Circumstellar dust around HR 4049 -
A critical test for theories of interstellar dust
L.B.F.M. Waters, H.J.G.M.L. Lamers, T.P. Snow, E. Mathlener,
N.R. Trams, P.A.M. van Hoof, C. Waelkens, C.G. Seab, R. Stanga
published in: A&A 211, 208 (1989)
The properties of the dust around the post-AGB star HR 4049 (B9.5Ib-II)
are derived. There is a large UV deficiency, which is approximately inversely
proportional to the wavelength, but does not show any sign of the
2175 Å absorption feature. The extinction law levels off at
longer wavelengths (larger than 4500 Å) producing a circumstellar
reddening of 0.19 <= E(B-V) <= 0.29 in the visual. The polarization angle
also changes near 4500 Å, suggesting the presence of two grain
populations. The IR excess as a function of wavelength (1 <= lambda <=
100 µm) can be explained by a simple optically thin dust model
with a 1/lambda-opacity and a 1/r2-density distribution. The grains
responsible for the UV extinction also cause the IR excess. There is evidence
for an extended cool (~30 K) dust component which is probably a remnant
of the AGB wind. The IRAS Low Resolution Spectrum reveals emission features
between 7 and 9 µm and at 11.3 µm, while ground based
observations show an emission at 3.3 µm, all indicating the presence
of polycyclic aromatic hydrocarbons (PAH's). These PAH's do not produce the
2175 Å absorption feature nor the diffuse bands at 5780 and
6614 Å. The consequences for the theories of the 2175 Å
absorption feature and the diffuse interstellar bands are discussed.
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Peter van Hoof
Royal Observatory of Belgium
Ringlaan 3
1180 Brussel
Belgium
email: p DOT vanhoof AT oma DOT be
