Chemical Abundances in Galactic HII Regions

Details Chemical Abundances in Galactic HII Regions Chemical Abundances in Galactic HII Regions

May 1969

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1969bott....5....3p&link_type=abstract

Boletín de los Observatorios de Tonantzintla y Tacubaya Vol. 5, pp. 3-22 (1969) (http://www.astroscu.unam.mx/bott/)

Astronomy and Astrophysics

Astronomy

346

Hii Regions, Abundances

Scientific paper

New photoelectric observations of Orion, M8, and M17 are presented; from these data the chemical abundances of He, C, N, O, Ne, and S with respect to hydrogen are derived, taking into account the effect of temperature fluctuations over the volumes considered. From the λ5876/λ4472 line-intensity ratio it is found that the assumption of collisional redistribution of the helium atomic levels for n > 12 does not apply to these nebulae: this effect reduces the helium chemical abundances. From the variation of the I(5876)/I(Hβ) ratio it is proven that there is a significant amount of neutral helium in the Orion Nebula, this effect increases the amount of helium derived from the recombination lines. From the I(4267)/I(5876) ratios there is some evidence that probably the λ4267 C II line is produced not only by recombination, but also by line absorption of stellar radiation. The helium abundances derived for the Orion Nebula, M8, and M17 are higher than the helium abundance derived from recent models of the sun, which implies, either that some helium enrichment has taken place since the sun was formed, or that the helium-to-hydrogen ratio was not homogeneous over the galaxy when the sun was formed.

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