Low- and high-velocity `water fountains': different evolutionary stages

TL;DR

The paper investigates whether the distinction between low-velocity and high-velocity water fountains (WFs) reflects intrinsic evolutionary stages rather than projection effects. It adopts near-infrared K-band spectroscopy of six WF targets plus one WF PN to classify spectral features and diagnose excitation mechanisms, distinguishing shocks from photoionization. The results show three cool-giant, low-velocity WFs with CO absorption and three hotter, high-velocity WFs with Brγ, He I, and H2 emission, with IRAS 18443-0231 appearing as a nascent PN; IRAS 15445-5449 also exhibits CO/Na I emission suggesting a circumstellar disk. Together the findings support an evolutionary sequence in WF jets, with jet ejection velocity increasing as post-AGB evolution proceeds, and highlight disk-related phenomena in at least one source, while noting potential sample biases that warrant larger surveys.

Abstract

`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 H$_{2}$O 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 $μ$m) spectra of these WFs, including the only WF PN known (IRAS 15103$-$5754) for comparison. Our spectra reveal two group of sources: (i) `low-velocity' WFs with H$_2$O maser velocity spread of $\lesssim$50 km s$^{-1}$ (IRAS 18019$-$2216, IRAS 18464$-$0140, and IRAS 18480+0008) showing the CO band at 2.29 $μ$m in absorption, typical of cool giant stars, and no emission lines; and (ii) `high-velocity' WFs, velocity spread of $\gtrsim$50 km s$^{-1}$ (IRAS 15103$-$5754, IRAS 15445$-$5449, and IRAS 18443$-$0231), exhibiting emission lines of Br$γ$, He I, and H$_2$, consistent with hotter central stars and/or shock-excited emission. The emission line ratios of these lines in IRAS 18443$-$0231 indicates that it may be a nascent PN. The spectrum of IRAS 15445$-$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.

Figures (2)

• Figure 1: K-band near-infrared spectra of our targets. The top panels are the objects without emission lines (Group A) and the bottom panels are the objects with emission lines (Group B). Other possible emission lines are are difficult to identify due to the low resolution of the spectra.
• Figure 2: Emission-line ratio diagnostic diagram for recombination lines of Br$\gamma$ at 2.165$\mu$m and He i lines at 2.058 and 2.120$\mu$m. The region occupied by the black points corresponds to the values expected from the shock models of sut17. The filled stars are IRAS 15103$-$5754 (blue), IRAS 15445$-$5445 (green), and IRAS 18443$-$0231 (red). The sizes of the symbols are larger than their errors. The arrows indicate upper limits to the flux of the He i 2.120 $\mu$m emission line (see Fig. \ref{['fig:wf_spectra']} and Table \ref{['tab:lines']}).