University of California, Santa Cruz

About: University of California, Santa Cruz is a education organization based out in Santa Cruz, California, United States. It is known for research contribution in the topics: Galaxy & Population. The organization has 15541 authors who have published 44120 publications receiving 2759983 citations. The organization is also known as: UCSC & UC, Santa Cruz.

Nucleosynthesis and Evolution of Massive Metal-Free Stars

Abstract: The evolution and explosion of metal-free stars with masses 10-100 M ? are followed, and their nucleosynthetic yields, light curves, and remnant masses determined. Such stars would have been the first to form after the big bang and may have left a distinctive imprint on the composition of the early universe. When the supernova yields are integrated over a Salpeter initial mass function (IMF), the resulting elemental abundance pattern is qualitatively solar, but with marked deficiencies of odd-Z elements with 7 ? Z ? 13. Neglecting the contribution of the neutrino wind from the neutron stars that they form, no appreciable abundances are made for elements heavier than germanium. The computed pattern compares favorably with what has been observed in metal-deficient stars with [Z] ?3. The amount of ionizing radiation from this generation of stars is ~2.16 MeV per baryon (4.15 B per M ?; where 1 B = 1 Bethe = 1051 erg) for a Salpeter IMF, and may have played a role in reionizing the universe. Neglecting rotation, most of the stars end their lives as blue supergiants and form supernovae with distinctive light curves resembling SN 1987A, but some produce primary nitrogen due to dredge-up and become red supergiants. These make brighter supernovae like typical Type IIp's. For the lower mass supernovae considered, the distribution of remnant masses clusters around typical modern neutron star masses, but above 20-30 M ?, with the value depending on explosion energy, black holes are copiously formed by fallback, with a maximum hole mass of ~40 M ?. A novel automated fitting algorithm is developed for determining optimal combinations of explosion energy, mixing, and IMF in the large model database to agree with specified data sets. The model is applied to the low-metallicity sample of Cayrel et?al. and the two ultra-iron-poor stars HE0107-5240 and HE1327-2326. Best agreement with these very low metallicity stars is achieved with very little mixing, and none of the metal-deficient data sets considered show the need for a high-energy explosion component. In contrast, explosion energies somewhat less than 1.2 B seem to be preferred in most cases.

The formation of massive star systems by accretion.

Abstract: Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to prevent them from growing by accretion. We present three-dimensional radiation-hydrodynamic simulations of the collapse of a massive prestellar core and find that radiation pressure does not halt accretion. Instead, gravitational and Rayleigh-Taylor instabilities channel gas onto the star system through nonaxisymmetric disks and filaments that self-shield against radiation while allowing radiation to escape through optically thin bubbles. Gravitational instabilities cause the disk to fragment and form a massive companion to the primary star. Radiation pressure does not limit stellar masses, but the instabilities that allow accretion to continue lead to small multiple systems.

Towards complete and error-free genome assemblies of all vertebrate species

Abstract: High-quality and complete reference genome assemblies are fundamental for the application of genomics to biology, disease, and biodiversity conservation. However, such assemblies are available for only a few non-microbial species1-4. To address this issue, the international Genome 10K (G10K) consortium5,6 has worked over a five-year period to evaluate and develop cost-effective methods for assembling highly accurate and nearly complete reference genomes. Here we present lessons learned from generating assemblies for 16 species that represent six major vertebrate lineages. We confirm that long-read sequencing technologies are essential for maximizing genome quality, and that unresolved complex repeats and haplotype heterozygosity are major sources of assembly error when not handled correctly. Our assemblies correct substantial errors, add missing sequence in some of the best historical reference genomes, and reveal biological discoveries. These include the identification of many false gene duplications, increases in gene sizes, chromosome rearrangements that are specific to lineages, a repeated independent chromosome breakpoint in bat genomes, and a canonical GC-rich pattern in protein-coding genes and their regulatory regions. Adopting these lessons, we have embarked on the Vertebrate Genomes Project (VGP), an international effort to generate high-quality, complete reference genomes for all of the roughly 70,000 extant vertebrate species and to help to enable a new era of discovery across the life sciences.

Fourteen- through 18-month-old infants differentially imitate intentional and accidental actions

Abstract: This study explored infants' ability to discriminate between, and their tendency to reproduce, the accidental and intentional actions of others. Twenty 14- through 18-month-olds watched an adult perform a series of two-step actions on objects that made interesting results occur. Some of the modeled actions were marked vocally as intentional (“There!”), some were marked vocally as accidental (“Woops!”). Following each demonstration, infants were given a chance to make the result occur themselves. Overall, infants imitated almost twice as many of the adult's intentional actions as her accidental ones. Infants before age 18 months thus may understand something about the intentions of other persons. This understanding represents infants' first step toward adult-like social cognition and underlies their acquisition of language and other cultural skills.

Formation and Evolution of Galaxy Dark Matter Halos and Their Substructure

Abstract: We use the "Via Lactea" simulation to study the co-evolution of a Milky Way-sized ΛCDM halo and its subhalo population. While most of the host halo mass is accreted over the first 6 Gyr in a series of major mergers, the physical mass distribution [not Mvir(z)] remains practically constant since z = 1. The same is true in a large sample of ΛCDM galaxy halos. Subhalo mass loss peaks between the turnaround and virialization epochs of a given mass shell, and the abundance of substructure within the shell freezes afterward. Of the z = 1 subhalos, 97% have a surviving bound remnant at the present epoch. The retained mass fraction is larger for initially lighter subhalos: today satellites with maximum circular velocities Vmax = 10 km s-1 at z = 1 have about 40% of the mass they had back then. At the first pericenter passage a larger average mass fraction is lost than during each following orbit. Tides remove mass in the substructure from the outside in, leading to higher concentrations compared to field halos of the same mass. This effect, combined with the earlier formation epoch of the inner satellites, results in strongly increasing subhalo concentrations toward the Galactic center. We present individual evolutionary tracks and present-day properties of the likely hosts of the dwarf satellites around the Milky Way. The formation histories of "field halos" that lie beyond the Via Lactea host today are found to strongly depend on the density of their environment. This is caused by tidal mass loss that affects many field halos on eccentric orbits.