4,309 research outputs found
Origins of the H, He I, and Ca II Line Emission in Classical T Tauri Stars
We perform local excitation calculations to obtain line opacities and
emissivity ratios and compare them with observed properties of H, He I, O I, Ca
II, and Na I lines to determine the density, temperature, and photon ionization
rate. We find that UV photoionization is the most probable excitation mechanism
for generating the He I 10830 opacities that produce all the associated
absorption features. We also calculate the specific line flux at an observed
velocity of v_obs = +/- 150 km/s for both radial wind and infall models. All
the model results, together with observed correlations between absorption and
emission features and between narrow and broad emission components, are used to
deduce the origins of the strong H, He I, and Ca II broad line emission. We
conclude that the first two arise primarily in a radial outflow that is highly
clumpy. The bulk of the wind volume is filled by gas at a density ~10^9 cm^-3
and optically thick to He I 10830 and H alpha, but optically thin to He I 5876,
Pa gamma, and the Ca II infrared triplet. The optically thick He I 5876
emission occurs mostly in regions of density greater than or equal to 10^11
cm^-3 and temperature greater than or equal to 1.5x10^4 K, while the optically
thick H alpha and Pa gamma emission occur mostly in regions of density around
10^11 cm^-3 and temperature between 8750 and 1.25x10^4 K. In producing the
observed line fluxes at a given v_obs, the covering factor of these emission
clumps is sufficiently small not to incur significant absorption of the stellar
and veiling continua in either He I or H lines. The strong Ca II broad line
emission likely arises in both the magnetospheric accretion flow and the disk
boundary layer, where the gases dissipate part of their rotational energies
before infalling along magnetic field lines. The needed density and temperature
are ~10^12 cm^-3 and less than or equal to 7500 K.Comment: Accepted to MNRAS, 88 pages, 24 figure
Non-Equilibrium Ionization Model for Stellar Cluster Winds and its Application
We have developed a self-consistent physical model for super stellar cluster
winds based on combining a 1-D steady-state adiabatic wind solution and a
non-equilibrium ionization calculation. Comparing with the case of collisional
ionization equilibrium, we find that the non-equilibrium ionization effect is
significant in the regime of a high ratio of energy to mass input rate and
manifests in a stronger soft X-ray flux in the inner region of the star
cluster. Implementing the model in X-ray data analysis softwares (e.g., XSPEC)
directly facilitates comparisons with X-ray observations. Physical quantities
such as the mass and energy input rates of stellar winds can be estimated by
fitting observed X-ray spectra. The fitted parameters may then be compared with
independent measurements from other wavelengths. Applying our model to the star
cluster NGC 3603, we find that the wind accounts for no more than 50% of the
total "diffuse" emission, and the derived mass input rate and terminal velocity
are comparable to other empirical estimates. The remaining emission most likely
originate from numerous low-mass pre-main-sequence stellar objects.Comment: 29 pages, 17 figures. accepted by MNRA
Redshifted Absorption at He I 10830 as a Probe of the Accretion Geometry of T Tauri Stars
We probe the geometry of magnetospheric accretion in classical T Tauri stars
by modeling red absorption at He I 10830 via scattering of the stellar and
veiling continua. Under the assumptions that the accretion flow is an
azimuthally symmetric dipole and helium is sufficiently optically thick that
all incident 1-micron radiation is scattered, we illustrate the sensitivity of
He I 10830 red absorption to both the size of the magnetosphere and the filling
factor of the hot accretion shock. We compare model profiles to those observed
in 21 CTTS with subcontinuum redshifted absorption at He I 10830 and find that
about half of the stars have red absorptions and 1-micron veilings that are
consistent with dipole flows of moderate width with accretion shock filling
factors matching the size of the magnetospheric footpoints. However, the
remaining 50% of the profiles, with a combination of broad, deep absorption and
low 1-micron veiling, require very wide flows where magnetic footpoints are
distributed over 10-20% of the stellar surface but accretion shock filling
factors are < 1%. We model these profiles by invoking large magnetospheres
dilutely filled with accreting gas, leaving the disk over a range of radii in
many narrow "streamlets" that fill only a small fraction of the entire infall
region. In some cases accreting streamlets need to originate in the disk
between several stellar radii and at least the corotation radius. A few stars
have such deep absorption at velocities greater than half the stellar escape
velocity that flows near the star with less curvature than a dipole trajectory
seem to be required.Comment: 26 pages, emulateapj format, Accepted by ApJ, to appear 2008 November
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An integrated urban systems model with GIS
The purpose of the research is to develop an integrated urban systems model, which will assist in formulating a better land use-transportation policy by simulating the relationships between land use patterns and travel behavior, integrated with geographic information systems (GISs). In order to make an integrated land use-transportation model possible with the assistance of GISs technologies, the following four sub-systems have been developed: (1) an effective traffic analysis zone generation system; (2) an iterative land use and transportation modeling system; (3) efficient interfaces between GIS and land use, and GIS and transportation models; and (4) a user-friendly graphic user interface (GUI) system. By integrating these sub-systems, a variety of alternative land use-transportation policies can be evaluated through the modification of input parameters in each simulation. Eventually, the developed model using a GIS will assist in formulating an effective land use policy by obtaining robust simulation results for both land use-transportation planners and decision makers. The model has been applied to the Urbana-Champaign area as well as to the Seoul region in Korea for a demonstration of the workings of the model.
The Absorption of Trapped Line Photons by Dust
We derive the rate at which photons in an optically thick line are absorbed by cold dust. This rate is approximately equal to the dust optical depth to the cloud center times the rate at which the photons escape from the cloud. Our derivation is in response to a recent article by Strel'nitskii in which he incorrectly criticized our previous application of this result to models for the pumping of cosmic masers. Strel'nitskii now agrees that his criticism was unjustified
Radiative Trapping and Hyperfine Structure: HCN
The anomalous weakness of the F = 1 → 1 hyperfine component in the J = 1 → 0 emission of interstellar HCN can be caused by radiative trapping in the J = 2 → 1 lines. The anomaly is readily produced if the J = 1 levels are populated largely by collisional excitation from J = 0 to J = 2 followed by radiative decay to J = 1 with the J = 2 → 1 lines optically thick. Regions where the anomaly is found probably have H_2 densities less than 10s^5 cm^(-3) and optical depths in the J =1 → 0 lines greater than 50
On the Origin of the 10 Micron Depressions in the Spectra of Compact Infrared Sources
The 10 µ depression observed in the spectrum of a compact infrared object is usually ascribed to absorption by intervening cold silicate grains, and the underlying source spectrum is taken to be either a blackbody or a blackbody with superposed excess 10 µ emission. We question this assumption of the underlying source spectrum for optically thick compact sources. We find, upon modeling both the objects BN and W3 IRS5, that the source actually emits less at the 10 µ resonance than outside the resonance, so that a depression at 10 µ already exists in the source spectrum. This difference in emission arises because, due to the higher opacity in the resonance, the observed 10 µ radiation is produced further out in the source than is the radiation just outside the resonance. And the lower dust temperature further out gives rise to a weaker emission at 10 µ than in the continuum. An observed 10 µ depression can be largely due to this effect, and little or no intervening extinction is required.
This explanation of the 10 µ depression leads to a correlation such that the magnitude of depression will increase with decreasing color temperature of the source. It also predicts no depression at 20 µ for sources with color temperatures greater than 200 K. Observations at 20 µ would then be able to decide on the validity of this explanation
Modeling T Tauri Winds from He I λ10830 Profiles
The high opacity of He I λ10830 makes it an exceptionally sensitive probe of the inner wind geometry of accreting T Tauri stars. In this line, blueshifted absorption below the continuum results from simple scattering of stellar photons, a situation that is readily modeled without definite knowledge of the physical conditions and recourse to multilevel radiative transfer. We present theoretical line profiles for scattering in two possible wind geometries, a disk wind and a wind emerging radially from the star, and compare them to observed He I λ10830 profiles from a survey of classical T Tauri stars. The comparison indicates that subcontinuum blueshifted absorption is characteristic of disk winds in ~30% of the stars and of stellar winds in ~40%. We further conclude that for many stars the emission profile of helium likely arises in stellar winds, increasing the fraction of accreting stars inferred to have accretion-powered stellar winds to ~60%. Stars with the highest disk accretion rates are more likely to have stellar wind than disk wind signatures and less likely to have redshifted absorption from magnetospheric funnel flows. This suggests the possibility that when accretion rates are high, disks can extend closer to the star, magnetospheric accretion zones can be reduced in size, and conditions can arise that favor radially outflowing stellar winds
Modeling T Tauri Winds from He I 10830 Profiles
The high opacity of He I 10830 makes it an exceptionally sensitive probe of
the inner wind geometry of accreting T Tauri stars. In this line blueshifted
absorption below the continuum results from simple scattering of stellar
photons, a situation which is readily modeled without definite knowledge of the
physical conditions and recourse to multi-level radiative transfer. We present
theoretical line profiles for scattering in two possible wind geometries, a
disk wind and a wind emerging radially from the star, and compare them to
observed He I 10830 profiles from a survey of classical T Tauri stars. The
comparison indicates that subcontinuum blueshifted absorption is characteristic
of disk winds in ~30% of the stars and of stellar winds in ~40%. We further
conclude that for many stars the emission profile of helium likely arises in
stellar winds, increasing the fraction of accreting stars inferred to have
accretion-powered stellar winds to ~60%. Stars with the highest disk accretion
rates are more likely to have stellar wind than disk wind signatures and less
likely to have redshifted absorption from magnetospheric funnel flows. This
suggests the possibility that when accretion rates are high, disks can extend
closer to the star, magnetospheric accretion zones can be reduced in size and
conditions arise that favor radially outflowing stellar winds.Comment: 41 pages, 11 figures. Accepted by Astrophysical Journa
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