Issue |
A&A
Volume 493, Number 2, January II 2009
|
|
---|---|---|
Page(s) | 385 - 397 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361:200809976 | |
Published online | 20 November 2008 |
The influence of scattering on the extinction of stars
Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: p309ekr@mpifr-bonn.mpg.de
Received:
15
April
2008
Accepted:
2
October
2008
Aims. In photometric measurements of stars, one usually assumes that all photons have reached the observer on a direct path and neglects the fraction that has been scattered. However, in observations of distant stars, for instance when they are in another galaxy, this fraction is not negligible. We investigate how scattered light enhances the flux and affects the extinction curve.
Methods. We compute the radiative transfer in a dusty medium using various techniques at
wavelengths where dust emission is absent. Among the configurations considered
are: a) homogeneous spheres with a central star; b) an infinite layer of stars
behind or mixed with an infinite layer of dust which may be homogeneous or
clumpy; c) one star in or behind a homogeneous or clumpy block of dust. In all
cases, we define in analogy to the extinction optical depth τ an effective
optical thickness such that the flux attenuation is given by
the factor
.
Results. a) We compute and analyze how it depends, in different
geometries, on the extinction optical depth, the dust albedo, the scattering
phase function and the spatial resolution of the measurement; b) we calculate
effective extinction curves towards single stars in clumpy and homogeneous media
adopting standard optical dust properties. They show marked differences to the
standard reddening curve; c) when determining
for a collection
of spatially unresolved stars, we find discrepancies with some of the results in
the literature.
Conclusions. When one converts the apparent magnitude of a star or a star cluster into an
absolute magnitude, scattered light should be taken into account whenever the
spatial resolution with which the measurement was performed corresponds to a
linear scale over which the scattering optical depth around the source is
greater than the relative observational error. One may try to determine
by solving the radiative transfer for an assumed distribution
of stars and dust, or search for representative calculations in the literature,
for example in the figures of this paper.
Key words: ISM: dust, extinction / radiative transfer
© ESO, 2009
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