A Herschel study of NGC 650^⋆,⋆⋆

¹ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
e-mail: p.vanhoof@oma.be
² Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
³ Dept of Physics & Astronomy, University College London, Gower St, London WC1E 6BT, UK
⁴ Dept. of Physics and Astronomy, University of Denver, Mail Stop 6900, Denver, CO 80208, USA
⁵ School of Physics and Astronomy, Cardiff University, 5 The Parade, Cardiff, Wales CF24 3YB, UK
⁶ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁷ Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK
⁸ Dept. of Astronomy, Stockholm University, AlbaNova University Center, Roslagstullsbacken 21, 10691 Stockholm, Sweden
⁹ Department of Physics, University of Lethbridge, Lethbridge, Alberta, T1J 1B1, Canada

Received: 31 December 2012
Accepted: 26 July 2013

Abstract

As part of the Herschel guaranteed time key project Mass loss of Evolved StarS (MESS) we have imaged a sample of planetary nebulae. In this paper we present the Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) images of the classical bipolar planetary nebula NGC 650. We used these images to derive a temperature map of the dust. We also constructed a photoionization and dust radiative transfer model using the spectral synthesis code Cloudy. To constrain this model, we used the PACS and SPIRE fluxes and combined them with hitherto unpublished International Ultraviolet Explorer (IUE) and Spitzer InfraRed Spectrograph (IRS) spectra as well as various other data from the literature. A temperature map combined with a photoionization model were used to study various aspects of the central star, the nebula, and in particular the dust grains in the nebula. The central star parameters are determined to be T_eff = 208 kK and L = 261 L_⊙ assuming a distance of 1200 pc. The stellar temperature is much higher than previously published values. We confirm that the nebula is carbon-rich with a C/O ratio of 2.1. The nebular abundances are typical for a type IIa planetary nebula. With the photoionization model we determined that the grains in the ionized nebula are large (assuming single-sized grains, they would have a radius of 0.15 μm). Most likely these large grains were inherited from the asymptotic giant branch phase. The PACS 70/160 μm temperature map shows evidence of two radiation components heating the grains. The first component is direct emission from the central star, while the second component is diffuse emission from the ionized gas (mainly Lyα). We show that previous suggestions of a photo-dissociation region surrounding the ionized region are incorrect. The neutral material resides in dense clumps inside the ionized region. These may also harbor stochastically heated very small grains in addition to the large grains.