Spectroscopy of two PN candidates in IC10

We present the results of the first spectroscopic observations of two planetary nebula (PN) candidates in the Local Group dwarf irregular galaxy IC10. Using several spectral classification diagrams, we show that the brightest PN candidate (PN7) is not a PN, but rather a compact HII region consisting of two components with low electron number densities. After the rejection of this PN candidate, the IC10 PN luminosity function cut-off becomes very close to the standard value. With the compiled spectroscopic data for a large number of extragalactic PNe, we analyse a series of diagnostic diagrams to generate quantitative criteria for separating PNe from unresolved HII regions. We show that, with the help of the diagnostic diagrams and the derived set of criteria, PNe can be distinguished from HII regions with an efficiency of ~99.6 per cent. With the obtained spectroscopic data, we confirm that another, 1.7 mag fainter PN candidate (PN9) is a genuine PN. We argue that, based on all currently available PNe data, IC10 is located at a distance of 725+63-33 kpc [distance modulus (m - M) = 24.30+0.18-0.10]. Based on observations obtained at the 6-m SAO RAS (Special Astrophysical Observatory of Russian Academy of Science) telescope. E-mail: akniazev@saao.ac.za (AYK); sap@sao.ru (SAP); zucker@ast.cam.ac.uk (DBZ