4 Results

At the distance of M33 (840 kpc, Freedman et al. 1991) and with a typical seeing of 1 $.\!\!^{\prime\prime}$0, we expect our observed PN candidates to appear as point sources, with strong emission in H$\alpha$ and [O III]$\lambda$5007 and no counterpart in the continuum regions (unless the central star is very luminous or is a binary, or unless the position of the M33 PNe coincide with foreground stars).

We found 48 objects with these characteristics (presented in Table 3). In the first column we give the identification and the J2000.0 coordinates are presented in Cols. 2 and 3. The H$\alpha$ and [O III] integrated fluxes (in units of 10^-15 erg cm^-2 s^-1) are presented in Cols. 4 and 5. In Col. 6 we give the m(5007) magnitudes, derived from the expression by Jacoby (1989), $m(5007)= -2.5 \log F(5007) - 13.74$, where F(5007) is the flux given in Col. 5. Finally, in Cols. 7 and 8 we give the identification of the sources in the list of PNc and the m(5007)reported by Magrini et al. (2000) with the corrected coordinates reported in Magrini et al. (2001).

Using the relationship between S/N ratio and completeness given by Ciardullo et al. (1987) we obtain a limit of m(5007)= 23.7 for the sample. However, we have made no attempt to study the PNLF because it is clear that our sample is contaminated with compact H II regions. Twenty objects only show emission in H$\alpha$, and 14 more have R =[OIII]/(H$_\alpha$) < 1 (this can be also appreciated in the way that our PNc follow the spiral arms). The other 14 objects have R>1 and are then bona fide PNc. The comparatively large contamination can be understood because we are in a region of strong H$\alpha$ emission (remember that our original purpose was to look for young objects, such as LBVs). Note however that we identify all objects detected by Magrini et al. (2000) in this region. As can be seen in Table 3 and Fig. 2 our derived [OIII] magnitudes for these objects differ from those reported by Magrini et al. (2000), and also their H$\alpha$ fluxes tend to be larger than ours for objects in common. This discrepancy can be partly attributed to the fact that we do not correct for extinction, and to the slightly poorer seeing conditions, the larger spatial scale of the images and the use of the Strömgren y filter as continuum in the Magrini et al. (2000) observations.

Taking all these problems into account we think we have to wait for follow-up spectroscopy and confirmation of the PN nature of the candidates before going on to work with the PNLF.

Figure 2: Comparision of [O III] magnitudes for objects in common with Magrini et al. (2000). Our measurements tend to be fainter than those of Magrini et al. ( 2000) by about 0.6 mag in average.

Finding charts for the PNc found in this paper are shown in Figs. 3 to 11. We present the H$\alpha$- and [O III]-subtracted images in the upper part of the panel and the red and blue continuum images in the lower part of the panels. The PNc are marked with a circle and the horizontal bar is $\sim$10'' long. The orientation of the images is N up and E to the left.