A Direct View of the Chemical Properties of Water from Another Planetary System: Water D/H in 3I/ATLAS

Abstract

All detected water reservoirs in the solar system exhibit a deuterium enrichment that links back to the physical environment at the time of stellar birth. Gas-phase and ice-grain D-enrichments occur through chemical processes that operate at low temperatures ($<$~30~K) pointing towards an origin in the pre-stellar molecular cloud or in the outer parts of the protoplanetary disk. However, not all stars are born in environments similar to our Sun, nor do their subsequent evolutionary histories follow the same path. These environmental differences can be traced by the water D/H ratio. Here we use ALMA observations of the interstellar comet 3I/ATLAS to constrain the water D/H ratio in extrasolar cometary material. With a water D/H value of [D/H]$_{\mathrm{H_2O}} > 6.6\times10^{-3}$, 3I/ATLAS shows a deuterium enrichment exceeding Earth's ocean value by more than a factor of $\gtrsim40$ and typical Solar System cometary values by more than a factor of $\gtrsim30$. The elevated deuterium enrichment points to water that formed under colder, less irradiated conditions and from less thermally processed material, consistent with an origin in a planetary system that formed under different physical and chemical conditions than our own.

Figures (10)

• Figure 1: Integrated intensity (moment 0) maps of the detected species. From left to right, HDO, CH$_3$OH in Band 6 and CH$_3$OH in Band 5. For each image, the ellipse in the bottom left shows the size of the ALMA beam and in the bottom right, the direction of the Sun and comet trail are signaled with the corresponding arrows. The white cross marks the location of the comet nucleus.
• Figure 1: Corner plot of the posterior distributions obtained from the simultaneous MCMC SUBLIME 1D fit to the HDO, H$_2$O and CH$_3$OH ALMA data. Dashed vertical lines indicate the 16th and 84th percentiles, and the red vertical line indicates the median (50th percentile). Together, the fitted parameters provide a snapshot of the physical and chemical state of the coma.
• Figure 1: Rotational diagram for the observed methanol lines, compared to the best-fit joint model (HDO+H$_2$O+CH$_3$OH). Left panel shows the results for Band 5 ($J = 4-3$) and right panel for Band 6 data ($J = 5-4$). Black points and gray uncertainty range correspond to the observational results. The red circles and dashed line show the results from the model analysis.
• Figure 2: 3I/ATLAS ALMA spectra and best-fit models for HDO, H$_2$O, and CH$_3$OH in Bands 5 and 6. The 3I/ATLAS spectra (gray) were extracted at the nucleus position, and the HDO, H$_2$O, and CH$_3$OH spectral windows were fitted simultaneously using an MCMC SUBLIME 1D retrieval to constrain the physical and chemical properties of the coma. The best-fit models are shown in red. Velocity scales are shown in the comet rest frame. For HDO, H$_2$O, CH$_3$OH Band 6 and CH$_3$OH Band 5, the reference frequencies are 241561.550, 183310.087, 241806.524, and 193454.358 MHz, respectively.
• Figure 2: CH$_3$OH Band 6 and 5 spectra compared with the best-fit MCMC SUBLIME nominal model. All panels share the same x- and y-axis scales. For each panel, the corresponding Band and the reference frequency of the window (in GHz) are given. Vertical dotted lines mark the positions of the CH$_3$OH transitions within each window. Labels are ordered from top to bottom to match the dotted lines from left to right.