The Spatio‐Kinematical Structure and Distance of the Preplanetary Nebula IRAS 19134+2131

Abstract

Using the Very Long Baseline Array at six epochs, we have observed H2O maser emission in the preplanetary nebula IRAS 19134+2131 (I19134), in which the H2O maser spectrum has two groups of emission features separated in radial velocity by ~100 km s-1. We also obtained optical images of I19134 with the Hubble Space Telescope to locate the bipolar reflection nebula in this source for the first time. The spatio-kinematical structure of the H 2O masers indicates the existence of a fast, collimated (precessing) flow having a projected extent of ~140 mas and an expansion rate of ~1.9 mas yr-1 on the sky plane, which gives a dynamical age of only ~40 yr. The two detected optical lobes are also separated by ~150 mas in almost the same direction as that of the collimated flow. The good agreement between the extent and orientation of the H2O maser outflow and optical lobes suggests that the lobes have been recently formed along the collimated fast flow. Thus, the circumstellar envelope around the evolved star has apparently been penetrated by the fast flow and has been cleared for the emergence of the starlight in the directions of the fast flow. The positions of all of the detected maser features have been measured with respect to the extragalactic reference source J1925+2106 over one year. Therefore, we analyzed maser feature motions that consist of the combination of an annual parallax, a secular motion following Galactic rotation, and the intrinsic motions within the flow. We obtain an annual-parallax distance to I19134 of D=8.0+0.9-0.7 kpc and estimate its location in the Galaxy to be (R, θ, z)=(7.4+0.4-0.3 kpc, 62deg+/-5deg, 0.65+0.07-0.06 kpc). From the mean motion of the blueshifted and redshifted clusters of maser features, we estimate the three-dimensional (3D) secular motion of I19134 to be (VR,Vθ,Vz)=(3+53-46, 125+20-28, 8+48-39) km s-1. From the height from the Galactic plane, z, and the velocity component perpendicular to the Galactic plane, Vz, we estimate a rough upper limit of ~9 Msolar to the stellar mass of I19134's progenitor.