Water Detection in the Interstellar Object 3I/ATLAS (2508.04675v1)

Abstract: We report the first detection of water activity in the third confirmed interstellar object, 3I/ATLAS, based on ultraviolet imaging with the \emph{Neil Gehrels-Swift Observatory}. Observations acquired with the Ultraviolet/Optical Telescope on 2025 July 31st - Aug 1st revealed OH (A$^2\Sigma$ -- X$^2\Pi$) emission near 3085~\AA. The water production rate results highly depend on the reddening assumption. For a reddening of 38.6\% between 5437.8~\AA\ and 3325.7~\AA, the water production rate is $(1.35 \pm 0.27) \times 10^{27} $ molecules\,s$^{-1}$ (40~kg\,s$^{-1}$) at a heliocentric distance of 3.51~au. This places 3I/ATLAS among the few comets with confirmed OH emission beyond 3~au, where water ice sublimation is typically inefficient. The inferred production rate is consistent with an active area of at least 19~km$^2$, assuming equilibrium sublimation. Based on current upper limits of the nucleus' radius, this requires that over 20\% of the surface is active, which is larger than activity levels observed in most solar system comets. Contemporaneous near-infrared spectroscopy indicates the presence of large icy grains in the coma, which may serve as an extended source of water vapor. The detection of OH emission prior to any CN detection is unusual and may reflect differences in grain-driven outgassing or volatile inventory compared to typical comets. While similar behavior has been observed in solar system comets, the mechanisms controlling distant activity and the storage and release of volatiles remain poorly understood. If 3I/ATLAS' coma continues to be dominated by H$_2$O, supporting the early and low-metallicity formation hypothesis, the derived large size of the nucleus could be indicative of a key knowledge gap in low-metallicity system planetesimal formation and loss mechanisms.

• The paper presents the first robust detection of OH emission in 3I/ATLAS using UVOT imaging, quantifying water production rates at 3.51 au.
• Observations indicate a high active fraction (at least 20% of the nucleus) and suggest significant contributions from sublimating icy grains in the coma.
• The study offers insights into volatile retention and planetesimal formation in low-metallicity environments through comparative cometary analysis.

Water Activity in the Interstellar Object 3I/ATLAS: Detection, Analysis, and Implications

Introduction

The detection of water activity in the third confirmed interstellar object, 3I/ATLAS, represents a significant addition to the comparative paper of interstellar and solar system comets. This work presents ultraviolet imaging and photometric analysis using the Neil Gehrels-Swift Observatory's Ultraviolet/Optical Telescope (UVOT), leading to the first robust detection of OH (A$^2\Sigma$–X$^2\Pi$) emission in 3I/ATLAS. The paper provides a quantitative assessment of water production rates, explores the dependence on coma reddening, and contextualizes the findings within the broader framework of cometary activity at large heliocentric distances. The implications for planetesimal formation in low-metallicity systems and the mechanisms of volatile retention and release in interstellar objects are discussed.

Observational Strategy and Data Reduction

The UVOT observations were conducted on July 31 and August 1, 2025, when 3I/ATLAS was at 3.51 au from the Sun. The UVW1 filter (centered at 2600 Å) was used to capture the OH emission band near 3085 Å, while the V-band filter (5468 Å) provided a measure of the dust continuum. Due to the high apparent motion of the comet, UVW1 exposures were acquired in event mode, enabling time-tagged photon collection and subsequent motion correction by slicing exposures into 30-second intervals and aligning on the predicted nucleus position. V-band images, acquired in image mode, were stacked after discarding frames contaminated by background stars.

Figure 1: Stacked UVOT images of 3I/ATLAS in V (left) and UVW1 (right), with photometric apertures and background annuli indicated.

Aperture photometry was performed with a 10-arcsec radius, and background subtraction utilized a star-masked annulus at 30–40 arcsec. The V-band flux was converted to apparent magnitude and used to estimate the continuum contribution in the UVW1 band, allowing isolation of the OH emission component.

Water Production Rate Determination and Reddening Dependence

The derivation of the water production rate from the observed OH emission is highly sensitive to the assumed reddening between the V and UVW1 effective wavelengths. The continuum subtraction relies on the ratio $\beta(S)$, which is a function of the spectral slope (reddening) of the dust. The paper systematically explores the impact of different reddening assumptions, leveraging published measurements of 3I/ATLAS's spectral slope across various wavelength intervals.

Figure 2: Left: Water production rate as a function of assumed reddening between V and UVW1. Middle: Literature reddening measurements for 3I/ATLAS versus central wavelength. Right: Reddening values converted to the UVOT effective wavelengths.

For a reddening of 38.6% per 1000 Å (as measured by Alvarez-Candal et al.), the derived water production rate is $(1.35 \pm 0.27) \times 10^{27}$ molecules s$^{-1}$ (∼40 kg s$^{-1}$) at 3.51 au. The detection is robust (SNR = 5.0) for reddening values above 26%, and the trend of increasing reddening at shorter wavelengths is consistent with both prior cometary studies and the observed steepening of 3I/ATLAS's reflectance spectrum in the UV.

Physical Interpretation: Active Area and Grain-Driven Activity

The inferred water production rate implies a minimum active area of 19 km$^2$ under equilibrium sublimation, corresponding to at least 20% of the nucleus surface (assuming the upper limit of 2.8 km for the nucleus radius from HST imaging). This active fraction is significantly higher than the typical 3–5% observed in most solar system comets, suggesting either unusually efficient surface activity or a substantial contribution from sublimating icy grains in the coma.

Contemporaneous near-infrared spectroscopy (Yang et al.) indicates the presence of large icy grains, which could serve as an extended source of water vapor. The detection of OH emission prior to any CN detection is atypical and may reflect differences in the volatile inventory or the efficiency of grain-driven outgassing compared to solar system analogs.

Comparative Context: Distant Activity and Interstellar Comet Diversity

The detection of OH emission at 3.51 au places 3I/ATLAS among a small subset of comets with confirmed water activity at large heliocentric distances, such as C/1980 E1 (Bowell) and C/2009 P1 (Garradd). In these cases, extended sources of water, likely from icy grains, have been invoked to explain elevated production rates and non-monotonic activity profiles. The high Af$\rho$/Q(H$_2$O) ratio for 3I/ATLAS is consistent with a dust-rich coma, comparable to hyperactive comets like Hale-Bopp.

The observed behavior contrasts with that of 2I/Borisov, which exhibited a transition from H$_2$O/CO-dominated activity pre-perihelion to CO-dominated post-perihelion. The current data for 3I/ATLAS support a scenario in which water remains the dominant volatile, at least inbound, consistent with formation in a low-metallicity, water-rich environment.

Implications for Planetesimal Formation and Interstellar Object Populations

The large inferred active area and the possibility of a water-dominated volatile inventory in 3I/ATLAS have implications for models of planetesimal formation and ejection in low-metallicity systems. If 3I/ATLAS is representative of an older, water-rich population, this may indicate that ejection mechanisms in such systems preferentially produce large, H$_2$O-rich interstellar objects. This scenario is supported by dynamical models suggesting that planetary interactions or stellar flybys in low-metallicity environments can efficiently eject massive planetesimals.

The paper proposes two testable hypotheses for the volatile evolution of 3I/ATLAS: (1) if H$_2$O remains dominant near perihelion with low abundances of CO, CO$_2$, and CN, this supports the low-metallicity, water-rich formation scenario; (2) if hypervolatiles become dominant post-perihelion, a more recent formation or different dynamical history is implied, analogous to 2I/Borisov.

Conclusion

The detection of water activity in 3I/ATLAS via UVOT imaging and photometry provides direct evidence for the presence of H$_2$O in the coma of an interstellar object at large heliocentric distance. The derived production rate, active area, and dust-to-gas ratio are all at the upper end of the distribution for both solar system and interstellar comets. The results highlight the importance of grain-driven activity and the need for multiwavelength, time-resolved monitoring to disentangle the contributions of nucleus sublimation and extended sources. The findings have significant implications for the understanding of planetesimal formation and volatile retention in extrasolar environments, and motivate continued observational campaigns to track the compositional evolution of 3I/ATLAS as it approaches and recedes from perihelion.