AI 中文总结
对星际彗星3I/ATLAS过近日点后的多望远镜光谱与测光监测显示，其碳成分由贫乏转为典型，颜色偏红且稳定，核半径约0.45-0.65公里。
AI 中文摘要
第三颗星际天体——彗星3I/ATLAS，在其过近日点后的演化过程中，使用印度多台望远镜进行了观测。我们利用物理研究实验室的0.5米、1.2米和2.5米望远镜，以及印度天体物理研究所运营的2米喜马拉雅钱德拉望远镜（HCT），在多个历元对彗星进行了成像和光谱观测。此外，使用造父变星天文台的0.132米折射望远镜在7个历元获得了时间序列测光。3I/ATLAS的光谱观测揭示了CN、C$_2$和C$_3$分子发射的特征，这与太阳系彗星发射光谱中常见的情况一致。产率和产率比表明，3I/ATLAS的成分从碳贫乏（近日点前）转变为碳典型（近日点后）。碳成分的变化可能是由于近日点附近地下层新鲜物质的暴露所致。我们还进行了测光观测以确定宽带颜色和自转周期。测得的颜色为$B-V=$1.03$\pm$0.13星等，$V-R=$0.59$\pm$0.11星等，$R-I=$0.08$\pm$0.04星等，$g-r=$0.52$\pm$0.09星等，$r-i=$0.13$\pm$0.08星等。这些颜色在$B-V$和$V-R$上比太阳系长周期彗星和太阳更红，但在$R-I$上接近太阳。在整个观测期间，测得的颜色几乎保持不变，并与已报道的近日点前数值一致。颜色和反射率梯度与太阳系中富含“超红物质”天体的测光颜色相匹配。由非引力推导出的核半径在0.45至0.65公里之间。
英文摘要
The third interstellar object, comet 3I/ATLAS, was observed using various Indian telescopes during its post-perihelion evolution. We carried out imaging and spectroscopic observations of the comet at multiple epochs using the 0.5-m, 1.2-m, and 2.5-m telescopes of the Physical Research Laboratory and the 2-m Himalayan Chandra Telescope (HCT) operated by the Indian Institute of Astrophysics. Additionally, time-series photometry was obtained on 7 epochs using the 0.132-m refractor of Cepheid Observatory. The spectroscopic observations of 3I/ATLAS reveal signatures of molecular emission from CN, C$_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 broadband 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.
Comments14 pages, 11 figures, 8 tables. Accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS)
