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Investigating the Abundance Enrichment Pattern of Heavy Elements 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 star 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 solar system, however, the value is not sufficiently high to favor the formation of a lead 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 stars, which indicates that the r-process contributions is critical during heavy element enrichment. This star has a metallicity of [Fe/H] = −1.34, which is larger than that of Galaxy halo CEMP-r/sstars.The reason may be because of the different history of metallicity enrichment between the SMC and the Galaxy halo. stellar nucleosynthesis, 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 accretion-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 asymptotic giant branch (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 gravity 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.
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