  B[e] supergiants (B[e]SGs) are evolved massive stars in a short-lived
transition phase. During this phase, these objects eject large amounts of
material, which accumulates in a circumstellar disk-like structure. The
expelled material is typically dense and cool, providing the cradle for
molecule and dust condensation and for a rich, ongoing chemistry. Very little
is known about the chemical composition of these disks, beyond the emission
from dust and CO revolving around the star on Keplerian orbits. As massive
stars preserve an oxygen-rich surface composition throughout their life, other
oxygen-based molecules can be expected to form. As SiO is the second most
stable oxygen compound, we initiated an observing campaign to search for
first-overtone SiO emission bands. We obtained high-resolution near-infrared
L-band spectra for a sample of Galactic B[e]SGs with reported CO band emission.
We clearly detect emission from the SiO first-overtone bands in CPD-52 9243 and
indications for faint emission in HD 62623, HD 327083, and CPD-57 2874. From
model fits, we find that in all these stars the SiO bands are rotationally
broadened with a velocity lower than observed in the CO band forming regions,
suggesting that SiO forms at larger distances from the star. Hence, searching
for and analyzing these bands is crucial for studying the structure and
kinematics of circumstellar disks, because they trace complementary regions to
the CO band formation zone. Moreover, since SiO molecules are the building
blocks for silicate dust, their study might provide insight in the early stage
of dust formation.
