Contemporaneous Optical and Near-Infrared Observations of the Interstellar Comet 3I/ATLAS Pre- and Post-Perihelion

Abstract

Interstellar objects provide a unique view into the formation of other star systems. Here we present spectroscopic observations of the recently discovered interstellar object 3I/ATLAS between a heliocentric distance of $3.7$ to $1.8$~au on either side of its travels through perihelion. We obtained several observations with the Keck-I/LRIS, Keck-II/NIRES, Gemini/GMOS, and UH88/SNIFS spectrographs, covering a wavelength range of $0.3 - 2.5~\mathrm{μm}$. We report the continued emission of both Ni and CN, along with post-perihelion detections of Fe and a weak detection of $\mathrm{C_3}$. We determine the spectral slope across optical and NIR wavelengths and find a positive spectral slope in the optical, with values ranging from $\sim 21 - 27\%$ in the blue regions ($0.4 - 0.55~\mathrm{μm}$) to $\sim 6 - 10\%$ in the red ($0.65 - 0.9~\mathrm{μm}$) regions. In contrast, the NIR showed a negative spectral slope of $\sim -0.9 \%$ between $0.9 - 1.5~\mathrm{μm}$ and $\sim -2.3\%$ between $1.9 - 2.5~\mathrm{μm}$. 3I/ATLAS shows a clear turnover in its spectral shape at $\sim 1.1~\mathrm{μm}$, corresponding to scattered light from the dusty coma. Finally, in the NIR, we do not find an increase in the depth of the water features identified in an earlier NIR observation of 3I/ATLAS. Our observations of 3I/ATLAS in the NIR show a similar shape to the NIR spectrum of 2I/Borisov as it approached perihelion.

Figures (4)

• Figure 1: Panchromatic reflectance spectra of 3I/ATLAS. All epochs show a clear rising continuum in the optical, peaking around $1.0$, indicative of a dusty coma. Between the second and third epochs, strong Ni and CN emission lines appear blueward of $\sim 0.4$, with Fe emission lines emerging as 3I/ATLAS made its perihelion passage. All spectra are normalized to reflectance at $0.5$, and the telluric regions are denoted by the vertical shaded regions. A zoomed in view on the cometary emission region $0.33 - 0.5$ is shown in Figure \ref{['fig:emission_features']}.
• Figure 2: Emission lines associated with Ni, Fe, CN, and $\mathrm{C_3}$ that emerge in the LRIS and SNIFS spectra of 3I/ATLAS once it had moved within $\mathrm{r_h} \sim 3$ au. The Fe and $\mathrm{C_3}$ emission lines evolve as 3I/ATLAS approached and past perihelion suggesting a change in the emitting surface. This may have been caused by the Fe-rich material being buried deeper in the surface than the Ni-rich compounds.
• Figure 3: Evolution of the optical and NIR spectral slopes of 3I/ATLAS as it approaches perihelion. The dashed vertical line indicates heliocentric distance where 3I/ATLAS crossed perihelion. While the optical slope showed a slight increase in spectral slope between the two Keck-I/LRIS observations, the NIR showed a declining evolution between the initial NIR observation Yang2025 and the Keck-II/NIRES data. The average optical spectral slope of 2I/Borisov is also included Jewitt2023ARAA. The spectral slope of 3I/ATLAS is similar to 2I/Borisov between $0.65 - 0.9$.
• Figure 4: Comparison of 3I/ATLAS's NIR spectrum with the binned NIR spectrum of 2I/Borisov Lee_2020_2I_Perihelion taken with FLAMINGOS-2 at a heliocentric distance of $\sim 2.0$ au. Both spectra show a negative spectral slope with no indication of the $1.5$ and $2.0$$\mathrm{H_2O}$ ice absorption features, which have been highlighted for clarity. The telluric region has been removed from both spectra and marked by the shaded region.