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First star signatures on high-z GRBs and DLAs
First star signatures on high-z GRBs and DLAs
First stars were born after the dark age era of the Universe. They produced the first light and metal pollution in the Universe. This was predicted in theory a long time ago, but no direct observation is available. In this thesis I study the first star signatures in high-z Gamma-ray Bursts from Population II stars (GRBIIs) and in Damped Laymanα Absorbers (DLAs), using numerical N-body/hydrodynamic simulations that include atomic and molecular cooling, star formation and metal enrichment from stellar populations with different initial mass functions. I first use one simulation based on a very massive first star model (m∗ = [100, 500]M⊙) to study the GRBIIs hosted in a medium enriched by first stars (PopIII-dominated). I find that a high fraction of high-z GRBIIs are PopIII-dominated, e.g. ∼10% at z = 10, but this fraction decreases quickly with redshift, e.g. only ∼1% at z = 6. Then I study the possibility to distinguish different first star models using observations of high-z GRBs by running three simulations with different mass ranges and the corresponding lifetimes and metal yields for first stars. I find that the fraction of GRBs that are PopIII-dominated is independent from the first star models. Similarly, PopIII-dominated GRBIIs are mostly hosted in galaxies with low metallicity, star formation rate and stellar mass. Because of the special metal yields of metal-free stars, the first star signals could be identified with the metal abundance ratios derived from the absorption lines in the GRBII afterglow spectra, e.g. the [Si/O] vs [C/O] or [Fe/C] vs [Si/C] would distinguish an imprint from very massive and massive first stars. However, the currently observed high-z GRBs, e.g. GRB 130606A at z = 5.91 and GRB 111008A at z = 5.0, do not display obvious metal features from first stars, which is also consistent with the low fraction of PopIII-dominated GRBs in this redshift range. I also study the imprint of first stars on high-z DLAs, and explore how to use the observations of DLAs to constrain the first star models. Since DLAs are characterized by very high neutral hydrogen density, low gas temperature and ionization fraction, a high fraction of DLAs at z > 5.5 could contain first stars signatures. From my simulations, I find that ∼ 40% of DLAs contain such imprint if they are selected as galaxies with temperature < 104 K, while the percentage drops to ∼ 3% if DLAs are defined based on the cross-section of neutral hydrogen. By comparing the simulation results with current observations, I find that DLAs observed at z > 5 are more consistent with a model in which the first stars have a mass in the range m∗ = [0.1, 100]M⊙, rather than one with very massive first stars, although the comparison is not conclusive and still subject to some uncertainty.
Not available
Ma, Qingbo
2017
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Ma, Qingbo (2017): First star signatures on high-z GRBs and DLAs. Dissertation, LMU München: Faculty of Physics
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Abstract

First stars were born after the dark age era of the Universe. They produced the first light and metal pollution in the Universe. This was predicted in theory a long time ago, but no direct observation is available. In this thesis I study the first star signatures in high-z Gamma-ray Bursts from Population II stars (GRBIIs) and in Damped Laymanα Absorbers (DLAs), using numerical N-body/hydrodynamic simulations that include atomic and molecular cooling, star formation and metal enrichment from stellar populations with different initial mass functions. I first use one simulation based on a very massive first star model (m∗ = [100, 500]M⊙) to study the GRBIIs hosted in a medium enriched by first stars (PopIII-dominated). I find that a high fraction of high-z GRBIIs are PopIII-dominated, e.g. ∼10% at z = 10, but this fraction decreases quickly with redshift, e.g. only ∼1% at z = 6. Then I study the possibility to distinguish different first star models using observations of high-z GRBs by running three simulations with different mass ranges and the corresponding lifetimes and metal yields for first stars. I find that the fraction of GRBs that are PopIII-dominated is independent from the first star models. Similarly, PopIII-dominated GRBIIs are mostly hosted in galaxies with low metallicity, star formation rate and stellar mass. Because of the special metal yields of metal-free stars, the first star signals could be identified with the metal abundance ratios derived from the absorption lines in the GRBII afterglow spectra, e.g. the [Si/O] vs [C/O] or [Fe/C] vs [Si/C] would distinguish an imprint from very massive and massive first stars. However, the currently observed high-z GRBs, e.g. GRB 130606A at z = 5.91 and GRB 111008A at z = 5.0, do not display obvious metal features from first stars, which is also consistent with the low fraction of PopIII-dominated GRBs in this redshift range. I also study the imprint of first stars on high-z DLAs, and explore how to use the observations of DLAs to constrain the first star models. Since DLAs are characterized by very high neutral hydrogen density, low gas temperature and ionization fraction, a high fraction of DLAs at z > 5.5 could contain first stars signatures. From my simulations, I find that ∼ 40% of DLAs contain such imprint if they are selected as galaxies with temperature < 104 K, while the percentage drops to ∼ 3% if DLAs are defined based on the cross-section of neutral hydrogen. By comparing the simulation results with current observations, I find that DLAs observed at z > 5 are more consistent with a model in which the first stars have a mass in the range m∗ = [0.1, 100]M⊙, rather than one with very massive first stars, although the comparison is not conclusive and still subject to some uncertainty.