The population of planetary nebulae and H II regions in M 81

A study of radial metallicity gradients and chemical evolution^*

¹ National Optical Astronomy Observatories, Tucson, AZ 85719, USA e-mail: lstanghellini@noao.edu
² INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: [laura;galli]@arcetri.astro.it
³ Universidad Autónoma de Madrid, Departamento de Física Teórica C-XI, 28049 Madrid, Spain e-mail: eva.villaver@uam.es

Received: 1 May 2010
Accepted: 14 June 2010

Abstract

Context. M 81 is an ideal laboratory to investigate the galactic chemical and dynamical evolution through the study of its young and old stellar populations.

Aims. We analyze the chemical abundances of planetary nebulae and H ii regions in the M 81 disk for insight on galactic evolution, and compare it with that of other galaxies, including the Milky Way.

Methods. We acquired Hectospec/MMT spectra of 39 PNe and 20 H ii regions, with 33 spectra viable for temperature and abundance analysis. Our PN observations represent the first PN spectra in M 81 ever published, while several H ii region spectra have been published before, although without a direct electron temperature determination. We determine elemental abundances of helium, nitrogen, oxygen, neon, sulfur, and argon in PNe and H ii regions, and determine their averages and radial gradients.

Results. The average O/H ratio of PNe compared to that of the H ii regions indicates a general oxygen enrichment in M 81 in the last ~10 Gyr. The PN metallicity gradient in the disk of M 81 is Δ log(O/H)/Δ R_G = -0.055 ± 0.02 dex/kpc. Neon and sulfur in PNe have a radial distribution similar to that of oxygen, with similar gradient slopes. If we combine our H ii sample with the one in the literature we find a possible mild evolution of the gradient slope, with results consistent with gradient steepening with time. Additional spectroscopy is needed to confirm this trend. There are no type I PNe in our M 81 sample, consistently with the observation of only the brightest bins of the PNLF, the galaxy metallicity, and the evolution of post-AGB shells.

Conclusions. Both the young and the old populations of M 81 disclose shallow but detectable negative radial metallicity gradient, which could be slightly steeper for the young population, thus not excluding a mild gradients steepening with the time since galaxy formation. During its evolution M 81 has been producing oxygen; its total oxygen enrichment exceeds that of other nearby galaxies.