CHEMICAL ABUNDANCES FOR A SAMPLE OF SOUTHERN OB STARS. 1 II. THE OUTER DISK

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1 The Astrophysical Journal, 606: , 2004 May 1 # The American Astronomical Society. All rights reserved. Printed in U.S.A. CHEMICAL ABUNDANCES FOR A SAMPLE OF SOUTHERN OB STARS. 1 II. THE OUTER DISK Simone Daflon Observatório Nacional, Rua General José Cristino 77, CEP , Rio de Janeiro, Brazil; daflon@on.br Katia Cunha Department of Physics, University of Texas at El Paso, El Paso, TX ; and Observatório Nacional, Rua General José Cristino 77, CEP , Rio de Janeiro, Brazil; katia@on.br and Keith Butler Institut für Astronomie und Astrophysik der Universität München, Scheinerstrasse 1, D Munich, Germany; butler@usm.uni-muenchen.de Received 2003 December 5; accepted 2004 January 18 ABSTRACT Non-LTE abundances of carbon, nitrogen, oxygen, aluminum, magnesium, silicon, and sulfur are derived for a sample of nine OB stars that are members of seven clusters or H ii regions located outside the solar circle, with Galactocentric distances between 9 and 13 kpc. The abundance distribution for these stars is found to have a peak roughly 0.3 dex lower than the average of the abundance distribution for OB stars from the inner disk. This result is in general agreement with the disk metallicity discontinuity claimed from observations of open clusters and Cepheids. The abundances obtained for this outer disk sample are all subsolar, with an average for C, N, O, Mg, and S of 0:39 0:05. For Si and Al, we find even lower average abundances of 0.56 and 0.58, respectively. Subject headings: Galaxy: abundances Galaxy: disk stars: abundances stars: early-type 1. INTRODUCTION The present-day metallicity distribution across the Galactic disk is an important constraint in understanding galaxy formation and evolution. Leaving for the moment the discussion of radial abundance gradients aside, several studies in the literature find that the metallicity distribution across the Galactic disk seems to be bimodal with Galactocentric distance, R g. For example, Twarog, Ashman, & Anthony-Twarog (1997) analyzed a sample of open clusters and found that the disk appears to break up into two distinct populations. In the region between 6.5 and 10 kpc, they obtained a tight and approximately constant distribution of metallicities, with a mean value of ½Fe=HŠ ¼0. On the other hand, they found larger metallicity dispersions within open clusters located beyond 10 kpc, with a mean metallicity of ½Fe=HŠ ¼ 0:3 andan apparent abundance discontinuity at R g ¼ 10 kpc. Caputo et al. (2001) analyzed a sample of Cepheids on the basis of pulsating models and found a metallicity distribution consistent with two distinct metallicity zones with a discontinuity at R g ¼ 10 kpc. Andrievsky et al. (2002a, 2002b) and Luck et al. (2003) derived Cepheid abundances in the Galactic disk and found that the outer disk abundance distribution was clearly distinct from the middle disk, also suggesting a break at R g ¼ 10 kpc. In previous papers we have presented non-lte abundances for a large sample of OB-star members of associations, open clusters, and H ii regions, located at different positions within the inner Galactic disk (Daflon, Cunha, & Becker 1999; Daflon et al. 2001a, 2001b, 2003; Daflon, Cunha, & Butler 2004; Papers I V, respectively). We now look outward in the Galactic disk, the ultimate goal being to define the abundance 1 Based on observations collected with the 1.52 m telescope at the European Southern Observatory (La Silla, Chile) under agreement with the Observatório Nacional, Brazil. 514 distribution across the disk of the Milky Way. In this study we concentrate on a southern sample of distant targets that lie in the outer parts of the disk. The same set of seven elements from our previous papers, carbon, nitrogen, oxygen, magnesium, aluminum, silicon, and sulfur, and the same samples of atomic lines are studied here, so that the present results are on the same scale and add to a homogeneous database of non- LTE abundances of early-type stars. Being the last abundance paper of a series, we assemble and discuss here all of our previously published results. In particular, we investigate possible differences between the abundance distributions of the inner and outer disk OB stars and compare them with the nebular abundances derived for the H ii regions associated with the studied star clusters. 2. OBSERVATIONS High-resolution echelle spectra (k=k ¼ 48; 000) were obtained with the 1.52 m telescope plus the Fiber-fed Extended Range Optical Spectrograph (FEROS) at the European Southern Observatory, La Silla, Chile. The observations have full wavelength coverage between 3900 and The data were reduced with the FEROS pipeline reduction. The latter has been tested earlier via comparisons with independent data sets and is deemed adequate for our abundance analyses, as discussed in Paper V. The target stars were selected to have Galactic longitudes between l 130 and 230, pointing in the direction of the Galactic anticenter. We observed a total of 26 stars, although 17 of them were discarded because of the presence of strong emission lines or rapid rotation (high v sin i), for which an accurate abundance analysis could not be done. Most of the target stars are relatively faint (V magnitudes greater than 11.0), close to the limiting magnitude observable with the 1.52 m telescope + FEROS. Our final sample has two stellar members each of the open clusters NGC 1893 and NGC 2414, one target

2 Southern OB STARS. II. The Outer Disk 515 TABLE 1 Observational Data Star Cluster MK V Date t exp (minutes) S/ N S2R3N09... NGC 1893 B1.5 III Feb S2R2N43... NGC 1893 B1 III Feb LS NGC 2414 B1 V Nov LS NGC 2414 B0.5 V Nov Sh Sh O9 V Feb LS Sh B1.5 V Nov HD Sh B0.5 IV Nov BD Sh B0 V Nov HD Mon OB2 O9 V Nov from the Monoceros OB2 association, and four targets belonging to distant H ii regions. Information on the sample stars and observations is gathered in Table 1. The signal-to-noise (S/N) ratios listed in the last column refer to the region around In Figure 1 we display the spectral region between 4635 and for the nine target stars. 3. EFFECTIVE TEMPERATURES, SURFACE GRAVITIES, AND NON-LTE ABUNDANCES The determination of stellar parameters followed the same methodology adopted in our previous study of OB stars located in the inner parts of the Galactic disk (Paper V). For the two brightest targets, effective temperatures and surface gravities could be derived iteratively from the dereddened Strömgren Fig. 1. Example spectra of the nine stars in our sample. From top to bottom, the spectral types vary from O9 to B1.5. indices c 0,[c 1 ], and, coupled with fits to the wings of H profiles. Alternatively, for the fainter targets, T eff s had to rely on the Q parameter and the calibration defined in Paper I. Given a particular temperature, a grid of H profiles was computed and log g s were derived from the best fit to the observations. The final effective temperatures were also verified against simple spectroscopic criteria as consistency checks. We inspected the relative intensities of O ii lines and C iii lines in the region around , Niii lines at 4634 and , and line ratios of He i/he ii. For all targets, except Sh , the derived temperatures were compatible with the observed stellar spectra. For Sh , the adoption of the photometric indices in Haug (1970) and Lahulla (1987) yields T(Q) ¼ 33; 730 K and 32,870 K, respectively. However, these high temperatures seemed inconsistent with the observed intensity of the He ii line at , which was rather weak. Furthermore, Sh was also studied by Gummersbach et al. (1998), who derived T ea ¼ 30; 800 K from Balmer lines combined with the ionization equilibrium of Si ii, Siiii, andsiiv lines. Taking all this into consideration, we elected to adopt a correction of 4% (corresponding to the magnitude of our estimated uncertainties) in the T eff derived for this star. The final adopted stellar parameters and photometric indices (mean values when more than one measurement was available) are listed in Table 2. Our abundances were derived using LTE model atmospheres from Kurucz (1992) and a non-lte theory of line formation with spectrum syntheses of selected transitions of C ii,nii,oii, Mg ii, Aliii, Siiii, andsiii. The abundance calculations are described in detail elsewhere and are not repeated here (see Papers III and IV). Our results and adopted microturbulent velocities for the studied stars are shown in Table 3. The estimated abundance uncertainties are smaller than 0.15 dex for C, N, Al, and S. For oxygen, magnesium, and silicon, errors are larger at 0.20 dex. 4. DISCUSSION 4.1. Inner Disk versus Outer Disk Abundances In an effort to build a homogeneous database of OB stellar abundance distributions within the Galactic disk, we add to this discussion all the sample stars and abundances from our previous studies. As a first step, we simply segregate the sample into members of the inner disk or outer disk. (We defer the detailed presentation and discussion of abundance gradients to a future paper). The elemental abundance distributions for our sample of outer disk stars (this study) are displayed as solid-line histograms in Figure 2, which also shows the abundance distributions obtained for 60 OB stars in the inner parts of the

3 516 DAFLON, CUNHA, & BUTLER Vol. 606 TABLE 2 Stellar Parameters Star T eff (K) log g B V U B b y c 1 S2R3N S2R2N LS LS Sh LS HD a 4.10 a BD HD a 4.20 a a T eff and log g derived iteratively. Galactic disk (dashed-line histograms; studied in Papers I V). Although it should be kept in mind that the sample sizes are different, it seems clear from these two distributions that there is a tendency of lower elemental abundances in those stars farther away from the Galactic center compared to ones from the inner disk. Moreover, the abundances of the outer disk stars in our sample fall well below solar, as indicated by the solar abundances displayed on the histograms. Inspection of Figure 3, where we plot the individual stellar abundances as a function of effective temperature, also shows that the outer disk stellar abundances ( filled circles) are lower than the inner disk ones (open circles). Average elemental abundances computed for our inner and outer samples are listed in Table 3; we find that the average abundances for the outer disk are lower than the inner disk by roughly the same amount for all studied elements with a mean inner outer difference of 0:27 0:03. A similar abundance pattern for the disk was found by Andrievsky et al. (2002a, 2002b) and Luck et al. (2003), who studied abundance distributions of Cepheids and found that the metallicities (iron) from outer-disk Cepheids are on average lower by roughly 0.2 dex than abundances from Cepheids in the inner disk. Their results were based on samples of stars both in the solar neighborhood and beyond 10 kpc from the Galactic center, and suggest the existence of a discontinuity in the disk metallicity distribution at about 10 kpc. Although the same general trend for lower abundances in the outer disk was found here, there are significant differences in the absolute abundances of OB stars and Cepheids. In Figure 3 we also show the average C, O, Mg, Al, Si, and S Cepheid abundances for the middle (short-dashed line) andouterdisk(long-dashed line), as listed in Table 6 of Luck et al. (2003). A comparison of the open circles with the short-dashed horizontal lines indicates that for the inner disk, the mean Cepheid abundances are above the OB stars abundances, except for Mg and C. As expected, we find the same general pattern of underabundance in the OB stars for the outer disk. The discrepancies between the OB star and Cepheid abundances are probably due to two effects: the surface abundances of C, O, and possibly Al and Mg in the Cepheids may have been modified through nucleosynthesis and mixing, but there are also abundance differences due to systematic errors that are not currently accounted for. Focusing only on the inner disk sample, our results for OB stars can be compared with those for nearby field F and G dwarfs from Reddy et al. (2003). In the cooler atmospheres of late-type stars, the state of ionization is very different from OB stars, and such absolute abundance comparisons can potentially provide an assessment of systematic errors in the analyses. Since the OB stars are young, we only compared abundances with those F and G dwarfs closest to the zero-age main sequence (ZAMS), as determined by Reddy et al. (there are eight stars with evolutionary ages of less than 2.2 Gyr). The average abundances derived for both the OB stars and the subset of F and G dwarfs compare favorably with each other, with typical differences of 0.1 dex for O, Mg, Al, and S. Larger differences are found for carbon and silicon (roughly 0.2 dex) and nitrogen (roughly 0.4 dex). Of course, this straightforward comparison neglects abundance differences that might result from Galactic birthplace and age differences of 1 Gyr; however, these differences are probably fairly TABLE 3 Non-LTE Abundances Star (km s 1 ) log (C) log (N) log (O) log (Mg) log (Al) log (Si) log (S) S2R3N S2R2N LS LS Sh LS HD BD HD Average: Outer Disk Average: Inner Disk

4 No. 1, 2004 Southern OB STARS. II. The Outer Disk 517 Fig. 2. Abundance distributions for the sample stars located on the outer (solid histogram) and inner(dashed histogram) parts of the Galactic disk. The arrows represent the solar abundances: C ¼ 8:41 0:03, N ¼ 7:80 0:04, and O ¼ 8:66 0:03 (Asplund 2003); Mg ¼ 7:538 0:060 and Si ¼ 7:536 0:049 (Holweger 2001); Al ¼ 6:47 0:07 and S ¼ 7:33 0:11 (Grevesse & Sauval 1998). small over the rather restricted range in Galactic radial position spanned by these samples Comparisons with Abundances from H ii Regions A comparison between abundances from young stars and abundances from the associated gas in H ii regions is of obvious interest in understanding the larger picture of star formation and chemical evolution in the Galaxy. This comparison focuses on those elements that are usually present in both stellar and nebular analyses, i.e., N, O, and S. At present, a direct comparison of our stellar results is possible for the clusters NGC 6611 and NGC 6604, the association of Cyg OB7, as well as the H ii regions Sh and Sh 2-285, lying at the edge of the Galaxy. Their nebular abundances from the literature are assembled in Table 4. These are from different studies, which adopt different methods to derive the physical parameters and abundances of the gas. A few comments can be made from an inspection of the compilation of stellar and nebular abundance results in the Table 4. For NGC 6611, there is generally good agreement between stellar and nebular abundances. For oxygen, in particular, there are a variety of nebular studies in the literature: the oxygen abundances derived span the range between 8.53 and The lowest oxygen abundance found is from Deharveng et al. (2000), whose oxygen abundances were based on a compilation of radio temperatures combined with new temperatures from [O iii] lines. For this cluster, we quote an average stellar abundance (computed from four stars) of 8:58 0:07, which falls within the nebular abundance range and is just slightly higher than the value obtained by Deharveng et al. (2000). We note that the abundances in Deharveng et al. are systematically lower than other studies in all cases. Their lower oxygen results are probably influenced by the adopted electron temperatures. In the case of NGC 6604 and its associated H ii region Sh 2-54, there is also good agreement, within the errors, between the oxygen abundance from Deharveng et al. (2000) and that of our target star BD (This is the only case where we find oxygen abundances lower than the nebular results from Deharveng et al.) For N, the only

5 Fig. 3. Elemental abundances of C, N, O, Mg, Al, Si, and S as a function of T eff, for the B stars in the inner (open circles) andouterdisk(filled circles). The abundances are averages of individual line abundances and the error bars are the line to line scatter when more than one line was used in the analysis. Dashed lines are the mean abundances for Cepheids in the middle (short-dashed line) and outer disk (long-dashed line), from Luck et al. (2003). TABLE 4 Stellar and Nebular Abundances Stellar Nebular Object log (N) log (O) log (S) log (N) log (O) log (S) Source NGC 6611 (M16) , 8.68, NGC 6604 (Sh 2-54) , Cyg OB7 (Sh 2-119) Sh Sh :10 þ0: þ0:14 0:13 5 References. (1) Hawley 1978; (2) Shaver et al. 1983; (3) Deharveng et al. 2000; (4) Pilyugin 2001; (5) Fich & Silkey 1991

6 Southern OB STARS. II. The Outer Disk 519 line) of dex. The only significant discrepancy is the sulfur determination for Sh This straightforward comparison indicates that both nebular and stellar abundance determination techniques are in near agreement at levels of 0.2 dex, although some differences still exist for some objects. This tight abundance agreement also constrains the amount of depletion onto grains from the nebular gas for the volatile elements N, O, and S. Fig. 4. Comparison between OB star abundances of N, O, and S and the nebular abundances (average values from Table 4) of the associated H ii regions. The two sets of abundances show reasonable agreement within dex. possible comparison is with Shaver et al. (1983), who find higher nebular values, but we note that the oxygen results of Shaver et al. are systematically higher than those of Deharveng et al. The association between Sh 2-54 and NGC 6604 is questioned by Caplan et al. (2000), who note that these objects seem to be spatially disconnected. For Cyg OB7, we computed average abundances from three stars (studied in Paper II and III) and find that the N and O abundances are just slightly higher than the nebular ones. This is also the case for Sh 2-284, for which both Shaver et al. (1983) and Deharveng et al. (2000) derive nebular oxygen abundances that are slightly lower than the stellar value, while the nebular nitrogen abundance is lower than the stellar by a larger amount (0.24 dex). For Sh 2-285, a comparison is only possible for nitrogen and sulfur; for the latter, the results are discrepant by roughly 0.6 dex. The comparisons between the nebular and OB-star abundances are summarized visually in Figure 4, where the nebular abundances are plotted versus the stellar abundances (from Table 4); for the nebular abundances, an average abundance value for each cluster or H ii region is used if multiple measurements are available. Figure 4 suggests that, in general, the nebular and OB-star abundances are in reasonable agreement, with a scatter about the line of perfect agreement (solid 5. SUMMARY AND CONCLUSIONS We present chemical compositions for nine stars lying in the outer Galactic disk and add to the construction of a database of abundances for OB stars derived by means of non-lte spectrum syntheses and based upon a homogeneous and selfconsistent methodology. Our abundance database has a total of 69 stars and has a larger number of inner disk stars than outer disk stars. We find two distinct abundance distributions for the inner and outer parts of the Galactic disk, with the stars farther away from the Galactic center having relatively lower abundances by roughly 0.3 dex than the inner disk stars. A direct comparison of our stellar N, O, and S non-lte results with abundances from H ii regions associated with the OB targets was possible in five cases. When the nebular abundance results from the different studies in the literature are averaged, it is found that the stellar abundances are similar to the nebular abundances, with typical differences of dex (similar to what is expected from the uncertainties in the abundance analyses themselves). However, there are systematic differences between the different nebular studies. It is perhaps significant that in 8 out of 11 cases, there is at least one nebular abundance result in the literature that is lower than the associated stellar abundance. For sulfur, in particular, the stellar abundances are found to be larger than the nebular results. These slightly higher stellar abundances than gas abundances are in general agreement with very modest depletion into dust grains for the volatile elements. We thank Celso Batalha and Cláudio Bastos Pereira for observations in service mode of targets in Sh and NGC 1893, respectively, and Rolando Vega for technical support during the observing run of 1999 November. S. D. thanks CAPES for a Ph.D. fellowship grant. We thank the DPG/ON for supporting the observations at La Silla, Chile. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. Andrievsky, S. M., Kovtyukh, V. V., Luck, R. E., Lépine, J. R. D., Maciel, W. J., & Beletsky, Yu., V. 2002b, A&A, 392, 491 Andrievsky, S. M., et al. 2002a, A&A, 381, 32 Asplund, M. 2003, in ASP Conf. Ser. 304, CNO in the Universe, ed. C. Charbonnel, D. Schaerer, & G. Meynet (San Francisco: ASP), 275 Caplan, J., Deharveng, L., Peña, M., Costero, R., & Blondel, C. 2000, MNRAS, 311, 317 Caputo, F., Marconi, M., Musella, I., & Pont, F. 2001, A&A, 372, 544 Daflon, S., Cunha, K., & Becker, S. 1999, ApJ, 522, 950 (Paper I) Daflon, S., Cunha, K., Becker, S., & Smith, V. 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