Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae

Context. ISOPHOT multi-aperture photometry is an efficient method to resolve compact sources or to detect extended emission down to relatively faint levels with single detectors in the wavelength range 3 to 100  μ m. Aims. Using ISOPHOT multi-aperture photometry and complementary ISO spectra and IR spectral energy distributions we discuss the nature of the extended IR emission of the two PNe NGC 6543 and NGC 7008. Methods. In the on-line appendix we describe the data reduction, calibration and interpretation methods based on a simultaneous determination of the IR source and background contributions from the on-source multi-aperture sequences. Normalized profiles enable direct comparison with point source and flat-sky references. Modelling the intensity distribution offers a quantitative method to assess source extent and angular scales of the main structures and is helpful in reconstructing the total source flux, if the source extends beyond a radius of 1 arcmin. The photometric calibration is described and typical accuracies are derived. General uncertainty, quality and reliability issues are addressed, too. Transient fitting to non-stabilised signal time series, by means of combinations of exponential functions with different time constants, improves the actual average signals and reduces their uncertainty. Results. The emission of NGC 6543 in the 3.6  μ m band coincides with the core region of the optical nebula and is homogeneously distributed. It is comprised of 65% continuum and 35% atomic hydrogen line emission. In the 12  μ m band a resolved but compact double source is surrounded by a fainter ring structure with all emission confined to the optical core region. Strong line emission of [ArIII] at 8.99  μ m and in particular [SIV] at 10.51  μ m shapes this spatial profile. The unresolved 60  μ m emission originates from dust. It is described by a modified (emissivity index $\beta = 1.5$) blackbody with a temperature of 85 K, suggesting that warm dust with a mass of 6.4 $\times$ 10 -4  $M_{\odot}$ is mixed with the ionised gas. The gas-to-dust mass ratio is about 220. The 25  μ m emission of NGC 7008 is characterised by a FWHM of about 50´´ with an additional spot-like or ring-like enhancement at the bright rim of the optical nebula. The 60  μ m emission exhibits a similar shape, but is about twice as extended. Analysis of the spectral energy distribution suggests that the 25  μ m emission is associated with 120 K warm dust, while the 60  μ m emission is dominated by a second dust component with 55 K. The dust mass associated with this latter component amounts to 1.2 $\times$ 10 -3  $M_{\odot}$, significantly higher than previously derived. The gas-to-dust mass ratio is 59 which, compared to the average value of 160 for the Milky Way, hints at dust enrichment by this object.