Observation of an interstellar line in absorption against the cosmic 2.7 K background is an unusual phenomenon. The intensity, I
, of a line generated in an interstellar cloud, with homogeneous excitation conditions, is given by
is the intensity of the continuum against which the line is observed, τ
the optical depth of the line, and S
the source function, which is the Planck's function at the excitation temperature T
, i.e., S
). Observation of an interstellar line in absorption against the cosmic 2.7 K background, obviously, implies 0 K < T
< 2.7 K, which requires rather peculiar physical conditions in the molecule, generating the line.