, C. Joblin, A&A, 426, 105, 2004

[8] Venkataraman V, Arijit Roy, Bhalamurugan Sivaraman, Shashikiran Ganesh, Nigel Mason, EPSC2020-1072, 2020

[9] K K Rahul, E Shivakarthik, J K Meka, A Das, V Chandrasekaran, B N Rajasekhar, J I Lo, B M Cheng, P Janardhan, A Bhardwaj, N J Mason, B Sivaraman, SpecChimica Acta Part A, 231, 117797, 2020.