1,480 research outputs found
Modeling Molecular-Line Emission from Circumstellar Disks
Molecular lines hold valuable information on the physical and chemical
composition of disks around young stars, the likely progenitors of planetary
systems. This invited contribution discusses techniques to calculate the
molecular emission (and absorption) line spectrum based on models for the
physical and chemical structure of protoplanetary disks. Four examples of
recent research illutrate these techniques in practice: matching resolved
molecular-line emission from the disk around LkCa15 with theoertical models for
the chemistry; evaluating the two-dimensional transfer of ultraviolet radiation
into the disk, and the effect on the HCN/CN ratio; far-infrared CO line
emission from a superheated disk surface layer; and inward motions in the disk
around L1489 IRS.Comment: 6 pages, no figures. To appear in "The Dense Interstellar Medium in
Galaxies", Procs. Fourth Cologne-Bonn-Zermatt-Symposiu
Effects of accretion flow on the chemical structure in the inner regions of protoplanetary disks
We have studied the dependence of the profiles of molecular abundances and
line emission on the accretion flow in the hot (\ga 100K) inner region of
protoplanetary disks. The gas-phase reactions initiated by evaporation of the
ice mantle on dust grains are calculated along the accretion flow. We focus on
methanol, a molecule that is formed predominantly through the evaporation of
warm ice mantles, to show how the abundance profile and line emission depend on
the accretion flow. Our results show that some evaporated molecules keep high
abundances only when the accretion velocity is large enough, and that methanol
could be useful as a diagnostic of the accretion flow by means of ALMA
observations at the disk radius of \la 10AU.Comment: 6 pages, 5 figures, Accepted for publication in A&
Warm Molecular Layers in Protoplanetary Disks
We have investigated molecular distributions in protoplanetary disks,
adopting a disk model with a temperature gradient in the vertical direction.
The model produces sufficiently high abundances of gaseous CO and HCO+ to
account for line observations of T Tauri stars using a sticking probability of
unity and without assuming any non-thermal desorption. In regions of radius R >
10 AU, with which we are concerned, the temperature increases with increasing
height from the midplane. In a warm intermediate layer, there are significant
amounts of gaseous molecules owing to thermal desorption and efficient
shielding of ultraviolet radiation by the flared disk. The column densities of
HCN, CN, CS, H2CO, HNC and HCO+ obtained from our model are in good agreement
with the observations of DM Tau, but are smaller than those of LkCa15.
Molecular line profiles from our disk models are calculated using a
2-dimensional non-local-thermal-equilibrium (NLTE) molecular-line radiative
transfer code for a direct comparison with observations. Deuterated species are
included in our chemical model. The molecular D/H ratios in the model are in
reasonable agreement with those observed in protoplanetary disks.Comment: 11 pages, Latex (aa.cls), to be published in Astronomy and
Astrophysic
Two-dimensional Distributions and Column Densities of Gaseous Molecules in Protoplanetary Disks II
We have investigated the two-dimensional (R,Z) distribution of deuterated
molecular species in circumstellar disks around young stellar objects. The
abundance ratios between singly deuterated and normal molecules (``D/H
ratios'') in disks evolve in a similar way as in molecular clouds.
Fractionation is caused by rapid exchange reactions that are exothermic because
of energy differences between deuterated and normal species. In the midplane
region, where molecules are heavily depleted onto grain surfaces, the D/H
ratios of gaseous molecules are higher than at larger heights. The D/H ratios
for the vertical column densities of NH3, H2O, and HCO+ are sensitive to the
temperature, and decrease significantly with decreasing radial distance for R <
300 AU. The analogous D/H ratios for CH4 and H2CO, on the other hand, are not
very sensitive to the temperature in the range (T=10-50 K) we are concerned
with, and do not decrease with decreasing R at R > 50 AU. The D/H
column-density ratios also depend on disk mass. In a disk with a larger mass,
the ratios of deuterated species to normal species are higher, because of
heavier depletion of molecules onto grains. In the second part of the paper, we
report molecular column densities for disks embedded in ambient cloud gas. Our
results suggest that CN and HCO+ can be tracers of gaseous disks, especially if
the central object is a strong X-ray source. Our results also suggest that the
radial distributions of CN, C2H, HCN, and H2CO may vary among disks depending
on the X-ray luminosity of the central star.Comment: 13 page
Line Emission from Gas in Optically Thick Dust Disks around Young Stars
We present self-consistent models of gas in optically-thick dusty disks and
calculate its thermal, density and chemical structure. The models focus on an
accurate treatment of the upper layers where line emission originates, and at
radii AU. We present results of disks around stars where we have varied dust properties, X-ray luminosities and
UV luminosities. We separately treat gas and dust thermal balance, and
calculate line luminosities at infrared and sub-millimeter wavelengths from all
transitions originating in the predominantly neutral gas that lies below the
ionized surface of the disk. We find that the [ArII] 7m, [NeII]
12.8m, [FeI] 24m, [SI] 25m, [FeII] 26m, [SiII] 35 m,
[OI] 63m and pure rotational lines of H, HO and CO can be quite
strong and are good indicators of the presence and distribution of gas in
disks. We apply our models to the disk around the nearby young star, TW Hya,
and find good agreement between our model calculations and observations. We
also predict strong emission lines from the TW Hya disk that are likely to be
detected by future facilities. A comparison of CO observations with our models
suggests that the gas disk around TW Hya may be truncated to AU,
compared to its dust disk of 174 AU. We speculate that photoevaporation due to
the strong stellar FUV field from TW Hya is responsible for the gas disk
truncation.Comment: Accepted to Astrophysical Journa
Cold CO Gas in Protoplanetary Disks
In a disk around DM Tau, previous observation of 13CO (J=2-1 and 1-0
transitions) derived the 13CO gas temperature of \sim 13-20K, which is lower
than the sublimation temperature of CO (20 K). We argue that the existence of
such cold CO can be explained by a vertical mixing of disk material. As the gas
is transported from a warm layer to a cold layer, CO is depleted onto dust
grains with a timescale of \sim 10^3 yr. Because of the steep temperature
gradient in the vertical direction, an observable amount of CO is still in the
gas phase when the fluid parcel reaches the layer of \sim 13 K. Apparent
temperature of CO decreases as the maximum grain size increases from
micron-size to mm-size.Comment: 11 pages, 2 figures, accepted to ApJ
Breakdown of `phase rigidity' and variations of the Fano effect in closed Aharonov-Bohm interferometers
Although the conductance of a closed Aharonov-Bohm interferometer, with a
quantum dot on one branch, obeys the Onsager symmetry under magnetic field
reversal, it needs not be a periodic function of this field: the conductance
maxima move with both the field and the gate voltage on the dot, in an apparent
breakdown of `phase rigidity'. These experimental findings are explained
theoretically as resulting from multiple electronic paths around the
interferometer ring. Data containing several Coulomb blockade peaks, whose
shapes change with the magnetic flux, are fitted to a simple model, in which
each resonant level on the dot couples to a different path around the ring
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