Neutral Helium Lines and the Helium Anomaly in Hot Stars
Abstract
Using our previously calculated departures from LTE for He r in hot stars we have calculated nonLTE and LTE line profiles and equivalent widths for the 2SAP, 2P-4S, 2P-5S, and 2P-4D transitions in the singlet and triplet systems of neutral helium in model atmospheres of 20000 and 30000 K with log g = 4. In comparing the LTE and non-LTE calculations we found that: (1) the cores of the stronger lines are deepened by including non-LTE; (2) for some of the weaker lines non-LTE raises the core of the line; (3) the wings of the lines are formed in LTE, as are the forbidden components of the 2P-4D transitions; and (4) the equivalent widths of the lines are almost unaffected by non-LTE. Because the forbidden components of the 2P-4D transitions are formed in LTE and the allowed cores of these lines are strongly affected by non-LTE, we have formed a ratio of line depth in the allowed component to line depth in the forbidden component to compare the non-LTE effect wfth observations. A comparison of LTE, non-LTE, and observed ratios shows that only the non-LTE results agree with the observations. Since the equivalent widths of the lines are not affected by non-LTE, we have calculated LTE widths for a series of model atmospheres and have found that the helium anomaly can be explained by LTE alone. In fact, the helium anomaly is basically a strong line/weak line effect. A comparison of our equivalent widths with observations indicates that the helium abundance in early-type dwarfs is about He/H = 0.12.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 1970
- DOI:
- 10.1086/150456
- Bibcode:
- 1970ApJ...160..609P