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- W1908136301 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 (5p3Po→4s3S) flux permits an assessment of Te in the He+ region. Its flux relative to the flux of He I 4.0490 μm (5g3G→4f3Fo, 5g1G→4f1Fo blend) is roughly ∝Te (for Te range applicable to most H II regions and planetary nebulae) and depends little on Ne. From our measured ratio, using Ne = 104 cm-3, Te 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." @default.
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- W1908136301 date "1998-07-10" @default.
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- W1908136301 title "The H[CLC]e[/CLC][TSUP]+[/TSUP]/H[TSUP]+[/TSUP] Abundance in the Orion Nebula from [ITAL]Infrared Space Observatory[/ITAL] Measurements" @default.
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- W1908136301 doi "https://doi.org/10.1086/311470" @default.
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