Population III Stars (population + iii_star)

Distribution by Scientific Domains

Selected Abstracts

Evolutionary models for two hyper-iron-poor low-mass stars

Gregory J. Harris
ABSTRACT The possible origin of two low-mass hyper-iron-poor (HIP) stars, HE0107-5240 and HE1327-2326 are investigated. The three scenarios tested are as follows. (i) The star forms metal free (Population III) and accretes metals throughout its lifetime. (ii) A Population III binary star system forms, and the lower mass star accretes metals from the higher mass star during its asymptotic giant branch phase. (iii) An HIP Population II star is formed and evolves with no further pollution. Using the ng-elms code, stellar evolution models of these three scenarios are computed and fitted to the observed colours. Models are computed at 0.8 and 0.7 M,, and for several metal accretion rates. It is not possible to unambiguously identify the origin, evolutionary stage or mass for either star with the present observational data. Our evolutionary models indicate that HE0107,5240 is either a red giant or a subgiant close to the base of the red giant branch. In line with the study of Aoki et al., HE1327,2326 is found to be either a main-sequence or a subgiant star. If HE1327,2326 is a main-sequence star the fits indicate that it must have a mass less than 0.8 M,; if the star is assumed to have an age of around 12.5 Gyr it will have a mass closer to 0.7 M,. Main-sequence low-mass model Population III stars which have been polluted with metals are found to closely resemble Population II stars. However, the case is different for subgiants. The polluted and unpolluted Population III subgiants have luminosity up to twice that of a comparable Population II model. If HE1327,2326 is a subgiant, its distance is measured and if its mass can be reasonably well constrained, it should be possible to determine if it is a Population II or III star. [source]

The halo mass function from the dark ages through the present day

Darren S. Reed
ABSTRACT We use an array of high-resolution N -body simulations to determine the mass function of dark matter haloes at redshifts 10,30. We develop a new method for compensating for the effects of finite simulation volume that allows us to find an approximation to the true ,global' mass function. By simulating a wide range of volumes at different mass resolution, we calculate the abundance of haloes of mass 105,12 h,1 M,. This enables us to predict accurately the abundance of the haloes that host the sources that reionize the Universe. In particular, we focus on the small mass haloes (,105.5,6 h,1 M,) likely to harbour Population III stars where gas cools by molecular hydrogen emission, early galaxies in which baryons cool by atomic hydrogen emission at a virial temperature of ,104K (,107.5,8 h,1 M,), and massive galaxies that may be observable at redshift ,10. When we combine our data with simulations that include high-mass haloes at low redshift, we find that the best fit to the halo mass function depends not only on the linear overdensity, as is commonly assumed in analytic models, but also on the slope of the linear power spectrum at the scale of the halo mass. The Press,Schechter model gives a poor fit to the halo mass function in the simulations at all epochs; the Sheth-Tormen model gives a better match, but still overpredicts the abundance of rare objects at all times by up to 50 per cent. Finally, we consider the consequences of the recently released WMAP 3-yr cosmological parameters. These lead to much less structure at high redshift, reducing the number of z= 10,mini-haloes' by more than a factor of two and the number of z= 30 galaxy hosts by nearly four orders of magnitude. Code to generate our best-fitting halo mass function may be downloaded from http://icc.dur.ac.uk/Research/PublicDownloads/genmf_readme.html. [source]

Forming supermassive black holes by accreting dark and baryon matter

Jian Hu
ABSTRACT Given a large-scale mixture of self-interacting dark matter (SIDM) particles and baryon matter distributed in the early Universe, we advance here a two-phase accretion scenario for forming supermassive black holes (SMBHs) with masses around ,109 M, at high redshifts z(,6). The first phase is conceived to involve a rapid quasi-spherical and quasi-steady Bondi accretion of mainly SIDM particles embedded with baryon matter on to seed black holes (BHs) created at redshifts z, 30 by the first generation of massive Population III stars; this earlier phase rapidly gives birth to significantly enlarged seed BH masses of during z, 20,15, where ,0 is the cross-section per unit mass of SIDM particles and Cs is the velocity dispersion in the SIDM halo referred to as an effective ,sound speed'. The second phase of BH mass growth is envisaged to proceed primarily via baryon accretion, eventually leading to SMBH masses of MBH, 109 M,; such SMBHs may form either by z, 6 for a sustained accretion at the Eddington limit or later at lower z for sub-Eddington mean accretion rates. In between these two phases, there is a transitional yet sustained diffusively limited accretion of SIDM particles which in an eventual steady state would be much lower than the accretion rates of the two main phases. We intend to account for the reported detections of a few SMBHs at early epochs, e.g. Sloan Digital Sky Survey (SDSS) 1148+5251 and so forth, without necessarily resorting to either super-Eddington baryon accretion or very frequent BH merging processes. Only extremely massive dark SIDM haloes associated with rare peaks of density fluctuations in the early Universe may harbour such early SMBHs or quasars. Observational consequences are discussed. During the final stage of accumulating a SMBH mass, violent feedback in circumnuclear environs of a galactic nucleus leads to the central bulge formation and gives rise to the familiar empirical MBH,,b correlation inferred for nearby normal galaxies with ,b being the stellar velocity dispersion in the galactic bulge; in our scenario, the central SMBH formation precedes that of the galactic bulge. [source]

Searching for the reionization sources

T. Roy Choudhury
ABSTRACT Using a reionization model simultaneously accounting for a number of experimental data sets, we investigate the nature and properties of reionization sources. Such a model predicts that hydrogen reionization starts at z, 15, is initially driven by metal-free (Population III) stars, and is 90 per cent complete by z, 8. We find that a fraction f, > 80 per cent of the ionizing power at z, 7 comes from haloes of mass M < 109 M, predominantly harbouring Population III stars; a turnover to a Population II dominated phase occurs shortly after, with this population, residing in M > 109 M, haloes, yielding f,, 60 per cent at z= 6. Using Lyman-break broad-band dropout techniques, J -band detection of sources contributing to 50 per cent (90 per cent) of the ionizing power at z, 7.5 requires reaching a magnitude J110,AB= 31.2 (31.7), where , 15 (30) (Population III) sources arcmin,2 are predicted. We conclude that z > 7 sources tentatively identified in broad-band surveys are relatively massive (M, 109 M,) and rare objects which are only marginally (,1 per cent) adding to the reionization photon budget. [source]

Unsolved problems in observational astronomy.


Abstract We present the highlights of current observational programs in stellar optical spectroscopy carried out with 8-10 m class telescopes as well as with smaller telescopes. Topics discussed include: 1. light elements abundances and their cosmological implications; 2. search for Population III stars and spectroscopy of extremely metal deficient stars; 3. abundances of different stellar populations in the Galaxy; 4. spectroscopy of resolved stars in Local Group galaxies; 5. Li and Be abundances and internal mixing in stars; 6. spectroscopy of very-low mass stars and brown dwarfs; 7. radial velocity search of extrasolar planets; 8. stellar oscillations and asteroseismology; 9. stellar magnetic activity and Doppler imaging of stellar surface features. We also highlight the role that dedicated 1-2 m automatic telescopes with spectroscopic capabilities can play in several fields of stellar optical spectroscopy. ( 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]