12,157 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
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
1
Elementary Hadronic Interactions at the CERN SPS
New data from the NA49 experiment at the CERN SPS concerning hadron+proton
and hadron+nucleus interactions are discussed and compared to nucleus+nucleus
collisions. Detailed information on baryonic momentum distributions and nuclear
enhancement factors for kaons and cascade baryons are extracted.Comment: 11 pages, 9 figures, to be published in the Proceedings of the XVIth
International Conference on Ultrarelativistic Nucleus-Nucleus Collisions
"Quark Matter 2002", Nantes, France, July 200
Developing a multi-metric habitat index for wadeable streams in Illinois (T-25-P-001). Annual Segment Report to the Illinois Department of Natural Resources.
Illinois Department of Natural Resources Grant/Contract No: (T-25-P-001)This project was initiated to describe key aquatic habitat characteristics and their association to anthropogenic disturbance by developing a field based, rapid assessment method for qualitatively monitoring instream conditions using a multi-metric habitat index. We have developed and applied a method for rating disturbance in wadeable streams throughout Illinois and collected information on physical habitat at 299 sites to date. Index development is in the preliminary stages with field work to continue during the summer of 2008. This report summarizes work performed for the period ending April 30, 2008 (Appendix A contains Eastern Illinois University subcontract annual report).INHS Technical Report Prepared for Illinois Department of Natural Resource
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