Presence of C2 emission lines in late pre-perihelion HATPI observations of 3I/ATLAS

HATPI uses a broad optical bandpass (430–890 nm) to obtain time-series photometry of moving objects, including the interstellar comet 3I/ATLAS. The authors find a steeper heliocentric brightness increase in the HATPI data compared to narrower-band measurements and consider whether gas emission lines within the HATPI bandpass could contribute to this difference.

Pre-perihelion spectroscopic reports showed Ni, Fe, and CN lines, with CN features near 387 nm lying outside HATPI’s blue cutoff, and no detections of redder C3 or C2 lines at that time. Post-perihelion observations, however, reported C2 emission lines within HATPI’s bandpass. The authors explicitly state that it is unclear whether such C2 emission was present during the latest pre-perihelion HATPI observations of 3I/ATLAS in mid-September 2025.