Adaptive Optics Imaging of IRAS 18276-1431: A Bipolar Preplanetary Nebula with Circumstellar ``Searchlight Beams'' and ``Arcs''

Abstract

We present high angular resolution images of the post-AGB nebula IRAS 18276-1431 (also known as OH 17.7-2.0) obtained with the Keck II adaptive optics (AO) system in its natural guide star (NGS) mode in the K_p, L_p, and M_s near-infrared bands. We also present supporting optical F606W and F814W HST images as well as interferometric observations of the ¹²CO J=1-0, ¹³CO J=1-0, and 2.6 mm continuum emission with OVRO. The envelope of IRAS 18276-1431 displays a clear bipolar morphology in our optical and NIR images with two lobes separated by a dark waist and surrounded by a faint 4.5^''×3.4^'' halo. Our K_p-band image reveals two pairs of radial ``searchlight beams'' emerging from the nebula center and several intersecting, arclike features. From our CO data we derive a mass of M>0.38(D/3 kpc)² M_solar and an expansion velocity V_exp=17 km s^-1 for the molecular envelope. The density in the halo follows a radial power law ~r^-3, which is consistent with a mass-loss rate increasing with time. Analysis of the NIR colors indicates the presence of a compact central source of ~300-500 K dust illuminating the nebula in addition to the central star. Modeling of the thermal IR suggests a two-shell structure in the dust envelope: (1) an outer shell with inner and outer radius R_in~1.6×10¹⁶ and R_out>~1.25×10¹⁷ cm, dust temperature T_dust~105-50 K, and a mean mass-loss rate of M˙~10^-3 M_solar yr^-1 and (2) an inner shell with R_in~6.3×10¹⁴ cm, T_dust~500-105 K, and M˙~3×10^-5 M_solar yr^-1. An additional population of big dust grains (radius a>~0.4 mm) with T_dust=150-20 K and mass M_dust=(0.16-1.6)×10^-3(D/3 kpc)² M_solar can account for the observed submillimeter and millimeter flux excess. The mass of the envelope enclosed within R_out=1.25×10¹⁷ cm derived from SED modeling is ~1(D/3 kpc)² M_solar.