Reliability of NH₃ as the temperature probe of cold cloud cores

¹ Department of Physics, PO Box 64, 00014 University of Helsinki, Finland
e-mail: mika.juvela@helsinki.fi
² Finnish Centre for Astronomy with ESO (FINCA)  

Received: 12 October 2011
Accepted: 15 December 2011

Abstract

Context. The temperature is a central parameter affecting the chemical and physical properties of dense cores of interstellar clouds and their potential evolution towards star formation. The chemistry and the dust properties are temperature dependent and, therefore, interpretation of any observation requires the knowledge of the temperature and its variations. Direct measurement of the gas kinetic temperature is possible with molecular line spectroscopy, the ammonia molecule, NH₃, being the most commonly used tracer.

Aims. We want to determine the accuracy of the temperature estimates derived from ammonia spectra. The normal interpretation of NH₃ observations assumes that all the hyperfine line components are tracing the same volume of gas. However, in the case of strong temperature gradients they may be sensitive to different layers and this could cause errors in the optical depth and gas temperature estimates.

Methods. We examine a series of spherically symmetric cloud models, 1.0 and 0.5 M_⊙ Bonnor-Ebert spheres, with different radial temperature profiles. We calculate synthetic NH₃ spectra and compare the derived column densities and temperatures to the actual values in the models.

Results. For high signal-to-noise observations, the estimated gas kinetic temperatures are within  ~0.3 K of the real mass averaged temperature and the column densities are correct to within  ~10%. When the S/N ratio of the (2, 2) spectrum decreases below 10, the temperature errors are of the order of 1 K but without a significant bias. Only when the density of the models is increased by a factor of a few, the results begin to show significant bias because of the saturation of the (1, 1) main group.

Conclusions. The ammonia spectra are found to be a reliable tracer of the real mass averaged gas temperature. Because the radial temperature profiles of the cores are not well constrained, the central temperature could still be different from this value. If the cores are optically very thick, there are no longer guarantees of the accuracy of the estimates.