L. Magrini

The chemistry of planetary nebulae and HII regions in the dwarf galaxies Sextans A and B from deep VLT spectra

Spectroscopic observations obtained with the VLT of one planetary nebula (PN) in Sextans A and of five PNe in Sextans B and of several H uf769uf769 regions in these two dwarf irregular galaxies are presented. The extended spectral coverage, from 320.0 to 1000.0 nm, and the large telescope aperture allowed us to detect a number of emission lines, covering more than one ionization stage for several elements (He, O, S, Ar). The electron temperature diagnostic [O uf769uf769uf769] line at 436.3 nm was measured in all six PNe and in several H uf769uf769 regions allowing for an accurate determination of the ionic and total chemical abundances by means of the Ionization Correction Factors method. For the time being, these PNe are the farthest ones where such a direct measurement of the electron temperature is obtained. In addition, all PNe and H uf769uf769 regions were also modelled using the photoionization code CLOUDY (Ferland et al. 1998, PASP, 110, 761). The physico-chemical properties of PNe and H uf769uf769 regions are presented and discussed. A small dispersion in the oxygen abundance of H uf769uf769 regions was found in both galaxies: 12 + log (O/H) = 7.6 ± 0. 2i n Sextans A, and 7.8 ± 0.2 in Sextans B. For the five PNe of Sextans B, we find that 12 + log (O/H) = 8.0 ± 0.3, with a mean abundance consistent with that of H uf769uf769 regions. The only PN known in Sextans A appears to have been produced by a quite massive progenitor, and has a significant nitrogen overabundance. In addition, its oxygen abundance is 0.4 dex larger than the mean abundance of H uf769uf769 regions, possibly indicating an efficient third dredge-up for massive, low-metallicity PN progenitors. The metal enrichment of both galaxies is analyzed using these new data.

The Local Group Census: Planetary nebulae in IC 10, Leo A and Sextans A

In the framework of our narrow-band survey of the Local Group galaxies, we present the results of the search for planetary nebulae (PNe) in the dwarf irregular galaxies IC 10, Leo A and Sextans A. Using the standard on-band/off-band technique, sixteen new candidate PNe have been discovered in the closest starburst galaxy, IC 10. The optical size of this galaxy is estimated to be much larger than previously thought, considering the location of the new PNe in an area of 3.6 kpc × 2.7 kpc. We also confirm the results of previous studies for the other two dwarf irregular galaxies, with the detection of one candidate PN in Leo A and another one in Sextans A. We review the number of planetary nebulae discovered in the Local Group to date and their behaviour with metallicity. We suggest a possible fall in the observed number of PNe when (Fe/H) � -1.0, which might indicate that below this point the formation rate of PNe is much lower than for stellar populations of near Solar abundances. We also find non-negligible metallicity effects on the (O uf769uf769uf769) luminosity of the brightest PN of a galaxy.

Chemical abundances of Planetary Nebulae in M 33

Using spectroscopic data presented in Magrini et al. (2003), we have analyzed with the photoionization code CLOUDY 94.00 (Ferland et al. 1998) 11 Planetary Nebulae belonging to the spiral galaxy M 33. Central star temperatures and nebular parameters have been determined. In particular the chemical abundances of He/H, O/H, N/H, Ar/H, and S/H have been measured and compared with values obtained via the Ionization Correction Factors (ICF) method, when available. Chemical abundance relationships have been investigated; in particular, a correlation between N/ Ha nd N/O similar to the Galactic one (Henry 1990), and a feeble anti-correlation between O/ Ha nd N/O have been found. A gradient in O/H across the disc of M 33 is consistent with the one found from H uf769uf769 regions in this galaxy (Vilchez et al. 1988). Further studies in the outer parts of M 33 are however needed to ascertain this point. The present result shows that oxygen and helium abundances (with lower accuracy also nitrogen, argon and sulphur) can be actually estimated from the brightest PNe of a galaxy, even if the electron temperature cannot be measured. We also found that the oxygen abundance is quite independent of the absolute magnitude of the PN and consequently the brightest PNe are representative of the whole PN population. This represents an important tool for measuring the metallicity of galaxies at the time of the formation of PNe progenitors.

The Local Group Census: planetary nebulae in the spheroidal galaxies NGC 147, NGC 185 and NGC 205

In the framework of our narrow-band survey of Local Group galaxies, we present the results of the search for planetary nebulae (PNe) in three spheroidal companions to the Andromeda galaxy. We find 9 candidate PNe in NGC 147, 5 in NGC 185, and 75 in the ∼0.4 square degree area searched around NGC 205, increasing the number of PNe known in these galaxies. It is shown that in the crowded regions of these galaxies continuum-subtracted images are more effective in detecting PNe than colour-colour diagrams obtained via automatic photometry. For NGC 205, the degree of contamination of PNe belonging to the halo of M 31 is estimated; taking it into account, 35 PNe within 1.5 tidal radii from the centre of NGC 205 have been used to build its PN luminosity function. Candidate PNe in NGC 185 are systematically brighter than those in NGC 147. Considering that star formation is thought to have been much stronger in NGC 185 than in NGC 147 in the last 3 Gyr, this might suggest that the bright end of the PN luminosity function is populated by relatively massive stars, as predicted by some recent theoretical models. This result, however, has to be taken with some caution, given the small PN population size of these galaxies and a rather incomplete knowledge of their star formation history.

Spectroscopy of planetary nebulae in M 33

Spectroscopic observations of 48 emission-line objects of M 33 have been obtained with the multi-object, wide field, fibre spectrograph AF2/WYFFOS at the 4.2m WHT telescope (La Palma, Spain). Line intensities and logarithmic extinction, cβ, are presented for 42 objects. Their location in the Sabbadin & DOdorico diagnostic diagram (H�/(S ii) vs H�/(N ii)) suggests that >70% of the candidates are Planetary Nebulae (PNe). Chemical abundances and nebular physical parameters have been derived for the three of the six PNe where the 4363u (O iii) emission line was measurable. These are disc PNe, located within a galactocentric distance of 4.1 kpc, and, to date, they are the farthest PNe with a direct chemical abundance determination. No discrepancy in the Helium, Oxygen and Argon abundances has been found in comparison with corresponding abundances of PNe in our Galaxy. Only a lower limit to the sulphur abundance has been obtained since we could not detect any (S iii) line. N/H appears to be lower than the Galactic value; some possible explanations for this under-abundance are discussed.

The Local Group Census: Planetary nebulae in Sextans B ?

Five planetary nebulae (PNe) have been discovered in the nearby dwarf irregular galaxy. Emission line images were obtained using the Wide Field Camera of the 2.5 m Isaac Newton Telescope (INT) at La Palma (Spain). The candidate PNe were identied by their point-like appearance and relatively strong (O iii) emission-line fluxes. They are located within a galactocentric distance of 2.8 arcmin, corresponding to 1.1 kpc at the distance of Sextans B. Luminosities are in the range 1800{5600 L. Sextans B is one of the smallest dwarf irregular galaxies with a PN population. The number of PNe detected suggest an enhanced star formation rate between 1 and 5 Gyr ago.

New candidate planetary nebulae in M 81

A3 4 0 34 0 eld centred on the spiral galaxy M 81 has been searched for emission-line objects using the prime focus wide eld camera (WFC) of the 2.54 m Isaac Newton Telescope (La Palma, Spain). A total of 171 candidate planetary nebulae (PNe) are found, 54 of which are in common with the ones detected by Jacoby et al. (1989). The behaviour of PNe excitation as a function of galactocentric distance is examined, and no signicant variations are found. The PNe luminosity function is built for the disk and bulge of M 81, separately. A distance modulus of 27:92 0:23 mag is found for disk PNe, in good agreement with previous distance measurements for M 81 (Jacoby et al. 1989; Huterer et al. 1995).

Planetary nebulae in the dwarf galaxy NGC 6822: Detection of new candidates

We have discovered 13 new candidate Planetary Nebulae (PNe) in the dwarf irregular galaxy NGC 6822, increasing its total number to 17 objects. Our sample of candidate PNe is complete down to 3.5 mag below the brightest PN.

The Census of Planetary Nebulae in the Local Group

A review of the imaging searches for planetary nebulae (PNe) in the Local Group (LG), with an updated list of references, is presented. Emphasis is put on the results of the Local Group Census, a recent survey carried out at the 2.5m INT telescope at La Palma. PNe have been searched and found in all LG galaxies with log(LV /L� ) > 6.7; very few PNe are expected to be found in less luminous galaxies. The PN population size, when corrected for the different completeness limits of the various surveys, scales pretty well with the V luminosity of a galaxy, but it is suggested that it is somewhat sensitive to the relative amount of star formation in the last 2 to 8 Gyr. Some possible evidence of the effect of star formation in the last few Gyr on the absolute magnitude of the bright cutoff of the PN luminosity function is also presented.

Local Group Census: The Dwarf Irregular Galaxy NGC6822

In the framework of the Local Group Census (LGC), an imaging pro- gramme to search for emission line objects in the Local Group, we present the detec- tion of 13 new candidate planetary nebulae (PNe) and follow-up optical spectroscopic confirmation in the dwarf irregular galaxy NGC 6822. This substantially increases the PNe sample of NGC 6822 to 17. Our sample of PNe is complete within 3.5 mag below the brightest PN. Using the PN luminosity function (PNLF), we find a distance mod- ulus of m − M =2 3.32 ± 0.45. The spectroscopic data help to discriminate between the different emission line objects (PNe, SNR, HII regions). The spectra with the highest S/N ratio allow us to determine abundances for 6 PNe and 4 HII regions. For the first time, we can compare the PN abundances of helium, nitrogen and oxygen in many LG galaxies. We find that the nitrogen is not enhanced in NGC 6822 (nor in M 33) as much as was expected from their respective metallicities. In contrast, the helium is largely enhanced in all the galaxies. The oxygen abundances in NGC 6822 are high (close to those of the H II regions), which supports the idea that most PNe originated from relatively massive and young progenitors.