2011A&A...530A..57S

We have selected the Galactic HII region M43, a close-by apparently spherical nebula ionized by a single star (HD37061, B0.5V) to investigate several topics of recent interest in the field of HII regions and massive stars. In a series of two papers, we perform a combined, comprehensive study of the nebula and its ionizing star by using as many observational constraints as possible. We collected for this study a set of high-quality observations, including the optical spectrum of HD37061, along with nebular optical imaging and long-slit spatially resolved spectroscopy. The first part of our study comprises a quantitative spectroscopic analysis of the ionizing star, and the empirical analysis of the nebular images and spectroscopy. All the information obtained here will be used to construct a customised photoionization model of the nebula in Paper II. We determine the stellar parameters of HD37061 and the total number of ionizing photons emitted by the star. We find observational evidence of scattered light from the Huygens region (the brightest part of the Orion nebula) in the M43 region. We show the importance of an adequate correction of this scattered light in both the imagery and spectroscopic observations of M43 in accurately determining the total nebular Hα luminosity, the nebular physical conditions. and chemical abundances. We perform a detailed nebular empirical analysis of nine apertures extracted from a long-slit located to the west of HD37061 (east-west direction), obtaining the spatial distribution of the physical conditions and ionic abundances. For three of the analyzed elements (O, S, and N), we determine total abundances directly from observable ions (no ionization correction factors are needed). The comparison of these abundances with those derived from the spectrum of the Orion nebula indicates the importance of the atomic data and, specially in the case of M42, the considered ionization correction factors.