Diode laser heterodyne observations of silicon monoxide in sunspots: A test of three sunspot models

Absorption features from the 8 μm SiO fundamental (v = 1--0) and hot bands (v = 2--1) have been observed in sunspots at sub-Doppler resolution using a ground-based tunable diode laser heterodyne spectrometer. The observed line widths suggest an upper limit of 0.5 km s-1 for the microturbulent velocity in sunspot umbrae. Since the silicon monoxide abundance is very sensitive to sunspot temperature, the measured equivalent widths permit an unambiguous determination of the temperature-pressure relation in the upper layers of the umbral atmosphere. In the region of SiO line formation (log P/sub g/ = 3.0--4.5), the results support the sunspot model suggested by Stellmacher and Wiehr