Continuous absorption of He I λ10830 in planetary nebulae

Several recent studies have found that the observed intensity of He I λ10830 in planetary nebulae is roughly one-half of that predicted by theory. This implies that the mechanisms populating the 2 ³S level of neutral helium are not yet understood. This discrepancy must be accounted for in order to assess the importance of collisional effects in He I and to accurately determine helium abundances. In this paper, we examine the extent to which internal dust in planetary nebulae can affect the emitted λ10830 intensity. We present approximate correction factors for the λ10830 intensity, and examine how the line ratio λλ10830/5876 and the IR excess vary as a function of the dust fraction and the electron density. We find that destruction by dust is very significant for nebulae which are optically thick in λ10830, and that the size of this effect is strongly dependent on the dust fraction. We also briefly discuss the serious observational uncertainties in the measurement of the λ10830 line. Given these observational uncertainties, and the uncertainties in the dust-to-gas ratio, we conclude that there is no strong evidence of an extra depopulation mechanism for the 2 ³S level.