450 research outputs found
Disc fragmentation and the formation of Population III stars
No full textOur understanding of Population III star formation is still in its infancy. They are formed in dark matter minihaloes of 10(5)-10(6) M-circle dot at z = 20-30. Recent high-resolution cosmological simulations show that a protostellar disc forms as a consequence of gravitational collapse and fragments into multiple clumps. However, it is not entirely clear if these clumps will be able to survive to form multiple stars as simulations are unable to follow the disc evolution for longer times. In this study, we employ a simple analytical model to derive the properties of marginally stable steady-state discs. Our results show that the stability of the disc depends on the critical value of the viscous parameter alpha. For alpha(crit) = 1, the disc is stable for an accretion rate of = 100 au in the presence of an accretion rate of 10(-2) M-circle dot yr(-1). For 0.06 < alpha(crit) < 1, the disc can be unstable for both accretion rates. The comparison of the migration and the Kelvin-Helmholtz time-scales shows that clumps are expected to migrate inwards before reaching the main sequence. Furthermore, in the presence of a massive central star, the clumps within the central 1 au will be tidally disrupted. We also find that UV feedback from the central star is unable to disrupt the disc, and that photoevaporation becomes important only once the accretion rate has dropped to 2 x 10(-4) M-circle dot yr(-1). As a result, the central star may reach a mass of 100 M-circle dot or even higher.European Research Council under the European Community [614199
A genetic algorithm for designing microarray experiments
No full textHeuristic techniques of optimization can be useful in designing complex experiments, such as microarray experiments. They have advantages over the traditional methods of optimization, particularly in situations where the search space is discrete. In this paper, a search procedure based on a genetic algorithm is proposed to find optimal (efficient) designs for both one- and multi-factor experiments. A genetic algorithm is a heuristic optimization method that exploits the biological evolution to obtain a solution of the problem. As an example, optimal designs for factorial microarray experiments are presented for different numbers of arrays and for various sets of research questions. Comparisons between different operators of the genetic algorithm are performed by simulation studies
(m) Sabīl de 'Ā'isha as-Suṭūhiyya
No full textSimaïka Marcus H., Shafik Muhammad, 'Amrusi Ahmad Fahmi al-, Mahbub Mahmud Sabri, Sayed Metoualli, Pauty Edmond. (m) Sabīl de 'Ā'isha as-Suṭūhiyya. In: Comité de Conservation des Monuments de l'Art Arabe. Fascicule 37, exercice 1933-1935, 1940. p. 144
THE FORMATION OF MASSIVE PRIMORDIAL STARS IN THE PRESENCE OF MODERATE UV BACKGROUNDS
No full textRadiative feedback produced by stellar populations played a vital role in early structure formation. In particular, photons below the Lyman limit can escape the star-forming regions and produce a background ultraviolet (UV) flux, which consequently may influence the pristine halos far away from the radiation sources. These photons can quench the formation of molecular hydrogen by photodetachment of H-. In this study, we explore the impact of such UV radiation on fragmentation in massive primordial halos of a few times 10(7) M-circle dot. To accomplish this goal, we perform high resolution cosmological simulations for two distinct halos and vary the strength of the impinging background UV field in units of J(21) assuming a blackbody radiation spectrum with a characteristic temperature of T-rad = 10(4) K. We further make use of sink particles to follow the evolution for 10,000 yr after reaching the maximum refinement level. No vigorous fragmentation is observed in UV-illuminated halos while the accretion rate changes according to the thermal properties. Our findings show that a few 10(2)-10(4) solar mass protostars are formed when halos are irradiated by J(21) = 10-500 at z > 10 and suggest a strong relation between the strength of the UV flux and mass of a protostar. This mode of star formation is quite different from minihalos, as higher accretion rates of about 0.01-0.1 M-circle dot yr(-1) are observed by the end of our simulations. The resulting massive stars are potential cradles for the formation of intermediate-mass black holes at earlier cosmic times and contribute to the formation of a global X-ray background
The implications of dust for high-redshift protogalaxies and the formation of binary disks
No full textNumerical simulations suggest that the first galaxies are formed in protogalactic halos with virial temperatures >= 10(4) K. It is likely that such halos are polluted with trace amounts of metals produced by the first generation of stars. The presence of dust can significantly change the chemistry and dynamics of early galaxies. In this article, we aim to assess the role of dust on the thermal and dynamical evolution of the first galaxies in the presence of a background UV flux, and its implications for the observability of Lyman-alpha emitters and sub-mm sources. We have performed high resolution cosmological simulations using the adaptive mesh refinement code FLASH to accomplish this goal. We have developed a chemical network appropriate for these conditions and coupled it with the FLASH code. The main ingredients of our chemical model include the formation of molecules (both in the gas phase and on dust grains), a multi-level treatment of atomic hydrogen, line trapping of Ly-alpha photons and, photoionization and photodissociation processes in a UV background. We found that the formation of molecules (H-2 and HD) is significantly enhanced in the presence of dust grains as compared to only gas phase reactions by up to two orders of magnitude. The presence of dust may thus establish a molecular ISM in high-redshift galaxies. The presence of a background UV flux strongly influences the formation of molecules by photodissociating them. We explore the evolution after a major merger, leading to the formation of a binary disk. These disks have gas masses of similar to 10(7) M-circle dot at a redshift of 5.4. Each disk lies in a separate subhalo as a result of the merger event. The disks are supported by turbulent pressure due to the highly supersonic turbulence present in the halo. For values of J(21) = 1000 (internal flux), we find that fragmentation may be enhanced due to thermal instabilities in the hot gas. The presence of dust does not significantly reduce the Ly-alpha emission. The emission of Ly-alpha is extended and originates from the envelope of the halo due to line trapping effects. We also find that dust masses of a few x10(8) M-circle dot are required to observe the dust continuum emission from z similar to 5 galaxies with ALMA
How realistic UV spectra and X-rays suppress the abundance of direct collapse black holes
No full textObservations of high-redshift quasars at z > 6 indicate that they harbour supermassive black holes (SMBHs) of a billion solar masses. The direct collapse scenario has emerged as the most plausible way to assemble SMBHs. The nurseries for the direct collapse black holes are massive primordial haloes illuminated with an intense UV flux emitted by Population II (Pop II) stars. In this study, we compute the critical value of such a flux (J(21)(crit)) for realistic spectra of Pop II stars through three-dimensional cosmological simulations. We derive the dependence of J(21)(crit) on the radiation spectra, on variations from halo to halo, and on the impact of X-ray ionization. Our findings show that the value of J(21)(crit) is a few times 10(4) and only weakly depends on the adopted radiation spectra in the range between T-rad = 2 x 10(4) and 10(5) K. For three simulated haloes of a few times 10(7) M-circle dot, J(21)(crit) varies from 2 x 10(4) to 5 x 10(4). The impact of X-ray ionization is almost negligible and within the expected scatter of J(21)(crit) for background fluxes of J(X, 21) <= 0.1. The computed estimates of J(21)(crit) have profound implications for the quasar abundance at z = 10 as it lowers the number density of black holes forming through an isothermal direct collapse by a few orders of magnitude below the observed black hole density. However, the sites with moderate amounts of H-2 cooling may still form massive objects sufficient to be compatible with observations
Robustness considerations in selecting efficient two-color microarray designs
No full textThe main goal of microarray experiments is to select a small subset of genes that are differentially expressed among competing mRNA samples. For a given set of such mRNA samples, it is possible to consider a number of two-color cDNA microarray designs with a fixed number of arrays. Appropriate criteria can be used to select an efficient design from such a set of alternative experimental designs. In practice, however, microarray expression data often contain missing observations and the most efficient design ( with complete observations) for a specific setup may not be efficient in the presence of missing observations. In this article, we propose two criteria to address the robustness of microarray designs against missing observations. We demonstrate the simultaneous use of efficiency and robustness criteria to select good microarray designs for both one-factor and multi-factor experiments
Massive black hole factories: Supermassive and quasi-star formation in primordial halos
No full textContext. Supermassive stars and quasi-stars (massive stars with a central black hole) are both considered as potential progenitors for the formation of supermassive black holes. They are expected to form from rapidly accreting protostars in massive primordial halos. Aims. We explore how long rapidly accreting protostars remain on the Hayashi track, implying large protostellar radii and weak accretion luminosity feedback. We assess the potential role of energy production in the nuclear core, and determine what regulates the evolution of such protostars into quasi-stars or supermassive stars. Methods. We followed the contraction of characteristic mass shells in rapidly accreting protostars, and inferred the timescales for them to reach nuclear densities. We compared the characteristic timescales for nuclear burning with those for which the extended protostellar envelope can be maintained. Results. We find that the extended envelope can be maintained up to protostellar masses of 3.6 x 10(8) m(3) M-circle dot, where m. denotes the accretion rate in solar masses per year. We expect the nuclear core to exhaust its hydrogen content in 7 x 106 yr. If accretion rates m >> 0.14 can still be maintained at this point, a black hole may form within the accreting envelope, leading to a quasi-star. Alternatively, the accreting object will gravitationally contract to become a main-sequence supermassive star. Conclusions. Due to the limited gas reservoir in typical 10(7) M-circle dot dark matter halos, the accretion rate onto the central object may drop at late times, implying the formation of supermassive stars as the typical outcome of direct collapse. However, if high accretion rates are maintained, a quasi-star with an interior black hole may form
8° Monographies de M. Monneret de Villard
No full textSimaïka Marcus H., Greg Robert Hyde, Verrucci Ernesto, Mahbub Mahmud Sabri, Ahmad Ali Hasan, Sayed Metoualli, Pauty Edmond. 8° Monographies de M. Monneret de Villard. In: Comité de Conservation des Monuments de l'Art Arabe. Fascicule 36, exercice 1930-1932, 1936. p. 268
m) Tombeau d'Ibrahim et Guirguis al-Gôhari
No full textSimaïka Marcus H., Greg Robert Hyde, Verrucci Ernesto, Mahbub Mahmud Sabri, Ahmad Ali Hasan, Sayed Metoualli, Pauty Edmond. m) Tombeau d'Ibrahim et Guirguis al-Gôhari. In: Comité de Conservation des Monuments de l'Art Arabe. Fascicule 36, exercice 1930-1932, 1936. p. 273
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