A Starfish Preplanetary Nebula: IRAS 19024+0044

Abstract

Using the Hubble Space Telescope, we have imaged the OH/IR star IRAS 19024+0044 (I19024) at 0.6, 0.8, 1.1, and 1.6 μm, as part of our surveys of candidate preplanetary nebulae. The images show a multipolar nebula of size ~3farcs7 × 2farcs3, with at least six elongated lobes emanating from the center of the nebula. Two of the lobes show limb-brightened tips having point-symmetric structure with respect to the expected location of the central star. The central region shows two dark bands southwest and northeast of a central shallow maximum that may be either two inclined dusty toroidal structures or the dense parts of a single wide, inhomogeneous, toroid. A very faint, surface brightness-limited, diffuse halo surrounds the lobes. Long-slit/echelle optical spectroscopy obtained at the Mount Palomar and Keck observatories shows a spatially compact source of Hα emission; the Hα line shows a strong, narrow, central core with very broad (±1000 km s^(-1)), weak wings, and a narrower blueshifted absorption feature signifying the presence of a ~100 km s^(-1) outflow. The spectrum is characterized by a strong, relatively featureless, continuum and lacks the strong forbidden emission lines characteristic of planetary nebulae, confirming that IRAS 19024 is a preplanetary nebula; the spectral type for the central star, although uncertain, is most likely early G. Interferometric observations of the CO J = 1-0 line emission with the Owens Valley Radio Interferometer show a marginally resolved molecular envelope (size 5farcs5 × 4farcs4) with an expansion velocity of 13 km s^(-1), resulting from the asymptotic giant granch (AGB) progenitor's dense, slow wind. We derive a kinematic distance of 3.5 kpc to I19024, based on its radial velocity. The bolometric flux is 7.3 × 10^(9-) ergs s^(-1) cm^(-2), and the luminosity 2850 L_☉. The relatively low luminosity of I19024, in comparison with stellar evolutionary models, indicates that the initial mass of its central star was ~1-1.5 M_☉. The lobes, which appear to be hollow structures with dense walls, have a total mass greater than or equal to about 0.02 M_☉. The dusty tori in the center have masses of a few times 10^(-3) M_☉. The faint halo has a power-law radial surface brightness profile with an exponent of about -3 and most likely represents the remnant spherical circumstellar envelope formed as a result of constant mass loss during the AGB phase over the past several thousand years. From the CO data we infer a molecular mass ≳ 0.025 M_☉ and an expansion age ≾ 2870 yr, giving a mass-loss rate ≳ 10^(-5) M_☉ yr^(-1). The far-infrared fluxes of I19024 indicate the presence of a large mass of dust in the nebula; from a simple model we infer the presence of cool (109 K) and warm (280 K) components of dust mass 5.7 × 10^(-4) and 1.5 × 10^(-7) M_☉. We discuss our results for I19024 in the light of past and current ideas for the dramatic transformation of the morphology and kinematics of mass-ejecta as AGB stars evolve into planetary nebulae.