We present our analysis of the spectra of NGC 7027 taken with the PACS and SPIRE instruments of the Herschel satellite

Barlow, MJ

Exter, KM

Ferland, GJ

Ottensamer, R

Porter, RL

Van de Steene, GC

van Hoof, PAM

Wesson, R

English

UCL Discovery

Planetary Nebulae: an Eye to the FutureProceedings IAU Symposium No. 283, 2012, eds.c© 2012 International Astronomical UnionDOI: 00.0000/X000000000000000XHerschel Observations of NGC 7027K.M. Exter1, P.A.M van Hoof2, G.C. Van de Steene2, R.Ottensamer3, R. Wesson4, M.J. Barlow4, G.J. Ferland5, R.L. Porder6,and the MESS consortium1IvS, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Beglium2 Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussels, Belgium3 University Vienna, Dept. of Astronomy, Turkenschanzstrasse 17, A-1180 Wien, Austria4 Dept. of Physics and Astronomy, Univ. College London, Gower St, London WC1E6BY, UK5 Dept. of Physics and Astronomy, Univ. of Kentucky, Lexington, KY 40506, USA6 Physics and Astronomy and center for Simulational Physics, Univ. of Georgia, Athens, GA30602-2451, USAAbstract. We present our analysis of the spectra of NGC 7027 taken with the PACS and SPIREinstruments of Herschel.Keywords. planetary nebulae: individual (NGC 7027), infrared: ISM1. IntroductionAs part of the Herschel Guaranteed Time Key Project MESS (Mass loss of EvolvedStarS) (PI Martin Groenewegen) we have gotten (will get) spectra of a sample of plan-etary nebulae (PNe) with the PACS (Poglitsch et al. 2010) and SPIRE (Griffin et al.2010) instruments of the Herschel satellite (Pillbratt et al. 2010). A detailed descriptionof the programme can be found in Groenewegen et al. (2011) and an overview of theHerschel observations for PNe in van Hoof et al. (2012).NGC 7027 is a young bipolar PN with a fairly massive and fast-evolving, hot centralstar, and a C-rich nebular chemistry. It has an elliptical morphology. The inner nebulais prolate spheriodal shell with an equatorial density enhancement, outside of this arevarious layers of nebula including a larger spherical CO envelope. In the inner parts,molecular regions interface with the ionised regions with an hourglass morphology, wherethe wide waist corresponds to the equatorial torus.2. The SPIRE dataThe SPIRE spectra of NGC 7027 were presented in Wesson et al. (2010). They detectedlines of CO, H20 and CH+, from which they determine a range of rotational temperatures:50–130K from CO, 70–260K from CH+. This range reflects a complexity in the nebula.Note the presence of water, despite a C-rich chemistry: these result from the PDR beingwarm and dense, where the harsh radiation field of the central star keep CO partiallydissociated.3. The PACS dataThe PACS 70µm image, deconvolved, is shown in Fig. 1. It is a limb-brightened shell,similar (at least qualitatively: an exact comparison still had to be done) to other IR andradio images.12The entire nebula fits inside the centre of the field-of-view of the PACS spectrograph(3x3 spaxels of 9.4” each). From these we have extracted a spectrum, which is alsopresented in Fig. 1. Comparison to the ISO spectrum show the improvement PACSoffers when observing such sources. For a more detailed explanation of the reduction ofthese data and the production of the spectrum, see a future paper on NGC 7027 (Exteret al.).4. ModellingWe have a preliminary model of NGC 7027 based on the Herschel data combined withradio, ISO-SWS, near-IR, optical, and UV data, and using the photoionisation/PDR codeCloudy, last described in Ferland et al. (1998). The model was created by varying variousinput parameters such as elemental abundances, the density of the gas and the dust-to-gas mass ratio, the inner radius of the circumstellar shell and the effective temperatureand luminosity of the central star. The outer radius was determined by reaching theobserved CO 12-11 line flux. Since NGC 7027 has a carbon-rich chemistry, we assumedgraphite dust and PAHs to be present in the circumstellar shell. We initially tried aconstant density model, but found that it could not produce a satisfactory fit to theobservations from the ionised region and the PDR. We then tried a constant pressuremodel, but found that also did not produce a satisfactory fit. We then tried a modelwhere the ionised region and the PDR had a different constant density (except for anarrow region at the ionisation front where there was a continuous change from onedensity to the other). This produced a good fit. The model indicates total hydrogennumber density of 3.3 × 104cm−3 in the ionised region and 7.9 × 105cm−3 in the PDR.The most plausible explanation is that when the central star heated up and started tophotoionise the circumstellar shell, that part of the shell started to expand. This did nothappen in the PDR, or at least the effect was much less there. Also, this process was sofast that to this date the shell has not been able to attain pressure equilibrium.ReferencesFerland, G.J., Korista, K.T., Verner, D.A., et al., 1998, PASP, 110, 761Griffin, M.J., Abergel, A., Abreu, A. et al., 2010 A&A, 518, L3Pilbratt, G.L., Riedinger, J.R., Passvogel, T. et al., 2010, A&A, 518, L1Poglitsch, A., Waelkens, C., Geis, N. et al., 2010 A&A, 518, L2van Hoof, P.A.M., Barlow, M.J., Van de Steene, G.C., et al., 2012 these proceedingsWesson, R., Cernicharo, J., Barlow, M.J., et al., 2010 A&A, 518, 144Figure 1. Left: PACS 70µm image of NGC 7027, deconvolved. Right: PACS spectrum com-pared to the (lower) offset ISO spectrum. Upper panel compares the PACS and ISO spectrawithout any offset.

Herschel observations of NGC 7027

https://discovery.ucl.ac.uk/10115780/1/Barlow_Exter_NGC7027_IAU283.pdf

Herschel observations of NGC 7027

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