At present the observation of normal stars in the thermal infrared has been a little exploited field and, although sensational results are not to be expected, the range is of great interest for atmospheric models. In the 20 micron window normal stars tend to be a good approximation to a blackbody continuum. However, in the 10-micron window different types of stars show important spectral bands which show large deviations from a blackbody, particularly emission and absorption by silicates.
In O-rich stars it is
usually possible to detect a very broad band centred at 9 micron (SiO),
although the line extends from 7.5-11 micron. This band can be seen strongly
either in absorption, or in emission, in the IRAS LRS spectra. Ultraluminous
stars may also show this band either in absorption, or in emission. This
line is only well-studied from very good observing sites, due to the need
to enter deep into the wings of the 10-micron window for continuum subtraction.
In T Tauri stars this band is particularly strong in emission. Carbon stars,
in contrast, show an important band at 11 micron (from graphite) which
is always seen in emission. The diffraction-limited seeing capability of
the instrument on the GTC is of interest for the study of the physical
characteristics of nearby red supergiant stars, such as Betelgeuse and
Mira Ceti. Betelgeuse is surrounded by a huge cloud of warm dust, ejected
from the star during its supergiant phase. Study of the circumstellar regions
of such stars will allow the dust-ejection history of these stars to be
studied in detail. the star, permitting the dust-shell emitting history
of the star to be studied.