摘要
Context. Water is a key volatile that provides insight into the initial stages of planet formation. The low water abundances inferred from water observations toward low-mass protostellar objects may point to a rapid locking of water as ice by large dust grains during star and planet formation. However, little is known about the water vapor abundance in newly formed planet-forming disks. Aims. We aim to determine the water abundance in embedded Keplerian disks through spatially-resolved observations of H <sub>2</sub> <sup>18</sup> O lines to understand the evolution of water during star and planet formation. Methods. We present H <sub>2</sub> <sup>18</sup> O line observations with ALMA and NOEMA millimeter interferometers toward five young stellar objects. NOEMA observed the 3 <sub>1,3</sub> -2 <sub>2,0</sub> line (E <sub>up</sub> ? k <sub>B</sub> = 203.7 K) while ALMA targeted the 4 <sub>1,4</sub> -3 <sub>2,1</sub> line (E <sub>up</sub> ? k <sub>B</sub> = 322.0 K). Water column densities were derived considering optically thin and thermalized emission. Our observations were sensitive to the emission from the known Keplerian disks around three out of the five Class I objects in the sample. Results. No H <sub>2</sub> <sup>18</sup> O emission is detected toward any of our five Class I disks. We report upper limits to the integrated line intensities. The inferred water column densities in Class I disks are N <sub>H</sub> <sub>O</sub> < 10 <sup>15</sup> cm <sup>-2</sup> on 100 au scales, which include both the disk and envelope. The upper limits imply a disk-averaged water abundance of ? 10 <sup>-6</sup> with respect to H <sub>2</sub> for Class I objects. After taking the physical structure of the disk into account, the upper limit to the water abundance averaged over the inner warm disk with T > 100 K is between ~10 <sup>-7</sup> and 10 <sup>-5</sup> . Conclusions. Water vapor is not abundant in warm protostellar envelopes around Class I protostars. Upper limits to the water vapor column densities in Class I disks are at least two orders of magnitude lower than values found in Class 0 disk-like structures.