We present deep 3500 - 10000 Ã Â spectra of HÂ II regions and planetary nebulae ( PNe ) in the starburst irregular galaxy NGCÂ 4449 , acquired with the Multi Object Double Spectrograph at the Large Binocular Telescope .
Using the “ direct ” method , we derived the abundance of He , N , O , Ne , Ar , and S in six HÂ II regions and in four PNe in NGCÂ 4449 .
This is the first case of PNe studied in a starburst irregular outside the Local Group .
Our HÂ II region and PN sample extends over a galacto-centric distance range of \approx 2 kpc and spans \approx 0.2 dex in oxygen abundance , with average values of 12 + \log ( O / H ) = 8.37 \pm 0.05 and 8.3 \pm 0.1 for HÂ II regions and PNe , respectively .
PNe and HÂ II regions exhibit similar oxygen abundances in the galacto-centric distance range of overlap , while PNe appear more than \sim 1 dex enhanced in nitrogen with respect to HÂ II regions .
The latter result is the natural consequence of N being mostly synthesized in intermediate-mass stars and brought to the stellar surface during dredge-up episodes .
On the other hand , the similarity in O abundance between HÂ II regions and PNe suggests that NGCÂ 4449 ’ s interstellar medium has been poorly enriched in \alpha -elements since the progenitors of the PNe were formed .
Finally , our data reveal the presence of a negative oxygen gradient for both HÂ II regions and PNe , whilst nitrogen does not exhibit any significant radial trend .
We ascribe the ( unexpected ) nitrogen behaviour as due to local N enrichment by the conspicuous Wolf-Rayet population in NGCÂ 4449 .