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On the metallicity of open clusters

I. Photometry

E. Paunzen¹ - U. Heiter² - M. Netopil^1,3 - C. Soubiran⁴

1 - Institut für Astronomie der Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
2 - Department of Physics and Astronomy, Uppsala University, Box 515, 75120 Uppsala, Sweden
3 - Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kaciceva 26, 10000 Zagreb, Croatia
4 - Université Bordeaux 1, CNRS, Laboratoire d'Astrophysique de Bordeaux, BP 89, 33270 Floirac, France

Received 25 January 2010 / Accepted 22 March 2010

Abstract
Context. Metallicity is one of four free parameterstypically considered when fitting isochrones to the cluster sequence.Unfortunately, this parameter is often ignored or assumed to be solarin most papers. Hence an unknown bias is introduced in the estimationof the other three cluster parameters (age, reddening and distance).Furthermore, studying the metallicity of open clusters allows us notonly to derive the Galactic abundance gradient on a global scale, butalso to trace the local solar environment in more detail.
Aims. In a series of three papers, we investigate the currentstatus of published metallicities for open clusters from widelydifferent photometric and spectroscopic methods. A detailed comparisonof the results allows us to establish more reliable photometriccalibrations and corrections for isochrone fitting techniques. Wellestablished databases such as WEBDA help us to perform a homogeneousanalysis of available measurements for a significant number of openclusters.
Methods. The literature was searched for [Fe/H] estimates on thebasis of photometric calibrations in any available filter system. Onthe basis of results published by Tadross, we demonstrate the caveatsof the calibration choice and its possible impact. In total, we find406 individual metallicity values for 188 open clusters within 64publications. The values were, finally, unweightedly averaged.
Results. Our final sample includes [Fe/H] values for 188 open clusters. Tracing the solar environment within pc²we identify a patchy metallicity distribution as an extension to theLocal Bubble that significantly influences the estimation of theGalactic metallicity gradient, even on a global scale. In addition,further investigations of more distant open clusters are clearly neededto obtain a more profound picture at Galactocentric distances beyond10 000 pc.
Conclusions. Only a combination of all available photometric andspectroscopic data will shed more light on how the local and globalGalactic properties are correlated with metallicity.

Key words:Galaxy: abundances - open clusters and associations: general - stars: abundances

1 Introduction

One of the most important key parameters for our understanding of stellar formation and evolution,is the intrinsic metallicity of (proto-)stars of a given mass. Even in theearly stages of stellar evolution, the metallicity severely influences the cooling and collapse of ionized gas (Jappsen et al.2007). By comparing detailedsimulations with observations, it has been shown that clouds of lower metallicity have a higher probability of fragmentation, indicating that the binary frequency is a decreasing function of the cloud metallicity(Machida2008).

Looking at the global properties of our Milky Way, a radialmetallicity gradient throughout the Galactic disk was discovered several decades ago, which provides strongconstraints on the mechanism of galaxy formation. Models now show that the stellar formation as a function of Galactocentric distance strongly influence the appearance andthe development of the metallicity gradients (Chiappini et al.2001).Our knowledge is based on stellar data, for example those of Cepheids (Cescutti et al.2007), or open clusters (Chen et al.2003) as well asglobular clusters (Yong et al.2008). However, these approaches are stillnot satisfactory. All individual stellar estimates are limited to the accurate distance estimation of fieldstars and the uncertainties in the spectroscopic abundance analysis for verydistant and therefore faint objects. The metallicity compilations of open clusters (for example Chen et al.2003) are normally based on inhomogeneous data sets.

In a series of papers, we concentrate on the metallicity ofopen clusters using the results of various techniques and methods.Our final goals are 1) to derive homogeneous metallicities of allavailable and published data; 2) to establish a more robust photometriccalibration on the basis of various filter systems; and 3) to investigatethe influence of metallicity on isochrone fitting techniques. There have been several studies (e.g., Twarog et al.1997; Magrini et al.2009) of this nature, but none have taken advantage of all available photometricas well as spectroscopic data.

Besides the investigation of the global Galactic properties, weare also able to shed more light on the validity of the automatic open cluster parameter estimations performed by Kharchenko et al. (2005).Their estimation of the age, reddening and distance of 650 open clustersand the follow-up conclusions (Kharchenko et al.2009) are all based onsolar metallicity. However, the classical technique of isochrone fitting invarious colour-magnitude diagrams, also incorporates the metallicity as ana-priori free parameter.

We present our extensive investigation in a series of three papers dividedinto a photometric, a spectroscopic, and a ``calibration'' part.

2 Target selection and literature assessment

Metallicities, for photometric observations, are oftenlisted either as [Fe/H] orZ values. If not statedotherwise, these parameters can be transformed using the helium-to-metal enrichment relationand a solar valueZ = 0.019, as given in Sect. 2.1 of Girardi et al. (2000).Almost all available photometric calibrations derive [Fe/H]as the default standard parameter for the metallicity.This is because the iron lines, besides hydrogen andhelium lines, dominate the optical spectrum for a wide variety of main sequence stars. Therefore, if integrating over one optical filter, the abundance of iron can be used as some kind of standard candle.Another advantage to using this element is that it is normally unaffected by the main sequence evolution.

The metallicity determinations obtained on the basis of isochrone fitting only,were not taken into consideration because the grid of isochrones arenormally only sparsely available for theZ parameter (Schaller et al.1992).Therefore, authors only consider whether the isochrones with aZ value higher or lower than that of the Sun, fit the observationsmore closely. One typical example is the paper by Piatti et al. (2006) who publishedZ = 0.040, corresponding to [Fe/H] = +0.37 dex,for the open cluster NGC 5288. Unfortunately, they do not include a corresponding plot with isochrones for the different metallicities.

We searched the literature for metallicity estimates of open cluster memberson the basis of photometric calibrations using WEBDAas a starting point. The apparent double clusters NGC 2451 A/B were excluded from our investigation because there is an unsolved and constant confusion about the true nature of these aggregates (Platais et al.2001).

In total, we found 406 individual metallicity values for 188 openclusters in 64 publications.Table 1 lists the values and the numberof stars used to derive the metallicities, if available, and the employedphotometric filter systems, which are:

• Caby: Strömgren and the Ca filters measuring the Ca II HK lines (Anthony-Twarog et al.1991);
• DDO: David Dunlap Observatory system (McClure & van den Bergh1968);
• Johnson:UBVRI colors (Bessell1995);
• Strömgren:photometry (Strömgren1966);
• Vilnius: a seven filter system (Smriglio et al.1990);
• Washington: a four filter system introduced by Canterna (1976).

The cluster parameters (age, reddening, and distance) were taken from Paunzen & Netopil (2006). The only exceptions are Loden 807, NGC 7209, Ruprecht 20, and Ruprecht 32, for which the values were taken from the updated list of Dias et al. (2002) as well as Berkeley 1 and Ruprecht 46, for which the parameters from WEBDA were used.The distance from the Galactic centre was calculated in the standardway using 8 kpc for the Sun (Groenewegen et al.2008). An error propagationwas applied assuming that the uncertainties in the Galactic coordinates for theindividual open clusters are negligible.

As a next step, we compared the results of two references using identicaldata sets, but different photometric calibrations.

3 A test case from the literature

The pitfalls of using two different photometric calibrations foridentical data sets are clearly illustrated in the papers by Tadross (2001,2003).

This author performed robust derivations of the clusterage, reddening, distance, and metallicity on the basis of JohnsonUBV photometry.In the first paper, he used average metallicity values derived from the calibrations ofCarney (1979) and Cameron (1985a), whereas in the second, only values from Cameron (1985a),were applied (Tadross, private communication). The second study wasfound to provide far more reliable results compared to the literature.

Both calibrations are based on the normalized ultravioletexcess,introduced by Sandage (1969), which is compared to spectroscopicallydetermined elemental abundances. While Carney (1979) used a list ofpublished abundances of subdwarfs, Cameron (1985a) constructed a grid consisting of blanketing lines and lines of constant metallicity based on theoretical as well as empirical data. The determined excess values were then transformed to [Fe/H] using a second order polynomial in bothreferences.

Figure1 compares the [Fe/H] values from both Tadrosspapers. It is obvious that there are seventeen clusters with values of less than -1.0 dex and seven with less than -1.5 dex in Tadross (2001), whereas there is only one cluster with [Fe/H] less than -1.0 dex in Tadross (2003). We investigated the plausibility of these cases of low metallicity.Using data of Cepheids, Pedicelli et al. (2009) showed that [Fe/H] values significant below -0.5 dex are not expected in the Galactic disk where open clusters are located, even at distances of 17 kpc. The compilation of open cluster data by Chen et al. (2003) includes only three aggregates with -0.5 > [Fe/H] > -1.0 dex, which is compatible withthe results by Magrini et al. (2009).

	Figure 1: A comparison of the [Fe/H] values derived via two different photometriccalibrations as published by Tadross (2001,2003). The dashed lines represent0.25 dexoffsets. Note the wide range of apparent underabundant aggregates in the first paper.
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Table 2:  The open clusters with very low [Fe/H] estimates from Tadross (2001) and the comparison with those from Tadross (2003).

In Table 2, we list the seven extreme cases together with the cluster parametersbased on the list by Paunzen & Netopil (2006). For two clusters, Berkeley 7 and Berkeley 30, respectively, the [Fe/H] determinations from Tadross (2003) seem to be well in the expected range.For the remaining five clusters, the new determinations are still in the range -0.46 > [Fe/H] > -0.85 dex, which is surprisingly low for the given agesand distances of the aggregates. For Berkeley 60, a large error in the distance and reddeningfrom the literature is evident. Ann et al. (2002) list a near solar metallicity (+0.07 dex) forthis cluster based on isochrone fitting to JohnsonUBVI measurements. For NGC 581 (Sanner et al.1999) and NGC 2244 (Bonatto & Bica2009),we also find a published solar metallicity. The cluster NGC 1750 isvery interesting in the sense that it overlaps with NGC 1758 (Straizyset al.2003).Therefore the choice of the members is essential to derive correctvalues. In summary, we find no evidence that the low metallicities aresupported by any other source.

The remaining values (Fig. 1)are within a range of+0.3 > [Fe/H] > -1.0 dex, which is theexpected one for open clusters (Chen et al.2003). For the following analysis, we intend to use the discussed metallicities in a proper way because they fill an important gapwithin the published investigations. Because the later paper by Tadross supersedes the previousone, we considered only the values from Tadross (2003) for our further analysis.

4 Mean [Fe/H] metallicities and analysis

The basic metallicity calibrations for the individual photometricsystems are, in general, very similar to each other. The starting point isa set of so-called ``standard objects'' for which the metallicity is clearly determined. The effective temperature and the surface gravity are normallynot used in the calibration process.

As a next step, a metallicity sensitive index within a specific photometricsystem has to be traced and correlated with the standard set of objects. The indices used are listed, in more details, below. There are only veryfew examples in which the whole variety of available photometric indices areused to correlate the metallicity. Martell & Laughlin (2002) presented third order polynomials of Strömgrenb-y,m₁, andc₁ with the complete permutation of these indices resulting in 20 coefficients with a listed precisionof six digits. However, these heuristic calibrations are not widely used.

The metallicity values presented in Table 1 are based on variousphotometric calibrations within six photometric systems listed in Sect. 2. These calibrations are all based on the individual indices described in the following.

Caby: thehk₀ index measures the Ca II HK line strengths and thus the abundances (Anthony-Twarog et al.1991). The values may be correlated with [Fe/H] depending on the temperature sensitive index (b-y).

DDO: themeasures the strength of the CN 4216 Å anomaly for red giants (Janes1975). The position of anobject in theC(45-48) versusC(42-45) diagram is an indicator of the luminosityfor solar metallicity. This location is used to predict the expected value ofC(41-42), a luminosity-dependent metallicity indicator. Theis then the difference between the observed and predicted value that can be transformed via a linear relation to [Fe/H]. For example, in the series of papers by Clariá and coworkers, the calibration of Clariá & Lapasset (1983) is used until 1991. From 1994 on, the calibration of Piatti et al. (1993) is used, based on a new definition of the cyanogen anomaly, now designatedCN. A comparative test published by Clariá et al. (2003, Ref. [21] in Table 1) indicates that the new calibration can result in a metallicity that differs by up to 0.1 dex from the previous one.

Johnson: the normalized ultraviolet excess,,is defined and calibrated relative to the Hyades main sequence in the two colour(U-B) versus (B-V) diagram. It is zero at (B -V) = 0.6 mag for themetallicity of the Hyades, which is higher than solar, and sensitive to the effect of line blanketing (Sandage1969).

Strömgren: them₁ index is the standard line blanketing indicator for almost thecomplete temperature range (Strömgren1966).

Vilnius: (P-X) can be directly correlated with [Fe/H] for cool-type starslater than G0 (Straizys & Bartkevicius1982).

Washington: starting with the solar-abundance relationsM-T₁ versusC-Mand usingT₁-T₂ as a temperature sensitive index, the blanketing in theM-T₁index is correlated with [Fe/H]. The blanketing in theC-M index is correlatedwith the CN 4216 Å anomaly of giants (Canterna1976). Geisler et al. (1991) published a new empirical abundance calibration for the Washington system, based on five metallicity-sensitive indices, which were used by all subsequent references given in Table1.

The standard procedure for deriving the metallicity of a cluster isalways very similar. After a proper selection of cluster members, which is especially important in studies using a rather limited number of stars (e.g. red giants), the objects have to be dereddened. In principle, the dereddening procedure has to be appliedindividually for each star rather than using a mean value, to take into account a possible differential reddening. Finally, using the respective metallicity calibrations applied to all selected objects, a mean value for the aggregate is calculated.

	Figure 2: The error distribution of the individual measurements and means for open clusters with estimates from the literature.
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	Figure 3: The Galactic distribution of the final open cluster sample. We excluded Berkeley 22, Berkeley 29, and Tombaugh 2 because they are located far away from the Sun with a very large distance error. The open circles mark open clusters with only oneavailable metallicity measurement whereas filled ones are those with more than one value.The Galactic centre is located at [+8000:0].
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The values from the literature listed in Table 1were averagedand the standard deviation of the mean was calculated. If only onevalue was available,the error from the literature was taken. If there is none available, weset theerror to 0.1 dex, respectively. No weights were introduced in ourprocedure.We note that the metallicities are on a logarithmic scale, so a plainaveraging results in a geometric and not in an arithmetic mean.

In total, [Fe/H] values for 188 open clusters listed in Table3 areavailable for our further analysis, among which 88 have more than one measurement available. The metallicities are comparable to those found forfield Population I type stars in the vicinity of the Sun.

To test whether the listed errors in the individual metallicity estimates arerealistic, we compared them to the errors in the means for open clusters withavailable values. The latter choice guarantees that no outliers influencethe results. In total, 29 open clusters with 144 individual measurements are used. Figure 2 shows the error distributions for these two samples.We find that 90% of all values are below 0.15 dex for both samples. We thereforeconclude that the individual errors are well within the range of those for the means,and that we do not introduce any bias by calculating the averages from the literature. There are, however, a few cases where the errors of the individual measurements might have been underestimated, notably NGC 2660 and NGC 6791.

Two of the most interesting and well studied topics are the metallicity distributionin the vicinity of the Sun and the Galactic metallicity gradient. With the current data set,we looked at both issues in more detail.

	Figure 4: A contour plot of the metallicity distribution within pc2 around the Sun in the [X:Y] plane. The patchy structure is consistent with results for the Local Bubble which has a diameter of up to 500 pc.
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The Galactic distribution within the [X:Y] plane is shown in Fig. 3.The Galactic centre is located at [+8000:0]. The open clustersBerkeley 22, Berkeley 29, and Tombaugh 2 are excludedbecause they are located far away from the Sun with a very largedistance error. The distribution of available metallicity values is nothomogeneous within the whole area. The contours were thereforeinterpolated in a 3D manner using cubic splines. From the plot, it isobvious that the vicinity of pc² can be traced quite reliably. This is an excellent extension to similar efforts for the Local Bubble. Lallement et al. (2003) present maps of the density distribution of cold gas within250 pc from the Sun (supplemented by a small number of more distant observations) and find a radius for the Local Bubble of 60 pc, with extensions of up to 250 pc and narrow ``interstellar tunnels'' out to at least 500 pc (Welsh & Shelton2009). Figure 4 shows a contour plot of the metallicity distribution using our derived mean [Fe/H] values. The patchy structure and the length scale resemblethose of the Local Bubble very well. They continue out to 2000 pc from the Sun.On the other hand, Luck et al. (2006), investigated a far more extended region of pc², derived from abundances of 54 Cepheids. Their resolution is lower than ours but the general trends are comparable.

The metallicity gradient of the Milky Way has been the subject of detailed investigation. Observational data of not only star clusters, but also different star groupssuch as Cepheids and Population II type stars have been used to place constraints on Galacticevolutionary models. The published metallicity gradients differ widely. Pedicelli et al. (2009),for example, concluded that there are two different gradients for the inner andouter Galactic disc, respectively. Figure 5 shows the [Fe/H] abundances versus the Galactocentric distance for our sample. The data are consistent with a rather wide range of metallicity gradients (0.1 to 0.3 dex kpc^-1). The estimate depends cruciallyon the data points with > 10 kpc.But there is an apparent lack of open clusters in this region with morethan one metallicity estimate. Keeping in mind the patchy structure ofthe local environment, the metallicity gradient should be estimated fordifferent linesof sight within the Galactic plane. This analysis has, to ourknowledge, never been done. We willtackle this topic in the final paper of this series.

	Figure 5: The [Fe/H] values versus the Galactocentric distance for our sample.The straight lines represent three different metallicity gradients of 0.1, 0.2, and 0.3 dex kpc-1, respectively. The Sun is at = 8 kpc.The open circles mark open clusters with only one available metallicitymeasurement, whereas filled ones are those with more than one value.
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5 Conclusions and outlook

We have presented our plans to complete a critical assessment of open cluster metallicities. In the first step presented here, we havecollected all metallicity determinations based on photometric calibrations.

In total, 406 published values for 188 open clusters in 64 references were found that were averaged not taking into accountany weights. A comparison of the papers by Tadross (2001,2003), which are based on the same datasets but different applied calibrations, demonstrate how crucial the choice of thecalibration is. The results, sometimes, widely diverge, in the sense that toolow metallicities are derived.

Using published cluster parameters, we investigated the metallicity distributionin the vicinity of the Sun and the Galactic metallicity gradient in more detail.We are able to confirm and extend the patchy structure of the Local Bubble out to2000 pc from the Sun. The current data set does not allow us to draw anyfurther conclusions about more distant Galactic regions. For the Galactic metallicitygradient, no conclusive value can be estimated with the current data set,because of the patchy local distribution and the lack of very distant open clusters.Both topics will be investigated in forthcoming studies.

On the basis of the current sample, we propose to search for and analyse elementalabundances of open cluster members. A detailed comparison of the photometric andspectroscopic results will essentially help to find the most reliable calibrations.In addition, apparent off-sets will be able to be identified.

Our last step will be to investigate the influence of the metallicity on open cluster parameter estimations that only take into account solar metallicity,and to provide correction values.

This work was supported by the financial contributions of the Austrian Agency for International Cooperation in Education and Research (WTZ CZ-11/2008, CZ-10/2010 and HR-14/2010), the City of Vienna (Hochschuljubiläumsstiftung project: H-1930/2008), the Forschungsstipendium der Universität Wien (F-416), a MOEL grant of the ÖFG (Project #388), a travel grant from the Swedish Research Council and the Swedish National Space Board.This research has made use of the WEBDA database, operated at the Institute for Astronomy of the University of Vienna.

References

Online Material

Table 1:  The [Fe/H] values from photometric calibrations for open clusters found in the literature.

Table 3:  Unweighted averaged [Fe/H] values for 188 Galactic open clusters, 88 of them have more than one measurement.

Footnotes

... Photometry

Tables 1 and 3 are only available in electronic form athttp//www.aanda.org

... WEBDA

http://www.univie.ac.at/webda

All Tables

Table 2:   The open clusters with very low [Fe/H] estimates from Tadross (2001) and the comparison with those from Tadross (2003).

Table 1:   The [Fe/H] values from photometric calibrations for open clusters found in the literature.

Table 3:   Unweighted averaged [Fe/H] values for 188 Galactic open clusters, 88 of them have more than one measurement.

All Figures

	Figure 1: A comparison of the [Fe/H] values derived via two different photometriccalibrations as published by Tadross (2001,2003). The dashed lines represent0.25 dexoffsets. Note the wide range of apparent underabundant aggregates in the first paper.
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In the text

	Figure 2: The error distribution of the individual measurements and means for open clusters with estimates from the literature.
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In the text

	Figure 3: The Galactic distribution of the final open cluster sample. We excluded Berkeley 22, Berkeley 29, and Tombaugh 2 because they are located far away from the Sun with a very large distance error. The open circles mark open clusters with only oneavailable metallicity measurement whereas filled ones are those with more than one value.The Galactic centre is located at [+8000:0].
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In the text

	Figure 4: A contour plot of the metallicity distribution within pc2 around the Sun in the [X:Y] plane. The patchy structure is consistent with results for the Local Bubble which has a diameter of up to 500 pc.
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In the text

	Figure 5: The [Fe/H] values versus the Galactocentric distance for our sample.The straight lines represent three different metallicity gradients of 0.1, 0.2, and 0.3 dex kpc-1, respectively. The Sun is at = 8 kpc.The open circles mark open clusters with only one available metallicitymeasurement, whereas filled ones are those with more than one value.
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In the text

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