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 demonstrates water outgassing in 3I/ATLAS through UV OH emission observations, yielding a production rate of (1.35 ± 0.27) ×10^27 molecules s⁻¹ at 3.51 au.
• The paper applied precise photometric corrections and reddening measurements with UVOT event mode imaging to accurately isolate the OH emission signal.
• The paper infers a high active fraction and nucleus activity, suggesting grain-driven outgassing and unique volatile composition in low-metallicity extrasolar environments.

Water Detection and Activity in the Interstellar Object 3I/ATLAS

Introduction

The detection of water activity in the third confirmed interstellar object, 3I/ATLAS, represents a significant advance in the compositional characterization of extrasolar planetesimals. This paper utilizes ultraviolet imaging from the Neil Gehrels-Swift Observatory's Ultraviolet/Optical Telescope (UVOT) to identify OH (A$^2\Sigma$ -- X$^2\Pi$) emission near 3085 Å, a direct tracer of H$_2$O outgassing. The analysis is contextualized by comparisons to both previous interstellar objects (1I/`Oumuamua, 2I/Borisov) and Oort Cloud comets, with particular attention to the mechanisms driving activity at large heliocentric distances and the implications for planetesimal formation in low-metallicity systems.

Observational Strategy and Data Reduction

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 (central wavelength 2600 Å) was used to capture OH emission, while the V-band filter (5468 Å) provided continuum reference. Due to the high apparent motion of the comet, UVW1 images were acquired in event mode and segmented into 30-second slices to mitigate motion blur, followed by alignment and stacking. V-band images, acquired in image mode, were similarly stacked but exhibited elongation due to motion blur.

Figure 1: Stacked UVOT images of 3I/ATLAS in V and UVW1 bands, showing the spatial extent of the coma and the apertures used for photometry.

Aperture photometry was performed with a 10-arcsec radius, and background subtraction utilized a star-masked annulus. The V-band flux was converted to magnitude using a linearly reddened solar spectrum, yielding $V = 17.29 \pm 0.04$ mag. The continuum contribution to the UVW1 flux was estimated and subtracted to isolate the OH emission.

Water Production Rate Determination

The derivation of the water production rate is highly sensitive to the assumed reddening between the V and UVW1 effective wavelengths. The count rate ratio $\beta(S)$, dependent on the spectral slope (reddening $S$), was computed by convolving a reddened solar spectrum with the UVOT filter response. The OH count rate was converted to flux and then to the number of OH molecules using fluorescence efficiencies and the vectorial model, accounting for photodissociation lifetimes and branching ratios.

Figure 2: Water production rate as a function of reddening and compilation of reddening measurements for 3I/ATLAS from the literature.

Reddening measurements from other telescopes were forward-modeled to the UVOT effective wavelengths, revealing a trend of increasing reddening at shorter wavelengths. The most relevant measurement, $38.6\%$ per 1000 Å from Alvarez et al., yields a water production rate of $(1.35 \pm 0.27) \times 10^{27}$ molecules s$^{-1}$ ($\sim$40 kg s$^{-1}$) at 3.51 au, with SNR = 5.0. This places 3I/ATLAS among the few comets with confirmed OH emission beyond 3 au, where water ice sublimation is typically inefficient.

Coma Properties and Activity Fraction

The measured $Af\rho(0)$ of 174 cm (V-band) and 180 cm (R-band) indicates a dust-rich coma, comparable to hyperactive comets such as Hale-Bopp and Garradd. Sublimation modeling suggests a minimum active area of 19 km$^2$, corresponding to a circular region of radius 2.47 km. HST observations constrain the nucleus radius to $<2.8$ km, implying an active fraction $>20\%$, which is significantly higher than the typical 3-5\% for solar system comets.

The detection of OH emission at large heliocentric distance is rare; only a handful of comets (e.g., Bowell, Garradd) have shown similar behavior, often attributed to extended sources such as large icy grains in the coma. Near-infrared spectroscopy of 3I/ATLAS supports the presence of such grains, which may serve as an extended source of water vapor and explain the early detection of OH prior to CN.

Compositional and Dynamical Implications

The absence of CN emission, despite clear OH detection, is atypical and may reflect differences in volatile inventory or grain-driven outgassing. This behavior is consistent with scenarios where large, dark icy grains are preferentially heated and lose their more volatile components, as observed in comet 67P/Churyumov-Gerasimenko. The compositional evolution of 3I/ATLAS will provide a direct test of the hypothesis that interstellar objects from low-metallicity systems are water-rich and depleted in high-metallicity volatiles (CO, CO$_2$, CN).

Two testable scenarios are proposed:

1. If 3I/ATLAS is H$_2$O-rich, the water production rate will peak near perihelion, with only trace amounts of CO, CO$_2$, CN, and hydrocarbons.
2. If hypervolatiles dominate, the water production rate will decline post-perihelion as more volatile species are released from depth, analogous to 2I/Borisov.

The large size and high activity fraction of 3I/ATLAS may indicate a knowledge gap in planetesimal formation and ejection mechanisms in low-metallicity systems. Overabundance of large, H$_2$O-rich interstellar objects could serve as a diagnostic of ejection conditions, such as planetary interactions or stellar flybys.

Methodological Considerations

The analysis demonstrates the critical importance of accurate reddening determination for UV photometry of cometary comae. The conversion of literature reddening measurements to the UVOT filter set, accounting for spectral steepening at short wavelengths, is essential for robust water production rate estimates. The use of event-mode imaging and motion correction is necessary for high-velocity targets to avoid photometric dilution.

Conclusion

The detection of OH emission in 3I/ATLAS confirms active water outgassing in the third known interstellar object, with a production rate of $(1.35 \pm 0.27) \times 10^{27}$ molecules s$^{-1}$ at 3.51 au. The inferred active area and fraction are anomalously large compared to solar system comets, and the early detection of OH without CN suggests grain-driven activity or compositional differences. These results provide a direct probe of planetesimal formation and volatile retention in extrasolar, low-metallicity environments. Continued multiwavelength monitoring, including simultaneous gas and ice observations, will be essential to elucidate the thermal and compositional evolution of interstellar comets and refine models of their origin and ejection.