2025/10/02 by Roldán A. Cala, L. F. Miranda, Cala, Roldán A. +5
Earth and Planetary Sciences · #Arctic and Antarctic ice dynamics #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Geophysics and Gravity Measurements #Solar and Stellar Astrophysics (astro-ph.SR) #Underwater Acoustics Research
paper · pdf · doi:10.48550/arxiv.2510.01856
openalex publication_date 2025/10/02 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/01
`Water fountains' (WFs) are optically obscured evolved stars, most of them thought to be in the post-asymptotic giant branch (post-AGB) phase, 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 potential 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 group of sources: (i) `low-velocity' WFs with H2O maser velocity spread of \lesssim50 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 \gtrsim50 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 indicates 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 reflects 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.