130 research outputs found
Evolution of Hydromagnetic Disturbances in Low Ionized Cosmic Plasmas
We consider the propagation of hydromagnetic waves generated by a compact
turbulent source in low ionized plasmas, applying the Lighthill theory. We
assume the plasma to be isothermal, and adopt a uniform, stationary medium
thread by ordered magnetic fields as an initial condition. Then, the distinct
properties of the hydromagnetic waves originating from a source oscillating
with a fixed frequency are studied in the linear regime. As is well known, in
low ionized plasmas, the generated waves dissipate due to ion-neutral damping.
In this paper, the dependence of the dissipation rate on the frequency of the
oscillating source is investigated. The larger the frequency becomes, the more
substantial is the wave dissipation. Implications of our results on the energy
source in molecular clouds are also discussed. Interestingly, since the outflow
lobes associated with young stellar objects act as compact turbulent sources,
hydromagnetic waves are generated by them. From our order-estimations, about
70% of the energy of the outflow itself propagates as waves or turbulences,
while the remaining 30% dissipates and heats the neutrals via ion-neutral
damping. Then, we confirm that the outflows are significant energy sources in
molecular clouds in the context of the Lighthill theory.Comment: 17 pages LaTeX, 3 PostScript figures, accepted, PASJ (Vol. 51, No. 3,
pp. 337 - 344, 1999
Opacity of fluffy dust aggregates
Context. Dust grains coagulate to form dust aggregates in protoplanetary
disks. Their porosity can be extremely high in the disks. Although disk
emission may come from fluffy dust aggregates, the emission has been modeled
with compact grains. Aims. We aim to reveal the mass opacity of fluffy
aggregates from infrared to millimeter wavelengths with the filling factor
ranging from 1 down to . Methods. We use Mie calculations with an
effective medium theory. The monomers are assumed to be 0.1 sized
grains, which is much shorter than the wavelengths that we focus on. Results.
We find that the absorption mass opacity of fluffy aggregates are characterized
by the product , where is the dust radius and is the filling
factor, except for the interference structure. The scattering mass opacity is
also characterized by at short wavelengths while it is higher in more
fluffy aggregates at long wavelengths. We also derive the analytic formula of
the mass opacity and find that it reproduces the Mie calculations. We also
calculate the expected difference of the emission between compact and fluffy
aggregates in protoplanetary disks with a simple dust growth and drift model.
We find that compact grains and fluffy aggregates can be distinguished by the
radial distribution of the opacity index . The previous observation of
the radial distribution of is consistent with the fluffy case, but more
observations are required to distinguish between fluffy or compact. In
addition, we find that the scattered light would be another way to distinguish
between compact grains and fluffy aggregates.Comment: 16 pages, 17 figures, published in A&A, 568, A4
Analytical Formulas of Molecular Ion Abundances and N2H+ Ring in Protoplanetary Disks
We investigate the chemistry of ion molecules in protoplanetary disks,
motivated by the detection of NH ring around TW Hya. While the ring
inner radius coincides with the CO snow line, it is not apparent why NH
is abundant outside the CO snow line in spite of the similar sublimation
temperatures of CO and N. Using the full gas-grain network model, we
reproduced the NH ring in a disk model with millimeter grains. The
chemical conversion of CO and N to less volatile species (sink effect
hereinafter) is found to affect the NH distribution. Since the
efficiency of the sink depends on various parameters such as activation
barriers of grain surface reactions, which are not well constrained, we also
constructed the no-sink model; the total (gas and ice) CO and N abundances
are set constant, and their gaseous abundances are given by the balance between
adsorption and desorption. Abundances of molecular ions in the no-sink model
are calculated by analytical formulas, which are derived by analyzing the
full-network model. The NH ring is reproduced by the no-sink model, as
well. The 2D (R-Z) distribution of NH, however, is different among the
full-network model and no-sink model. The column density of NH in the
no-sink model depends sensitively on the desorption rate of CO and N, and
the flux of cosmic ray. We also found that NH abundance can peak at the
temperature slightly below the CO sublimation, even if the desorption energies
of CO and N are the same.Comment: accepted to Ap
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