The extinction and dust-to-gas structure of the planetary nebula NGC 7009 observed with MUSE^⋆

¹ European Southern Observatory. Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
e-mail: jwalsh@eso.org
² GEPI, Observatoire de Paris, PSL Research University, CNRS, Université Paris-Diderot, Sorbonne Paris Cité, Place Jules Janssen, 92195 Meudon, France
³ Dept. of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁴ Department of Physics and Astronomy, University of Denver, 2112 E. Wesley Ave., Denver, CO 80210, USA
⁵ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago, Chile
⁶ Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester, Manchester M13 9PL, UK

Received: 17 December 2015
Accepted: 17 February 2016

Abstract

Context. Dust plays a significant role in planetary nebulae. Dust ejected with the gas in the asymptotic giant branch (AGB) phase is subject to the harsh environment of the planetary nebula (PN) while the star is evolving towards a white dwarf. Dust surviving the PN phase contributes to the dust content of the interstellar medium.

Aims. The morphology of the internal dust extinction has been mapped for the first time in a PN, the bright nearby Galactic nebula NGC 7009. The morphologies of the gas, dust extinction and dust-to-gas ratio are compared to the structural features of the nebula.

Methods. Emission line maps in H Balmer and Paschen lines were formed from analysis of MUSE cubes of NGC 7009 observed during science verification of the instrument. The measured electron temperature and density from the same cube were employed to predict the theoretical H line ratios and derive the extinction distribution across the nebula. After correction for the interstellar extinction to NGC 7009, the internal A_V/N_H has been mapped for the first time in a PN.

Results. The extinction map of NGC 7009 has considerable structure, broadly corresponding to the morphological features of the nebula. The dust-to-gas ratio, A_V/N_H, increases from 0.7 times the interstellar value to >5 times from the centre towards the periphery of the ionized nebula. The integrated A_V/N_H is about 2× the mean ISM value. A large-scale feature in the extinction map is a wave, consisting of a crest and trough, at the rim of the inner shell. The nature of this feature is investigated and instrumental and physical causes considered; no convincing mechanisms were identified to produce this feature, other than AGB mass loss variations.

Conclusions. Extinction mapping from H emission line imaging of PNe with MUSE provides a powerful tool for revealing the properties of internal dust and the dust-to-gas ratio.