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

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

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...501l.209r&link_type=abstract

Astrophysical Journal Letters v.501, p.L209

Astronomy and Astrophysics

Astronomy

5

Ism: H Ii Regions, Ism: Abundances, Ism: Atoms, Ism: Individual Name: Orion Nebula

Scientific paper

Using the Short Wavelength Spectrometer on the Infrared Space Observatory, we measured in the Orion Nebula several components of the He I Br alpha transition, the first detections to the best of our knowledge in 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 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 mu m ( 5p\3Po-->4s\3S ) flux permits an assessment of Te in the He+ region. Its flux relative to the flux of He I 4.0490 mu m ( 5g\3G-->4f\3Fo , 5g\1G-->4f\1Fo 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.

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