1996A&A...308..913T

We report the first results of PIROG 7, a balloon-borne platform equipped with a 60cm telescope and a heterodyne receiver designed for observations of the 557GHz (1₁₀->1₀₁) line of ortho-H₂O. We describe the experiment, atmospheric transmission measurements, and astronomical observations of Orion. The lack of broad, centrally peaked line emission in this source, compared to the recently detected line of H₂¹⁸O at 548GHz, indicates that the abundance of water is large and its emission optically thick. On the other hand, a weak and narrow absorption feature was detected toward Orion. While the signal-to-noise ratio of the observed absorption is such that it should only be considered as a tentative detection at the present moment, the result is consistent with the available information on the physical characteristics of the gas in this region. The feature can be interpreted as the result of absorption by a foreground ridge-like component of bright and wide emission arising from the warm gas in the high-velocity core of Orion.