  We report a search for the diatomic hydrides SiH, PH, and FeH along the line
of sight toward the chemically rich circumstellar envelopes of IRC+10216 and VY
Canis Majoris. These molecules are thought to form in high temperature regions
near the photospheres of these stars, and may then further react via gas-phase
and dust-grain interactions leading to more complex species, but have yet to be
constrained by observation. We used the GREAT spectrometer on SOFIA to search
for rotational emission lines of these molecules in four spectral windows
ranging from 600 GHz to 1500 GHz. Though none of the targeted species were
detected in our search, we report their upper limit abundances in each source
and discuss how they influence the current understanding of hydride chemistry
in dense circumstellar media. We attribute the non-detections of these hydrides
to their compact source sizes, high barriers of formation, and proclivity to
react with other molecules in the winds.
