Small grains in nebulae: The case of IC 418 |
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| Authors: | J P Phillips A Mampaso J M Vilchez P Gomez |
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| Institution: | (1) Physics Department, Queen Mary College, London, England;(2) Instituto de Astrofisica de Canarias, La Laguna, Tenerife, Spain |
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| Abstract: | We present near infrared maps of the planetary nebula IC 418 taken at 5 arc sec spatial resolution. These show an extremely compact source structure at 1.65  m, and very much broader and more elliptical emission distributions at 2.2 and 3.6 m. The possibility that the near infrared continuum derives from small grains with low thermal capacity is discussed, and synthetic spectra are derived. These indicate a very much slower variation of flux with wavelength than would be deduced for grains at unitary temperature, and imply a near infrared continuum which is significantly cooler than maximum grain temperatures. Fits to the spectrum of IC 418 suggest that peak grain temperatures may approachT
p0 103 K (for    –2), for instance, compared to the observed colour temperature of 650 K.Finally, the relevant grains are required to be refractory and, given the large C/O ratio in this nebula, probably consist of graphite. Their mass is also required to be small, and is probably of order 2×10–7
M
 . This compares with the probable total mass for large dust grains of 10–4
M
. Although they are both small and warm, such grains would be expected to persist over the lifetime of the nebula, and may also be responsible for various NIR band emission features. |
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