Other
Scientific paper
Nov 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt........45p&link_type=abstract
PhD Thesis, Univ. of Oporto, (1996)
Other
2
Scientific paper
A set of spectra made at high resolution for a sample of 43 T Tauri stars at Hα are presented, in total 198 spectra; repeated observations were made of 22 stars on time scales of a day to several years. We discuss the profile types and propose a new classification scheme, using four classes and two subclasses. The statistical distribution of the profile types shows those with two peaks where the blue one is the lower, but still higher than 50% of the red one, is the most common type (14 stars) closely followed by objects with symmetrical line profiles (11 stars). The profiles show a large span in equivalent widths, from 1.4 Å to 277 Å with the red wing dominating for stars with Weq higher than 70 Å. The extended wings of the lines were also examined, using the height of Imax / 40 as reference level. The profiles with symmetrical line wings dominate, but two thirds of the stars have asymmetrical line wings. The extent of the blue and red wings have the same distribution centered around 300 km/s. We have also followed the behaviour of the lines with time, with regard to changes of profile type and of equivalent width, both on short time scales with a maximum time interval of 1 week, and on longer time scales of months to years. The profile type remains rather stable on short time scales, 78% do not change type, while on longer time scales the number decreases to 50%. With regard to the equivalent width, some stars show little variation, while others are significantly variable. A model was developed to try to explain the line profiles observed, in the context of a spherically symmetric wind and using the code RAMIDUS. We used the Sobolev method to compute the radiation field at the line transitions, but a new term was introduced to account for the continuum radiation produced by the envelope, and a very simple but accurate procedure was used to evaluate the escape probabilities. The conditions in the wind are computed at predefined points by solving the system of equations of statistical equilibrium. To evaluate the continuum field, needed also to compute the photoionization and induced recombination rates, the envelope was divided in layers limited by those points where the conditions were to be computed. These rates were evaluated by a formal integration over the entire atmosphere. The continuum was established by the bound-free processes, bremsstrahlung, the negative ion of hydrogen and the angle dependent Thomson scattering. We used an atmosphere extending for 10 stellar radii, isothermal and composed only of hydrogen. For the atomic model, a hydrogen atom of 15 levels of the principal quantum number with all sub-levels was used. The solutions were then investigated for a wind with a rapidly increasing velocity, with gas temperatures between 6000 K and 12000 K, a density at the base around 1012 cm-3 and T* between 4000 K and 5000 K. The results showed that the ionization fraction of the wind is highly sensitive to the parameters involved, and can be fully ionized or neutral, depending on the assumed density, electron temperature and velocity field. The profiles were computed by a formal integration of the equation of radiative transfer. Special attention was paid to the line broadening mechanisms, Doppler, Stark, Van der Waals, resonance and electron scattering. The resulting line profiles are symmetric or of P Cygni type with extended wings that in many cases reach far beyond the maximum wind velocity.
No associations
LandOfFree
The use of Hα emission for the analysis of the atmospheres of T Tauri stars does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with The use of Hα emission for the analysis of the atmospheres of T Tauri stars, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and The use of Hα emission for the analysis of the atmospheres of T Tauri stars will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1750513