1,825 research outputs found
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&
Narrow-line magneto-optical trap for erbium
We report on the experimental realization of a robust and efficient
magneto-optical trap for erbium atoms, based on a narrow cooling transition at
583nm. We observe up to atoms at a temperature of about
. This simple scheme provides better starting conditions for direct
loading of dipole traps as compared to approaches based on the strong cooling
transition alone, or on a combination of a strong and a narrow kHz transition.
Our results on Er point to a general, simple and efficient approach to laser
cool samples of other lanthanide atoms (Ho, Dy, and Tm) for the production of
quantum-degenerate samples
Molecular Line Emission from Gravitationally Unstable Protoplanetary Disks
In the era of high resolution submillimeter interferometers, it will soon be
possible to observe the neutral circumstellar medium directly involved in gas
giant planet (GGP) formation at physical scales previously unattainable. In
order to explore possible signatures of gas giant planet formation via disk
instabilities, we have combined a 3D, non-local thermodynamic equilibrium (LTE)
radiative transfer code with a 3D, finite differences hydrodynamical code to
model molecular emission lines from the vicinity of a 1.4 M_J self-gravitating
proto-GGP. Here, we explore the properties of rotational transitions of the
commonly observed dense gas tracer, HCO+. Our main results are the following:
1. Very high lying HCO+ transitions (e.g. HCO+ J=7-6) can trace dense planet
forming clumps around circumstellar disks. Depending on the molecular
abundance, the proto-GGP may be directly imageable by the Atacama Large
Millimeter Array (ALMA). 2. HCO+ emission lines are heavily self-absorbed
through the proto-GGP's dense molecular core. This signature is nearly
ubiquitous, and only weakly dependent on assumed HCO+ abundances. The
self-absorption features are most pronounced at higher angular resolutions.
Dense clumps that are not self-gravitating only show minor self-absorption
features. 3. Line temperatures are highest through the proto-GGP at all assumed
abundances and inclination angles. Conversely, due to self-absorption in the
line, the velocity-integrated intensity may not be. High angular resolution
interferometers such as the Submillimeter Array (SMA) and ALMA may be able to
differentiate between competing theories of gas giant planet formation.Comment: 10 pages, 13 figures; Accepted by Ap
The Effects of UV Continuum and Lyman alpha Radiation on the Chemical Equilibrium of T Tauri Disks
We show in this Letter that the spectral details of the FUV radiation fields
have a large impact on the chemistry of protoplanetary disks surrounding T
Tauri stars. We show that the strength of a realistic stellar FUV field is
significantly lower than typically assumed in chemical calculations and that
the radiation field is dominated by strong line emission, most notably Lyman
alpha radiation. The effects of the strong Lyman alpha emission on the chemical
equilibrium in protoplanetary disks has previously been unrecognized. We
discuss the impact of this radiation on molecular observations in the context
of a radiative transfer model that includes both direct attenuation and
scattering. In particular, Lyman alpha radiation will directly dissociate water
vapor and may contribute to the observed enhancements of CN/HCN in disks.Comment: 14 pages, 4 figures, accepted by ApJ Letter
Indications for grain growth and mass decrease in cold dust disks around Classical T Tauri stars in the MBM 12 young association
We report detection of continuum emission at 850 and 450 micron from disks
around four Classical T Tauri stars in the MBM 12 (L1457) young association.
Using a simple model we infer masses of 0.0014-0.012 M_sun for the disk of LkHa
263 ABC, 0.005-0.021 M_sun for S18 ABab, 0.03-0.18 M_sun for LkHa 264 A, and
0.023-0.23 M_sun for LkHa 262. The disk mass found for LkHa 263 ABC is
consistent with the 0.0018 M_sun inferred from the scattered light image of the
edge-on disk around component C. Comparison to earlier 13CO line observations
indicates CO depletion by up to a factor 300 with respect to dark-cloud values.
The spectral energy distributions (SED) suggest grain growth, possibly to sizes
of a few hundred micron, but our spatially unresolved data cannot rule out
opacity as an explanation for the SED shape. Our observations show that these T
Tauri stars are still surrounded by significant reservoirs of cold material at
an age of 1-5 Myr. We conclude that the observed differences in disk mass are
likely explained by binary separation affecting the initial value. With
available accretion rate estimates we find that our data are consistent with
theoretical expectations for viscously evolving disks having decreased their
masses by ~30%.Comment: 15 pages, 3 figures, uses aastex. ApJ Letters, in pres
Physical and Chemical Structure of Protoplanetary Disks with Grain Growth
We calculate the physical structure of protoplanetary disks by evaluating the
gas density and temperature self-consistently and solving separately for the
dust temperature. The effect of grain growth is taken into account by assuming
a power-law size distribution and varying the maximum radius of grains a_max.
In our fiducial model with a_max=10um, the gas is warmer than the dust in the
surface layer of the disk, while the gas and dust have the same temperature in
deeper layers. In the models with larger a_max, the gas temperature in the
surface layer is lower than in the fiducial model because of reduced
photo-electric heating rates from small grains, while the deeper penetration of
stellar radiation warms the gas at intermediate height. A detailed chemical
reaction network is solved at outer radii (r \ge 50 AU). Vertical distributions
of some molecular species at different radii are similar, when plotted as a
function of hydrogen column density Sigma_H from the disk surface.
Consequently, molecular column densities do not much depend on disk radius. In
the models with larger a_max, the lower temperature in the surface layer makes
the geometrical thickness of the disk smaller, and the gaseous molecules are
confined to smaller heights. However, if we plot the vertical distributions of
molecules as a function of Sigma_H, they do not significantly depend on a_max.
The dependence of the molecular column densities on a_max is not significant,
either. Notable exceptions are HCO+, H3+ and H2D+, which have smaller column
densities in the models with larger a_max.Comment: 29 pages, 10 figures, accepted to Ap
Influence of pH on soil washing and phytoextraction of Cd from soil, a case study using the paddy-field soil with a high pH (Biological Interactions in Arable Land-Grassland-Forest Continuums and their Impact on the Ecosystem Functions, 7th International Symposium on Integrated Field Science)
Molecular Hydrogen Emission from Protoplanetary Disks II. Effects of X-ray Irradiation and Dust Evolution
Detailed models for the density and temperature profiles of gas and dust in
protoplanetary disks are constructed by taking into account X-ray and
ultraviolet (UV) irradiation from a central T Tauri star, as well as dust size
growth and settling toward the disk midplane. The spatial and size
distributions of dust grains in the disks are numerically computed by solving
the coagulation equation for settling dust particles. The level populations and
line emission of molecular hydrogen are calculated using the derived physical
structure of the disks. X-ray irradiation is the dominant heating source of the
gas in the inner disk region and in the surface layer, while the far UV heating
dominates otherwise. If the central star has strong X-ray and weak UV
radiation, the H2 level populations are controlled by X-ray pumping, and the
X-ray induced transition lines could be observable. If the UV irradiation is
strong, the level populations are controlled by thermal collisions or UV
pumping, depending on the properties of the dust grains in the disks. As the
dust particles evolve in the disks, the gas temperature at the disk surface
drops because the grain photoelectric heating becomes less efficient, while the
UV radiation fields become stronger due to the decrease of grain opacity. This
makes the H2 level populations change from local thermodynamic equilibrium
(LTE) to non-LTE distributions, which results in changes to the line ratios of
H2 emission. Our results suggest that dust evolution in protoplanetary disks
could be observable through the H2 line ratios. The emission lines are strong
from disks irradiated by strong UV and X-rays and possessing small dust grains;
such disks will be good targets in which to observe H2 emission.Comment: 33 pages, accepted for publication in the Astrophysical Journa
AKARI observations of ice absorption bands towards edge-on young stellar objects
To investigate the composition and evolution of circumstellar ice around low-mass young stellar objects (YSOs), we observed ice absorption bands in the near infrared (NIR) towards eight YSOs ranging from class 0 to class II, among which seven are associated with edge-on disks. We performed slit-less spectroscopic observations using the grism mode of the InfraRed Camera (IRC) on board AKARI, which enables us to obtain full NIR spectra from 2.5 mu m to 5 mu m, including the CO2 band and the blue wing of the H2O band, which are inaccessible from the ground. We developed procedures to carefully process the spectra of targets with nebulosity. The spectra were fitted with polynomial baselines to derive the absorption spectra. The molecular absorption bands were then fitted with the laboratory database of ice absorption bands, considering the instrumental line profile and the spectral resolution of the grism dispersion element. Towards the class 0-I sources (L1527, IRC-L1041-2, and IRAS 04302), absorption bands of H2O, CO2, CO, and XCN are clearly detected. Column density ratios of CO2 ice and CO ice relative to H2O ice are 21-28% and 13-46%, respectively. If XCN is OCN-, its column density is as high as 2-6% relative to H2O ice. The HDO ice feature at 4.1 mu m is tentatively detected towards the class 0-I sources and HV Tau. Non-detections of the CH-stretching mode features around 3.5 mu m provide upper limits to the CH3OH abundance of 26% (L1527) and 42% (IRAS 04302) relative to H2O. We tentatively detect OCS ice absorption towards IRC-L1041-2. Towards class 0-I sources, the detected features should mostly originate in the cold envelope, while CO gas and OCN-could originate in the region close to the protostar, where there are warm temperatures and UV radiation. We detect H2O ice band towards ASR41 and 2MASSJ 1628137-243139, which are edge-on class II disks. We also detect H2O ice and CO2 ice towards HV Tau, HK Tau, and UY Aur, and tentatively detect CO gas features towards HK Tau and UY Aur
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