Chemical abundances of metal-poor stars as probes of neutron capture nucleosynthesis

Abstract

After more than six decades of the pioneering work of Burbidge, Burbidge, Fowler and Hoyle (1957), which first suggested the production channels and the evolution of neutron-capture elements, many details still remain far from being fully understood. As we go more and more towards the metal-poor regime, the abundances of heavy elements show larger and larger scatter that still can not be explained either by abundance estimates uncertainties or based on the existing theories of nucleosynthesis and evolution. The diverse abundance ratios of neutron-capture elements observed in metal-poor stars demand a comprehensive study of the origin and evolution of heavy elements and the abundance pattern exhibited by these elements. The problem can be best addressed through chemical composition studies of stars that exhibit high abundances of heavy elements. Prompted by this idea, we have undertaken to probe the origin and evolution of heavy elements produced by neutron-capture nucleosynthesis processes from a detailed chemical composition study by considering a large sample of potential metal-poor star candidates. Such studies are currently lacking. We have selected twenty potential metal-poor star candidates from various catalogues of metal-poor stars that include the following objects: BD+75 348, BD+09 3019, CD–27 14351, HD 87853, HD 145777, HD 147609, HD 154276, HD 238020, HE 0017+0055, HE 0308–1612, HE 0319– 0215, HE 0401–0138, HE 0507–1653, HE 0930–0018, HE 1005–1439, HE 1023–1504, HE 1153–0518, HE 1246–1344, HE 2144–1832 and HE 2339–0837. For this sample, the stellar atmospheric parameters, the effective temperature Tef f , the microturbulent velocity ξ, the surface gravity log g, and the metallicity [Fe/H] are derived from local thermodynamic equilibrium analyses using model atmospheres. The sample is found to cover metallicity in the range −0.28 to −3.5 and one object with near solar metallicity. Elemental abundances of C, N, O, α-elements, iron-peak elements, and several neutron-capture elements are estimated using the equivalent width measurement technique as well as spectrum synthesis calculations in some cases. In cases when the estimates of abundances could not be determined, the upper limits of abundances are estimated whenever possible. From a detailed chemical analysis, we have identified in our sample, one normal metal-poor star, five Ba stars, one CH star, six Carbon-Enhanced Metal-Poor (CEMP)-s stars, three CEMP-r/s stars, one CEMP-no star, two Extremely metal-poor stars, and one unique CEMP star that may be called as CEMP-i/s star. The results presented in this thesis are based on our investigations that include the first time chemical composition studies of twelve objects, updates on abundance estimates of several key elements based on high resolution (R ≥ 48,000) spectra and complemented by a comprehensive analysis of compiled literature abundance data of a few hundred objects. The high-resolution spectra were obtained using HCT/HESP, SUBARU/HDS and the Fiber-fed Extended Range Optical Spectrograph (FEROS). Among the CEMP stars, the so-called CEMP-r/s stars are known to exhibit enhancement of both slow (s-) and rapid (r-) process elements in their surface chemical composition. For these stars, the heavy element abundances cannot be explained either by s-process or r-process nucleosynthesis alone. These two processes produce distinct abundance patterns, as the production sites of s- and r-process elements are very different. Our analysis shows three objects in our sample, HE 0017+0055, HE 2144– 1832 and HE 2339–0837, to be bonafide CEMP-r/s stars. These three objects and the objects CD−27 14351, HE 0308–1612, HE 0319–0215, HE 0507–1653 and HE 0930– 0018 were previously claimed as potential CH star candidates based on low-resolution spectroscopy. To better understand their formation mechanisms using the chemical signatures, we have performed a detailed systematic follow-up spectroscopic study based on high-resolution spectra. In the context of the double enhancement observed in the CEMP-r/s stars, we have discussed all the possible formation scenarios and critically examined if the i-process (intermediate process) nucleosynthesis, that occurs at a high neutron-density (n ∼1015cm−3) could explain the observed abundance patterns. The required high neutron density can be attained during proton-ingestion from a H-rich envelope to the intershell region of an AGB star and is capable of producing both r- and s-process elements in a single stellar site. We have further extended this study to a sample of eight CEMP-r/s stars from literature to trace the origin of the observed double enhancement. Our analysis shows that the observed abundance patterns of all the CEMP-r/s stars, under this study, could be reproduced fairly well using the i-process model yields. Among the CEMP stars, distinguishing the CEMP-s and CEMP-r/s stars is crucial in order to understand the physical and nucleosynthetic processes responsible for the abundance patterns of the two subclasses. The CH, CEMP-s and CEMP-r/s stars in our sample show enhanced abundance of Ba and seven of them exhibit enhanced abundance of Eu ([Eu/Fe] > 0.70). Based on our analysis, while three stars are found to show characteristic properties of CEMP-r/s stars, HE 0308–1612 shows characteristic properties of CH stars, and the rest show characteristic properties of CEMP-s stars. However, the object HD 145777 is found to fall into different subclasses when classified based on the different classification schemes put forward by different groups of authors. To remove the uncertainties and clear the confusion regarding its classification, we have revisited the classification schemes of CEMP-s and CEMP-r/s stars found in the literature. From a detailed investigation of different classifiers of CEMP stars, we have seen that none of the existing classification criteria could clearly distinguish the CEMP-s and CEMP-r/s stars. We have put forward a new classification scheme to distinguish the two subclasses. We found that for both CEMP-s and CEMP-r/s stars, [Ba/Eu] and [La/Eu] exhibit positive values and [Ba/Fe] ≥ 1.0. However, CEMPr/s stars satisfy [Eu/Fe] ≥ 1.0, 0.0 ≤ [Ba/Eu] ≤ 1.0, and/or 0.0 ≤ [La/Eu] ≤ 0.7. CEMP-s stars normally show [Eu/Fe] < 1.0 with [Ba/Eu] > 0.0 and/or [La/Eu] > 0.5. If [Eu/Fe] ≥ 1.0, then the condition on [Ba/Eu] and/or [La/Eu] for a star to be a CEMP-s star is [Ba/Eu] > 1.0 and/or [La/Eu] > 0.7. Using a large sample of CEMP-s and CEMP-r/s stars from the literature we have examined whether the ratio of heavy-s to light-s process elements [hs/ls] can be used as a classifier. In particular, we have examined if there are any limiting values for [hs/ls] that can be used to distinguish between CEMP-s and CEMP-r/s stars. We have critically examined the heavy elements’ abundance ratios and found that, the CEMP-s and CEMP-r/s stars peak at different values of [hs/ls]. However, there is an overlap in the range 0.0 < [hs/ls] < 1.5, and hence, this ratio cannot be used to distinguish between CEMP-s and CEMP-r/s stars. We have noticed a similar overlap in the case of [Sr/Ba] ratios as well in the range −1.6 < [Sr/Ba] < −0.5, and hence this ratio also cannot be used to separate the two subclasses. Understanding the surface chemical composition of CEMP stars with enhanced abundances of heavy elements still remained problematic. The peculiar abundance pattern observed in the carbon-enhanced extremely metal-poor (EMP) object HE 1005–1439 in our sample was surprisingly found to be enriched with products of both s-process and i-process nucleosynthesis. Observation of such a unique abundance pattern is the first of its kind and certainly points to the discovery of a new class of objects. Nevertheless, understanding the origin of the surface chemical composition of this object posed a real challenge. To address this problem, we have performed a detailed high-resolution spectroscopic analysis of the object using SUBARU/HDS spectra at a resolution of (R ∼ 50,000). We have measured the abundances of all the elements that we could estimate for this object. The elemental abundance estimates include ten light elements from C through Ni and twelve heavy elements Sr, Y, Ba, La, Ce, Pr, Nd, Eu, Dy, Er, Hf, and Pb. In order to understand the origin of the observed abundance pattern, we have performed a parametric-model-based analysis of the abundances of the heavy elements. The results of our analysis clearly showed that the surface chemical composition of the object has contributions from both the s-process and i-process nucleosyntheses. We have critically examined the observed abundances and carefully investigated the formation scenarios involving the s-process and the i-process that are available in the literature. We found that neither s-process nor i-process nucleosynthesis alone could explain the observed abundances. We have noted that the radial velocity estimates obtained from several epochs showed variations, and that may be an indication of the presence of a binary companion. Hence, considering a binary system, we proposed a formation scenario for this object involving effective proton ingestion episodes (PIEs) triggering i-process nucleosynthesis followed by s-process nucleosynthesis with a few third-dredge-up (TDU) episodes in the now extinct companion AGB star. Although based on the CEMP stars’ classification criteria, the star is found to belong to CEMP-s category; the observed abundance pattern could not be explained based on the theoretical s-process model predictions. On the contrary, our parametric-model based analysis clearly indicates that the surface chemical composition of the object is influenced by similar contributions from both the s- and i-processes. Among the fundamental stellar parameters, the mass of a star is the most fundamental parameter and plays a crucial role in its structure, evolution and final fate. We have estimated the masses of the barium stars and the masses of the primary companions of both the barium and the CEMP-s stars in our sample. All the stars show enhanced abundances of heavy elements. We have classified five objects BD+75 348, BD+09 3019, HD 147609, HD 154276 and HD 238020, as Ba stars and CD–27 14351, HD 145777, HE 0319–0215, HE 0507–1653, HE 0930–0018 and HE 1023–1504 as CEMP-s stars. Out of the five Ba stars, HD 154276 and HD 238020 are found to be mild Ba stars, and the other three as strong Ba stars. Using the HR diagram, we have estimated the mass distribution of Ba stars, compiling the data of 205 Ba stars found in the literature along with the barium stars studied here. The average value of the mass distribution is found to be at 1.9 M . We have also estimated the initial masses of the companion AGBs of the programme stars with the help of a parametric-model-based analysis using FRUITY models. The same analysis is used to estimate the masses of the companion AGBs of 159 Ba and 36 CEMP-s stars found in the literature. The mass distribution of companion AGBs of Ba stars shows different peaks for mild and strong Ba stars. While the primary mass distribution of mild Ba stars peaks at 3.7 M , for the strong Ba stars, the peak appears at 2.5 M . We, therefore, propose that the initial masses of the progenitor AGBs of the Ba stars dominantly control the formation of the mild and the strong Ba stars. However, there is a clear overlap between the progenitor masses of both the subclasses of Ba stars in the mass range 1.3 to 4.0 M . So, the contributions of other factors such as metallicities and the orbital periods in the formation of the mild and the strong Ba stars may not be negligible. The progenitor AGBs’ mass distribution of CEMP-s stars is found to peak at 2.04 M . The main results obtained from the above studies appeared in parts in several research papers: Purandardas et al., MNRAS, 486, 3266 (2019), Shejeelammal et al., MNRAS, 492, 3708 (2020), Goswami & Goswami, JAA, 41, 47 (2020), Goswami et al., A&A, 649, A49 (2021), Goswami & Goswami, A&A, 657, A50 (2022), Goswami & Goswami, AJ, (2022, Accepted for publication).