  Low-mass pre-main sequence (PMS) stars are strong and variable X-ray
emitters, as has been well established by EINSTEIN and ROSAT observatories. It
was originally believed that this emission was of thermal nature and primarily
originated from coronal activity (magnetically confined loops, in analogy with
Solar activity) on contracting young stars. Broadband spectral analysis showed
that the emission was not isothermal and that elemental abundances were
non-Solar. The resolving power of the Chandra and XMM X-ray gratings
spectrometers have provided the first, tantalizing details concerning the
physical conditions such as temperatures, densities, and abundances that
characterize the X-ray emitting regions of young star. These existing high
resolution spectrometers, however, simply do not have the effective area to
measure diagnostic lines for a large number of PMS stars over required to
answer global questions such as: how does magnetic activity in PMS stars differ
from that of main sequence stars, how do they evolve, what determines the
population structure and activity in stellar clusters, and how does the
activity influence the evolution of protostellar disks. Highly resolved
(R>3000) X-ray spectroscopy at orders of magnitude greater efficiency than
currently available will provide major advances in answering these questions.
This requires the ability to resolve the key diagnostic emission lines with a
precision of better than 100 km/s.
