Issue |
A&A
Volume 434, Number 3, May II 2005
|
|
---|---|---|
Page(s) | 971 - 986 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20042080 | |
Published online | 18 April 2005 |
Dust coagulation in protoplanetary disks: A rapid depletion of small grains
Max Planck Institut für Astrophysik, PO Box 1317, 85741 Garching, Germany e-mail: dullemon@mpia.de Sterrenkundig Instituut “Anton Pannekoek”, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: dominik@science.uva.nl
Received:
28
September
2004
Accepted:
29
November
2004
We model the process of dust coagulation in protoplanetary disks
and calculate how it affects their observational appearance. Our model
involves the detailed solution of the coagulation equation at every location
in the disk. At regular time intervals we feed the resulting 3D dust
distribution functions into a continuum radiative transfer code to obtain
spectral energy distributions. We find that, even if only the very basic –
and well understood – coagulation mechanisms are included, the process of
grain growth is much too quick to be consistent with infrared observations
of T Tauri disks. Small grains are removed so efficiently that, long before
the disk reaches an age of 106 years typical of T Tauri stars, the SED
shows only very weak infrared excess. This is inconsistent with observed
SEDs of most classical T Tauri stars. Small grains must be
replenished, for instance by aggregate fragmentation through high-speed
collisions. A very simplified calculation shows that when aggregate
fragmentation is included, a quasi-stationary grain size distribution is
obtained in which growth and fragmentation are in equilibrium. This
quasi-stationary state may last 106 years or even longer, depending on
the circumstances in the disk, and may bring the time scales into the right
regime. If this is indeed the case, or if other processes are responsible
for the replenishment of small grains, then the typical grain sizes inferred
from infrared spectral features of T Tauri disks do not necessarily reflect
the age of the system (small grains young, larger grains
older), as is often proposed. Indeed, there is evidence
reported in the literature that the typical inferred grain sizes do not
correlate with the age of the star. Instead, it is more likely that the
typical grain sizes found in T Tauri star (and Herbig Ae/Be star and Brown
Dwarf) disks reflect the state of the disk in some more complicated way,
e.g. the strength of the turbulence, the amount of dust mass transformed
into planetesimals, the amount of gas lost via evaporation etc. A simple
evolutionary scenario in which grains slowly grow from pristine
m
grains to larger grains over a period of a few Myr is most likely
incorrect.
Key words: accretion, accretion disks / stars: circumstellar matter / stars: formation / stars: pre-main sequence / infrared: stars / ISM: dust, extinction
© ESO, 2005
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