This thesis describes the results of two spectral studies of T Tauri stars. T Tauri stars
are young, low mass, stars which have yet to evolve to the point where Hydrogen fusion
occurs in their cores. They are normally found in groups associated with gas clouds in
locations called star forming regions. Much work has been done to explain the bizarre
spectral features of T Tauri stars, and this has led to the development of a standard
model for their structures. Classical T Tauri stars are thought to be surrounded by large
discs of accreting material, which slowly adds to the mass of the star. This accretion
process results in large U.V. and I.R. fluxes for the stars when compared to their main
sequence counterparts. Weak-line T Tauri stars do not display any accretion features
and are thought to be more evolved than Classical T Tauri stars. Observations show
that both types of T Tauri star have active chromospheres and large cool spots, which
are both thought to be a result of strong magnetic fields. This thesis is motivated
by two studies, one spectroscopic, one photometric, both of which suggest that some
T Tauri stars display photospheric features from regions of different temperatures, or
multiple spectral types (MST).The first spectral study discussed is of near-I.R. data taken with UKIRT. The
spectra are of high resolution data from the region around 1.63 fim, and are of T Tauri,
giant and dwarf stars of known spectral type. It is shown that a ratio of selected OH
to Fe lines gives a good trend with the optically measured Te//> derived from spectral
types, for both dwarfs and giants. Whilst some of the T Tauri stars fit this trend, it is
found that 4 stars clearly lie above it. This is taken as an indication of cooler regions
in the spectra than had previously been detected, an MST effect.The second spectral study discussed is of optical data taken with the FLAIR system
on the UKST. The spectra span the entire optical region observable with this instrument, and are intended to show variation in temperature sensitive features between
the blue and red optical regions. Spectra are presented from 2 consecutive nights of
observations and previously discovered MST stars are found to show deeper G bands
than would be predicted from the TiO bands in the red spectra. It is found that the
G band of one object has varied both on a night to night basis and since the previous
observations from the mid-1980’s. It is also shown that no variation has occurred in
the red spectraAll of the information on MST stars is then compared in order to determine the best
physical model for the effect. It is shown that none of the regions in the standard model
of Classical T Tauri stars can produce the MTS features. The first model considered
that could display MTS features is of binary systems of stars with differing spectral
type, but equivalent flux in the optical. It is found that these systems would be too
rare to explain all the MTS results. The large cool spots seen on T Tauri stars are also
considered as the cause of the MST effect. It is shown that the size of spots required
is much greater than the size of observed cool regions. The current theories of solar
active regions are discussed, revealing that the dominant photospheric regions are in
fact the small, bright faculae. Whilst the spectra of solar faculae are still not fully
understood it is found that a faculae+photosphere+cool spot model of T Tauri spectra
can produce all the observed MST effects.Finally, the systematic errors in the H-R diagram introduced by the MST effect
are considered. By comparison with research on dwarf stars it is found that the MST
effects do not significantly alter the stellar Tef f and luminosity. The largest errors
are introduced if only a small wavelength region is considered when determining the
spectral type of the star. Studies that derive a Te/ / from an apparent spectral type
and ignore the MST effect can both over and underestimate Te/ / ; leading to a possible
factor of 2 error in the stellar mass. As T Tauri spectral parameters are also used to
derive the overall features of star forming regions, this can lead to the miscalculation
of the mass function and ages of these regions
