Issue |
A&A
Volume 491, Number 3, December I 2008
|
|
---|---|---|
Page(s) | 663 - 670 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361:20077493 | |
Published online | 01 October 2008 |
Coagulation of small grains in disks: the influence of residual infall and initial small-grain content
1
Sterrenkundig Instituut “Anton Pannekoek”, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: dominik@science.uva.nl
2
Afdeling Sterrenkunde, Radboud Universiteit Nijmegen, Postbus 9010, 6500 GL Nijmegen, The Netherlands
3
Leader of Junior Research Group “Formation of planetary building blocks” at the Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: dullemon@mpia.de
Received:
16
March
2008
Accepted:
4
August
2008
Turbulent coagulation in protoplanetary disks is known to
operate on a timescale far shorter than the lifetime of the
disk. In the absence of mechanisms that replenish the small
dust grain population, protoplanetary disks would rapidly lose
their continuum opacity-bearing dust. This is inconsistent with
infrared observations of disks around T Tauri stars and Herbig Ae/Be
stars, which are usually optically thick at visual wavelengths and
show signatures of small (a ≲ 3 μm) grains. A plausible
replenishing mechanism of small grains is collisional fragmentation
or erosion of large dust aggregates, which model calculations
predict to play an important role in protoplanetary disks. If
optically thick disks are to be seen as proof for ongoing
fragmentation or erosion, then alternative explanations for the
existence of optically thick disks must be studied carefully. In
this study we explore two scenarios. First, we study the effect of
residual, low-level infall of matter onto the disk surface. We find
that infall rates as low as 10-11 /yr can, in
principle, replenish the small grain population to a level that
keeps the disk marginally optically thick. However, it remains to
be seen if the assumption of such an inflow is realistic for star+disk
systems at the age of several Myrs, at which winds and jets are
expected to have removed any residual envelope. The effectiveness
of even a low level infall can be understood by the strongly
non-linear behavior of the coagulation equation: a high, fine-grain,
dust density at any given time leads to very fast, effective removal of
these small grains, while a low fine-grain density lasts for a far
longer time. We then consider a second scenario in which, during
the buildup phase of the disk, an intermediate fine-grain dust
abundance is generated that is sufficiently low to ensure longevity
of the state yet sufficiently high for the disk to remain optically
thick. While our models confirm that such an “initial condition” can be
constructed, we argue that these special initial conditions
cannot be achieved during the disk build-up phase. In summary,
fragmentation or erosion still appear to be the most promising
processes to explain the abundant presence of small grains in old
disks.
Key words: accretion, accretion disks / stars: circumstellar matter / stars: formation / stars: pre-main-sequence / infrared: stars
© ESO, 2008
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