Reliability of NH3 as the temperature probe of cold cloud cores

Abstract

The temperature is a central parameter affecting the chemical and physical properties of dense cores of interstellar clouds and their evolution to star formation. The chemistry and the dust properties are temperature dependent and the interpretation of observation requires the knowledge of the temperature and its variations. Measurement of the gas kinetic temperature is possible with molecular line spectroscopy, the ammonia molecule, NH3, being the most commonly used tracer. 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 gas volume. In the case of temperature gradients they may be sensitive to different layers and cause errors in the optical depth and gas temperature estimates. We examine a series of spherical cloud models, 1.0 and 0.5 M_Sun Bonnor-Ebert spheres, with different radial temperature profiles. We calculate synthetic NH3 spectra and compare the derived column densities and temperatures to the true values. 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 1K but without a significant bias. 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. The ammonia spectra are found to be a reliable tracer of the mass averaged gas temperature. Because the radial temperature profiles of the cores are not well constrained, the central temperature could still differ from this value. If the cores are optically very thick, there are no guarantees of the accuracy.

7 pages, accepted to A&A

References in corpus (5)

• Cold Dark Clouds: The Initial Conditions for Star Formation
• Observing the gas temperature drop in the high-density nucleus of L 1544
• On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B
• On The Gas Temperature of Molecular Cloud Cores
• Detection of 6 K gas in Ophiuchus D

Cited by in corpus (12)

• Benchmarking spin-state chemistry in starless core models
• Empirical Temperature Measurement in Protoplanetary Disks
• The Onset of Massive Star Formation: The Evolution of Temperature and Density Structure in an Infrared Dark Cloud
• Deuteration of ammonia in the starless core Ophiuchus/H-MM1
• Collisional excitation of NH3 by atomic and molecular hydrogen
• The Comparison of Physical Properties Derived from Gas and Dust in a Massive Star-Forming Region
• Star-forming regions of the Aquila rift cloud complex. II. Turbulence in molecular cores probed by NH3 emission
• The physical state of selected cold clumps
• Magnetic fields and outflows in the large Bok globule CB 54
• The physical and chemical structure of Sagittarius B2, VIII. Full molecular line survey of hot cores
• ALMAGAL VI. The spatial distribution of dense cores during the evolution of cluster-forming massive clump
• Synthetic ngVLA line observations of a massive star-forming cloud