[P-PS01] Outer Solar System Exploration Today, and Tomorrow

[PPS01-P15] An Improved Upper Limit of Cyanodiacetylene (HC₅N) in the Atmosphere of Titan

*Takahiro IINO^2,1, Hideo Sagawa⁵, Kotomi Taniguchi³, Hikaru Kubota⁵, Shigeru Takahashi⁴, Martin Cordiner⁶ (1.Information Technology Center, the University of Tokyo, 2.University of Yamanashi, 3.NAOJ, 4.University of Hyogo, 5.Kyoto Sangyo University, 6.NASA Goddard Space Flight Center)

Keywords:

Titan,Planetary Atmosphere,Astrobiology

In the complex Titan atmospheric chemistry, photochemical reactions involving cyanodiacetylene (HC₅N) may be important as reaction pathways that could lead to the production of amines and aromatic molecules. We carried out a new comprehensive search of HC₅N in Titan’s atmosphere using Band 6 (215 – 275 GHz or 1.1 – 1.4 mm in wavelength)spectroscopic archival data from the Atacama Large Millimeter/submillimeter Array (ALMA). To search for a faint spectral line feature of HC₅N, we used a spectral stacking method that integrates different transition data. Integrating six independent observation datasets with a total integration time of ∼15000 seconds, we found no statistically significant detection of an HC5N spectral feature. Using the Planetary Spectrum Generator (PSG) for radiative transfer modeling and employing the previously predicted vertical distribution proposed by Vuitton et al. (2019), Lellouch et al. (2010a), Loison et al. (2015) and Krasnopolsky (2009), we derived the upper limit column density of HC5N, corresponding to 3–σ noise level, to be 0.3 – 10×10¹³ molecules/cm² above ∼60 km altitude (depending on the assumed vertical profile). The corresponding upper limit of [HC₅N]/[HC₃N] ratio value derived from three of four models seem to be lower than that measured in High-mass Protostellar Objects (HMPOs), possibly due to the lower abundance of C₄H₂.