The Cosmic Production of Helium

Publications of the Astronomical Society of Australia, 2004

K-dwarfs are very long lived, slowly evolving stars, so that their present day helium Y and metal content (metallicity Z) is essentially the same as when they were born. K-dwarfs thus contain a fossil record of the amount of helium and metals which has been produced in successive stellar generations over the lifetime of the Galaxy. We here estimate the amount of helium compared to the amount of metals produced via stellar fusion (Δ Y/ΔZ). We use K-dwarfs in the Hipparcos catalogue for which accurate metallicities and luminosities are available. By including recently measured K-dwarfs with super-solar metallicities we are able to obtain a very significant improvement on previous studies. The best fitting value is ΔY/ΔZ = 2.4 ± 0.4 at the 68% confidence level. Values as low as 1 or as high as 4 are excluded with more than 99% confidence.

Monthly Notices of the Royal Astronomical Society, 2007

We use nearby K dwarf stars to measure the helium-to-metal enrichment ratio ∆Y /∆Z, a diagnostic of the chemical history of the Solar Neighbourhood. Our sample of K dwarfs has homogeneously determined effective temperatures, bolometric luminosities and metallicities, allowing us to fit each star to the appropriate stellar isochrone and determine its helium content indirectly. We use a newly computed set of Padova isochrones which cover a wide range of helium and metal content.

Publications of the Astronomical Society of the Pacific, 2011

Monthly Notices of the Royal Astronomical Society

The structural stratification of a solar-type main-sequence star primarily depends on its mass and chemical composition. The surface heavy element abundances of the solar-type stars are reasonably well determined using conventional spectroscopy; however, the second most abundant element helium is not. This is due to the fact that the envelope temperature of such stars is not high enough to excite helium. Since the helium abundance of a star affects its structure and subsequent evolution, the uncertainty in the helium abundance of a star makes estimates of its global properties (mass, radius, age etc.) uncertain as well. The detections of the signatures of the acoustic glitches from the precisely measured stellar oscillation frequencies provide an indirect way to estimate the envelope helium content. We use the glitch signature caused by the ionization of helium to determine the envelope helium abundance of 38 stars in the Kepler seismic LEGACY sample. Our results confirm that atomic diffusion does indeed take place in solar-type stars. We use the measured surface abundances in combination with the settling predicted by the stellar models to determine the initial abundances. The initial helium and metal mass fractions have subsequently been used to get the preliminary estimates of the primordial helium abundance, Y p = 0.244 ± 0.019, and the Galactic enrichment ratio, Y/ Z = 1.226 ± 0.849. Although the current estimates have large errorbars due to the limited sample size, this method holds great promises to determine these parameters precisely in the era of upcoming space missions.

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

Monthly Notices of the Royal Astronomical Society, 2010

To study the possible origin of the huge helium enrichment attributed to the stars on the blue main sequence of ω Centauri, we make use of a chemical evolution model that has proven able to reproduce other major observed properties of the cluster, namely, its stellar metallicity distribution function, age-metallicity relation and trends of several abundance ratios with metallicity. In this framework, the key condition to satisfy all the available observational constraints is that a galactic-scale outflow develops in a much more massive parent system, as a consequence of multiple supernova explosions in a shallow potential well. This galactic wind must carry out preferentially the metals produced by explosive nucleosynthesis in supernovae, whereas elements restored to the interstellar medium through low-energy stellar winds by both asymptotic giant branch (AGB) stars and massive stars must be mostly retained. Assuming that helium is ejected through slow winds by both AGB stars and fast rotating massive stars (FRMSs), the interstellar medium of ω Centauri's parent galaxy gets naturally enriched in helium in the course of its evolution. throughout this paper are by number, except for Y and Z, that indicate the mass fraction of helium and total metals, respectively. As usual, log ε(X) ≡ 12 + log (X/H), c 2009 RAS

2013

List of Tables ix List of Figures x 3 He 4  12 C 12 C(α,γ) 16 O C

Presentamos modelos de evolución química para el disco de nuestra galaxia. También presentamos una nueva determinación de X, Y , y Z para M17, una región H ii de nuestra galaxia rica en elementos pesados. Comparamos nuestros modelos del disco galáctico con las abundancias de las regiones H ii. El valor predicho por nuestro modelo para ∆Y /∆O es muy similar al valor que obtenemos por medio de las observaciones de M17 y la abundancia primordial de helio, Y p. A partir de M17 y Y p obtenemos que ∆Y /∆Z = 1.97 ± 0.41, resultado que concuerda con dos determinaciones de ∆Y /∆Z, obtenidas a partir de observaciones de estrellas enanas K de la vecindad solar, que corresponden a 2.1 ± 0.4 y 2.1 ± 0.9 respectivamente. Nuestros modelos ajustan razonablemente bien el valor de O/H con el que se formó el Sol.

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 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.