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Temperature Variations from HST Spectroscopy of NGC 1976

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Author(s): Baldwin , J. A. | Blagrave , K. P. M. | Dufour , R. J. | Ferland , G. J. | Liu , X. W. | Martin , P. G. | Nguyen , J. F. | Rubin , R. H.

Journal: Revista Mexicana de Astronomía y Astrofísica : Universidad Nacional Autónoma de México. Instituto de Astronomía
ISSN 0185-1101

Issue: 18;
Start page: 113;
Date: 2002;
Original page

Keywords: H II Regions - ISM: Abundances - ISM: Individual (Orion Nebula)

ABSTRACT
We present HST/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line ux ratio [O III] 4364/5008 and thereby evaluate the electron temperature (Te) and the fractional mean-square Te variation (t2A) across the nebula. We also measure the intrinsic line ux ratio [N II] 5756/6585 in order to estimate Te and t2A 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 (Ne). 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; t2A varies from 0.00682 to 0.0176. When we use Ne 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 Te reported here are an average along each line of sight. Therefore, despite finding remarkably low t2A, we cannot rule out significantly larger temperature uctuations along the line of sight. The result that the average [N II] Te exceeds the average [O III] Te confirms what has been previously found for Orion and what is expected on theoretical grounds.
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