The He⁺/H⁺ Abundance in the Orion Nebula from Infrared Space Observatory Measurements

Abstract

Using the Short Wavelength Spectrometer on the Infrared Space Observatory, we measured in the Orion Nebula several components of the He I Brα transition, the first detections to the best of our knowledge in an H II region, and the H I Brα line. A value of He⁺/H⁺=0.085+/-0.003 is inferred from these data and is typical of previous values found for Orion. The IR method to obtain He⁺/H⁺ is very insensitive to extinction. The measurement of the weak He I 2.855 μm (5p³P^o-->4s³S) flux permits an assessment of T_e in the He⁺ region. Its flux relative to the flux of He I 4.0490 μm (5g³G-->4f³F^o, 5g¹G-->4f¹F^o blend) is roughly ~T_e (for T_e range applicable to most H II regions and planetary nebulae) and depends little on N_e. From our measured ratio, using N_e = 10⁴ cm^-3, T_e is slightly less than 10⁴ 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 ratios relative to H⁺. This has usually not been taken into account in the past when deriving abundances of other ions/elements relative to H. Errors in the velocity and line width inferred may also occur by failure to consider the blended He component. Because the strongest He I lines 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.

Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the UK), and with the participation of ISAS and NASA.