Temperature variations from Hubble Space Telescope spectroscopy of the Orion Nebula

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Monthly Notices of the Royal Astronomical Society, ISSN: 0035-8711, Vol: 340, Issue: 2, Page: 362-374

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Rubin, R. H.; Martin, P. G.; Dufour, R. J.; Ferland, Gary J.; Blagrave, K. P. M.; Liu, X.-W.; Nguyen, J. F.; Baldwin, J. A.
Oxford University Press (OUP)
Physics and Astronomy; Earth and Planetary Sciences; ISM: abundances; ISM: atoms; HII regions; ISM: individual: NGC 1976; ISM: individual: proplyd P159-350; Astrophysics and Astronomy; Physics
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We present Hubble Space Telescope (HST)/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (T) and the fractional mean-square T variation (t) across the nebula. We also measure the intrinsic line ratio [N II] 5756/6585 in order to estimate T and t 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 as well as a possible contribution to the [N II] 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 [O III] temperature for our four HST/STIS slits varies from 7678 K to 8358 K; t varies from 0.00682 to at most 0.0176. For our preferred solution, the average [N II] temperature for each of the four slits varies from 9133 to 10 232 K; t varies from 0.00584 to 0.0175. The measurements of T reported here are an average along each line of sight. Therefore, despite finding remarkably low t, we cannot rule out significantly larger temperature fluctuations along the line of sight. The result that the average [N II] T exceeds the average [O III] T confirms 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 θ Ori C; and indirectly, evidence of high electron density.