Bibcode
Fernández-Martín, A.; Pérez-Montero, E.; Vílchez, J. M.; Mampaso, A.
Bibliographical reference
Astronomy and Astrophysics, Volume 597, id.A84, 19 pp.
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1
2017
Journal
Citations
51
Refereed citations
45
Description
Context. The study of the radial variations of metallicity across the
Galactic disc is a powerful method for understanding the history of star
formation and chemical evolution of the Milky Way. Although several
studies about gradients have been performed so far, the knowledge of the
Galactic antincentre is still poor. Aims: This work aims to
determine accurately the physical and chemical properties of a sample of
H ii regions located at RG > 11 kpc and to study the
radial distribution of abundances in the outermost part of the Galaxy
disc. Methods: We carried out new optical spectroscopic
observations of nine H ii regions with the William Herschel Telescope
covering the spectral range from 3500 Å to 10 100 Å. In
addition, we increased the sample by searching the literature for
optical observations of regions towards the Galactic anticentre,
re-analysing them to obtain a single sample of 23 objects to be
processed in a homogeneous and consistent manner. The total sample
distribution covers the Galactocentric radius from 11 kpc to 18 kpc. Results: Emission line ratios were used to determine accurate
electron densities and temperatures of several ionic species in 13 H ii
regions. These physical parameters were applied to the spectra to
determine direct total chemical abundances. For those regions without
direct estimations of temperature, chemical abundances were derived by
performing tailor-made photoionisation models and/or by using an
empirical relation obtained from radio recombination and optical
temperatures. We performed weighted least-squares fits to the
distribution of the derived abundances along the Galactocentric
distances to study the radial gradients of metallicity across the
outermost part of the MW. The distributions O/H, N/H, S/H, and Ar/H
towards the anticentre can be represented by decreasing linear radial
gradients, while in the case of N/O abundances the radial distribution
is better fitted with a two-zone model. The He/H radial gradient is
presented here for the first time; we find a slope that is not
significantly different from zero. The derived gradient for oxygen shows
a clear decrease with distance with a slope of -0.053 ± 0.009 dex
kpc-1. Although a shallower slope at large Galactocentric
distances is suggested by our data, the flattening of the distribution
cannot be confirmed and more objects towards the anticentre need to be
studied in order to establish the true form of the metallicity gradient.
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