SCIENCE CHINA Physics, Mechanics & Astronomy

. Article . June 2014 Vol.57 No. 6: 1201–1205 doi: 10.1007/s11433-014-5475-4

Investigating the abundance enrichment pattern of heavy elements in the only observed CEMP-r/s J004441.04-732136.4 of the SMC† CUI WenYuan1,2*, ZHANG Bo2 &ZHAOGang1,3

1School of Space Science and Physics, Shandong University at Weihai, Weihai 264209, China; 2College of Physics Science and Information Engineering, Hebei Normal University, Shijiazhuang 050024, China; 3National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China

Received March 19, 2014; accepted March 31, 2014; published online April 15, 2014

The post-AGB star J004441 is the first and the only one CEMP-r/s star found in SMC. Herein, we investigate the observed abun- dance pattern of the heavy elements using our parametric model. A consistent fitting results was obtained for the sample star. Based on the low r = 0.08, the s-process nucleosynthesis occurred in the interior is supposed to belong to the single neutron-exposure τ = . T9 1/2 −1 ffi event. The median value of 0 0 44( 0.348 ) mbarn supports a higher e ciency of the s-process nucleosynthesis relative to J004441 than that of the , however, the value is not sufficiently high to favor the formation of a star. Thus, J004441 does not belong to lead star group. The large Cs value of J004441 supports the intrinsic characteristic of the s-enrichment. The Cr value is similar with that found in halo CEMP-r/s , which indicates that the r-process contributions is critical during heavy element enrichment. This star has a of [Fe/H] = −1.34, which is larger than that of halo CEMP-r/sstars.The reason may be because of the different history of metallicity enrichment between the SMC and the Galaxy halo.

, abundances, chemically peculiar stars

PACS number(s): 26.20.+f, 97.10.Cv, 97.10.Tk, 97.30.Fi

Citation: Cui W Y, Zhang B, Zhao G. Investigating the abundance enrichment pattern of heavy elements in the only observed CEMP-r/s star J004441.04-732136.4 of the SMC. Sci China-Phys Mech Astron, 2014, 57: 1201–1205, doi: 10.1007/s11433-014-5475-4

1 Introduction [1]. The site for the r-process, however, is unclear. Although there are many suggestions such as -induced col- lapse [2], the mergers of neutron stars [3], and Type 1.5 su- Neutron-capture process nucleosynthesis is responsible for pernovae [4], the ν-driven wind of Type II supernovae (here- the production of elements heavier than iron, which is divided after SN II) is still regarded as the most possible candidate into slow (s)-process and rapid (r)-process, depending on the [5]. Almost all heavy elements are produced by both the s- competition between the β decay and the next neutron cap- and r-process. For example, the s-process contributes about ture event. These two neutron-capture process need to occur 85 percent of solar Ba and 3 percent of solar Eu, which are in different astrophysical sites because of the different physi- usually regarded as the represent elements of the s- and r- cal conditions they required. The s-process primarily occurs process, respectively [6]. in low- and intermediate-mass (M < 7M) stars when they In low-mass AGB stars, the main neutron source is the evolve through their (AGB) phase 12C(p,γ)13N(β)13C(α, n)16O reaction, which has been con- firmed by theory and observations [7,8]. The 13C-pocket is *Corresponding author (email: [email protected]) formed in a narrow region of the top of He-rich intershell †Recommended by HAN ZhanWen (Associate Editor)

c Science China Press and Springer-Verlag Berlin Heidelberg 2014 phys.scichina.com link.springer.com 1202 Cui W Y, et al. Sci China-Phys Mech Astron June (2014) Vol. 57 No. 6 between the H-rich convective envelope and the degenerate abundance profile of this star. Herein we present the com- carbon core during the interpulse phases. Many mechanisms parisons between our calculated results and the observational have been suggested to explain how the proton are mixed into abundances of the post-AGB star J004441. the He intershell from the bottom of the H-rich convective en- velope, such as the partial mixing of protons, rotation shear, 2Results partially activated and the effect of internal wave mix- ing [8–11]. However, the physical mechanisms of both the In order to investigate the s-process nucleosynthesis related to third dredge-up and the partial mixing in the intershell are the SMC post-AGB star J004441 and understand the neutron- still poorly understood. capture process in the metal-poor conditions well, we have The 13C neutron source has the primary-like characteris- studied the abundance pattern of the sample star using the tic, which indicates the 13C production is independent of the parametic model for low-mass AGB stars [22,23]. In the metallicity. The s-process nucleosynthesis, however, is pre- model, the theoretical abundance Ni of the ith element was dicted to depend strongly on metallicity, because the den- calculated based on the following formula with the r-process sity of iron seed nuclei decreased with the metallicity, then contributions considered, more neutrons are available for each iron seed in metal- = + [Fe/H], deficient environments ([Fe/H] < −1). It can be expected that Ni(Z) CsNi, s Cr Ni, r10 (1) high overabundances of Pb with respect to other s-elements where Z is the metallicity of the star, N , and N , are the need to be produced, which locates at the terminal point of i s i r abundances of the ith element produced by the s- and r- the s-process nucleosynthesis path [8,12]. Although some process (per Si = 106 at Z = Z) respectively, C and C are metal-poor objects with strong Pb enhancement have been s r the component coefficients representing the contributions of observed, others without strong Pb overabundance have also the s- and the r-process respectively. Because of the large Eu been reported simultaneously [13,14]. Much research effort overabundance of J004441, we assumed that it was formed has been done in order to explain the large spread of Pb over- from a gas cloud that had been polluted by SN II with r-rich abundance found in metal-poor stars [9,15]. material. In the model, larger value of C indicates larger Post-asymptotic giant branch (post-AGB) stars are the fi- r contributions from r-process for the abundance enrichment nal evolution of low- and intermediate-mass stars. When a pattern. During the calculations, we use the observed abun- star evolves in the AGB phase, the s-process material synthe- dances of J004441 as the constraint, and obtain the best fitting sized interior is dredged up to the surface, then it shows in- results based on a minimum value of χ2. trinsiclly s-enriched even in the post-AGB. More studies on The best fitting results for J004441are presented in Fig- the post-AGB are needed in order to shed some light on the ure 1, in which it can be seen that all observed abundances s-process nucleosynthesis. Currently, J004441.04-732136.4 of heavy elements in J004441 are fitted well in the error (hereafter J004441) is the only one post-AGB found in the ranges. Even the up limit of Pb is included, the parametric Small Magellanic Cloud (SMC) and with a detailed abun- model is consistent. Moreover, the validity of the physical dances reported [16]. J004441 shows strong enhancements of both [La/Fe] = 2.84 ± 0.32 and [Eu/Fe] = 1.93 ± 0.24 with a low metallicity of [Fe/H] = −1.34 ± 0.32. La usually is re- ferred as a representative element of the s-process. Combin- ing with the strong carbon enhancement [C/Fe] = 1.67±0.36, 4

J004441 belongs to the group named carbon-enhanced metal- La poor (CEMP)-r/s star [17]. The abundance distribution of 3 the heavy elements in J004441 has been compared with two Y Pb theoretical model results, that is, STAREVOL code [18] and

Cristallo’s model [16,19]. Most of the light and heavy ele- 2 ment abundances can been predicted by both models, how- [X/Fe] ever, the lower abundances of the r-process elements such as Eu Δτ = 0.6 (mbarn−1) r = 0.08 Eu, Gd, Er and higher Pb were predicted than the observed 1 Cr = 0.08 abundances of J004441 [16]. Cs = 0.0115 It is still unclear that how the protons diffuse from the χ2 = 0.58697 bottom of the convective envelope into the He intershell to 0 form the 13C-pocket [9]. Thus, the parametric model is still 30 40 50 60 70 80 90 Z valid in studying the abundance profile of the s-process pecu- liar stars [20,21]. In this paper, using the parametric model Figure 1 Best fit to observational results of metal-deficient star J004441. we restudy the abundance distribution of heavy elements in Filled circles with appropriate error bars and downward arrows denote the observed element abundances, the solid lines represent predictions from s- J004441 [22,23] in order to investigate what is the neces- process calculations, in which the r-process contribution is considered simul- sary physical conditions to reproduce the observed s-process taneously. Standard unit of Δτ is mbarn−1. Cui W Y, et al. Sci China-Phys Mech Astron June (2014) Vol. 57 No. 6 1203 parameters obtained by the AGB model calculations is Δτ, which are calculated by the parametric model with other strongly supported. In the AGB model, a fundamental pa- physical parameters fixed such as r = 0.08, Cs = 0.0115 rameter is the overlap factor r, which represents the fraction and Cr = 51.7. The observed abundance with proper errors of material in the He intershell that could experience subse- of the sample star are also plotted in Figure 2 for compar- quent neutron exposures. At the solar metallicity, the r value ison. From Figure 2, we can see that the abundance ratios is reported between 0.4and0.7 based on a low-mass (1.5– [Y/Fe] and [La/Fe] are sensitive to Δτ,and[Eu/Fe] is some- 3.0M) AGB model [8]. The r value deduced for J004441 is what sensitive. It can be seen that there is only one over- . Δτ = . +0.01 0 08, which is lower than the solar ones. As described else- lap region in Figure 2, that is, 0 60−0.09, in which all where [20,21], such small r values r  0.1 are also reported observed abundance ratios referred above can be fitted well. for some halo s-rich stars including both CEMP-s and -r/s It can be noted that the minimum value of χ2 is 0.58697 at stars. Δτ = 0.60 mbarn−1, and the physical parameter Δτ is con- Another fundamental parameter is the neutron exposure strained consistently. per pulse Δτ,whichis0.6mbarn−1 deduced for J004441. This value is almost consistent to the mean Δτ value 0.63, which is obtained from 24 metal-poor halo stars including 3 Discussion 15 CEMP-r/s and eight CEMP-s stars [21]. The mean neu- Based on the formation mechanisms, the s-rich stars are di- tron exposure is denoted as τ = −Δτ/lnr.Thevalueofτ 0 0 vided into two groups, that is, extrinsic and intrinsic one. For is τ = 0.44( T9 )1/2 mbarn−1 (T = 0.1, in units of 109 0 0.348 9 the intrinsic group, their s-process material is directly syn- K) deduced for J004441, which larger than the solar value thesized in interior of themselves, and then dredged up to the τ = (0.30 ± 0.01)( T9 )1/2 mbarn−1 [24]. This value lies 0 0.348 surface, thus they look s-enriched. For example, the low- and in the range from 0.15 to 4.64 deduced for metal-poor stars intermediate-mass stars belong to the intrinsic group when [21]. The physical parameter C represents the dilution de- s they fast evolves into their AGB or post AGB phase. The gree of the observed abundance for the s-process material number of this group is small, because of the short timescale compared with that in the region of s-process nucleosynthe- of AGB and post AGB evolution phase. The extrinsic group sis. The C value obtained for the post-AGB star J004441 is s forms by the pollution through mass transfer from their mas- 0.0115, which is larger than any one of the values obtained sive AGB companion in a binary system. Most of the Galaxy for the metal-poor halo s-rich stars [21]. The physical param- halo s-rich stars belong to the extrinsic group. eter Cr represents the enrichment times of r-process contribu- It can be seen that the post-AGB star J004441 belongs to tions in the sample star relative to the initial abundance where the intrinsic group of s-rich stars. The small r value, that is, the sample star formed. The Cr value deduced for J004441 r = 0.08, indicates that the s-process nucleosynthesis in this is 51.7, which lies between the obtained range from 17.5 to star belongs to the single neutron exposure event. This indi- / 86.4 for halo CEMP-r s stars and is close to the associated cates that the observed abundance pattern of the s-process in typical value of about 60.0 [21]. J004441 is produced almost by the first neutron exposure, and The uncertainty of the obtained physical parameters for then a more efficient dredge-up event occurs. Almost all ma- the s-process in the post-AGB star J004441 need to be in- terial in the s-process nucleosynthesis region is dredged up vestigated in order to get the reliable conclusions. Figure 2 to the surface, and the next neutron exposure event can not shows the predicted abundance ratios [X/Fe] and the reduced change the abundance pattern any longer. In fact, a more effi- χ2 values as a function of the neutron exposure per pulse cient third dredge-up corresponding to a very small r values, could be expected at low metallicity environment, because of the low mass-loss rate of star wind and then the large core 3 2 mass of AGB star [20,25]. The single neuron-exposure event 1 is also found in SMC stars. 0 4 The large neutron exposure per pulse Δτ and mean neu- 3 τ 2 ron exposure 0 compared to those of solar one show that 1 the s-process nucleosynthesis occurred in the post-AGB star 3 ffi 2 J004441 has higher e ciency than that in the solar system,

[Eu/Fe] [La/Fe]1 [Y/Fe] which supports that more heavier elements such as Ba, La 40 and Pb that need to be produced. This results is also con-

2 20 χ sistent with the theory for the s-process nucleosynthesis that 0 −0.2 0.40.20.0 0.80.6 1.0 1.2 1.4 1.6 more heavier elements need to be produced at low metal- Δτ licities [8]. However, it should be noted that the τ0 value Figure 2 Abundance ratios [X/Fe] of the post-AGB star J004441 (solid is lower than those obtained for most of halo CEMP-s and 2 curves) and and reduced χ (bottom) as a function of the neutron expo- -r/s stars, which is primarily because of its low r value [21]. sure per pulse Δτ computed by a model with Cr = 51.7, Cs = 0.0115, and r = 0.08. Observed abundances of J004441 (solid straight and dashed lines) Based on the theory of s-process nucleosynthesis, more ef- are also presented for comparing [16]. ficient s-process may aid in producing lead stars, which is 1204 Cui W Y, et al. Sci China-Phys Mech Astron June (2014) Vol. 57 No. 6 denoted as stars [Pb/hs] > 1 where ‘hs’ referred as Ba, La r/s star found in SMC up to now. This also belongs to the and Ce [8,12]. The corresponding τ0 value should be larger small group of intrinsic s-rich stars. than 1.0 for lead stars, for example, τ0 = 1.26 for lead stars A consistent fitting of the results has been obtained for HD 196944 [21]. The value obtained for the post-AGB star J004441 using the parametric model, which strongly supports τ = . T9 1/2 −1 ffi the corrections of the obtained physical parameters. Based J004441, 0 0 44( 0.348 ) mbarn , is not su ciently high to support the forming of the lead stars. In fact, the value on the low r = 0.08, the s-process nucleosynthesis occurred of the abundance ratio [Pb/hs] is less than −0.11 in J004441 in its interior belongs to the single neutron-exposure event. τ = . T9 1/2 −1 [16,26]. Seemingly, J004441 does not belong to the lead star The median value of 0 0 44( 0.348 ) mbarn indicates group, which is consistent with our model results. that the efficiency of the s-process nucleosynthesis relative to The value of the s-process component coefficient Cs ob- J004441 is higher than that of the solar system because of the tained for the post-AGB star J004441 is 0.0115, which is low metallicity [Fe/H] = −1.34. However, this is not suffi- larger than that obtained for any one of the halo CEMP-s and ciently high to favor the formation of lead stars. This is the r/s stars. In fact, most of the CEMP-s and -r/s stars belong reason why J004441 does not belong to lead star group based to the extrinsic group of the s-rich stars. These binary have on its low [Pb/hs] = −0.11. The large Cs value of J004441 been confirmed by the observations [27]. As discussed else- supports the intrinsic characteristic of the s-enrichment. where [22], the Cs usually includes two parts, one represents The Cr value obtained for J004441 is similar with that the dilution in the convective envelope of an AGB star, and found in halo CEMP-r/s stars, which indicates that the the other represents the dilution in the convective envelope of r-process contributions is important during the history of the observed star when the accretion material from the AGB heavy-element enrichment. Then the possible formation stars arrived by the star wind. The lower value of Cs supports mechanism for J004441 is that it firstly forms in a gas cloud higher dilution encountered by the initial s-rich material in which then is polluted by one or a few SN II with r-rich the region of s-process nucleosynthesis. However, the post- material, and the s-material is synthesized in the interior AGB star J004441 belongs the intrinsic group of the s-rich and polluted by itself through the third dredge-up event dur- stars, which only encounters the dilution by the convective ing the AGB evolution phase. The special characteristic of / envelope during the AGB phase. Then, the large Cs value J004441 is that CEMP-r s star has the highest metallicity can be naturally explained. [Fe/H] = −1.34. The reason may be because of the differ- Based on the high abundance value of La and Eu ent history of metallicity enrichment between the SMC and ([La/Fe] = 2.84 ± 0.32, [Eu/Fe] = 1.93 ± 0.24), the post- the Galaxy halo. AGB star J004441 is also a CEMP-r/s star [16]. It is the only CEMP-r/s star found in SMC currently. The Cr value obtained for J004441 is 51.7, which is similar with the val- This work was supported by the National Natural Science Foundation of ues obtained from halo CEMP-r/s stars and larger than the China (Grant Nos. U1231119, 11273011, 11390371, 11003002, 11021504 typical values (< 3.0) obtained from halo CEMP-s stars [21]. and 10973016), the China Postdoctoral Science Foundation (Grant No. This indicates that the r-process contributions are also critical 2013M531587) and the Natural Science Foundation of Hebei Province compared with the s-process contributions for the abundance (Grant No. A2011205102). pattern formation of J004441. Thus, the most possible for- mation scenario for J004441 is that it firstly formed in a gas cloud which had been polluted by one or a few SN II with 1 Busso M, Gallino R, Wasserburg G J. Nucleosynthesis in asymptotic r-rich material, then passed through the AGB phase when the giant branch stars: Relevance for galactic enrichment and solar system s-rich material synthesized in its interior and dredged up to formation. Annu Rev Astron Astrophys, 1999, 37: 239–309 the surface by the third dredge-up event, and then the post 2 Qian Y Z, Wasserburg G J. Stellar sources for heavy r-process nuclei. Astrophys J, 2003, 588: 1099–1109 AGB phase as we observe currently. This is similar with the 3 Rosswog S, Davies M B, Thielemann F K, et al. Merging neutron stars: popular formation mechanism for the Galaxy halo CEMP-r/s Asymmetric systems. Astron Astrophys, 2000, 360: 171–184 stars. However, J004441 found in the SMC has a unique char- 4 Zijlstra A A. Low-mass supernovae in the early Galactic halo: Source acteristic, that is, with high metallicty [Fe/H] = −1.34 which of the double r/s-process enriched halo stars? Mon Not R Astron Soc, is larger than the typical [Fe/H] < −2.0 found 2004, 348: L23–L27 in Galaxy halo CEMP-r/s stars. The reason may be because 5 Woosley S E, Wilson J R, Mathews G J, et al. The r-process and neutrino-heated ejecta. Astrophys J, 1994, 433: 229–246 of the different history of metallicity enrichment between the 6 Burris D L, Pilachowski C A, Armandroff T, et al. Neutron-capture el- SMC and the Galaxy halo. ements in the early Galaxy: Insights from a large sample of metal-poor giants. Astrophys J, 2000, 544: 302–319 7 Abia C, Dom´ınguez I, Gallino R, et al. s-process nucleosynthesis in 4 Conclusions carbon stars. Astrophys J, 2002, 579: 817–831 8 Gallino R, Arlandini C, Busso M, et al. Evolution and nucleosynthesis In this paper, using our parametric model we investigate the in low-mass asymptotic giant branch stars. II. Neutron capture and the abundance pattern of the heavy elements observed in the post- s-process. Astrophys J, 1998, 497: 388–403 AGB star J004441, which is the first and the only one CEMP- 9 Straniero O, Gallino R, Cristallo S. s process in low-mass asymptotic Cui W Y, et al. Sci China-Phys Mech Astron June (2014) Vol. 57 No. 6 1205

giant branch stars. Nucl Phys A, 2006, 777: 311–339 19 Cristallo S, Piersanti L, Straniero O, et al. Evolution, nucleosynthe- 10 Denissenkov P A, Tout C A. Partial mixing and formation of the 13C sis, and yields of low-mass asymptotic giant branch stars at different pocket by internal gravity waves in asymptotic giant branch stars. Mon metallicities. II. the FRUITY database. Astrophys J Suppl S, 2011, Not R Astron Soc, 2003, 340: 722–732 197: 17 11 Piersanti L, Cristallo S, Straniero O. The effects of rotation on s-process 20 Aoki W, Ryan S G, Norris E, et al. Neutron capture elements in s- nucleosynthesis in asymptotic giant branch stars. Astrophys J, 2013, process-rich, very metal-poor stars. Astrophys J, 2001, 561: 346–363 774: 98 21 Cui W Y, Shi J R, Geng Y Y. The study of s-process nucleosynthesis 12 Goriely S, Mowlavi M. Neutron-capture nucleosynthesis in AGB stars. basedonbariumstars,CEMP-sandCEMP-r/s stars. Astrophys Space Astron Astrophys, 2000, 362: 599–614 Sci, 2013, 346: 477–492 13 Van Eck S, Goriely S, Jorissen A, et al. More lead stars. Astron Astro- 22 Cui W Y, Zhang J, Zhu Z Z, et al. Investigation for the enrichment phys, 2003, 404: 291–299 pattern of the element abundances in r+s star He 0338 3 945: A special 14 Behara N T, Bonifacio P, Ludwig H G, et al. Three carbon-enhanced r-II star? Astrophys J, 2010, 708: 51–57 metal-poor dwarf stars from the SDSS. Chemical abundances from 23 Zhang B, Ma K, Zhou G D. Neutron-capture elements in the s-and r- CO5BOLD 3D hydrodynamical model atmospheres. Astron Astro- process-rich stars: Constraints on neutron-capture nucleosynthesis pro- phys, 2010, 513: A72 cesses. Astrophys J, 2006, 642: 1075–1081 15 Cui W Y, Zhang B. The origin of the lead-rich stars in the Galactic 24 K¨appeler F, Beer H, Wisshak K. s-process nucleosynthesis-nuclear halo: Investigation of model parameters for the s-process. Mon Not R physics and the classical model. Rep Prog Phys, 1989, 52: 945– Astron Soc, 2006, 368: 305–309 1013 16 De Smedt K, Van Winckel H, Karakas A I. Post-AGB stars in the SMC 25 Ma K, Cui W Y, Zhang B. Investigation of the single neutron exposure as tracers of : The extreme s-process enrichment of the model for the s-process: The primary nature of the neutron source. 21 µm star J004441.04-732136.4. Astron Astrophys, 2012, 541: A67 Mon Not R Astron Soc, 2007, 375: 1418–1422 17 Jonsell K, Barklem P S, Gustafsson B, et al. The Hamburg/ESO R- 26 De Smedt K, Van Winckel H, Kamath D, et al. The lead discrepancy process enhanced star survey (HERES). III. HE 0338-3945 and the for- in intrinsically s-process enriched post-AGB stars in the Magellanic mation of the r + s stars. Astron Astrophys, 2006, 451: 651–670 Clouds. Astron Astrophys, 2014, in press 18 Siess L. Evolution of massive AGB stars II. model properties at non- 27 Lucatello S, Tsangarides S, Beers T C. The binary frequency among solar metallicity and the fate of super-AGB stars. Astron Astrophys, carbon-enhanced, s-process-rich, metal-poor stars. Astrophys J, 2005, 2007, 476: 893–910 625: 825–832