University of Hawaii 88-inch Telescope Observations of the Interstellar Comet 3I/ATLAS: Spectrophotometric Blue-Sensitive Spectral Time Series Spanning Two Months from Discovery

TL;DR

This study presents a spectrophotometric time series of the interstellar object 3I/ATLAS using the SNIFS instrument on the UH 2.2 m telescope, covering discovery to two months before perihelion. The authors derive synthetic photometry, monitor activity with CN production rates via a Haser model, and quantify spectral slopes, finding a predominantly red, dust-dominated continuum with colors stabilizing around g-r ≈ 0.69–0.75, r-i ≈ 0.26–0.30, and c-o ≈ 0.50–0.55. CN activity emerges in mid-August with production rates up to Q(CN) ≈ 3×10^{24} s^{-1} (depending on aperture), alongside Ni emission and marginal Fe signals, while the continuum slope shows a notable but limited variation, including a July 12 spike likely caused by systematics. The data support a two-phase pre-perihelion evolution: an early plateau with near-solar colors and a subsequent uniform brightening driven by dust production with a steep r_h dependence, consistent with old-comet-like activity and CO2-driven processes indicated by complementary observations. Overall, 3I/ATLAS appears redder than 1I/’Oumuamua and 2I/Borisov, with a persistent, dust-dominated coma that provides valuable constraints on the composition and evolutionary history of extrasolar small bodies and informs future follow-up studies.

Abstract

Interstellar objects are the ejected building blocks of other solar systems. As such, they enable the acquisition of otherwise inaccessible information about nascent extrasolar systems. The discovery of the third interstellar object, 3I/ATLAS, provides an opportunity to explore the properties of a small body from another solar system and to compare it to the small bodies in our own. To that end, we present spectrophotometric observations of 3I/ATLAS taken using the SuperNova Integral Field Spectrograph on the University of Hawaii 2.2-m telescope. Our data includes the earliest $λ\leq3800$ A spectrum of 3I/ATLAS, obtained $\sim$12.5 hours after the discovery announcement. Later spectra confirm previously reported cometary activity, including Ni and CN emission. The data show wavelength-varying spectral slopes ($S\approx($0\%-29\%)/1000 A, depending on wavelength range) throughout the pre-perihelion ($r_h=4.4$-$2.5$ au) approach of 3I/ATLAS. We perform synthetic photometry on our spectra and find 3I/ATLAS shows mostly stable color evolution over the period of our observations, with $g-r$ colors ranging from $\sim$0.69-0.75 mag, $r-i$ colors ranging from $\sim$0.26-0.30 mag, and $c-o$ colors ranging from $\sim$0.50-0.55 mag. Ongoing post-perihelion observations of 3I/ATLAS will provide further insight into its potentially extreme composition.

Figures (5)

• Figure 1: The spectrophotometric time series data of 3I/ATLAS from the UH 2.2-meter telescope taken with the SNIFS spectrograph. Grey curves are the original data, and black curves represent the Gaussian-smoothed data. The SNIFS dichroic boundary between $\sim$5000-5200 Å is shaded grey. The Nearby Supernova Factory pipeline improves the dichroic region; however, some spectra still exhibit low signal-to-noise ratios in that region. The Pan-STARRS (bottom) and ATLAS (top) filter transmission functions are also shown. The red channel of the first spectrum is contaminated by a stellar streak, leading to a slope that differs from those of the other spectra and is therefore excluded. The blue channel is unaffected by the stellar streak because the contaminating star is extincted by the Milky Way.
• Figure 2: Synthetic photometry in the Pan-STARRS $griz$ and ATLAS $co$ passbands. All magnitudes are reported in the AB system. Reported "m10" ATLAS magnitudes from Tonry2025 are shown as black lines, and the "m6" ATLAS magnitudes are shown as grey lines. The cyan filter is denoted using dotted lines, and the orange filter is denoted using dashed lines.
• Figure 3: The continuum-subtracted spectra showing emission lines from Ni (dotted red), Fe (dashed green), and CN (solid blue).
• Figure 4: The spectral slope evolution fits for several epochs of 3I/ATLAS SNIFS spectra. The different colors represent different fitted wavelength ranges.
• Figure 5: The spectral slope evolution for 3I/ATLAS from our SNIFS spectra. The slope is described in the text. We compute the slopes using a linear fit to the reflectance spectra, with values normalized at 5500 Å. The first spectrum is excluded because it lacks uncontaminated red-wavelength coverage. The crosshatched regions centered on the dashed red and dotted blue lines denote the aggregate slope values for 1I/ ' Oumuamua and 2I/Borisov, respectively, from Jewitt2023ARAA. The grey-shaded region shows the spectral slope reported by Rahatgaonkar2025 for their spectroscopic time-series data from $r_h\simeq4.4$ to 2.9 au. Only statistical uncertainties are plotted. See Section \ref{['sec:slopes_discussion']} for discussion about the steep red slopes between July 12-14.