Electron densities in the Orion Nebula.

Abstract

view Abstract Citations (1) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Electron densities in the Orion Nebula. Osterbrock, Donald E. Abstract Photometric measurements of the ratio of intensities of the two components of the EOiiJ X3727 emission line were made for 16 points in NGC 1976 and NGC 1982. The plates were taken with the Newtonian-focus spectrograph of the 100-inch telescope and were developed together with step-slit calibration plates taken with the calibration optics of the coude spectrograph. The probable error of the measurement of the ratio from a single plate is between 0.02 and 0.03 (as judged from the internal consistency) and there are two or three plates of each point. These measurements were undertaken in an effort to check the theoretical relation, recently published by Seaton (1954), between electron density and intensity ratio of the two components of X3727. For although the absolute densities are not known well, the steep gradient of surface brightness from the center outwards in the Orion Nebula shows that there is sharp decrease in density from the center outwards. Thus the relative run of density is known, at least qualitatively. The measured line ratios show good agreement with the theory. In the brightest regions of the nebula, near the Trapezium, ~3729 is considerably fainter than X3726 and the minimum observed intensity ratio is X3729/X3726 = 0.50 in an especially bright region. Further out in the nebula, at points of lower surface brightness, the two components are more nearly equal, while at the points observed in the present investigation which are furthest from the Trapezium the intensity ratio is X3729/X3726 = 1.25. Two points deviate from the otherwise good correlation between surface brightness and intensity ratio, but they are quite evidently regions of especially high absorption, one of them being in the bay just northeast of the Trapezium. The observations of the X3729/~3726 intensity ratio in the Orion Nebula thus fit the theory well. The densities derived from this ratio lie between a high of 3 X io4 electrons/cm3 near the Trapezium and a low of about 3 X 102 electrons/cm3 at the faintest point observed in the present investigation, about 15' southwest of the Trapezium. Seaton, M. J. `954, Ann. Astroph. 17, 74. Mount Wilson and Palomer Observatories, Pasadena, Cal. Publication: The Astronomical Journal Pub Date: June 1955 DOI: 10.1086/107203 Bibcode: 1955AJ.....60..175O full text sources ADS |