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We explore the appearance of light clusters at high densities of collapsing stellar cores. Special attention is paid to the unstable isotope 4H, which was not included in previous studies. The importance of light clusters in the calculation of rates for neutrino matter interaction is discussed. The main conclusion is that thermodynamic quantities are only weakly sensitive to the chemical composition. The change in pressure and hence the direct change in collapse dynamics will be minor. But the change in neutrino heating and neutronization processes can be significant. [ABSTRACT FROM AUTHOR]

56Ni is an important indicator of the supernova explosions, which characterizes light curves. Nevertheless, rather than 56Ni, explosion energy has often been paid attention from explosion mechanism community, since it is, at a glance, easier to estimate from numerical data than the amount of 56Ni. The final explosion energy, however, is difficult to estimate by detailed numerical simulations because current simulations cannot reach typical time-scale of saturation of the explosion energy. Instead, the amount of 56Ni converges within a short time-scale so that it would be a more robust probe of the explosion mechanism. We investigated the amount of 56Ni synthesized by explosive nucleosynthesis in supernova ejecta by means of numerical simulations and an analytic model. For numerical simulations, we employ a Lagrangian hydrodynamics code in which neutrino heating and cooling terms are taken into account by the light-bulb approximation. We additionally develop an analytic model, which gives a reasonable estimate of the amount of 56Ni. We find that, in order to produce enough amount of 56Ni, |$\mathcal {O}(1)$| Bethe s−1 of growth rate of the explosion energy is needed, which is much larger than those found in recent exploding simulations, typically |$\mathcal {O}(0.1)$| Bethe s−1. We also find that a progenitor model with a large compactness parameter leads to efficient synthesis of 56Ni, opposite to the explodability that prefers a small compactness. These two results may indicate that there is a limited parameter space in the properties of the progenitor stars, which can explain both the explodability and 56Ni production simultaneously. [ABSTRACT FROM AUTHOR]

Current Type Ia supernova (SN Ia) models can reproduce most visible+IR + UV observations. In the X-ray band, the determination of elemental abundance ratios in supernova remnants (SNRs) through their spectra has reached enough precision to constrain SN Ia models. Martínez-Rodríguez et al have shown that the Ca/S mass ratio in SNRs cannot be reproduced with the standard nuclear reaction rates for a wide variety of SN Ia models, and suggested that the 12C+16O reaction rate could be overestimated by a factor as high as ten. We show that the same Ca/S ratio can be obtained by simultaneously varying the rates of the reactions 12C + 16O, 12C + 12C, 16O + 16O, and 16O(γ, α)12C within the reported uncertainties. We also show that the yields of the main products of SN Ia nucleosynthesis do not depend on the details of which rates are modified, but can be parametrized by an observational quantity such as Ca/S. Using this SNR-calibrated approach, we then proceed to compute a new set of SN Ia models and nucleosynthesis for both Chandrasekhar and sub-Chandrasekhar mass progenitors with a 1D hydrodynamics and nucleosynthesis code. We discuss the nucleosynthesis of the models as a function of progenitor metallicity, mass, and deflagration-to-detonation transition density. The yields of each model are almost independent on the reaction rates modified for a common Ca/S ratio. [ABSTRACT FROM AUTHOR]

Recent detection of the isotope 7Be (which decays into 7Li) in the outbursts of classical novae confirms the suggestion made in the 1970s that novae could make 7Li. We reconsidered the role of novae as producers of 7Li by means of a detailed model of the chemical evolution of the Milky Way. We showed that novae could be the Galactic 7Li source accounting for the observed increase of Li abundances in the thin disc with stellar metallicity. The best agreement with the upper envelope of the observed Li abundances is obtained for a delay time of ≈1 Gyr for nova production and an effective 7Li yield of 1.8(± 0.6)×10−5 M⊙ over the whole nova lifetime. Lithium in halo stars is depleted by ≈0.35 dex, assuming the pristine abundance from standard big-bang nucleosynthesis. We elaborate a model that matches the pristine stellar abundances, assuming that all stars are depleted by 0.35 dex. In this case, the delay remains the same, but the Li yields are doubled compared with the previous case. This model also has the merit of matching the Li abundance in meteorites and young T Tauri stars. A thick disc model, adopting the parameters derived for the thin disc, can also explain the absence of an increase of Li abundance in its stars. The thick disc is old, but formed and evolved in a time shorter than that required by novae to contribute significantly to 7Li. Therefore, no 7Li enhancement is expected in thick disc stars. We show that the almost constant Li abundance in the thick disc results from the compensation of stellar astration by spallation processes. [ABSTRACT FROM AUTHOR]

I suggest a novel type of nuclear reactions in accreting neutron stars – neutron transfer, which is quantum tunnelling of weakly bounded neutron from one nucleus to another. I estimate the rate of this process for fixed nuclei separation and then average the result over realistic distribution of nuclei to get the rate value for astrophysical conditions. The neutron transfer can modify reaction chains in accreting neutron stars, thus affecting their heating and cooling. In particular, it can suppress cooling by URCA pairs of nuclei, which is supposed to be crucial for the hottest neutron stars. [ABSTRACT FROM AUTHOR]

We investigate the long-term evolution of black hole accretion discs formed in neutron star mergers. These discs expel matter that contributes to an r -process kilonova, and can produce relativistic jets powering short gamma-ray bursts. Here we report the results of a three-dimensional, general-relativistic magnetohydrodynamic (GRMHD) simulation of such a disc which is evolved for long enough (∼9 s, or ∼6 × 105 r g/ c) to achieve completion of mass ejection far from the disc. Our model starts with a poloidal field, and fully resolves the most unstable mode of the magnetorotational instability. We parametrize the dominant microphysics and neutrino cooling effects, and compare with axisymmetric hydrodynamic models with shear viscosity. The GRMHD model ejects mass in two ways: a prompt MHD-mediated outflow and a late-time, thermally driven wind once the disc becomes advective. The total amount of unbound mass ejected (⁠|$0.013\, \mathrm{M}_\odot$|⁠, or |${\simeq } 40{{\ \rm per\ cent}}$| of the initial torus mass) is twice as much as in hydrodynamic models, with higher average velocity (0.1 c) and a broad electron fraction distribution with a lower average value (0.16). Scaling the ejected fractions to a disc mass of |${\sim } 0.1\, \mathrm{M}_\odot$| can account for the red kilonova from GW170817 but underpredicts the blue component. About |${\sim } 10^{-3}\, \mathrm{M}_\odot$| of material should undergo neutron freezout and could produce a bright kilonova precursor in the first few hours after the merger. With our idealized initial magnetic field configuration, we obtain a robust jet and sufficient ejecta with Lorentz factor ∼1−10 to (over)produce the non-thermal emission from GW1708107. [ABSTRACT FROM AUTHOR]

Carbon-enhanced metal-poor (CEMP) stars with low abundances of neutron capture elements (CEMP-no stars) are ubiquitous among metal-poor stars in the Milky Way. Recent observations have uncovered their two subgroups that differ in the carbon to magnesium ([C/Mg]) abundance ratio. Here, we demonstrate that similar abundance patterns are also present in Milky Way-like galaxies in the eagle cosmological hydrodynamical simulation, where these patterns originate from the fact that stars may form from gas enriched predominantly by asymptotic giant branch (AGB) stars or by type II supernovae. This occurs when stars form in the poorly mixed interstellar medium of Milky Way progenitor galaxies that are undergoing bursty star formation. The CEMP-no stars with lower [C/Mg] form at the onset of a starburst from gas enriched by low-metallicity type II supernovae that power a strong outflow, quenching further star formation. When star formation resumes following cosmological gas accretion, the CEMP-no stars with higher [C/Mg] form, with enrichment by AGB ejecta evident in their higher abundance of barium and lower abundance of magnesium. This suggests that bursty star formation in the progenitors of the Galaxy leaves a permanent imprint in the abundance patterns of CEMP stars. [ABSTRACT FROM AUTHOR]

C Factor is alongside the topographic factor, one of the most influential factors in estimating soil losses by means of the USLE model. Starting from this reality, we have used three methodologies in this study in order to obtain a cover-management factor in a 291 km² territory located in North-West of Romania. The main objective of this comparative analysis is to highlight the best suited workflow for the medium-sized areas under the medium and high usage of data sets. The results were partly corroborated with data obtained from ESDAC which resulted from the application of the so-called LANDUM model. The best results have been obtained by using the Linear Spectral Unmixing technique on Landsat 8 OLI/TIRS to derive the vegetation and bare soil at the pixel level, and two more variants of built-up areas, namely one for the water, followed by the algorithm of the first four components and the evaluation of the C Factor (CLSU). [ABSTRACT FROM AUTHOR]

The chemical composition of helium-rich hot subluminous O stars plays an important role to understand and model their formation history. We present a spectroscopic analysis of four He-sdO stars,CD-31° 4800, [CW83] 0904- 02, LSS 1274 and LS IV +10° 9. The analysis is based on archival optical and UV high-resolution spectra. We used Tlusty200/Synspec48 to compute line blanketed non-LTE model atmospheres and their corresponding synthetic spectra and derive the atmospheric parameters as well as the abundances of the most prominent elements. All stars have helium-dominated atmospheres with hardly any hydrogen and temperatures between 42000 K and 47000 K while their surface gravity spans between log g = 5.4 and 5.7. CD-31° 4800 shows an enrichment of nitrogen and the characteristic pattern of hydrogen burning via the CNO-cycle, while the rest of the elements have about the solar abundance. This points to the slow merger of two helium white dwarfs as the most likely origin for this system. The other three stars are enriched in carbon, nitrogen and neon while their intermediate mass element's abundance scatters around the solar value. They were possibly formed in the deep mixing late hot flasher scenario. [ABSTRACT FROM AUTHOR]

We assess the prospects for attaining steady nuclear-flow equilibrium in expanding r-process environments where beta decay and/or neutrino capture determine the nuclear charge-changing rates. For very rapid expansions, we find that weak steady flow equilibrium normally cannot be attained. However, even when neutron-capture processes freeze out in such nonequilibrium conditions, abundance ratios of nuclear species in the r-process peaks might still mimic those attained in weak steady flow. This result suggests that the r-process yield in a regime of rapid expansion can be calculated reliably only when all neutron-capture, photodisintegration, and weak interaction processes are fully coupled in a dynamical calculation. We discuss the implications of these results for models of the r-process sited in rapidly expanding neutrino-heated ejecta. © 1997. The American Astronomical Society. All rights reserved.

Detections of deuterium in high-redshift Lyman limit absorption systems along the line of sight to QSOs promise to reveal the primordial deuterium abundance. At present, the deuterium abundances (D/H) derived from the very few systems observed are significantly discordant. Assuming the validity of all the data, if this discordance does not reflect intrinsic primordial inhomogeneity, then it must arise from processes operating after the primordial nucleosynthesis epoch. We consider processes that might lead to significant deuterium production or destruction and yet allow the cloud to mimic a chemically unevolved system. These processes include, for example, anomalous/stochastic chemical evolution and D/4He photodestruction. In general, we find it unlikely that these processes could have significantly altered D/H in Lyman limit clouds. We argue that chemical evolution scenarios, unless very finely tuned, cannot account for significant local deuterium depletion since they tend to overproduce 12C, even when allowance is made for possible outflow. Similarly, D/4He photodestruction schemes engineered to locally produce or destroy deuterium founder on the necessity of requiring an improbably large γ-ray source density. Future observations of D/H in Lyman limit systems may provide important insight into the initial conditions for the primordial nucleosynthesis process, early chemical evolution, and the galaxy formation process. © 1997. The American Astronomical Society. All rights reserved.

• In the northern Barents Sea there is a seasonal pattern of production and size distribution of planktonic foraminifers and pteropods, increasing from winter (March) to summer (July–August) and late autumn (December). • In general, pteropods dominate over planktonic foraminifera in the Arctic influenced stations. • In the study area, pteropods contribute the most (>80%) to carbon standing stocks and export production. • The highest values of carbon standing stocks and export production were found in the seasonal ice zone during all seasons. The Barents Sea is presently undergoing rapid warming and the sea-ice edge and the productive zones are retreating northward at accelerating rates. Planktonic foraminifers and shelled pteropods are ubiquitous marine calcifiers that play an important role in the carbon budget and being particularly sensitive to ocean biogeochemical changes and ocean acidification. Their distribution at high latitudes have rarely been studied, and usually only for the summer season. Here we present results of their distribution patterns in the upper 300 m in the water column (individuals m−3), protein content and size distribution on a seasonal basis to estimate their inorganic and organic carbon standing stocks (µg m−3) and export production (mg m−2 d−1). The study area constitutes a latitudinal transect in the northern Barents Sea from 76˚ N to 82˚ N including seven stations through both Atlantic, Arctic, and Polar surface water regimes and the marginal and seasonal sea-ice zones. The transect was sampled in 2019 (August and December) and 2021 (March, May, and July). The highest carbon standing stocks and export production were found at the Polar seasonally sea-ice covered shelf stations with the contribution from shelled pteropods being significantly higher than planktonic foraminifers during all seasons. We recorded the highest production of foraminifers and pteropods in summer (August 2019 and July 2021) and autumn (December 2019) followed by spring (May 2021), and the lowest in winter (March 2021). [ABSTRACT FROM AUTHOR]

We examine the process of electron neutrino and electron antineutrino capture on nuclei with masses A > 40 in the context of the post-core-bounce supernova environment. We discuss the influence of final-state electron blocking, extended distributions of Gamow-Teller strength, and the Coulomb wave correction factor on neutrino and antineutrino capture rates. We study the effect of discrete state transitions and the thermal population of excited states on these rates. We estimate the strength of forbidden capture channels and discuss their importance in neutrino capture rate estimates. We find that forbidden strength can dominate the anti-neutrino capture rates on neutron-rich nuclei which are blocked and have no allowed transitions. Forbidden weak strength may be important in other cases of neutrino or antineutrino capture as well, depending on the excitation energy distribution of this strength. In addition, the importance of neutrino capture relative to anti-neutrino capture on both heavy nuclei and free nucleons is discussed. Formulae for calculating these rates in the context of the post-core-bounce supernova environment are presented. Tables of rates are provided for some key nuclei. A FORTRAN code for calculating rates is available on request.

We investigate charged current neutrino and antineutrino capture on nuclei in the post-core bounce supernova environment. We point out that these processes may play an important, and heretofore overlooked, role in determining the nucleosynthesis in models of neutrino-heated supernova ejecta. In particular, we suggest that inclusion of these rates may help solve the problem in these models of overproduction of nuclides with neutron numbers near 50 and, in addition, enhance the production of the some of the light p-process nuclei in the α-process, particularly 92Mo. The neutrino capture rates on neutron-rich nuclei are found to be dominated by transitions to the Fermi (isobaric analog state) and Gamow-Teller resonances. In these cases, the neutrino capture thresholds are approximately just the nuclear Coulomb energy differences between nuclear parents and daughters, and the neutrino capture rates therefore exhibit only weak dependence on neutron and proton numbers compared to that of β--decay rates. We exploit this property to constrain the location of the r-process region in the post-core bounce supernova environment. We present analytic estimates for the rates of electron neutino and antineutrino capture on nuclei and nucleons.

We study big bang nucleosynthesis in the presence of large mass scale, nonlinear entropy fluctuations. Overdense regions, with masses above the local baryon Jeans mass, are expected to collapse and form condensed objects. Surviving nucleosynthesis products therefore tend to originate from underdense regions. We compute expected survival light-element (2H, 3He, 4He, and 7Li) abundance yields for a variety of stochastic fluctuation spectra. In general, we find that spectra with significant power in fluctuations on mass scales below that of the local baryon Jeans mass (∼ 105 M⊙) produce nucleosynthesis yields which are in conflict with observationally inferred primordial abundances. Only when this small-scale structure is absent or suppressed, and the collapse efficiency of overdense regions is high, can there exist a range of fluctuation spectral characteristics which concievably could meet all primordial abundance constraints. Even then, the primordial abundance of 7Li would have to be larger than 7Li/H ≈ 3 × 10-10. In such models, abundance constraints could be met even when the precollapse baryonic fraction of the closure density is Ωb ≈ 0.2 h-2 (h is the Hubble parameter in units of 100 km s-1 Mpc-1). Nucleosynthesis in these models is characterized by high 2H/H and low 4He mass fraction relative to a homogeneous big bang at a given value of Ωb h2. A potentially observable signature of these models is the production of intrinsic primordial abundance variations on baryon mass scales up to 1010-1012 M⊙.

After a description of recent developments in the physics of rapid neutron capture in Type II supernovae and a discussion of the detectability of supernovae ejecta, we present the data from a search for Ge, Kr, Yb, Os, and Hg in five stars behind or within the Vela remnant. Only Ge II was detected, but its column density and the upper limits of the other species show no excess above the estimated contribution of ambient gas. Finally, we discuss the extent that clumping and the improved performance of the GHRS may make r-process ejecta from Vela detectable in the future.

Abstract The determination of photospheric abundances in late-type stars from spectroscopic observations is a well-established field, built on solid theoretical foundations. Improving those foundations to refine the accuracy of the inferred abundances has proven challenging, but progress has been made. In parallel, developments on instrumentation, chiefly regarding multi-object spectroscopy, have been spectacular, and a number of projects are collecting large numbers of observations for stars across the Milky Way and nearby galaxies, promising important advances in our understanding of galaxy formation and evolution. After providing a brief description of the basic physics and input data involved in the analysis of stellar spectra, a review is made of the analysis steps, and the available tools to cope with large observational efforts. The paper closes with a quick overview of relevant ongoing and planned spectroscopic surveys, and highlights of recent research on photospheric abundances.

We show that a class of inhomogeneous big bang nucleosynthesis models exist which yield light-element abundances in agreement with observational constraints for baryon-to-photon ratios significantly smaller than those inferred from standard homogeneous big bang nucleosynthesis (HBBN). These inhomogeneous nucleosynthesis models are characterized by a bimodal distribution of baryons in which some regions have a local baryon-to-photon ratio η ≈ 3 × 10-10, while the remaining regions are baryon depleted. HBBN scenarios with primordial (2H + 3He)/H ≲ 9 × 10-5 necessarily require that most baryons be in a dark or nonluminous form, although new observations of a possible high deuterium abundance in Lyα clouds may relax this requirement somewhat. The models described here present another way to relax this requirement and can even eliminate any lower bound on the baryon-to-photon ratio.

We show that a class of inhomogeneous big bang nucleosynthesis models exist which yield light-element abundances in agreement with observational constraints for baryon-to-photon ratios significantly smaller than those inferred from standard homogeneous big bang nucleosynthesis (HBBN). These inhomogeneous nucleosynthesis models are characterized by a bimodal distribution of baryons in which some regions have a local baryon-to-photon ratio η ≈ 3 × 10-10, while the remaining regions are baryon depleted. HBBN scenarios with primordial (2H + 3He)/H ≲ 9 × 10-5 necessarily require that most baryons be in a dark or nonluminous form, although new observations of a possible high deuterium abundance in Lyα clouds may relax this requirement somewhat. The models described here present another way to relax this requirement and can even eliminate any lower bound on the baryon-to-photon ratio.

This paper reviews the results of our 40-year studies of Cyg X-1 using various methods and observational data: mainly optical photometric, spectroscopic, and polarimetric ones, with X-ray data also considered. We show that, while performing model calculations approximating observational data, it is vital to take into account a diversity of factors, for example, the accretion structure variations that influence the shape of the orbital light curve. We emphasize the necessity of comprehensive studies, development and usage of all possible methods.

In the course of a general study of electron capture and β-decay rates on fp shell nuclei which are abundant after core silicon burning in massive stars, we have found that the Fuller, Fowler and Newman β-decay rate are much stronger than generally has been realized. In fact, they can balance the capture rates during the conditions which are prevalent after core silicon burning, resulting in a new Urca process. The strength of the β-decay rates is a result of thermal population of the Gamow-Teller back resonance in the parent nucleus and the behavior of β-decay and electron capture Q-values for ensembles of nuclei in nuclear statistical equilibrium. All β-decay rate tabulations prior to Fuller, Fowler, and Newman neglected the contribution of back resonances and thus drastically underestimated the overall rate. We use a simple analytic model to explain this balancing. The full rates are coupled to a nuclear statictical equilibrium code to demonstrate the strength of the decays. One zone models are used to examine, in a self-consistent fashion, how these rates could affect the presupernova evolution of the iron core and hence the core collapse problem.

We present a detailed numerical study of inhomogeneous big bang nucleosynthesis where, for the first time, nuclear reactions are coupled to all significant fluctuation dissipation processes. These processes include neutrino heat transport, baryon diffusion, photon diffusive heat transport, and hydrodynamic expansion with photon-electron Thomson drag. Light-clement abundance yields are presented for broad ranges of initial amplitudes and length scales for spherically condensed fluctuations. The 2H, 3He, 4He, and 7Li nucleosynthesis yields are found to be inconsistent with observationally inferred primordial abundances for all but very narrow ranges of fluctuation characteristics. Rapid hydrodynamic expansion of fluctuations late in the nucleosynthesis epoch results in significant destruction of 7Li (7Be) only if the baryonic contribution to the closure density (Ωb) is less than or comparable to the upper limit on this quantity from homogeneous big bang nucleosynthesis. This implies that 7Li overproduction will preclude an increase on the upperlimit for Ωb for any inhomogeneous nucleosynthesis scenario employing spherically condensed fluctuations.

We show that primordial nucleosynthesis in baryon-inhomogeneous big-bang models can lead to significant heavy-element production while still satisfying all of the light-element abundance constraints including the low lithium abundance observed in Population II stars. The parameters which admit this solution arise naturally from the process of neutrino-induced expansion of baryon inhomogeneities prior to the epoch of nucleosynthesis. These solutions entail a small fraction of baryons (≲2%) in very high density regions with local baryon to photon ratio ηh ≃ 10-4, while most baryons are at a baryon-to-photon ratio which optimizes the agreement with light-element abundances. This model would imply a unique signature of baryon inhomogeneities in the early universe, evidenced by the existence of primordial material containing heavy-element products of proton and alpha-burning reactions with an abundance of [Z] ∼ -6 to -4.

The Multi-Unit Spectroscopic Explorer (MUSE) is a large integral field unit mounted on the ESO Very Large Telescope. Its spatial (60 arcsecond field) and wavelength (4800–9300Å) coverage is well suited to detailed imaging spectroscopy of extended planetary nebulae, such as in the Galaxy. An overview of the capabilities of MUSE applied to Planetary Nebulae (PNe) is provided together with the specific advantages and disadvantages. Some examples of archival MUSE observations of PNe are provided. MUSE datacubes for two targets (NGC 3132 and NGC 7009) are analyzed in detail, and they are used to show the advances achievable for planetary nebula studies. Prospects for further MUSE observations of PNe and a broader analysis of existing datasets are outlined.

We study several models for the origin and chemical evolution of compact irregular galaxies in order to determine the primordial 4He abundance, Yp, from the zero metallicity intercept of the observed Y versus Z correlations. This study confirms the suggestion that a straight-line fit to the observations does not necessarily give the correct primordial 4He abundance. This is especially true for the extrapolation of the Y versus N/H data which depends upon the relative contributions from secondary and primary nitrogen in low metallicity stars. The extrapolation of the O/H data is also slightly nonlinear even for primary oxygen in a closed-box model with instantaneous recycling due to the time dependence of the hydrogen mass fraction, the breakdown of the instantaneous recycling approximation, the production of helium, nitrogen, and oxygen in stars of different mass, and ejection of metal-poor material from low-mass stars. Our best fits to the data, even after excluding possible contamination of H II regions from neighboring Wolf-Rayet stars, gives Yp = 0.228 ± 0.005 for O/H and Yp = 0.223 ± 0.006 for N/H. These primordial helium abundances are as much as 2 σ below the minimum helium abundance which can be produced in the standard homogeneous big bang model with three light neutrino flavors. This discrepancy may be due to shortcomings of the chemical evolution models, additional systematic errors in the determination of the helium and/or metal abundances in extragalactic H II regions, or effects of nonstandard primordial nucleosynthesis.

We study several models for the origin and chemical evolution of compact irregular galaxies in order to determine the primordial 4He abundance, Yp, from the zero metallicity intercept of the observed Y versus Z correlations. This study confirms the suggestion that a straight-line fit to the observations does not necessarily give the correct primordial 4He abundance. This is especially true for the extrapolation of the Y versus N/H data which depends upon the relative contributions from secondary and primary nitrogen in low metallicity stars. The extrapolation of the O/H data is also slightly nonlinear even for primary oxygen in a closed-box model with instantaneous recycling due to the time dependence of the hydrogen mass fraction, the breakdown of the instantaneous recycling approximation, the production of helium, nitrogen, and oxygen in stars of different mass, and ejection of metal-poor material from low-mass stars. Our best fits to the data, even after excluding possible contamination of H II regions from neighboring Wolf-Rayet stars, gives Yp = 0.228 ± 0.005 for O/H and Yp = 0.223 ± 0.006 for N/H. These primordial helium abundances are as much as 2 σ below the minimum helium abundance which can be produced in the standard homogeneous big bang model with three light neutrino flavors. This discrepancy may be due to shortcomings of the chemical evolution models, additional systematic errors in the determination of the helium and/or metal abundances in extragalactic H II regions, or effects of nonstandard primordial nucleosynthesis.