The s-Process Nucleosynthesis in Extremely Metal-Poor Stars as the Generating Mechanism of Carbon Enhanced Metal-Poor Stars

The origin of carbon-enhanced metal-poor (CEMP) stars plays a key role in characterising the formation and evolution of the first stars and the Galaxy since the extremely-poor (EMP) stars with [Fe/H] \leq -2.5 share the common features of carbon enhancement in their surface chemical compositions. The origin of these stars is not yet established due to the controversy of the origin of CEMP stars without the enhancement of s-process element abundances, i.e., so called CEMP-no stars. In this paper, we elaborate the s-process nucleosynthesis in the EMP AGB stars and explore the origin of CEMP stars. We find that the efficiency of the s-process is controlled by O rather than Fe at [Fe/H] \lesssim -2. We demonstrate that the relative abundances of Sr, Ba, Pb to C are explained in terms of the wind accretion from AGB stars in binary systems.Comment: The Proceedings of the Nuclei in the Cosmos XIV, in pres

The Stellar Abundances for Galactic Archeology (SAGA) Database - Compilation of the Characteristics of Known Extremely Metal-Poor Stars

We describe the construction of a database of extremely metal-poor (EMP) stars in the Galactic halo whose elemental abundances have been determined. Our database contains detailed elemental abundances, reported equivalent widths, atmospheric parameters, photometry, and binarity status, compiled from papers in the recent literature that report studies of EMP halo stars with [Fe/H] < -2.5. The compilation procedures for this database have been designed to assemble the data effectively from electronic tables available from online journals. We have also developed a data retrieval system that enables data searches by various criteria, and permits the user to explore relationships between the stored variables graphically. Currently, our sample includes 1212 unique stars (many of which are studied by more than one group) with more than 15000 individual reported elemental abundances, covering all of the relevant papers published by December 2007. We discuss the global characteristics of the present database, as revealed by the EMP stars observed to date. For stars with [Fe/H] < -2.5, the number of giants with reported abundances is larger than that of dwarfs by a factor of two. The fraction of carbon-rich stars (among the sample for which the carbon abundance is reported) amount to ~30 % for [Fe/H] < -2.5. We find that known binaries exhibit different distributions of orbital period, according to whether they are giants or dwarfs, and also as a function of metallicity, although the total sample of such stars is still quite small.Comment: 24 pages, 10 figures, accepted by PASJ, final version. The SAGA database is available at http://saga.sci.hokudai.ac.j

Transition of the Stellar Initial Mass Function Explored with Binary Population Synthesis

The stellar initial mass function (IMF) plays a crucial role in determining the number of surviving stars in galaxies, the chemical composition of the interstellar medium, and the distribution of light in galaxies. A key unsolved question is whether the IMF is universal in time and space. Here we use state-of-the-art results of stellar evolution to show that the IMF of our Galaxy made a transition from an IMF dominated by massive stars to the present-day IMF at an early phase of the Galaxy formation. Updated results from stellar evolution in a wide range of metallicities have been implemented in a binary population synthesis code, and compared with the observations of carbon-enhanced metal-poor (CEMP) stars in our Galaxy. We find that applying the present-day IMF to Galactic halo stars causes serious contradictions with four observable quantities connected with the evolution of AGB stars. Furthermore, a comparison between our calculations and the observations of CEMP stars may help us to constrain the transition metallicity for the IMF which we tentatively set at [Fe/H] = -2. A novelty of the current study is the inclusion of mass loss suppression in intermediate-mass AGB stars at low-metallicity. This significantly reduces the overproduction of nitrogen-enhanced stars that was a major problem in using the high-mass star dominated IMF in previous studies. Our results also demonstrate that the use of the present day IMF for all time in chemical evolution models results in the overproduction of Type I.5 supernovae. More data on stellar abundances will help to understand how the IMF has changed and what caused such a transition.Comment: 8 pages, 2 figures, accepted by MNRAS Lette

A multilocular thymic cyst associated with mediastinal seminoma: evidence for its medullary epithelial origin highlighted by POU2F3-positive thymic tuft cells and concomitant myoid cell proliferation

Multilocular thymic cyst (MTC) and germ cell tumors are common diseases that impact the mediastinum. Correctly diagnosing these diseases can be difficult because several other conditions can mimic them. We report a male patient with MTC associated with mediastinal seminoma. A needle biopsy of the mediastinal tumor revealed numerous epithelioid cell granulomas that mimicked sarcoidosis or mycobacterial infection. However, large atypical cells positive for Oct3/4 and KIT were noted between the granulomas; thus, we diagnosed the patient with mediastinal seminoma. The resected tumor, after chemotherapy, consisted of multiple cystic lesions, and a residual germ cell tumor was first considered. However, thymic medulla-specific elements, namely, POU2F3-positive thymic tuft cells and rhabdomyomatous myoid cells accompanying the epithelium, led to the correct diagnosis of MTC. Our case underscores the importance of recognizing the histological features associated with mediastinal seminoma and provides novel findings for MTC pathogenesis, namely, the presence of thymic tuft cells