Pristine ices in a planet-forming disk revealed by heavy water

TL;DR

This study uses ALMA observations of the protoplanetary disk around V883 Ori to measure multiple water isotopologues, including the rarely detected p-D2O. The derived D2O/H2O and D2O/HDO/HDO/H2O ratios, together with the (D2O/HDO)/(HDO/H2O) value, are consistent with water ice inherited from the earliest phases of star formation, rather than being largely reprocessed in the disk. Comparisons to Class 0 protostars and the comet 67P, along with chemical-model predictions, support a scenario in which pristine interstellar ices are preserved into the disk and potentially into comets and exoplanetary bodies. The findings provide a crucial link between the cold molecular cloud phase and the ice reservoirs that seed planet formation, reinforcing the idea that Earth's water may trace back to the earliest building blocks of our solar system. Overall, the work demonstrates that water ice in disks can retain its pristine, interstellar origin through the planet-formation process, with implications for the delivery of water to nascent planetary systems.

Abstract

Water is essential to our understanding of the planet-formation process and habitability on Earth. Although trace amounts of water are seen across all phases of star and planet formation, the bulk of the water reservoir often goes undetected, hiding crucial parts of its journey from giant molecular clouds to planets. This raises the question of whether water molecules in comets and (exo-)planets is largely inherited from the interstellar medium or if the water molecules are destroyed and then reformed in the disk. Water isotopologue ratios involving doubly deuterated water (D$_2$O) are a sensitive tracer to answer this question. We present strong evidence of inheritance through an enhancement of D$_2$O in the outbursting V883 Ori disk. The high D$_2$O/H$_2$O ratio of $(3.2 \pm 1.2) \times 10^{-5}$ is consistent with values seen in protostellar envelopes and a comet and is two orders of magnitude higher than expected if water is reprocessed. The high deuteration of the heaviest isotopologues D$_2$O/HDO = $(2.3 \pm 1.0) \times $HDO/H$_2$O further establishes the inheritance of water. We conclude that water ice in disks originates from the earliest phases of star formation, providing the missing link between cold dark clouds and (exo-)comets.

Figures (15)

• Figure 1: Integrated spectrum of D2O in the V883 Ori disk. The spectrum is extracted from an elliptical region with a $0\overset{\prime\prime}{.}4$ semi-major axis and a $0.4"\times \cos(i)$ semi-minor axis centered on the continuum peak, with $i$ the disk inclination. The pixels in the image cube were shifted by the projected Keplerian velocity at that location in the disk before extracting the spectrum to correct for the projected Keplerian rotation of the disk and decrease line blending of D2O, indicated with the red vertical line, with the neighboring CH3OD and CH3OCH3 lines. The solid horizontal line indicates the 5$\sigma$ noise level measured in off-source regions.
• Figure 2: Channel maps of the D2O emission compared to that of HDO. The grey line indicates the emission above $3\sigma$ attributed to the HDO line at 225 GHz. The white contours indicate the 3 and 4$\sigma$ level for the emission of D2O and the neighbouring COMs.
• Figure 3: The D2O/H2O ratio across different stages of star and planet formation. The measurements in the V883 Ori disk are presented in black and those in the Class 0 objects NGC 1333 IRAS 2A, B335, L483, and the 67P comet in grey Coutens2014, Altwegg2015, Altwegg2017, Jensen2019, Jensen2021b. The latter are computed as D2O/HDO $\times$ HDO/H2O. The errorbars represent the 1$\sigma$ uncertainty (s.d.) on the measured column density ratio in each source. The colored background and the histograms on the side, each normalized to the peak number of fluid parcels, indicate the expected water isotopologue ratios for inheritance where $\lesssim 10$% of the H2O ice is destroyed (blue) or reset where $\gtrsim70$% of the H2O is expected to be destroyed through photodissociation and photodesorption in a model of a collapsing core (red; Furuya2017). The red histogram is smoothed using a Savitzky-Golay filter with a window of 10 and an order of 3.
• Figure 4: Continuum image of the V883 Ori disk after self-calibration. The white ellipse in the bottom left corner indicates the beam of the observations.
• Figure 5: The HDO/H2O ratio across different stages of star and planet formation. The measurement in the V883 Ori disk are presented in black and those in the Class 0 objects NGC 1333 IRAS 2A, NGC 1333 IRAS 4A-NW, NGC 1333 IRAS 4B, IRAS 16293-2422, B335, BHR71-IRS1, L483, the L 1551 IRS5 disk, and the comets in grey Jensen2019, Jensen2021b, Coutens2014, Tobin2023, Altwegg2017, Altwegg2015, Hagemann1970, Persson2014, Mandt2024, DeLaeter2003, BockeleeMorvan1998, Meier1998, Gibb2012, Villanueva2009, BockeleeMorvan2012, Biver2006, Biver2016, Lis2013, Hartogh2011, Lis2019, Andreu2023. The errorbars represent the 1$\sigma$ uncertainty (s.d.) on the measured column density ratio in each source. The colored background and the histograms each normalized to the peak number of fluid parcels on the side indicate the expected water isotopologue ratios for inheritance where $\lesssim 10$% of the H2O ice is destroyed (blue) or reset where $\gtrsim70$% of the H2O is expected to be destroyed through photodissociation and photodesorption in a model of a collapsing core (red; Furuya2017). The red histogram is smoothed using a Savitzky-Golay filter with a window of 10 and an order of 3.