No abstract

We calculate the evolution of zero-metallicity Population III (Pop III) stars whose mass grows from the initial mass of ∼1 M by accreting the surrounding gases. Our calculations cover whole evolutionary stages from the pre-main sequence, via various nuclear burning stages, through the final core-collapse or pair-creation instability phases. We adopt two different sets of stellar mass accretion rates as our fiducial models. One is derived from a cosmological simulation of the first generation (PopIII.1) stars, and the other is derived from a simulation of the second generation stars that are affected by radiation from PopIII.1 stars. The latter represents one case of PopIII.2 stars. We also adopt additional models that include radiative feedback effects. We show that the final mass of Pop III.1 stars can be as large as ∼1000 M , beyond the mass range (140-300 M ) for the pair-instability supernovae. Such massive stars undergo core-collapse to form intermediate-mass black holes, which may be the seeds for merger trees to supermassive black holes. On the other hand, Pop III.2 stars become less massive ( 40-60 M ), being in the mass range of ordinary iron core-collapse stars. Such stars explode and eject heavy elements to contribute to chemical enrichment of the early universe as observed in the abundance patterns of extremely metal-poor stars in the Galactic halo. In view of the large range of possible accretion rates, further studies are important to see if these fiducial models are actually the cases.

No abstract

With the successful launch of Chandra and XMM/Newton X-ray space missions combined with the lower-energy band observations, we are in the position where careful comparison of neutron star cooling theories with observations will make it possible to distinguish among various competing theories. For instance, the latest theoretical and observational developments already exclude both nucleon and kaon direct URCA cooling. In this way we can now have realistic hope for determining various important properties, such as the composition, degree of superfluidity, the equation of state and steller radius. These developments should help us obtain better insight into the properties of dense matter.Comment: 11 pages, 1 figur

We present new ASCA observations covering the 0.5-10 keV X-ray range of the cooling neutron star candidates PSR 0656ϩ14 and PSR 1055Ϫ52. Previous ROSAT observations had shown that two-component models, either two blackbodies or a blackbody plus a power-law, provided the best spectral fits to their X-ray emission. The combined ASCA and ROSAT spectrum of PSR 0656ϩ14 reveals two blackbody components with T 2 8 ϫ 10 5 K and T 2 1.5 ϫ 10 6 K and shows evidence that a power-law component is needed to account for higher energy photons. This three-component fit gives a reduced 2 that is half the value of a more conventional two component fit (1.3 as compared to 2.4). The fit to the combined spectrum for PSR 1055Ϫ52 yields a two-blackbody fit with T 2 8 ϫ 10 5 K and T 2 3.7 ϫ 10 6 K. Our results favor the existence of a hot polar cap in each of these pulsars with the ratio of the polar cap area to the neutron star surface area being 7 ϫ 10 Ϫ3 and 3 ϫ 10 Ϫ5 for PSR 0656ϩ14 and PSR 1055Ϫ52, respectively. The results are compared to models that make predictions of polar cap heating processes.