TWO BARIUM STARS IN THE OPEN CLUSTER NGC 5822^*

The stellar spectra of the stars analyzed in this work show many atomic absorption lines of Fe i and Fe ii as well as transitions due to Na i, Mg i, Al i, Si i, Ca i, Ti i, Cr i, Ni i, Y ii, Zr i, La ii, Ce ii, and Nd ii. We have chosen a set of lines sufficiently unblended to yield reliable abundances. Equivalent widths were measured using the task splot from IRAF. The selected lines are listed in Table 2 for the case of Fe i and Fe ii lines and in Table 3 for the other elements. Tables 2 and 3 also provide the lower excitation potential, χ (eV), of the transitions and the gf values. The gf values for the Fe i and Fe ii lines were taken from Lambert et al. (1996) and Castro et al. (1997), and for the other elements the sources of the gf values are given in Table 3.

To obtain the chemical abundances, it is necessary first to calculate the stellar parameters: effective temperature T_eff, surface gravity log g, metallicity [Fe/H] ([X/H] = log (N_X/N_H)_⋆ − log (N_X/N_H)_☉), and microturbulent velocity ξ. The atmospheric parameters were determined using the local thermodynamic equilibrium (LTE) model atmospheres of Kurucz (1993) and the spectral analysis code MOOG (Sneden 1973).

The effective temperature was derived by requiring that the abundances calculated for the Fe i lines do not show any dependence upon excitation potential. Thus, the solution thus found is unique, depending only on a set of Fe i and Fe ii lines and the employed atmospheric model, and yields as a by-product the metallicity of the star ([Fe/H]). The gravity was determined by forcing the Fe i and Fe ii lines to yield the same iron abundance at the selected effective temperature. The microturbulent velocity was determined by forcing the abundance determined from individual Fe i lines to show no dependence on the equivalent width. Results for the stellar parameters are shown in Table 4.

Previous determinations of the atmospheric parameters of NGC 5822-2 and -201 were done, respectively, by Luck (1994) and Smiljanic et al. (2009). The solutions are also shown in Table 4. The observations of Luck (1994) were done with medium resolution (R = 18,000) while Smiljanic et al. (2009) observed with the FEROS spectrograph. As can be observed in Table 4, our stellar parameters are in good agreement with previous determinations, except for the microturbulence velocity.

As was mentioned earlier, we have compared the spectra of NGC 5822-2 and -201 with the spectrum of barium star HD 65314 and normal giant HD 2114. HD 65314 was also selected as an s-process-enriched comparison star after analyzing our results for the stellar parameters and abundances obtained from the large high-resolution spectroscopic survey of the barium stars selected from the samples of MacConnell et al. (1972) and Bidelman (1981) as well as some stars from Gomez et al. (1997). This survey aims to obtain the atmospheric parameters, abundances, and the kinematical properties of these chemically peculiar stars and compare them with the non-s-process-enriched giant stars. Of the 230 surveyed stars, we have already discovered a new CH subgiant, BD−03°3668 (Pereira & Drake 2011) as well as a sample of metal-rich barium stars (Pereira et al. 2011). As will be seen in Section 4.2.3, HD 65314 displays a mean s-process abundance ([s/Fe]) similar to that of NGC 5822-2 and -201. Table 4 also shows the atmospheric parameters of HD 65314.

HD 2114 was included in our study after searching in the literature for a star with stellar parameters similar to those found for the cluster giants and with a spectrum that would be available in the ESO archives, so that a comparison between HD 2114 and the spectra of the stars studied here was possible. We obtained the following atmospheric parameters using the spectroscopic data from ESO: T_efff = 5200, log g = 2.3, [Fe/H] = −0.13, and ξ = 1.7. These are similar to those found by Hekker & Meléndez (2007): T_efff = 5160, log g = 2.55, [Fe/H] = −0.15, and ξ = 1.85.

The internal errors in our adopted effective temperatures (T_eff) and microturbulent velocities (ξ) can be determined from the uncertainty in the slopes of the Fe i abundance versus excitation potential and Fe i abundance versus reduced equivalent width (W_λ/λ). The standard deviation in log g was set by changing this parameter around the adopted solution until the difference between the Fe i and Fe ii mean abundance differed by exactly one standard deviation of the [Fe i/H] mean value. Based on the above description, we estimate typical uncertainties in atmospheric parameters of the order of ±100 K, ±0.20 dex, and ±0.2 km s⁻¹ for T_eff, log g, and ξ, respectively.

To test our gravities obtained from spectroscopy for NGC 5822-2 and -201, we have calculated evolutionary gravities from the equation below:

$$\begin{eqnarray} &&\log g_{\star }= \log (M_{\star }/M_{\odot }) + 0.4(V-A_{v}+BC) \nonumber \\ &&+ 4 \log T_{{\rm eff}} -2 \log r({\rm kpc}) - 16.5. \end{eqnarray} \tag{ 1 }$$

The turnoff mass (M = 2.1 M_☉), interstellar absorption (A_V = 0.45), and distance (r = 697 pc) for NGC 5822 were taken from Table 2 of Smiljanic et al. (2009), and bolometric corrections were taken from Alonso et al. (1999). We found a mean difference of approximately of 0.1 dex for both stars between the spectroscopic and evolutionary gravities.

The abundances of chemical elements were determined with LTE model atmosphere techniques. The equivalent widths were calculated by integration through a model atmosphere and were compared with the measured equivalent widths. The current version of the line-synthesis code moog (Sneden 1973) was used to carry out the calculations. Table 5 shows the derived abundances of the elements and also the number of lines employed (or number of spectral regions, in the case of carbon and nitrogen whose abundances were determined using molecular lines) for each species, n, the standard deviation, and in the notations [X/H] and [X/Fe] as well as the C/O ratio. The adopted abundances for the elements analyzed in this work were normalized to the solar abundances of Grevesse & Sauval (1998). For the solar iron abundance, we adopted log ε(Fe) = 7.52.

Carbon, nitrogen, and oxygen abundances were determined using the spectrum synthesis technique. For the oxygen abundance in NGC 5822-2 and -201, we used the line at 6363.78 Å because the line at 6300.31 Å  is contaminated by the telluric O₂ line in both stars. The gf value for the 6363.78 Å line was taken from Lambert (1978). In HD 65314, we used the forbidden line at 6300.63 Å  to obtain the oxygen abundance.

Since the abundances of the CNO elements are interdependent because of the association of carbon and oxygen in CO molecules in the atmospheres of cool giants, the CNO abundance determination procedure was iterated until all the abundances of these three elements agreed. The abundances of carbon and nitrogen were determined using the lines of the CN and C₂ molecules. The line lists used in this work are the same as used in previous studies done for some barium stars (Pereira & Drake 2009; Drake & Pereira 2011). The observed and synthetic spectra of NGC 5822-2, -201, and HD 65314 in the region around the C₂ molecule at 5635 Å  are shown in Figure 5.

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We do not measure the barium abundance in our stars because all barium lines have equivalent widths higher than 200 mÅ  and therefore will not be at the linear part of the curve of growth (Hill et al. 1995; Pereira et al. 2011). However, since we have measured several other lines of other elements synthesized by the s-process (Y, Zr, La, Ce, and Nd), we believe that we probed this nucleosynthesis process in NGC 5822-2 and NGC 5822-201 fairly well.

The abundance uncertainties of NGC 5822-2 and -201 are shown in Tables 6 and 7. HD 65314 also displays similar abundance uncertainties. The abundance uncertainties due to the errors in the effective temperatures and microturbulent velocities were determined by changing these parameters around the adopted solutions until the difference in the iron abundance became one standard deviation. The uncertainties in the abundances, due to errors in the stellar atmospheric parameters, were computed by changing these parameters by their standard deviations given in Table 4 and calculating the changes incurred in the element abundances. The final uncertainty of the abundance by number is found by composing quadratically the uncertainties due to atmospheric parameters and the W_λ's. The abundance uncertainties resulting from the errors in the equivalent width measurements were computed from the expression provided by Cayrel (1988). These errors we are set, essentially, by the S/N and the resolution of the spectra. In our case, having 48,000 and a typical S/N of 150, the expected uncertainties in the equivalent widths are about 2–3 mÅ. For all measured equivalent widths, these uncertainties led to errors in the abundances less than those due to the uncertainties in the stellar parameters. They were calculated by quadratically combining the different sources of errors under the assumption that the errors were independent. In the last column of Tables 6 and 7, we quote the observed abundance dispersion among the lines for those elements with more than three available lines.

We verify the well-known relations from Tables 6 and 7 that neutral elements are more sensitive to the temperature variations, while singly ionized elements are more sensitive to the variations in log g. For the elements whose abundance is based on stronger lines, such as the lines of calcium, yttrium, and cerium, the error introduced by the microturbulence is important.

For NGC 5822-2 and -201, the bolometric magnitude which results from the adopted turnoff mass, temperature, and gravity of the two stars is, respectively, M_bol⋆ = −0.6 ± 0.5 (with a luminosity of 140 L_☉) and M_bol⋆ = 0.0 ± 0.5 (with luminosity of 76 L_☉), assuming M_bol☉ = +4.74 for the Sun (Bessel et al. 1998). These luminosities are too low to consider NGC 5822-2 and -201 as AGB stars that started shell helium burning (via thermal pulses) and became self-enriched in the neutron-capture elements. In fact, theoretical calculations show that in order to develop the first thermal pulse, the star's luminosity should be ≃ 1800 L_☉ (M_bol = −3.4; Lattanzio 1986) or ≃ 1400 L_☉ (M_bol = −3.1; Vassiliadis & Wood, 1993).

Since we know the distance of the cluster of NGC 5822, we can also compare the masses of these giants with the masses of the field barium stars. The masses of barium stars have already been determined in the literature. The determinations were made either by the available parallax or by placing them in log g–log T_eff diagram with theoretical evolutionary tracks (Allen & Barbuy 2006; Antipova et al. 2003, 2004; Boyarchuk et al. 2002; Liang et al. 2003; Pereira et al. 2011; Smiljanic et al. 2007). In all these studies, the authors found that the masses of barium stars are in the range between 1.3 and 4.2 M_☉. To date, the most complete analysis of the determination of the mass of barium stars is the work of Mennesier et al. (1997). There, the authors, using the Hipparcos parallaxes and based on the Lu (1991) catalog, showed from the absolute magnitude versus intrinsic color index diagram that the mass of barium stars is in the range between 1.25 and 7.0 M_☉. However, some high values found for the mass of some barium stars should be viewed with caution since there are many stars in the Lu (1991) catalog that are not barium stars. As far as the two giants in NGC 5822 are concerned, their masses are also constrained by the cluster age and the turnoff mass. With a value of 2.1 M_☉, (Section 3.2), we can see that they also lie in the same range of the field barium stars.

4.2.1. Carbon, Nitrogen, and Oxygen

In Figure 6, we compare our CNO abundances obtained for NGC 5822-2 and -201 and HD 65314 with field giant stars analyzed by Mishenina et al. (2006) and Luck & Heiter (2007). We also included the [C/Fe] ratio for another barium star in the open cluster NGC 2420 from Smith & Suntzeff (1987). The [C/Fe] and [N/Fe] ratios for the field giants were computed using absolute abundances on the scale of log ε(H) = 12.0 obtained in these papers and then converted to [X/Fe] ratios using the solar abundances adopted in this work (Section 3.3). As seen in Figure 6, the [N/Fe] ratios for these two cluster giants have similar values of the giants analyzed by Mishenina et al. (2006) and Luck & Heiter (2007). Figure 6 also shows that nitrogen is overabundant with respect to the Sun in field giants as well as in NGC 5822-2 and -201 and in HD 65314. As a star becomes giant, due to deepening of its convective envelope, nuclear processed material is brought from the interior to the outer layers of the star, which changes the surface composition. As a consequence of the first dredge-up process, the abundance of ¹²C is reduced and the abundance of ¹⁴N is enhanced (Lambert 1981). Our results for the [N/Fe] ratio show the effects of the first dredge-up.

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We updated the metallicity of the two barium stars of NGC 2420 as well as the carbon and the heavy-element abundances. Smith & Suntzeff (1987) derived a metallicity of −0.55 and −0.68, respectively, for stars D and X. These observations were made at lower resolution and with a spectral coverage of 200 Å. As was discussed in Jacobson et al. (2011), NGC 2420 could be more metal-rich than −0.57, as was earlier derived by Smith & Suntzeff (1987). In fact, the authors obtained a mean metallicity of −0.20, which we adopted for stars X and D to obtain the [X/Fe] ratios ([C/Fe] and the [s-process/Fe]) seen in Figures 6, 7, 10, and 11.

The [O/Fe] ratios of NGC 5822-2 and -201 and HD 65314 are similar to those seen in field giants. The [O/Fe] ratio was also computed based on an absolute abundance and then converted to the [X/Fe] ratio using the solar abundances adopted in Section 3.3. For this reason, there is an offset of −0.13 dex in the [O/Fe] ratio between Figure 6 in this paper and Figure 13 in Mishenina et al. (2006), who adopted a solar oxygen abundance of 8.70. Luck & Heiter (2007) adopted a solar oxygen abundance (8.81) similar to this paper's, and because of that our normalized [O/Fe] ratio is equal to the ratio derived by these authors.

Figure 7 shows the CNO abundances of our target stars with some previous determinations done for some barium giant stars (Barbuy et al. 1992; Allen & Barbuy 2006; Pereira & Drake 2009). Our derived [C/Fe] ratios are in good agreement with these previous analyses. The barium stars also shown the effects of the first dredge-up episode, since we also observed nitrogen enrichment. As far as oxygen is concerned, our [O/Fe] is also in agreement with these previous determinations in the barium giants.

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4.2.2. Other Elements: Na to Ni

Sodium and aluminum, α-elements (Mg, Si, Ca, and Ti), and iron-peak elements (Cr and Ni) in NGC 5822-2 and -201 as well as in HD 65314 follow the general trend seen in the field giants analyzed by Luck & Heiter (2007). Figures 8 and 9 show the abundances of the above-mentioned elements for our studied stars.

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4.2.3. Heavy Elements: Neutron-capture Elements

Figure 10 shows the abundances of Y, La, Ce, and Nd for NGC 5822-2 and -201 as well as for the barium stars HD 65314 and NGC 2420-X and D (Smith & Suntzeff 1987). Clearly, these stars have higher abundances of these elements than field giants. This result is interesting because these differences are significant for the four elements. This fact is also noted at the bottom of Figure 10, where we also provide the mean abundance of these heavy elements, "s" in the notation [s/Fe]. We obtained [s/Fe] = 0.77 ± 0.12 and 0.83 ± 0.05, respectively, for NGC 5822-2 and -201. These values are not only much higher than the mean value for the field giants at similar metallicity ([s/Fe] ≈ 0.1 and [s/Fe] = 0.17 ± 0.12 for HD 2114), but also it is similar to the values found in other barium stars (as can be seen in Figure 11). For the barium star HD 65314, we have [s/Fe] = 0.74 ± 0.4. The abundance of zirconium (not shown in the figure) was also taken into account to calculate [s/Fe]; however, in the literature there are not many results for the zirconium abundance in field giants, except for barium stars (Antipova et al. 2005; Allen & Barbuy 2006; Smiljanic et al. 2007; Pereira et al. 2011).

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In Figure 12, we show the relationship between the orbital period and the [s/Fe] for a sample of field barium stars. We used the available period data from Jorissen et al. (1998) and the [s/Fe] ratios from different sources of the literature. Unfortunately, there are not many barium stars with determined binary periods, but there is a trend of decreasing [s/Fe] ratio with increasing orbit period—although the scatter seen in this figure would mean that the separation of the stars in the system is not the only parameter that influences the observed overabundances (Jorissen et al. 1998). Dilution effects of the accreted material as well as the metallicity are also important. The star NGC 5822-2 with a period of 1000 days (Mermilliod et al. 1989) displays dynamics and overabundances similar to other field barium stars.

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The previous determination of the heavy-element abundance in one of the cluster giant analyzed in this work, NGC 5822-2, was done by Luck (1994), who found [Zr/Fe] = 0.59, which is in good agreement with our results. For the [Nd/Fe] ratio, Luck (1994) found 0.19, which is different from our value of 0.80. This difference of results is probably related to the number of lines employed in the analysis. While our abundance ratio is based on eight lines, the result of [Nd/Fe] of Luck (1994) was based on a single line. Since Luck (1994) used a lower resolution than we used, this single line likely could be contaminated by transitions other than the Nd line.

We may speculate a few reasons why more than 20 yr have passed since the last discovery of a barium star in an open cluster. First, after 2008, when the large radial velocity survey of Mermilliod et al. (2008) was published, several new cluster giants were identified, thus allowing the study of new objects, some of which are binary systems. Second, another important point for the identification of barium stars in an open cluster is the determination of the heavy-element abundances. Few cluster giants have their heavy-element abundances determined; even when they are, the abundances were determined for one or two elements of the s-process based on a few lines (Maiorca et al. 2011). Therefore, with new high-resolution spectroscopic surveys to obtain the metallicities and abundances for giants in open clusters, other new barium stars will probably be discovered.

The discovery of these two barium stars in NGC 5822, and the two previously known in NGC 2420, raised another point for a comparison between the frequency of the barium stars to normal giants in field stars and in the cluster. In field stars, the percentage of barium stars is only 1 (MacConnell et al. 1972). In open clusters, the binary frequency of red giant spectroscopic binaries with periods less than 4000 days is 23% (Mermilliod & Mayor 1990). The barium star fraction, considering these two barium stars in NGC 5822 among 21 known giants and two in NGC 2420 among 22, is 10%, 10 times larger than that in the field stars.