THE HE+ H+ ABUNDANCE IN THE ORION NEBULA FROM INFRARED SPACE-OBSERVATORY MEASUREMENTS/

Using the Short Wavelength Spectrometer on the Infrared Space Observat ory, we measured in the Orion Nebula several components of the He I Br alpha transition, the first detections to the best of our knowledge i n an H II region, and the H I Br alpha line. A value of He+/H+ = 0.085 +/- 0.003 is inferred from these data and is typical of previous valu es found for Orion. The IR method to obtain He+/H+ is very insensitive to extinction. The measurement of the weak He I 2.855 mu m (5p P-3 de grees --> 4s S-3) flux permits an assessment of T-e in the He+ region. Its Aux relative to the flux of He I 4.0490 mu m (5g (3)G --> 4f(3)F degrees, 5g (1)G --> 4f(1)F degrees blend) is roughly proportional to T-e (for T-e range applicable to most H II regions and planetary nebul ae) and depends little on N-e. From our measured ratio, using N-e = 10 (4) cm(-3), T-e is slightly less than 10(4) K. We stress that many IR measurements of a hydrogen line are likely to also include one or more significant helium line contributions. Attributing all of the flux to the stronger H I line will result in underestimates for abundance rat ios relative to H+. This has usually not been taken into account in th e past when deriving abundances of other ions/elements relative to H. Errors in the velocity and line width inferred may also occur by failu re to consider the blended He component. Because the strongest He I li nes will generally be on the blue side of their counterpart H I line, interpreting a blended feature as only H I will cause a spurious bias to lower velocities.