Collisional excitation of hydrogen and the determination of the primordial helium abundance from H II regions (original) (raw)
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The Astrophysical Journal, 2000
We present results from photoionization models of low-metallicity HII regions. These nebulae form the basis for measuring the primordial helium abundance. Our models show that the helium ionization correction factor (ICF) can be non-negligible for nebulae excited by stars with effective temperatures larger than 40,000 K. Furthermore, we find that when the effective temperature rises to above 45,000 K, the ICF can be significantly negative. This result is independent of the choice of stellar atmosphere. However, if an HII region has an [O III] λ5007/[O I] λ6300 ratio greater than 300, then our models show that, regardless of its metallicity, it will have a negligibly small ICF. A similar, but metallicity dependent, result was found using the [O III] λ5007/Hβ ratio. These two results can be used as selection criteria to remove nebulae with potentially nonnegligible ICFs. Using our metallicity independent criterion on the data of Izotov & Thuan (1998) results in a 20% reduction of the rms scatter about the best fit Y − Z line. A fit to the selected data results in a slight increase of the value of the primordial helium abundance.
The primordial helium abundance from updated emissivities
Journal of Cosmology and Astroparticle Physics, 2013
Observations of metal-poor extragalactic H II regions allow the determination of the primordial helium abundance, Y p. The He I emissivities are the foundation of the model of the H II region's emission. Porter, Ferland, Storey, & Detisch (2012) have recently published updated He I emissivities based on improved photoionization cross-sections. We incorporate these new atomic data and update our recent Markov Chain Monte Carlo analysis of the dataset published by Izotov, Thuan, & Stasińska (2007). As before, cuts are made to promote quality and reliability, and only solutions which fit the data within 95% confidence level are used to determine the primordial He abundance. The previously qualifying dataset is almost entirely retained and with strong concordance between the physical parameters. Overall, an upward bias from the new emissivities leads to a decrease in Y p. In addition, we find a general trend to larger uncertainties in individual objects (due to changes in the emissivities) and an increased variance (due to additional objects included). From a regression to zero metallicity, we determine Y p = 0.2465 ± 0.0097, in good agreement with the Planck result of Y p = 0.2485 ± 0.0002. In the future, a better understanding of why a large fraction of spectra are not well fit by the model will be crucial to achieving an increase in the precision of the primordial helium abundance determination. Contents 1 Introduction 1 2 Revisiting AOS3 2 3 Investigating the updated emissivities 4 4 Reviewing the new sample 5
Chemical Abundances in Our Galaxy and Other Galaxies Derived from H ii Regions
The Evolution of Galaxies, 2001
We discuss the accuracy of the abundance determinations of H ii regions in our Galaxy and other galaxies. We focus on the main observational constraints derived from abundance determinations that have implications for models of galactic chemical evolution: a) the helium to hydrogen abundance ratio, He/H; b) the oxygen to hydrogen abundance ratio, O/H; c) the carbon to oxygen abundance ratio, C/O; d) the helium to oxygen and helium to heavy elements abundance ratios, ∆Y /∆O and ∆Y /∆Z; and e) the primordial helium abundance, Yp.
Uncertainties in 4He abundance determinations in extragalactic H II regions
New Astronomy, 2001
In order to provide a useful constraint on standard big bang nucleosynthesis predictions, the primordial helium abundance must be determined with an accuracy of a few percent. Here we investigate the estimation of errors in deriving and reporting nebular helium abundances from optical emission line spectra of H II regions. We first show that while an estimation of reddening and underlying stellar absorption in H Balmer emission lines can be made by solving for these quantities simultaneously, a minimization routine may underestimate the true errors in the solution due to the degeneracy of the sensitivities of the individual lines. We show that Monte Carlo modeling allows for a better estimate of the errors in underlying absorption and reddening which need to be propagated to all of the data. We emphasize that a comparison of the corrected Balmer line strengths relative to their theoretical values provides a robust test of the magnitude of their associated 4 errors. We conduct a detailed examination of ''self-consistent'' methods for determining not only the He abundance from H I and He I emission line ratios, but also other physical parameters. We show that there are strong degeneracies in the 4 sensitivity of the He abundance to various physical parameters. Because of this, Monte Carlo simulations of the data are 4 required to derive accurate estimates of both the He abundance and the appropriate errors. Typically, we find that the uncertainties derived from the Monte-Carlo simulations are a factor of | 2 larger than those obtained from a straight minimization procedure. This translates into a similar increase in the size of the uncertainty of the derived primordial abundance. For the first time, we demonstrate how to quantify the effects of stellar absorption underlying the He I emission lines (as opposed to assuming that the effects are negligible). We further show that He I l4026 is a sensitive diagnostic of underlying He I absorption, and we recommend adding it to minimization methods. Finally, we demonstrate that solving for physical parameters via a minimization routine opens up the possibility of incorrect solutions if there are any systematic problems with even one observed He I emission line.
Space Sciences Series of ISSI, 1998
Recent determinations of the primordial He abundance have given significantly different results. We are attempting to identify some of the causes of these differences and propose observational solutions. Here we identify a systematic difference in how the data are interpreted (differences in corrections for the presence of neutral helium) and the importance of a systematic bias towards lower derived helium abundances (underestimating the presence of underlying stellar absorption).
He i Emission in the Orion Nebula and Implications for Primordial Helium Abundance
The Astrophysical Journal, 2007
We apply a recently developed theoretical model of helium emission to observations of both the Orion Nebula and a sample of extragalactic H II regions. In the Orion analysis, we eliminate some weak and blended lines and compare theory and observation for our reduced line list. With our best theoretical model we find an average difference between theoretical and observed intensities I predicted /I observed − 1 = 6.5%. We argue that both the red and blue ends of the spectrum may have been inadequately corrected for reddening. For the 22 highest quality lines, with 3499Å ≤ λ ≤ 6678Å, our best model predicts observations to an average of 3.8%. We also perform an analysis of the reported observational errors and conclude they have been underestimated. In the extragalactic analysis, we demonstrate the likelihood of a large systematic error in the reported data and discuss possible causes. This systematic error is at least as large as the errors associated with nearly all attempts to calculate the primordial helium abundance from such observations. Our Orion analysis suggests that the problem does not lie in the theoretical models. We demonstrate a correlation between equivalent width and apparent helium abundance of lines from extragalactic sources that is most likely due to underlying stellar absorption. Finally, we present fits to collisionless case-B He I emissivities as well as the relative contributions due to collisional excitations out of the metastable 2s 3 S term.
Disentangling the metallicity and star formation history of H ii galaxies through tailor-made models
Monthly Notices of the Royal Astronomical Society, 2010
We present a self-consistent study of the stellar populations and the ionized gas in a sample of 10 HII galaxies with, at least, four measured electron temperatures and a precise determination of ionic abundances following the "direct method". We fitted the spectral energy distribution of the galaxies using the program STARLIGHT and Starburst99 libraries in order to quantify the contribution of the underlying stellar population to the equivalent width of Hβ (EW(Hβ)), which amounts to about 10 % for most of the objects. We then studied the Wolf-Rayet stellar populations detected in seven of the galaxies. The presence of these populations and the EW(Hβ) values, once corrected for the continuum contribution from underlying stars and UV dust absorption, indicate that the ionizing stellar populations were created following a continuous star formation episode of 10 Myr duration, hence WR stars may be present in all of objects even if they are not detected in some of them.
Precision abundance analysis of bright H ii galaxies
Monthly Notices of the Royal Astronomical Society, 2007
We present high signal-to-noise spectrophotometric observations of seven luminous Hii galaxies. The observations have been made with the use of a double-arm spectrograph which provides spectra with a wide wavelength coverage, from 3400 to 10400Å free of second order effects, of exactly the same region of a given galaxy. These observations are analysed applying a methodology designed to obtain accurate elemental abundances of oxygen, sulphur, nitrogen, neon, argon and iron in the ionized gas. Four electron temperatures and one electron density are derived from the observed forbidden line ratios using the five-level atom approximation. For our best objects errors of 1% in t e ([Oiii]), 3% in t e ([Oii]) and 5% in t e ([Siii]) are achieved with a resulting accuracy of 7% in total oxygen abundances, O/H. The ionisation structure of the nebulae can be mapped by the theoretical oxygen and sulphur ionic ratios, on the one side, and the corresponding observed emission line ratios, on the other -the η and η' plots -. The combination of both is shown to provide a means to test photo-ionisation model sequences currently applied to derive elemental abundances in Hii galaxies.
The Astrophysical Journal, 2007
We determine the primordial helium mass fraction Y p using 93 spectra of 86 low-metallicity extragalactic H ii regions. This sample constitutes the largest and most homogeneous high-quality data sets in existence for the determination of Y p. For comparison and to improve the statistics in our investigation of systematic effects affecting the Y p determination, we have also considered a sample of 271 low-metallicity H ii regions selected from the Data Release 5 of the Sloan Digital Sky Survey. Although this larger sample shows more scatter, it gives results that are consistent at the 2σ level with our original sample. We have considered known systematic effects which may affect the 4 He 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 Y p depends on the
Extragalactic Abundances of Hydrogen, Deuterium and Helium: New Steps, Missteps and Next Steps
1997
Estimates of the deuterium abundance in quasar absorbers are reviewed, including a brief account of incorrect claims published by the author and a brief review of the problem of hydrogen contamination. It is concluded that the primordial abundance may be universal with a value (D/H)Papprox10−4(D/H)_P\approx 10^{-4}(D/H)Papprox10−4, within about a factor of two, corresponding to OmegaBh0.72approx0.02\Omega_B h_{0.7}^2\approx 0.02OmegaBh0.72approx0.02 or eta10approx2.7\eta_{10}\approx 2.7eta10approx2.7 in the Standard Big Bang. This agrees with current limits on primordial helium, YPle0.243Y_P\le 0.243YPle0.243, which are shown to be surprisingly insensitive to models of stellar enrichment. It also agrees with a tabulated sum of the total density of baryons in observed components. Much lower primordial deuterium ($\approx 2\times 10^{-5}$) is also possible but disagrees with currently estimated helium abundances; the larger baryon density in this case fits better with current models of the Lyman-$\alpha$ forest but requires the bulk of the baryons to be in some currently uncounted form.