Work

Post-perihelion photometric and spectroscopic monitoring of the interstellar comet 3I/ATLAS

Year: 2026

Type: article

Abstract: ABSTRACT The third interstellar object, comet 3I/ATLAS, was observed using various Indian telescopes during its post-perihelion evolution. We carried out imaging and spectroscopic observations of the comet at multiple epochs using the 0.5-, 1.2-, and 2.5-m telescopes of the Physical Research Laboratory and the 2-m Himalayan Chandra Telescope (HCT) operated by the Indian Institute of Astrophysics. Additionally, time-series photometry was obtained on seven epochs using the 0.132-m refractor of Cepheid Observatory. The spectroscopic observations of 3I/ATLAS reveal signatures of molecular emission from CN, C$_2$, and C$_3$, as commonly observed in the emission spectra of Solar system comets. The production rates and rate ratios indicate that the composition of the 3I/ATLAS transitioned from carbon-depleted (pre-perihelion) to carbon-typical (post-perihelion). The change in carbon composition could be due to exposure of fresh material from subsurface layers near perihelion. We also performed photometric observations to determine broad-band colours and the spin period. The measured colours are $B-V=$ 1.03 $\pm$ 0.13 mag, $V-R=$ 0.59 $\pm$ 0.11 mag, $R-I=$ 0.08 $\pm$ 0.04 mag, $g-r=$ 0.52 $\pm$ 0.09 mag, and $r-i=$ 0.13 $\pm$ 0.08 mag. The colours are redder than the long-period comets of the Solar system and the Sun in $B-V$ and $V-R$, but near solar in $R-I$. The measured colours remain nearly constant throughout the observing period and are consistent with the reported pre-perihelion values. The colours and the reflectivity gradient match the photometric colours of the ‘ultrared-material rich’ objects in the Solar system. The nuclear radius derived from non-gravitational force lies between 0.45 and 0.65 km.

Source: Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag1800