The importance of non-LTE models for the interpretation of observations of interstellar NO

¹ LERMA and UMR 8112 of CNRS, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France e-mail: francois.lique@obspm.fr
² Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
³ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands e-mail: vdtak@sron.nl
⁴ Department of Physical Chemistry and Laser Centre, Vrije Universtiteit, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

Received: 23 June 2008
Accepted: 10 October 2008

Abstract

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 M_J 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 () state are readily detectable in typical conditions (N(NO) = 10¹³ cm^-2; T = 10–30 K), whereas lines in the excited state are observed only in warm ( 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.