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Metallicity distribution of bulge planetary nebulae and the [O/Fe] x [Fe/H] relation

The O/H metallicity distribution of different samples of planetary nebulae in the bulge of the Milky Way and M31 are compared. O/H abundances are converted into [Fe/H] metallicity by the use of theoretical [O/Fe] x [Fe/H] relationships both for the bulge and the solar neighbourhood. It is found that these relationships imply an offset of [Fe/H] abundances by a factor up to 0.5 dex for bulge nebulae. Systematic errors in the O/H abundances as suggested by some recent recombination line work, ON cycling and statistical uncertainties are unable to explain the observed offset, suggesting that the adopted relationship for the bulge probably overestimates the oxygen enhancement relative to iron.


Introduction
Recent chemical evolution models usually predict different relationships between the [α-elements/Fe] and the metallicity as measured by the [Fe/H] ratio for the different phases that comprise the Galaxy, namely the disk, bulge and halo (see for example Pagel 1997). These relationships basically reflect the rate at which these elements are produced in different scenarios, being usually faster in the bulge and halo compared to the disk in the framework of an inside-out model for Galaxy formation.
Metallicities of bulge stars are poorly known compared with disk objects, as only limited samples of well measured stars are available. As a conclusion, the derived metallicity distribution and the corresponding ratios between the α-elements and metalllicity are not well known. In this work, a sample of bulge planetary nebulae (PN) with relatively accurate abundances is used to shed some light on the [O/Fe] × [Fe/H] relationship adopted for the bulge. It will be shown that this relation probably exagerates the amount of oxygen produced at a given metallicity.   Cuisinier et al. (1999, thick line) and Ratag et al. (1997, thin line). Top panel: The same for the PN in the bulge of M31 (Jacoby & Ciardullo 1999) 2. Metallicity distribution of bulge PN Many bulge, or type V PN (Maciel 1989) are known, but only recently have accurate abundances been obtained. Recent work by Cuisinier et al. (1999) and Costa & Maciel (1999) has led to He, O, N, Ar and S abundances for about 40 bulge PN, with an uncertainty comparable to disk objects, namely up to 0.2 dex for O/H. The bulge O/H abundances are generally comparable with those of the disk, and the O/H, Ar/H and S/H ratios can be higher than the disk counterparts even though very metal rich PN are missing in the bulge. Since underabundant nebulae are also present, these results suggest that the bulge contains a mixed population, so that star formation in the bulge spans a wide time interval.
Chemical abundances of PN in the bulge of M31 have been recently studied by Jacoby & Ciardullo (1999), who have included in their analysis some results by Stasińska et al. (1998) and Richer et al. (1999). Fig. 1 shows a comparison of this sample (42 PN, top panel) with the galactic bulge objects from Ratag et al. (1997) (103 PN, lower panel, thin line) and Cuisinier et al. (1999) (30 PN, lower panel, thick line). The oxygen abundance is given in the usual notation, ǫ(O) = log(O/H) + 12. It can be seen that all the O/H abundance distributions are similar, peaking around 8.7 dex, and showing very few if any super metal rich objects with supersolar abundances.
In order to compare the PN metallicity distribution with the stellar distributions, it is necessary to convert the measured nebular O/H abundances into the usual [Fe/H] metallicities relative to the Sun. Direct measurements are of limited usefulness, for two main reasons. First, a very small number of planetary nebulae have measured iron lines, due to their weakness and the relatively large distances of the nebulae, so that the derived values are more uncertain than the usual O, N or Ar abundances. Second, all available measurements indicate a strong depletion usually attributed to grain formation, so that the measured Fe abundances should be considered as lower limits to the total abundances at the times of formation of the PN progenitor stars. Both these aspects are illustrated in Fig. 2 Matteucci et al. (1999) both for the solar neighbourhood and the galactic bulge. The corresponding relations are more suitably plotted in Fig  relation is taken into account the derived distribution is displaced towards lower metallicities by roughly 0.5 dex. The PN samples were carefully selected not to include disk objects (see a detailed discussion in Cuisinier et al. 1999), and their progenitor stars are clearly not massive enough to appreciably change their initial O/H composition. Therefore, the bulge PN metallicity distribution is expected to be similar to that of the K giants. This conclusion is strenghtened by the earlier measurements of Sadler et al. (1996) for Mg and by the results recently presented by Feast (2000), who has shown that the metallicity distribution of bulge Mira variables peaks around [Fe/H] ≃ 0, which is again higher by roughly 0.5 dex than the bulge PN shown by the solid histogram in Fig. 3. Since both classes of objects are basically the offspring of the evolution of intermediate to low mass stars, their metallicity distributions should also be similar. These objects may be located within a certain distance from the galactic centre, but, as discussed by Frogel (1999), any gradient between Baade's Window and the Centre must be small, amounting up to a few tenths of a dex.

Discussion
Several reasons can be considered in order to explain the discrepancy between the bulge PN distribution shown in  Matteucci et al. (1999). Also shown are: Four disk PN from Perinotto et al. (1999, squares with error bars), a disk PN from Pottasch & Beintema (1999, triangle with error bars), 6 bulge giants from McWilliam & Rich (1994, crosses), and a bulge star from Barbuy (1999, asterisk).

tainties in the adopted [O/Fe] × [Fe/H] relationships. Let us briefly consider each of these possibilities.
First, some recent work has raised the possibility that the O/H abundances of planetary nebulae may be underestimated, in view of the discrepancies between the forbidden line and recombination line values for a number of objects (Mathis & Liu 1999, Liu & Danziger 1993. In principle, that would be applied to all PN samples considered in this work, namely, the bulge PN of Ratag et al. (1997), Cuisinier et al. (1999) and the objects in the bulge of M31 of Jacoby & Ciardullo (1999). There are no detailed published (under)abundance analyses for a large number of objects, but in order to be able to explain the discrepancy shown in Fig. 3, the O/H abundances should have to be underestimated by about 0.5 dex, or a factor 3. This factor should also be applied to disk PN, as their abundances are derived using basically the same methods used for bulge PN. However, Cuisinier et al. (1999) have shown that their metallicity distribution for bulge PN is similar to the corresponding distribution of disk PN given by Maciel & Köppen (1994). This is confirmed by the recent (although smaller) sample of disk PN used by Maciel & Quireza (1999) Maciel (1996) within the usually adopted uncertainties in the PN abundances of up to 0.2 dex for oxygen. Therefore, if we were to apply a correction factor of about 0.5 dex to the O/H abundances, the derived disk PN distribution would imply a very large number of extremely metal-rich objects, whose nature it would be very difficult to explain. Moreover, the PN distribution would peak at about 9.2 dex, which means that most disk PN would be more oxygen rich than the Sun by about a factor 2. Even considering that the populations of PN and G-dwarfs have somewhat different ages, no known age-metallicity relation would be sufficient to explain such large overabundance (see for example the age metallicity relation by Twarog 1980 or the recent work by Rocha-Pinto et al. 1999 and references therein). As a conclusion, any underestimate of the O/H abundances in PN would have to be much lower than 0.5 dex, and could be well accommodated within an average uncertainty of 0.2 dex. Of course, this does not exclude the possibility that some individual nebulae may have strongly underestimated abundances, but the average factor is probably much lower than 0.5 dex. Further work is needed, possibly including a sizable sample of disk PN with well measured forbidden-line and recombination-line abundances.
Second, the possibility of ON cycling has often been mentioned for planetary nebulae (see for example Maciel 1992), basically due to some anticorrelation in the N/O ratio compared with He/H for disk PN. However, this phenomenon is only expected to occur in Type I PN (cf. Peimbert 1978), which are formed mainly from the higher mass progenitor stars. These objects have an excess of He/H and/or N/O, and their oxygen abundances are in fact slightly lower than the most common Type II objects (see for example Maciel 2000). However, the amount by which the O/H ratio is decreased is very small (roughly 0.1 dex), and Type I objects are explicitly excluded from the disk sample of Maciel & Quireza (1999). Regarding the bulge PN, almost all objects show no trace of He/H or N/O enrichment, so that this possibility cannot be used to explain the discrepancy shown in Fig. 3.
Third, statistical uncertainties are more difficult to analyze, since the considered samples are relatively small and may be affected by observational selection effects. However, all distributions can be understood in terms of general models for chemical evolution of the Galaxy, and the similarities of the metallicity distributions of different objects such as G-dwarfs and disk PN, or bulge PN, Mira variables and K giants fit rather nicely in the framework of galactic evolution, within the uncertainties of the derived abundances. Considering the bulge PN in particular, the similarity of the three different distributions shown in Fig. 1 is striking, even though they reflect different samples using different techniques. Small differences such as the lack of oxygen rich PN in the Cuisinier et al. (1999) sample may be explained by the smaller size of this sample. However, we are interested in very broad aspects of these distributions, and not in the detailed behaviour at a given metallicity of range of metallicities, so that it is unlikely that statistical uncertainties might produce the discrepancy shown in Fig. 3.
Fourth, we are left with the possibility that the bulge [O/Fe] × [Fe/H] relation as given by Matteucci et al. (1999) or, equivalently, the [Fe/H] × log O/H relation given by the solid line in Fig. 2 might be responsible for all or most of the discrepancy in the metallicity distributions shown in Fig. 3. Such relation assumes a faster evolution during the bulge formation, so that at a given metallicity the [O/Fe] ratio is higher than in the solar neighbourhood. Although this is correct in principle, the present results suggest that the amount of oxygen produced has been overestimated, leading to an excess of O/H for a given metallicity. In fact, the earlier models of Matteucci & Brocato (1990) Matteucci et al. (1999). These stars are also shown in Fig. 2 (crosses), and are clearly located to the left of the bulge curve, even allowing for some uncertainty in the O/H abundances. The average difference is about 0.5 dex, which is just what is needed to eliminate the offset of the bulge PN metallicity distribution shown in Fig. 3. Therefore, it can be concluded that the theoretical [O/Fe] × [Fe/H] relation for the bulge probably overestimates the oxygen enhancement relative to iron by 0.3 to 0.5 dex, at least for metallicities [Fe/H] ≥ −1.5 dex.