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

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

Intergenogroup Recombinant Sapovirus in Japan, 2007–2008

We investigated the incidence of sapovirus (SaV)–associated gastroenteritis in infants and children in Japan during 2007–2008 and characterized the diversity of SaV-positive strains. SaV was detected in 19 (4%) of 477 fecal specimens. The leading genogroup (79%, 15 cases) comprised intergenogroup recombinant SaVs (GII/GIV)

Transition of the stellar initial mass function explored using binary population synthesis

The stellar initial mass function (IMF) plays a crucial role in the determination of the number of surviving stars in galaxies, of the chemical composition of the interstellar medium and of the distribution of light in galaxies. A key unsolved question i

Early-onset and late-onset group B streptococcal disease in Japan: a nationwide surveillance study, 2004–2010

SummaryObjectivesTo clarify the incidence and prognosis of early-onset (EOD) and late-onset (LOD) GBS disease in Japan. To evaluate the influence of national guidelines issued in 2008 on the epidemiology of GBS disease.MethodsRetrospective nationwide questionnaire surveillance on culture-confirmed GBS infections between 2004 and 2010.ResultsEighty-eight EOD and 162 LOD cases were reported from 152 participating hospitals. The case fatality of EOD was 13.6% and of LOD was 8.0%. Premature birth <37 weeks (p<0.001) and low birth weight <2500g (p<0.001) were significantly associated with EOD mortality. A high rate of neurological sequelae was noted in meningitis in EOD (8/24) and LOD (29/85) cases. Based on a live-birth number of 438 359 and inborn case numbers of 36 EOD and 42 LOD, the incidence of EOD and LOD were estimated to be 0.08 (95% confidence interval (CI) 0.06–0.11)/1000 and 0.10 (95% CI 0.07–0.12)/1000 live-births, respectively. Before (2004–2008) and after (2009–2010) the issue of guidelines, the mortality of EOD (from 14.8% to 11.8%) and LOD (from 9.8% to 2.5%) improved, but the incidence was unchanged.ConclusionsThe incidence of EOD and LOD is apparently low in Japan, but the mortality and morbidity rates remain substantial. The issue of national guidelines did not affect the incidence