Interstellar Absorption Determined from Measures of the Total Hα Flux from the Orion Nebula.

Abstract

The intensities of the first three Balmer lines in the Orion Nebula have been studied as a function of position in the nebula. Intensities of Ha, Hft, and H7 have been determined for a few points by means of a photoelectric spectrometer at the University of Michigan. These intensities were put on an absolute scale by comparing to the standard stars, E Ori, ~ Gem, and p Ari, which were then compared to Vega. The scanner measures were made through a square aperture about one minute of arc on a side. Each position was observed several times on each of at least three nights. The mean error of one observation referred to the standard stars is about 3 % for the weakest signals measured and less than 2 % for the strongest. Observations of Ha and H~ for many more points across the nebula were made at the Lowell Observatory using narrow-band interference filters. The Hp filter has a half-intensity width of 60 A and a peak transmission of 46%. The Ha filter is particularly good, having a half-intensity width of 9 A and a peak transmission of 75%. It is centered at 6570 A and must be tilted 4 deg in order to be centered at Ha. Filters centered at 4700 and 6300 A were used to subtract the continuum. The filter measures were calibrated by fitting them to the scanner observations; thus, the filters were used for interpolation only. Except for the east side of the nebula, which includes the dark bay, the intensities show a good degree of circular symmetry. The measured Ha isophotes were extended using this assumption to eliminate the effect of the dark bay which is felt to be purely a local phenomenon. These extended isophotes were then integrated to give the total nebular radiation in Ha. Finally, this total flux in Ha was compared to that predicted on the basis of the radio frequency observations. The difference between the two, which should be a measure of the total optical absorption is 0.5 mag. at Ha. This amounts to a maximum total absorption at the wavelength of the V magnitude of 0.7 mag. which is considerably smaller than the absorption obtained by Johnson from infrared measures of stars.