Near-infrared polarimetry and modelling of the dusty young planetary nebula IRAS 19306+1407

Abstract

We present near-infrared polarimetric images of the dusty circumstellar envelope (CSE) of IRAS 19306+1407, acquired at the United Kingdom Infrared Telescope (UKIRT) using the UKIRT 1-5 μm Imager Spectrometer (UIST) in conjunction with the half-waveplate module IRPOL2. We present additional 450- and 850-μm photometry data obtained with the Sub-millimetre Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT), as well as archived Hubble Space Telescope (HST) F606W- and F814W-filter images. The CSE structure in polarized flux at J and K bands shows an elongation north of north-east and south of south-west with two bright scattering shoulders north-west and south-east. These features are not perpendicular to each other and could signify a recent 'twist' in the outflow axis. We model the CSE using an axisymmetric light scattering (ALS) code to investigate the polarization produced by the CSE, and an axisymmetric radiation transport (DART) code to fit the spectral energy distribution. A good fit was achieved with the ALS and DART models using silicate grains, 0.1-0.4 μm with a power-law size distribution of a -3.5 , and an axisymmetric shell geometry with an equator-to-pole ratio of 7:1. The spectral type of the central star is determined to be B 11 supporting previous suggestions that the object is an early planetary nebula. We have constrained the CSE and interstellar extinction as 2.0 and 4.2 mag, respectively, and have estimated a distance of 2.7 kpc. At this distance, the stellar luminosity is ∼4500 L ⊙ and the mass of the CSE is ∼0.2 M ⊙ . We also determine that the mass loss lasted for ∼5300 yr with a mass-loss rate of ∼3.4 x 10 -5 M ⊙ yr -1 .