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.

Miocene evolution of atmospheric carbon dioxide

Abstract: Changes in pCO2 or ocean circulation are generally invoked to explain warm early Miocene climates and a rapid East Antarctic ice sheet (EAIS) expansion in the middle Miocene. This study reconstructs late Oligocene to late Miocene pCO2 from ep values based on carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifera from Deep Sea Drilling Project sites 588 and 608 and Ocean Drilling Program site 730. Our results indicate that highest pCO2 occurred during the latest Oligocene (∼350 ppmv) but decreased rapidly at ∼25 Ma. The early and middle Miocene was characterized by low pCO2 (260–190 ppmv). Lower intervals of pCO2 correspond to inferred organic carbon burial events and glacial episodes with the lowest concentrations occurring during the middle Miocene. There is no evidence for either high pCO2 during the late early Miocene climatic optimum or a sharp pCO2 decrease associated with EAIS growth. Paradoxically, pCO2 increased following EAIS growth and obtained preindustrial levels by ∼10 Ma. Although we emphasize an oceanographic control on Miocene climate, low pCO2 could have primed the climate system to respond sensitively to changes in heat and vapor transport.

On Measuring Chemical Abundances in Distant Galaxies Using Global Emission Line Spectra

Abstract: The advent of 8--10 meter class telescopes enables direct measurement of the chemical properties in the ionized gas of cosmologically--distant galaxies with the same nebular analysis techniques used in local H II regions. We show that spatially unresolved (i.e., global) emission line spectra can reliably indicate the chemical properties of distant star-forming galaxies. However, standard nebular chemical abundance measurement methods (those with a measured electron temperature from [O III] lambda4363) may be subject to small systematic errors when the observed volume includes a mixture of gas with diverse temperatures, ionization parameters, and metallicities. To characterize these systematic effects, we compare physical conditions derived from spectroscopy of individual H II regions with results from global galaxy spectroscopy. We consider both low-mass, metal poor galaxies with uniform abundances and larger galaxies with internal chemical gradients. Well-established empirical calibrations using strong-line ratios can serve as reliable (~0.2 dex) indicators of the overall systemic oxygen abundance even when the signal-to-noise of the Hbeta and [O III] emission lines is as low as 8:1. We present prescriptions, directed toward high-redshift observers, for using global emission line spectra to trace the chemical properties of star-forming galaxies in the distant universe. [abridged]

Presupernova Evolution of Rotating Massive Stars. II. Evolution of the Surface Properties

Abstract: We investigate the evolution of the surface properties of models for rotating massive stars, i.e., their luminosities, effective temperatures, surface rotational velocities, and surface abundances of all isotopes, from the zero-age main sequence to the supernova stage. Our results are based on the grid of stellar models by Heger, Langer, & Woosley, which covers solar metallicity stars in the initial-mass range 8-25 M?. Results are parameterized by initial mass, initial rotational velocity and major uncertainties in the treatment of the rotational mixing inside massive stars. Rotationally induced mixing processes widen the main sequence and increase the core hydrogen-burning lifetime, similar to the effects of convective overshooting. It can also significantly increase the luminosity during and after core hydrogen burning, and strongly affects the evolution of the effective temperature. Our models predict surface rotational velocities for various evolutionary stages, in particular for blue supergiants, red supergiants, and for the immediate presupernova stage. We discuss the changes of the surface abundances due to rotationally induced mixing for main sequence and post-main-sequence stars. We single out two characteristics by which the effect of rotational mixing can be distinguished from that of massive close binary mass transfer, the only alternative process leading to nonstandard chemical surface abundances in massive stars. A comparison with observed abundance anomalies in various types of massive stars supports the concept of rotational mixing in massive stars and indicates that it is responsible for most of the observed abundance anomalies.

The Core Accretion Model Predicts Few Jovian-Mass Planets Orbiting Red Dwarfs

Abstract: The favored theoretical explanation for giant planet formation—in both our solar system and others—is the core accretion model (although it still has some serious difficulties). In this scenario, planetesimals accumulate to build up planetary cores, which then accrete nebular gas. With current opacity estimates for protoplanetary envelopes, this model predicts the formation of Jupiter-mass planets in 2-3 Myr at 5 AU around solar-mass stars, provided that the surface density of solids is enhanced over that of the minimum-mass solar nebula (by a factor of a few). Working within the core accretion paradigm, this Letter presents theoretical calculations that show that the formation of Jupiter-mass planets orbiting M dwarf stars is seriously inhibited at all radial locations (in sharp contrast to solar-type stars). Planet detection programs sensitive to companions of M dwarfs will test this prediction in the near future.