Adaptive Optics Imaging of IRAS 18276−1431: A Bipolar Preplanetary Nebula with Circumstellar “Searchlight Beams” and “Arcs”

We present high-angular resolution images of the post-AGB nebulaIRAS18276-1431 (also known as OH17.7-2.0) obtained with the Keck II AdaptiveOptics (AO) system in its Natural Guide Star (NGS) mode in the Kp, Lp, and Msnear-infrared bands. We also present supporting optical F606W and F814W HSTimages as well as interferometric observations of the 12CO(J=1-0), 13CO(J=1-0),and 2.6mm continuum emission with OVRO. The envelope of IRAS18276-1431 displaysa clear bipolar morphology in our optical and NIR images with two lobesseparated by a dark waist and surrounded by a faint 4.5"x3.4" halo. Our Kp-bandimage reveals two pairs of radial ``searchlight beams'' emerging from thenebula center and several intersecting, arc-like features. From our CO data wederive a mass of M>0.38[D/3kpc]^2 Msun and an expansion velocity v_exp=17km/sfor the molecular envelope. The density in the halo follows a radial power-lawproportional to r^-3, which is consistent with a mass-loss rate increasing withtime. Analysis of the NIR colors indicates the presence of a compact centralsource of ~300-500K dust illuminating the nebula in addition to the centralstar. Modeling of the thermal IR suggests a two-shell structure in the dustenvelope: 1) an outer shell with inner and outer radius R_in~1.6E16cm andR_out>~1.25E17cm, dust temperature T_d~105-50K, and a mean mass-loss rate ofMdot~1E-3Msun/yr; and 2) an inner shell with R_in~6.3E14cm, T_dust~500-105K,and Mdot~3E-5Msun/yr. An additional population of big dust grains (radiusa>~0.4mm) with T_dust=150-20K and mass M_dust=(0.16-1.6)E-3 [D/3kpc]^2 Msun canaccount for the observed sub-mm and mm flux excess. The mass of the envelopeenclosed within R_out=1.25E17cm derived from SED modeling is ~1[D/3kpc]^2 Msun.Comment: 46 pages, 14 figures, 3 tables, accepted for publication in ApJ. Figures 12 & 13 in low resolution. Full resolution versions are available upon request to the first autho