FLASHING: New high-velocity H${}_2$O masers in IRAS 18286$-$0959

Abstract

We discovered new high-velocity components of H${}_2$O maser emission in one of the "water fountain" sources, IRAS~18286$-$0959, which has been monitored using the Nobeyama 45 m telescope in the new FLASHING (Finest Legacy Acquisitions of SiO- and H${}_2$O-maser Ignitions by Nobeyama Generation) project since 2018 December. The maser spectra show new, extremely high expansion velocities ($>$200~km~s${}^{-1}$ projected in the line of sight) components, some of which are located symmetrically in the spectrum with respect to the systemic velocity. They were also mapped with KaVA (KVN and VERA Combined Array) in 2019 March. We located some of these maser components closer to the central stellar system than other high velocity components (50--200~km~s${}^{-1}$) that have been confirmed to be associated with the known bipolar outflow. The new components would flash in the fast collimated jet at a speed over 300~km~s${}^{-1}$ (soon) after 2011 when they had not been detected. The fastest of the new components seem to indicate rapid deceleration in these spectra, however our present monitoring is still too sparse to unambiguously confirm it (up to 50~km~s${}^{-1}$yr${}^{-1}$) and too short to reveal their terminal expansion velocity, which will be equal to the expansion velocity that has been observed ($v_{\exp }\sim$120~km~s${}^{-1}$). Future occurrences of such extreme velocity components may provide a good opportunity to investigate possible recurrent outflow ignitions. Thus sculpture of the parental envelope will be traced by the dense gas that is entrained by the fast jet and exhibits spectacular distributions of the relatively stable maser features.

11 pages, 5 figures, 2 tables

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