Context: The ALMA and Herschel missions promise to deliver data of high spatial and spectral resolution at far-infrared and sub-millimeter wavelengths. Modeling these data will require the knowledge of accurate radiative and collisional rates for species of astrophysical interest.
Aims: We calculate the rotational excitation rate coefficients of NO in collisions with He and check the validity of the LTE approach in interpreting observations of rotational lines of NO.
Methods: State-to-state rate coefficients between the 360 lowest hyperfine levels of NO were calculated using the MJ randomizing limit method for temperatures from 10 K to 350 K. We performed calculations of the excitation of NO using the new rate coefficients using a large velocity gradient (LVG) radiative transfer code.
Results: The critical densities of the lines are found to be at ⪆105 cm-3. The low dipole moment of NO ensures that the line emission is optically thin up to column densities of ~1015 cm-2. Lines in the ground (Ω=1/2) state are readily detectable in typical conditions (N(NO) = 1013 cm-2; T = 10-30 K), whereas lines in the excited Ω=3/2 state are observed only in warm (T>50 K) regions with higher column densities (N(NO) = 1014 cm-2). Line ratios of NO may well be used to constrain the ambient temperature and/or density.
Conclusions: The new rate coefficients will help significantly in interpreting NO emission lines observed with current and future telescopes, and enable this molecule to become a powerful astrophysical tool.
