Fundamental properties of nearby stars and the consequences on ΔY/ΔZ
Context. One of the greatest difficulties in astrophysics is the determination of the fundamental stellar parameters, one of which is the initial mass fraction of helium (Y). However, given that Y can be measured spectroscopically in only a small percentage of stars, a linear relationship is assumed...
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| Principais autores: | , , |
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| Formato: | article |
| Idioma: | English |
| Publicado em: |
Astronomy & Astrophysics
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| Endereço do item: | https://repositorio.ufrn.br/jspui/handle/123456789/29021 https://doi.org/10.1051/0004-6361/201220765 |
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| Resumo: | Context. One of the greatest difficulties in astrophysics is the determination of the fundamental stellar parameters, one of which is the initial mass fraction of helium (Y). However, given that Y can be measured spectroscopically in only a small percentage of stars, a linear relationship is assumed between Y and the mass fraction of metals (Z) from a canonical perspective of the chemical evolution of the galaxies. This Y–Z relation is generally represented as Y = Yp + ΔY/ΔZ × Z, with the value of the helium-to-metal enrichment ratio (ΔY/ΔZ) assumed as a constant. However, there is no fundamental reason for every star to have a Y value on a linear
scale with Z. Indeed, different ΔY/ΔZ values may be expected in different populations which have undergone different chemical enrichment histories. Aims. In this paper a new method for determining the fundamental stellar parameters of nearby stars is presented that uses at the same time Mbol, Teff, and log . One of these parameters is Y, which is used to determine the validity of the Y–Z relation. Methods. A new set of evolutionary tracks is created using the PGPUC stellar evolution code, which includes 7 masses (0.5 ≤ M/M ≤ 1.1), 7 helium abundances (0.230 ≤ Y ≤ 0.370), and 12 metallicities (1.6× 10−4 ≤ Z ≤ 6.0× 10−2) for solar-scaled chemical compositions ([α/Fe] = 0.0). The suggested method is tested using two different spectroscopic databases of nearby main sequence stars with precise parallaxes, and spectroscopic measurements of [Fe/H], Teff and. Results. The proposed method is compared to other techniques used to determine the fundamental stellar parameters, where one assumes an age of 5 Gyr for all nearby stars. This comparison demonstrates that the hypothesis regarding constant age leads to an underestimation of the Y value, especially for low metallicities. In addition, the suggested method is limited to masses above 0.60 M and requires high-precision measurements of spectroscopic surface gravities in order to obtain reliable results. Finally, estimating masses and Ages assuming a Y–Z relation rather than a free Y value may induce average errors of approximately 0.02 M and 2 Gyr, respectively. |
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