Helium abundances and its radial gradient from the spectra of H ii regions and ring nebulae of the Milky Way

Author:

Méndez-Delgado J E12ORCID,Esteban C12ORCID,García-Rojas J12ORCID,Arellano-Córdova K Z13ORCID,Valerdi M4ORCID

Affiliation:

1. Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain

2. Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain

3. Instituto Nacional de Astrofísica, Óptica y Electrónica. Apdo. Postal 51 y 216 Puebla, Mexico

4. Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70-264 Ciudad Universitaria, Mexico

Abstract

ABSTRACT We determine the radial abundance gradient of helium in the disc of the Galaxy from published spectra of 19 H ii regions and ring nebulae surrounding massive O-type stars. We revise the Galactocentric distances of the objects considering Gaia DR2 parallaxes (Gaia Collaboration 2018) and determine the physical conditions and the ionic abundance of He+ in a homogeneous way, using between 3 and 10 He i recombination lines in each object. We estimate the total He abundance of the nebulae and its radial abundance gradient using four different ionization correction factor (ICF; He) schemes. The slope of the gradient is always negative and weakly dependent on the ICF(He) scheme, especially when only the objects with log(η) < 0.9 are considered. The slope values go from −0.0078 to −0.0044 dex kpc−1, consistent with the predictions of chemical evolution models of the Milky Way and chemodynamical simulations of disc galaxies. Finally, we estimate the abundance deviations of He, O, and N in a sample of ring nebulae around Galactic Wolf–Rayet stars, finding a quite similar He overabundance of about +0.24 ± 0.11 dex in three stellar ejecta ring nebulae.

Funder

Albert Ellis Institute

European Regional Development Fund

Consejo Nacional de Ciencia y Tecnología

European Space Agency

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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