Planetary nebulae and HII regions in NGC 300.

We report a search for planetary nebulae and HII regions in the central part of the late-type spiral NGC 300, using a CCD and the on-band/off-band filter technique. The total exposure time for all frames was 4 hours. We have identified 34 PNs and 88 HII regions. We construct the cumulative [OIII] λ5007 PN luminosity function (PNLF) and obtain a distance modulus of 26.9+/-0.4, in satisfactory agreement with the cepheid distance modulus (26.7+/-0.1). We make a general comparison between cepheid and PNLF distances, showing that they are in excellent overall agreement. NGC 300 is in principle one of the best possible calibrators of the PNLF method of distance determination, to be preferentially applied, like the LMC, in cases of populations with recent star formation (as opposed to the bulge of M 31, which has served as calibrator for populations without recent star formation). Additional work will be necessary to confirm whether or not there is a sufficiently large difference between PNLFs of populations with and without recent star formation (typically, late-type spirals and irregulars as opposed to ellipticals and bulges of spirals) to justify in practice the use of different calibrators. We obtain for NGC 300 a specific PN formation rate which is closer to the specific evolutionary flux (dying stars per year per solar luminosity) than obtained in giant elliptical galaxies. This relatively higher PN formation rate in NGC 300 is consistent with either the low metallicity of this galaxy or with the presence of recent star formation (populations without recent star formation are predicted to have somewhat higher specific evolutionary fluxes, but apparently many of their dying stars fail to produce observable PNs if their ages are extremely old or their metallicities are very high). Concerning the HII regions in NGC 300, we have found a few high-excitation ones, as well as 7 ring-like objects. On the other hand we have not found any nebular emission at the position of the bright galactic nucleus, which is very probably an unresolved compact stellar cluster similar to the nucleus of M 33.