For eight distant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75, NGC 6192, NGC 6404, and NGC 6583, we determined the yttrium and barium abundances using the UVES, VLT spectra (ESO, Chile). The stars of one young cluster (Ruprecht 7) demonstrate significant barium overabundance( 3c0.55 dex) that can not be due to the determination error. We have considered the Ba abundance determination errors due to LTE approach, saturation of the lines, synthetic and observed barium line fitting, and the causes of the Ba overabundance associated with the Galactic disc enrichment or the origin of open clusters. Possible explanation for this overabundance can be the origin of n-capture elements enrichment of the clusters (galactic or extragalactic) or additional sources of the Ba production

Carraro, Giovanni

Korotin, S. A.

Kovtyukh, V. V.

Mishenina, T. V.

Yegorova, I. A.

Journal of Physics : Conference Series

English

For eight distant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75, NGC 6192, NGC 6404, and NGC 6583, we determined the yttrium and barium abundances using the UVES, VLT spectra (ESO, Chile). The stars of one young cluster (Ruprecht 7) demonstrate significant barium overabundance(∼0.55 dex) that can not be due to the determination error. We have considered the Ba abundance determination errors due to LTE approach, saturation of the lines, synthetic and observed barium line fitting, and the causes of the Ba overabundance associated with the Galactic disc enrichment or the origin of open clusters. Possible explanation for this overabundance can be the origin of n-capture elements enrichment of the clusters (galactic or extragalactic) or additional sources of the Ba production

CARRARO, GIOVANNI

Archivio istituzionale della ricerca - Università di Padova

Original Citation:On the subject of the Ba overabundance in the open clusters starsInstitute of Physics PublishingPublisher:Published version:DOI:Terms of use:Open Access(Article begins on next page)This article is made available under terms and conditions applicable to Open Access Guidelines, as described athttp://www.unipd.it/download/file/fid/55401 (Italian only)Availability:This version is available at: 11577/3211839 since: 2016-11-26T22:51:25Z10.1088/1742-6596/665/1/012025Università degli Studi di PadovaPadua Research Archive - Institutional RepositoryThis content has been downloaded from IOPscience. Please scroll down to see the full text.Download details:IP Address: 147.162.26.126This content was downloaded on 22/11/2016 at 15:39Please note that terms and conditions apply.You may also be interested in:CHEMICAL ABUNDANCES OF MEMBER STARS IN THE OPEN CLUSTER NGC 2632 (PRAESEPE)X. L. Yang, Y. Q. Chen and G. ZhaoOn the subject of the Ba overabundance in the open clusters starsView the table of contents for this issue, or go to the journal homepage for more2016 J. Phys.: Conf. Ser. 665 012025(http://iopscience.iop.org/1742-6596/665/1/012025)Home Search Collections Journals About Contact us My IOPscienceOn the subject of the Ba overabundance in the openclusters starsT V Mishenina1, S A Korotin1 , G Carraro2;3, V V Kovtyukh1, and IA Yegorova21 Astronomical Observatory of Odessa National University, Ukraine2 European Southern Observatory, Chile3 Dipartimento di Fisica e Astronomia, Universita di Padova, ItalyE-mail: tmishenina@ukr.netAbstract. For eight distant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25,Berkeley 73, Berkeley 75, NGC 6192, NGC 6404, and NGC 6583, we determined the yttriumand barium abundances using the UVES, VLT spectra (ESO, Chile). The stars of one youngcluster (Ruprecht 7) demonstrate signicant barium overabundance(0.55 dex) that can notbe due to the determination error. We have considered the Ba abundance determination errorsdue to LTE approach, saturation of the lines, synthetic and observed barium line tting, andthe causes of the Ba overabundance associated with the Galactic disc enrichment or the originof open clusters. Possible explanation for this overabundance can be the origin of n-captureelements enrichment of the clusters (galactic or extragalactic) or additional sources of the Baproduction.1. IntroductionYttrium and barium are the neutron capture elements, and at the solar metallicity, the mainsources of the n-capture element enrichment with interstellar and pre-stellar medium are theAGB stars of the small and moderate masses. The production of the n-capture elementsand their presence in the stars of dierent ages and location in the Galaxy play importantrole in the modeling of the Galactic chemical evolution. Among them barium is a particularneutron-capture element, it shows a remarkable overabundance with respect to iron in someopen clusters and disc stars of the Galaxy. This fact still has no clear interpretation, but thereare some possible reasons: 1) determination of reliable barium abundance (e.g. under the NLTEapproximation, taking the HFS into account, the saturation of the Ba lines); 2) dependence ofthe barium abundances on the age in the galactic disc; 3) additional sources of production ofbarium; 4) the origin of the open clusters. In this study we try to clarify the questions of theBa overabundance. To do that we measured the abundance of yttrium and barium in eightdistant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75,NGC 6192, NGC 6404, and NGC 6583.2. Observation and atmospheric parametersFor our study, we used the spectra obtained with the echelle spectrograph UVES on board VLTwith resolving power R = 40,000 for the wavelength range 4750{6800 A by [1] and [2]. In orderNuclear Physics in Astrophysics VI (NPA6) IOP PublishingJournal of Physics: Conference Series 665 (2016) 012025 doi:10.1088/1742-6596/665/1/012025Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd 1to check the atmospheric parameters obtained in [1], we compared the equivalent widths EWsof the lines and the temperatures derived by [1] with our data for two stars Ruprecht 4 3 andRuprecht 7 2. We measured the EWs using the DECH20 software package [3]. The eectivetemperatures were computed based on the calibration of the central depths ratio of the lineswith dierent potentials for the lower excitation level [4]. We have good agreement for both theequivalent widths and eective temperatures. And then we used the atmospheric parametersobtained by [1] and [2] to determine the yttrium and barium abundances in the studied stars.3. Abundance determinationThe models by [5] were used to determine the Y and Ba abundances. The LTE yttriumabundance was obtained with a new version of the STARSP software package [6] and the VALDatomic data [7] using three lines of Y II (4854.873, 4883.690 and 5087.426 A). The adoptedsolar yttrium abundance is (Y/H) = 2.24, where log A(H) = 12. The barium abundance wascomputed under the non-local thermodynamic equilibrium (NLTE) approach with a version ofMULTI code [8], modied by S.A. Korotin ([9]). To determine the barium abundance, we usedthree lines of Ba II (5853, 6141 and 6496 A). For line 6496 A we took the HFS into account.The HFS for other lines (5853 and 6141 A) is not signicant. Departures from the LTE do notsignicantly aect Ba abundances (the NLTE correction value is about 0.1 dex).-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0050100150200250300  EW (mA)lg A(Ba)6496 AFigure 1. The growth curve forthe barium line 6496 A.5853.4 5853.80.30.40.50.6RELATIVE FLUX6141.6 6141.80.30.40.50.6Rup 7 #46496.7 6496.9 λ0.30.40.50.6(A)Figure 2. The observed and synthetic spectra ttingwith dierent barium abundances  0.1 dex.3.1. Determination errorsSince the equivalent widths of barium are strong enough in the spectra of the studied stars,the line saturation can provoke a low accuracy of the barium content determination. Figs.1 and 2 show the growth curve for the barium line 6496 A and the observed and syntheticspectra tting. We can see from these Figures that the equivalent widths and proles are rathersensitive to the barium abundance. The lines with EW up to 200 mA are very sensitive tothe element abundance, while the stronger lines, obtained by the computed synthetic spectra,allow us to obtain abundance values with an accuracy of 0.1 dex. In this case, the totalerror of determination of the Y and Ba abundances does not exceed 0.2 dex accounting for theuncertainty in the stellar parameters and the accuracy of the synthetic spectrums tting.4. ResultsThe dependence of [Y/Fe] and [Ba/Fe] from the age is shown in Figs. 3, 4. The Y abundancesby [10] and Ba abundances by [11] are marked as black circles; those by [12] and [13] are markedas magenta triangles; those by [14] are marked as blue triangles; those by [15] are marked asgreen triangles; the Ba abundances by [16] as yellow triangles and by [17] as asterisks; by [18]- the thin disc (marked as black dots); for the Cepheids, the average values <[Y/Fe]> by [19]Nuclear Physics in Astrophysics VI (NPA6) IOP PublishingJournal of Physics: Conference Series 665 (2016) 012025 doi:10.1088/1742-6596/665/1/0120252and <[Ba/Fe]> by [20] are marked as empty circles, and those by the present study are markedas red circles.0 2 4 6 8 10-0,50,00,51,0  [Y/Fe]Age (Gyr)Figure 3. [Y/Fe] vs. Age.0 2 4 6 8 10-0,50,00,51,0  [Ba/Fe]Age ( Gyr)Figure 4. [Ba/Fe] vs. Age.As it is seen in Fig. 3, there is a slight trend for yttrium (from -0.2 to 0.2 dex) with increasingage. As one can see, this trend is similar for the clusters and the thin disc that supports theidea of a similarity of origin of the stars of the thin disc and open clusters. And also it can beindicative of the increase in contributions into the enrichment of the interstellar medium fromthe least massive AGB stars (M < 1.5 M). For barium (Fig. 4), we observe a signicant trendand a large spread of the values. The obtained average values for two youngest clusters Ruprecht4 and Ruprecht 7 in our example (with ages about 0.8 Gyr) are <[Ba/Fe]> = 0.33 and 0.56 dex,respectively. To assess the extent to which those values are consistent with the values of bariumcontent in young stars, we compared the data of the yttrium and barium abundances for youngerclusters with data of Galactic Cepheids (the disc young stars with ages around 0.2{0.4 Gyr).Good agreement is observed for the yttrium, but the average values of barium in the Cepheids( 0.2 dex, [20]) are signicantly lower than 0.6 dex. In (Figs. 5, 6) we show the behaviour of theyttrium and barium abundances in young clusters and Cepheids with galaxy-centric distances.Cepheids conrms the existence of the yttrium abundance gradient in the Galactic disc. Behaviorof the barium diers from that of yttrium - the barium abundance doesn't show any trend withmetallicity for the Cepheids, and four clusters exhibit signicant barium overabundances thatexceed the spread of values ([Ba/H] > 0.5 dex) in the Cepheids. Those are IC 2391, NGC 2324,IC 2602 [11] and Ruprecht 7 (our determinations). The overabundance, especially for Ruprecht7, remains an open question. It is not due to deviations from the LTE, since in our calculationswe took those eects into account. Moreover, as shown by [14], this overabundance cannotbe related to the chromospheric activity. The assumption that all these stars are the stars ofbarium class stars is also doubtful. We note that Ruprecht 7 has an abnormal orbit according to[21], see Fig. 7. However, the accuracy in the orbit determination (large proper motion errorsfor distant clusters and the adopted xed Galactic potential) does not allow us to consider theresult as reliable and it requires further investigation.5. ConclusionsWe found that the barium overabundance is real and it is not the result of determination errors.The origin of open clusters or the presence of an alternative way of enrichment (not AGB stars)could be possible reasons of that overabundance. New accurate determinations of barium andneutron-capture elements are needed.Nuclear Physics in Astrophysics VI (NPA6) IOP PublishingJournal of Physics: Conference Series 665 (2016) 012025 doi:10.1088/1742-6596/665/1/01202534 6 8 10 12 14 16 18-0,8-0,6-0,4-0,20,00,20,40,6  [Y/H]RGC (kpc)Figure 5. [Y/H] vs. RGC4 6 8 10 12 14 16-0,6-0,4-0,20,00,20,40,60,81,0  [Ba/H]RGC (kpc)Figure 6. [Ba/H] vs. RGC0 2 4 6 8 10 12 14 16 18-0,50,00,51,0  [Ba/Fe]Zmax (kpc)Figure 7. [Ba/Fe] vs. ZmaxAcknowledgmentsT.M. and S.K. thank for the support from the Swiss National Science Foundation, projectSCOPES No. IZ73Z0-12References[1] Carraro G Geisler D Villanova S et al 2007 A&A 476, 217[2] Magrini L Randich S and Zoccali M et al 2010, A&A, 523, id.A11[3] Galazutdinov G A 1992 Preprint SAO RAS., 92, 96[4] Kovtyukh V V Soubiran C Belik S I and Gorlova N I 2003, A&A 411, 559[5] Castelli F and Kurucz R L 2004 arXiv:astro-ph/0405087[6] Tsymbal V V 1996 ASP Conf. Ser., 108, 198[7] Kupka F Piskunov N E Ryabchikova T A et al 1999 A&AS, 138, 119[8] Carlsson M 1986 Uppsala Obs. Rep., 33[9] Korotin S A Mishenina T V Gorbaneva T I and Soubiran C 2011 MNRAS, 415, 2093[10] Maiorca E Randich S Busso M et al 2011 ApJ, 736, 120[11] D'Orazi V Magrini L Randich S et al 2009 ApJ, 693, 31[12] Pancino E Carrera R Rossetti E and Gallart C 2010 A&A, 511, id.A56[13] Carrera R and Pancino E 2011 A&A 535, 30[14] D'Orazi V Biazzo K Desidera S et al 2012 MNRAS, 423, 2789[15] Reddy A B S Giridhar S and Lambert D L 2012 MNRAS, 419, 1350[16] Bragaglia A Sestito P Villanova S et al 2008 A&A, 480, 79[17] Yong D Carney B W and Teixera de Almeida M-L 2005 AJ, 130, 597[18] Mishenina T V Pignatari M Korotin S A et al 2013, A&A, 552, 128[19] Luck R E and Lambert D L 2011 AJ, 142, 136[20] Andrievsky S M Lepine J R D Korotin S A et al 2013 MNRAS, 428,[21] Gozha M L Borkova T V and Marsakov V A 2012 Astron. Letters, 38, 506Nuclear Physics in Astrophysics VI (NPA6) IOP PublishingJournal of Physics: Conference Series 665 (2016) 012025 doi:10.1088/1742-6596/665/1/0120254

On the subject of the Ba overabundance in the open clusters stars

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On the subject of the Ba overabundance in the open clusters stars

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