Adaptive optics imaging of IRAS 18276-1431: A bipolar preplanetary nebula with circumstellar 'searchlight beams' and 'Arcs'

© 2007. The American Astronomical Society. All rights reserved. The authors thank the anonymous referee for his/ her critical comments and valuable suggestions that have significantly improved this paper. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. This work has been partially performed at the California Institute of Technology and the Department of Molecular and Infrared Astrophysics of the Instituto de Estructura de la Materia, CSIC, and has been partially supported by National Science Foundation grant 9981546 to Owens Valley Radio Observatory and the Spanish MCyT under project DGES/AYA2003- 02785. R. S. is thankful for partial financial support for this work from a NASA/ADP grant. A. G. d. P. is financed by the MAGPOP EU Marie Curie Research Training Network and partially by the Spanish Programa Nacional de Astronomía y Astrofísica under grant AYA2003-01676. M. M. and R. S. acknowledge financial support for this work from NASA LTSA and ADP awards RTOP 399-20-40-06 and RTOP 399-20-00-08, and HST/GO award GO 09101.01-A. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, and the NASA Astrophysics Data System.
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_☉.
W. M. Keck Foundation
National Science Foundation
Owens Valley Radio Observatory
Ministerio de Ciencia y Tecnología, (MCYT), España
Astrophysics Data Analysis program (ADP), NASA
Spanish Programa Nacional de Astronomía y Astrofísica
LTSA (Long Term Space Astrophysics program), NASA
National Aeronautics and Space Administration (NASA)
HST/GO award
Unión Europea. FP6
Depto. de Física de la Tierra y Astrofísica
Fac. de Ciencias Físicas
TRUE
pub