Contents of: VI/111/./abstract/PSARA_PRE_STAR.abs

The following document lists the file abstract/PSARA_PRE_STAR.abs from catalogue VI/111.
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SCIENTIFIC ABSTRACT

The main objective of the LWS proposals on Pre-Main Sequence evolution is the 
understanding of the matter evolution from the dilute, cold interstellar
state of the dark molecular clouds to the compact, hot stellar 
state of the Main Sequence stars. The present proposal deals with the 
study of the stars during their evolution toward the main sequence. 

The Pre-Main-Sequence contraction phase is generally identified with
optically visible young stellar objects of low to intermediate mass, viz. the 
T Tauri, Herbig Ae/Be stars and the FU Orionis the latter showing outbursts 
of more than five magnitudes in the optical. All these objects occupy a 
large part of the Hertzsprung-Russell diagram, with a spread in age 
ranging from about 10^5 to several 10^7 years.  They show experimental 
evidences of association with disks which are intimately related to the 
evolution of the central object and to the mass loss phenomenon exibited
by most of them.
 
Since most of the disk mass is at relatively low temperatures and the column 
densities of dust particles through the disk are large, the Pre-MS disks are 
obvious targets when studying the Pre-MS stellar evolution with ISO.
Shock diagnostic emission lines can be used to study the relation 
between the circumstellar  disks and the outflows from the stars. 
The optical forbidden lines observed in the spectra of these objects
(notably [O I] and [S II]) are most probably due to shock excitation by 
the stellar wind. These shocks produce also a number of strong infrared 
lines which are affected by extinction to a much lesser extent. 
Particularly in those cases, where substantial extinction arises
in a circumstellar disk, the line shape can be used to map the opacities 
as function of the wavelength and to estimate the physical scale of the disk.

OBSERVATION SUMMARY
Full spectral scans using the LWS in grating mode are planned for all our
targets. The "on source" integration time for LWS Grating spectra are  computed
in order to observe with S/N=10 lines of 2 10^-20 W/cm^2.  For the brightest
sources this limit is reduced in order to observe with  S/N=10 lines that are
1/20 of their 60 um continuum.

In presence of an "outflow" the "off source" observation is done carrying  out a
map (in "raster scan") of the flow with a full grating spectrum in  each point
of the map. The total integration time spent on all the  points of the map will
be similar to the one spent on the single  "on source" point; the reference "off
source" point will be therefore  built by binning together all the points of the
map. 

A full SWS grating scan is foreseen for all the objects  (with the exception of
the objects  that will be observed in the SWS guaranteed time) with a 25 um IRAS
flux sufficient to give S/N=10 on a line that is 1/30 of the continuum in the
SWSO1 mode.

We have chosen a few template objects representative of the evolutionary stages
addressed in this proposal for FP observations of lines expected to
be excited in the different physical regimes.


© Université de Strasbourg/CNRS

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