The Structure of NGC 1976 in the Radio Range, v2

TL;DR

This study uses high-resolution radio images of NGC 1976 at multiple frequencies to model its temperature structure, revealing a multi-layer temperature variation inconsistent with a single temperature value.

Contribution

It introduces a multi-layer model based on radio recombination line data to describe the temperature variation across NGC 1976, challenging simpler models.

Findings

Temperature varies from 8500K to 6000K across the nebula.

The estimated temperature fluctuation t2 is 0.003.

Radio data suggest lower temperature fluctuations than optical data.

Abstract

High angular resolution radio continuum images of NGC 1976 (M42, Orion A) at frequency=330 MHz (wavelength=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 Te. The best fit to the continuum data is a multi-layer model based on radio recombination line (RRL) data with a monotonic variation, from Te=8500K in the higher intensity, more compact region at the rear of NGC 1976 to Te =6000K in the low intensity, extended region in the foreground. An estimate of temperature fluctuations toward the peak from this model yields t2=0.003. This is a factor of 10 lower than fluctuation values from optical collisionally excited line data.