2003MNRAS.340..362R

We present Hubble Space Telescope (HST)/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line ratio [OIII] 4364/5008 and thereby evaluate the electron temperature (T_e) and the fractional mean-square T_evariation (t ²_A) across the nebula. We also measure the intrinsic line ratio [NII] 5756/6585 in order to estimate T_eand t ²_Ain the N⁺ region. The interpretation of the [NII] data is not as clear cut as the [OIII] data because of a higher sensitivity to knowledge of the electron density as well as a possible contribution to the [NII] 5756 emission by recombination (and cascading). We present results from binning the data along the various slits into tiles that are 0.5 arcsec square (matching the slit width). The average [OIII] temperature for our four HST/STIS slits varies from 7678 K to 8358 K; t ²_Avaries from 0.00682 to at most 0.0176. For our preferred solution, the average [NII] temperature for each of the four slits varies from 9133 to 10232 K; t ²_Avaries from 0.00584 to 0.0175. The measurements of T_ereported here are an average along each line of sight. Therefore, despite finding remarkably low t ²_A, we cannot rule out significantly larger temperature fluctuations along the line of sight. The result that the average [NII]T_eexceeds the average [OIII]T _econfirms what has been previously found for Orion and what is expected on theoretical grounds. Observations of the proplyd P159-350 indicate: large local extinction associated; ionization stratification consistent with external ionization by θ¹ Ori C; and indirectly, evidence of high electron density.