Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | A63 | |
Number of page(s) | 8 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/201014159 | |
Published online | 03 September 2010 |
Polarization disks in near-infrared high-resolution imaging
Max-Planck-Institut für Radioastronomie,Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: murakawa@mpifr-bonn.mpg.de
Received:
29
January
2010
Accepted:
29
April
2010
A polarization disk is a characteristic feature of optical and near-infrared
(NIR) polarimetric images of young stellar objects (YSOs) and is regarded as
convincing evidence that a dust disk is present. We analyze high-resolution
linear polarization maps of a sample of low-mass YSO disk models by means of
radiative transfer calculations to investigate the effects of the disk
geometry and grain sizes on polarization properties. Our modeling assumes
spherical grains with a power-law size distribution of
n(a) a-3.5; 0.005 μm ≤ a ≤
and
with a fixed
of 0.25 μm for the outer envelope and
a different
for the disk. The parameters to examine are
the disk height (i.e. the ratio of the disk height to the outer disk radius
H of 0.1 to 1.0) and the dust sizes in the disk (i.e.
of
0.25 to 1000.0 μm). In a near pole-on view, the polarization vectors are
centro-symmetrically aligned even towards the disk, but the degree of
polarization can be different from the envelope. We predict that the pole-on
disk can be distinguished from the envelope. In contrast, the model images
show a bipolar nebulosity and a polarization disk with a vector alignment in
edge-on view. The polarization is low (<10%) for large grains or low H
values and high (up to ~80%) for small grains and high H values.
In contrast, comparably constant polarizations (20–40%) are obtained
in the optical. The wavelength dependence in low NIR polarization cases is
often detected in many T Tauri stars, suggesting that grain growth or
an advanced disk accretion is expected in these objects. The opposite trend
in high NIR polarization cases, which is found in some low-mass protostars,
is reproduced with spherical grain models. To understand our results,
we developed a generalized scattering model, which is an extension of the
vector alignment mechanism. In the low-mass star disk case, multiple-scattered
light behaves as if it chooses paths of comparably low optical density region
(e.g. the disk surface), avoiding a high density, equatorial region, which we
call the roundabout effect. The single-scattered light does not reach
the observer, and the double-scattered light contributes the most flux.
However, the effect of the first scattering still appears in the final
polarization status. The higher the disk height in our models, the closer to
the scattering angle on the disk surface, resulting in a higher
polarization. The variety of the wavelength dependence on the polarization is
also an example of the roundabout effect. In the optical, only stray light
passed through the envelope reaches the observer. Thus, the optical
polarization is characterized by scattering by small grains in the envelope.
On the other hand, since the NIR photons can pass through a somewhat inner part
of the disk, the NIR polarization can still offer information on the dust and
geometry of the disk. We expect that a polarization disk analysis in
high-resolution data, such as the one we present, offers opportunities
to investigate the grain growth and dust settling in YSOs, and our new
scattering model is also fundamental for nonspherical grain models.
Key words: circumstellar matter / dust, extinction
© ESO, 2010
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