Temperature variations from HST spectroscopy of NGC 1976

We present HST/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line flux ratio [O III] 4364/5008 and thereby evaluate the electron temperature (T_e) and the fractional mean-square T _e variation (t²_A) across the nebula. We also measure the intrinsic line flux ratio [N II] 5756/6585 in order to estimate T_e and t²_A in the N⁺ region. The interpretation of the [N II] data is not as clear cut as the [O III] data because of a higher sensitivity to knowledge of the electron density (N _e). We present results from binning the data along the various slits into tiles that are 0.5" square. The average [O III] temperature for our four HST/STIS slits varies from 7678 K to 8358 K; t²_A varies from 0.00682 to 0.0176. When we use N_e of 8000 (for slits 1 and 2) and 3000 cm^-3 (for slits 4 and 5) with Lennon & Burke (1994) collision strengths, the average[N II] temperature for each of the four slits varies from 8979 K to 9761 K; t²_A varies from 0.00551 to 0.0172. The measurements of T_e reported here are an average along each line of sight. Therefore, despite finding remarkably low t²_A, we cannot rule out significantly larger temperature uctuations along the line of sight. The result that the average [N II] T _e exceeds the average [O III] T_e confirms what has been previously found for Orion and what is expected on theoretical grounds.