Determination Of Yp Research Articles

Primordial 4He abundance refinement using sample of SDSS DR15 galaxies

We present spectroscopic data on metal-deficient HII regions from dwarf galaxies of the Local Volume on the basis of Sloan Digital Sky Survey (SDSS) Data Release 15 observations. Using the data on the intensities of the optical emission lines of the HII regions, one can determine their chemical composition, as well as other physical properties, such as temperature and electron density. Oxygen abundance (and thus the metallicity) of the source is determined using direct method by using strong [OIII] lines. Here we present only sources with oxygen abundance below 12 + logO/H = 8.5. These data is used to determine primordial helium abundance, which value should be determined with as little errors as possibles. The main way to reduce errors is to use the spectroscopic data of the highest quality possible. On the other hand one can statistically decrease evaluation errors by using large dataset. Main focus of this paper is studying if we can decrease errors in Yp determination by combining HeBCD sample with SDSS spectra of lower quallity.

Primordial helium and the cosmic background radiation

The products of primordial nucleosynthesis, along with the cosmic microwave background (CMB) photons, are relics from the early evolution of the Universe whose observations probe the standard model of cosmology and provide windows on new physics beyond the standard models of cosmology and of particle physics. According to the standard, hot big bang cosmology, long before any stars have formed a significant fraction ( ∼ 25%) of the baryonic mass in the Universe should be in the form of helium-4 nuclei. Since current observations of 4He are restricted to low redshift regions where stellar nucleosynthesis has occurred, an observation of high redshift, prestellar, truly primordial 4He would constitute a fundamental test of the hot, big bang cosmology. At recombination, long after big bang nucleosynthesis (BBN) has ended, the temperature anisotropy spectrum imprinted on the CMB depends on the 4He abundance through its connection to the electron density and the effect of the electron density on Silk damping. Since the relic abundance of 4He is relatively insensitive to the universal density of baryons,but is sensitive to a non-standard, early Universe expansion rate, the primordial mass fraction of 4He, Yp, offers a test of the consistency of the standard models of BBN and the CMB and, provides constraints on non-standard physics. Here, the WMAP seven year data (supplemented by other CMB experiments), which lead to an indirect determination of Yp at high redshift, are compared to the BBN predictions and to the independent, direct observations of 4He in low redshift, extragalactic HII regions. At present, given the very large uncertainties in the CMB-determined primordial 4He abundance (as well as for the helium abundances inferred from HII region observations), any differences between the BBN predictions and the CMB observations are small, at a level ⪅1.5σ.

Revised Primordial Helium Abundance Based on New Atomic Data

We have derived a primordial helium abundance of Yp = 0.2477 +- 0.0029, based on new atomic physics computations of the recombination coefficients of He I and of the collisional excitation of the H I Balmer lines together with observations and photoionization models of metal-poor extragalactic H II regions. The new atomic data increase our previous determination of Yp by 0.0086, a very significant amount. By combining our Yp result with the predictions made by the standard Big Bang nucleosynthesis model, we find a baryon-to-photon ratio, \eta, in excellent agreement both with the \eta value derived by the primordial deuterium abundance value observed in damped Lyman-\alpha systems and with the one obtained from the WMAP observations.

The Primordial Abundance of4He: A Self‐consistent Empirical Analysis of Systematic Effects in a Large Sample of Low‐Metallicity HiiRegions

We determine the primordial helium mass fraction Yp using 93 spectra of 86 low-metallicity extragalactic H II regions. This sample constitutes the largest and most homogeneous high-quality data set in existence for the determination of Yp. For comparison, and to improve the statistics in our investigation of systematic effects affecting the Yp determination, we have also considered a sample of 271 low-metallicity H II regions selected from Data Release 5 of the Sloan Digital Sky Survey. Although this larger sample shows more scatter, it gives results that are consistent at the 2 σ level with our original sample. We have considered known systematic effects that may affect the 4He abundance determination. They include different sets of He I line emissivities and reddening laws, collisional and fluorescent enhancements of He I recombination lines, underlying He I stellar absorption lines, collisional excitation of hydrogen lines, temperature and ionization structure of the H II region, and deviation of He I and H emission-line intensities from case B. However, the most likely value of Yp depends on the adopted set of He I line emissivities. Using Monte Carlo methods to solve simultaneously the above systematic effects, we find a primordial helium mass fraction of Yp = 0.2472 ± 0.0012 when using the He I emissivities from Benjamin and coworkers and 0.2516 ± 0.0011 when using those from Porter and coworkers. The first value agrees well with the value given by standard big bang nucleosynthesis (SBBN) theory, while the value obtained with the likely more accurate emissivities of Porter and coworkers is higher at the 2 σ level. This latter value, if confirmed, would imply slight deviations from SBBN.

The Primordial Abundance of4He: An Update

We include new data in an updated analysis of helium in low-metallicity extragalactic H II regions with the goal of deriving the primordial abundance of 4He (YP). We show that the new observations of Izotov et al. are consistent with previous data. However, they should not be taken in isolation to determine YP, owing to the lack of sufficiently low-metallicity points. We use the extant data in a semi-empirical approach to bounding the size of possible systematic uncertainties in the determination of YP. Our best estimate for the primordial abundance of 4He assuming a linear relation between 4He and O/H is YP = 0.230 ? 0.003(stat) based on the subset of H II regions with the lowest metallicity; for our full data set we find YP = 0.234 ? 0.002(stat). Both values are entirely consistent with our previous results. We discuss the implications of these values for standard big bang nucleosynthesis (SBBN), particularly in the context of recent measurements of deuterium in high-redshift, low-metallicity QSO absorption-line systems.

The Primordial Helium Abundance: Systematic Effects and a New Determination

We present new high-quality spectrophotometric observations of 27 supergiant H II regions in 23 low-metallicity blue compact galaxies (BCGs) with oxygen abundance 12 + log (O/H) between 7.22 and 8.17 (Z☉/50 ≤ Z ≤ Z☉/6), most of which were selected from the first and second Byurakan objective-prism surveys. Combining this new data with our previous sample of 10 low-metallicity BCGs, we extract a subsample of 27 H II regions in 24 BCGs most appropriate for the determination of the primordial helium abundance Yp. We find that the most metal-deficient BCG known, I Zw 18 (Z ~ Z☉/50), cannot be used for this purpose because of its abnormally low He I line intensities. We have examined critically the systematic effects that may influence the determination of Yp. We find that the effects of the corrections for neutral helium and for underlying stellar absorption in the He I lines, possible deviations from case B recombination theory, fluorescent enhancement of the He I line intensities, temperature fluctuations in H II regions, Wolf-Rayet stellar winds, and supernova shock waves to be small. The main effect comes from the particular set of atomic data used. The best set of atomic data for use in the determination of Yp is composed of Smits's He I emissivities and Kingdon & Ferland's collisional enhancement correction factors. This set gives the smallest dispersion of the data in the Y-O/H and Y-N/H planes and corrects best the data for collisional enhancement effects. By extrapolating the Y versus O/H and Y versus N/H linear regressions to O/H = N/H = 0, we obtain Yp = 0.243 ± 0.003 for both regressions, considerably larger than the values derived before. In the framework of the standard hot big bang nucleosynthesis model with a number of neutrino families Nν = 3 and a neutron half-lifetime τn = 887 s, our new Yp gives a baryon-to-photon number ratio η=3.51.0−0.7 × 10-10, or a baryonic mass fraction Ωh250=0.05±0.01. Our Yp determination is fully consistent with measurements of the primordial 7Li in Galactic halo stars and of (D +3He) in the local interstellar medium and the solar system, and consistent at the 2 σ level with the primordial D abundance derived in a quasar absorption system by Tytler, Fan, & Burles. We derive a slope dY/dZ = 1.7 ± 0.9, considerably smaller than the slopes obtained before and consistent with chemical evolution models for star-forming dwarf galaxies with an outflow of well-mixed material. Extrapolation to solar metallicity with such a slope gives the correct solar helium mass fraction within the errors.

Polar motion results from Geos 3 laser ranging

The observability of polar motion from laser range data has been investigated, and the contributions from the dynamical and kinematical effects have been evaluated. Using 2‐day arcs with Geos 3 laser data, simultaneous solutions for pole position components, xp and yp, and orbit elements have been obtained for a 2‐week interval spanning August 27 to September 10, 1975, using three NASA Goddard Space Flight Center stations located at Washington, D.C. (Stalas), Bermuda, and Grand Turk. The results for yp from this limited data set differenced with the BIH linearly interpolated values yield a mean of 39 cm and a standard deviation of 1.07 m. Consideration of the variance associated with each estimate yields a mean of 20 cm and a standard deviation of 81 cm. The results for xp indicate that the mean value is in fair agreement with the BIH; however, the xp determination is weaker than the yp determination due to the distribution of laser sites (all three are between 77°W and 65°W) which results in greater sensitivity to the data distribution. In addition, the sensitivity of these results to various model parameters is discussed.