2025A&A...703L...7C

“Water fountains” (WFs) are optically obscured evolved stars. Most of them are thought to be in the post-asymptotic giant branch (post-AGB) phase, and they are characterized by H₂O maser emission tracing molecular jets. Interestingly, four WFs (IRAS 15445-5449, IRAS 18019-2216, IRAS 18443-0231, and IRAS 18464-0140) and one WF candidate (IRAS 18480+0008) are potentially planetary nebulae (PNe) because they exhibit radio continuum emission, suggesting the presence of a photoionized region characteristic of PNe. To classify these objects, we obtained K-band (2.0-2.3 μm) spectra of these WFs, including the only WF PN known (IRAS 15103-5754) for comparison. Our spectra reveal two groups of sources: (i) “low-velocity” WFs with an H₂O maser velocity spread of ≲50 km s^–1 (IRAS 18019-2216, IRAS 18464-0140, and IRAS 18480+0008) showing the CO band at 2.29 μm in absorption, typical of cool giant stars, and no emission lines, and (ii) “high-velocity” WFs, velocity spread of ≳50 km s^–1 (IRAS 15103-5754, IRAS 15445-5449, and IRAS 18443-0231), exhibiting emission lines of Brγ, He I, and H₂, consistent with hotter central stars and/or shock-excited emission. The emission line ratios of these lines in IRAS 18443-0231 indicate that it may be a nascent PN. The spectrum of IRAS 15445-5449 also shows a CO band and Na I doublet in emission, suggesting the presence of a compact circumstellar disk and/or active mass loss. These results favor the previously suggested notion that the difference between low- and high-velocity WFs is not simply a projection effect but a reflection of intrinsically different evolutionary stages. Moreover, the results are also consistent with the idea of an increase in the jet ejection velocity as the post-AGB evolution proceeds.