The role of the CO 67-186 μm lines in astrophysics

摘要

A spherical and large-velocity-gradient model is adopted for the theoretical calculations of NGC 7027, AFGL 2688 and IRC + 10216, whose emissions of CO far-infrared lines are comparatively strong. The statistical equilibrium calculations cover 107 energy levels. The upper energy limit is 4863 cm~(-1) above the energy level with v = 0 and J = 0. There are altogether 107 energy levels with v = 0, J = 0 - 50; v = 1, J = 0 - 37 and v = 2, J = 0 - 17. The relative flux distributions of the CO lines, as functions of the upper rotational quantum number J_u of the line, may be a good probe to examine the physical conditions of the region in which the CO line emissions arise. The CO far-infrared lines of IRC + 10216 are excited mainly by the central star, while for NGC 7027 and AFGL 2688 by the shocked warm and dense gases. The excitation mechanism is also is also discussed for other observed objects by using ISO. The CO spectra in the wavelength range 67-186 μm are important probes for detecting objects with kinetic temperatures of several hundred degrees, i.e. the envelopes of carbonor oxygen-rich stars, planetary nebulae, young stellar objects and star-forming complexes.