%0 Journal Article
%A Contreras, C. Sánchez
%A Le Mignant, D.
%A Sahai, R.
%A Gil de Paz, Armando
%A Morris, M.
%T Adaptive optics imaging of IRAS 18276-1431: A bipolar preplanetary nebula with circumstellar "searchlight beams" and "Arcs"
%D 2007
%@ 0004-637X
%U https://hdl.handle.net/20.500.14352/51767
%X 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 12^CO J = 1-0, 13^CO 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_☉ 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 × 10^16 and R_out ≥1.25 × 10^17 cm, dust temperature T_dust ~ 105-50 K, and a mean mass-loss rate of M ≈ 10^3 M_☉ yr^-1; and (2) an inner shell with R_in ~ 6.3 × 10 cm, T_dust ~ 500-105 K, and M ~ 3 × 10^-5 M_☉ 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_☉ can account for the observed submillimeter and millimeter flux excess. The mass of the envelope enclosed within R_out = 1.25 × 10^17 cm derived from SED modeling is ~1 (D/3 kpc)^2 M_☉.
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