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The Helium-to-Hydrogen Ratio of Stars in Young Clusters and Associations

  • P. E. Nissen (a1)

Abstract

A photoelectric narrow-band index, I(4026), of the He I λ 4026 line has been observed with an echelle spectrometer for B-type stars in young clusters and associations.

Model-atmosphere computations of the Strömgren [c 1] index, the Hβ index, and I(4026) as functions of T eff, g and the helium-to-hydrogen ratio have been used to derive these atmospheric parameters from the observed indices for each star.

For the OriOBIb, LacOBI and Sco-Cen associations 11 out of 48 stars in the temperature range from 14 000 to 20 000 K are found to be helium poor in the sense that the derived helium-to-hydrogen ratios lie in the range from 0.05 down to very low values. In the temperature range from 20 000 to 30 000 K no helium poor stars are found, but two stars in OriOBIb are helium rich. Both of these stars show rather strong variations of the I(4026) index.

All stars in h + χ Per and CepOBIII are apparently helium deficient compared to nearby field stars.

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Copyright

References

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Garrison, R. F.: 1967, Astrophys. J. 147, 1003.
Nissen, P. E.: 1974, Astron. Astrophys. 36, 57.
Strömgren, B.: 1966, Ann. Rev. Astron. Astrophys. 4, 433.
Thomsen, B.: 1974, Astron. Astrophys. 35, 479.

The Helium-to-Hydrogen Ratio of Stars in Young Clusters and Associations

  • P. E. Nissen (a1)

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