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

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 Δlog(O/H)/ΔR 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.

The oxygen abundance gradient in M81 and the robustness of abundance determinations in H ii regions

“…The values we find for Te([N ii]) in our observed regions are generally higher than those we find for the regions of Patterson et al (2012) by an amount that can explain the differences in this abundance ratio. On the other hand, we find similar values of Te([N ii]) for our regions and the regions observed by Stanghellini et al (2010). In this case the differences can be attributed to the large values of the [O ii] λ3727/Hβ measured by Stanghellini et al (2010) in several regions, which are higher than the ones observed by us and by Patterson et al (2012).…”

“…We also recalculated the galactocentric distances of these H ii regions using the same parameters stated above for M81. The results are presented in Tables 5 and 6. The direct method could be applied to 31 H ii regions of the final sample where the electron temperature can be estimated (Te ([N ii]), Te([O iii]), or both): 12 from this work, 13 from Stanghellini et al (2010) and six from Patterson et al (2012). The strong-line methods were applied to all the regions in the final sample.…”

“…The differences are mainly due to the fact that Stanghellini et al (2010) calculated the neutral oxygen abundance in several objects using [O i] emission and added it to the O + and O ++ abundances to get the total oxygen abundance as can be seen in their table 3, available online; see, for example, the results for their region number 5. This is not a procedure usually followed for H ii regions since the ionization potentials of O 0 and H 0 are both ≃ 13.6 eV, suggesting that [O i] emission should arise in regions close to the ionization front.…”

“…This galaxy belongs to an interacting group of galaxies and has well-defined spiral arms that contain a large number of H ii regions. The oxygen abundance gradient of M81 has been calculated in different studies using several methods (Stauffer & Bothun 1984;Pilyugin et al 2004;Stanghellini et al 2010;Patterson et al 2012;Stanghellini et al 2014;Pilyugin, Grebel & Kniazev 2014). These works find slopes that go from −0.093 to −0.011 dex kpc −1 , and some of them include H ii regions where it is possible to measure the electron temperature and calculate the metallicity with the direct method.…”

Extragalactic planetary nebulae: Tracers of the chemical evolution of nearby galaxies

“…If we examine the locus of M33 and M81 disk PNe on the He/H-N/O plane we can single out the Type I PNe, and then select the remaining PNe as the oldest population in either galaxy. It is worth recalling that the Type I PNe are metallicity dependent (the amount of nitrogen produced by hot bottom burning is dependent on the amount of carbon, and the oxygen abundance depends on the metallicity of the galaxy, see Stanghellini et al 2010 PNe in M33 are of Type I, and we detected HeII 4686 in many PNe. We conclude that the populations of the brightest PNe in galaxies strongly depend upon galactic metallicity.…”