VLT observations of the asymmetric Etched Hourglass Nebula, MyCn 18

¹ Centre for Astronomy, School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
e-mail: n.clyne1@nuigalway.ie; matt.redman@nuigalway.ie
² Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Received: 21 June 2013
Accepted: 1 July 2014

Abstract

Context. The mechanisms that form extreme bipolar planetary nebulae remain unclear.

Aims. The physical properties, structure, and dynamics of the bipolar planetary nebula, MyCn 18, are investigated in detail with the aim of understanding the shaping mechanism and evolutionary history of this object.

Methods. VLT infrared images, VLT ISAAC infrared spectra, and long-slit optical Echelle spectra are used to investigate MyCn 18. Morpho-kinematic modelling was used to firmly constrain the structure and kinematics of the source. A timescale analysis was used to determine the kinematical age of the nebula and its main components.

Results. A spectroscopic study of MyCn 18’s central and offset region reveals the detailed make-up of its nebular composition. Molecular hydrogen, atomic helium, and Bracket gamma emission are detected from the central regions of MyCn 18. ISAAC spectra from a slit position along the narrow waist of the nebula demonstrate that the ionised gas resides closer to the centre of the nebula than the molecular emission. A final reconstructed 3D model of MyCn 18 was generated, which provides kinematical information on the expansion velocity of its nebular components by means of position-velocity (P-V) arrays. A kinematical age of the nebula and its components were obtained by the P-V arrays and timescale analysis.

Conclusions. The structure and kinematics of MyCn 18 are better understood using an interactive 3D modelling tool called SHAPE. A dimensional and timescale analysis of MyCn 18’s major components provides a possible mechanism for the nebula’s asymmetry. The putative central star is somewhat offset from the geometric centre of the nebula, which is thought to be the result of a binary system. We speculate that the engulfing and destruction of an exoplanet during the asymptotic giant branch phase may have been a key event in shaping MyCn 18 and generating of its hypersonic knotty outflow.