SIMBAD references

High angular resolution radio continuum images of NGC 1976 (M42, Orion A) at ν = 330 MHz (λ = 91 cm), 1.5 GHz (20 cm), and 10.6 GHz (2.8 cm) have been aligned, placed on a common grid, smoothed to common resolutions of 80'' (=0.16 pc at 420 pc) and 90'' (=0.18 pc) and compared on a position-by-position basis. The results are not consistent with a single value of T_e. The best fit to the continuum data is a multi-layer model based on radio recombination line data with a monotonic variation, from T_e= 8500 K in the higher intensity, more compact region at the rear of NGC 1976 to T_e= 6000 K in the low-intensity, extended region in the foreground. An estimate of temperature fluctuations toward the peak from this model yields t ² = 0.003. This is a factor of 10 lower than fluctuation values from optical collisionally excited line data.