The He I 2.06 $\mu$m/Br$\gamma$ Ratio as a Nebular Diagnostic

Previous abstract Next abstract

Session 60 -- HII Regions
Display presentation, Thursday, 9:20-4:00, Pauley Room

[60.01] The He I 2.06 $\mu$m/Br$\gamma$ Ratio as a Nebular Diagnostic

J. C. Shields (Ohio State)

The strength of the He I 2.06 $\mu$m emission line relative to hydrogen recombination features has been previously proposed as a diagnostic of effective temperature $T_{eff}$ for stars powering HII regions. Such an indicator would be of particular interest for constraining the high-end initial mass function in starbursts. This talk will present new calculations bearing on the sensitivity of the 2.06 $\mu$m feature to nebular conditions. The results illustrate a number of complications for interpreting the He I/Br$\gamma$ ratio as a constraint on $T_{eff}$. The ratio is sensitive to the relative volumes of He$^+$ and H$^+$, as well as the relative ionization fractions of He and H within the He$^+$ zone, both of which respond to variation in $T_{eff}$. Predicted values of He I/Br$\gamma$ are also sensitive to nebular geometry, dust content, density, and helium abundance. The present examples suggest that caution is required in intepreting measured ratios as indices of ionizing continuum shape.

Thursday program listing