IRAS 18113-2503: THE WATER FOUNTAIN WITH THE FASTEST JET?

Abstract:
We present Expanded Very Large Array water maser observations at 22 GHz toward the source IRAS 18113-2503. Maser components span over a very high velocity range of similar or equal to 500 km s(-1), the second largest found in a Galactic maser, only surpassed by the high-mass star-forming region W49N. Maser components are grouped into a blueshifted and a redshifted cluster, separated by 0 ''.12. Further mid-IR and radio data suggest that IRAS 18113-2503 is a post-asymptotic giant branch star, thus a new bona fide member of the rare class of "water fountains" (WFs). It is the evolved object with the largest total velocity spread in its water masers and with the highest velocity dispersion within its redshifted and blueshifted lobes (similar or equal to 170 km s(-1)). The large total velocity range of emission probably indicates that IRAS 18113-2503 has the fastest jet among the known WF stars. On the other hand, the remarkably high velocity dispersion within each lobe may be i! nterpreted in terms of shocks produced by an episode of mass ejection whose velocity increased up to very high values or, alternatively, by projection effects in a jet with a large opening angle and/or precessing motions.