A large dispersion observed in the age-metallicity relation inferred from different populations within the solar neighbourhood, is interpreted as largely due to intrinsic scatter and compared to predictions from inhomogeneous simple models of chemical evolution with stochastic star formation (Caimmi 2000). More specifically, the interstellar medium is idealized as substructured into a discrete number of n identical active or quiescent regions where star formation can or cannot randomly occur, respectively. The evolution is idealized as a series of identical time steps where stellar nucleosynthesis takes place within active regions, and subdivided into an assembling stage where the volume undergoes substantial changes, and a subsequent relaxed stage where the volume remains more or less unaltered. At the end of any step, a mass fraction (including newly synthesised elements) from short-lived stars is returned to the interstellar medium, instantaneously mixed with remaining gas from active and quiescent regions, and a new set of identical n regions is made for the next step. The theoretica age-oxygen abundance relation (TAOR) and the theoretical differential oxygen abundance distribution (TDOD) are computed for three different runs where the state of a region (active or quiescent) is decided by different sequences of random numbers. For each Galactic environment, different cases involving low (n=10) and high (n=100) number of regions are considered together with the case of expected evolution (infinite n) exploited in an earlier attempt (Caimmi and Milanese 2009). Finally, the TAOR and the TDOD are compared with the empirical age-oxygen abundance relation (EAOR) and the empirical differential oxygen abundance distribution (EDOD), respectively, inferred from subsamples made of halo, bulge, thick disk, thin disk stars and, in addition, thick + thin disk stars calculated for selected thick to thin disk mass ratios. The main results are (1) the TAOR fits to an acceptable extent (for at least one computer run) to the EAOR for both the halo, the thick disk, and the thick + thin disk, regardless of the number of regions, while the same holds for the thin disk only if the number of regions is low i.e. via stochastic effects, and (2) the TDOD fits to an acceptable extent (for at least one computer run) to the EDOD for all the above mentioned environments, regardless of the number of regions, where the fit is further improved for the thick + thin disk if the number of regions is low i.e. via stochastic effects. The results need to be confirmed using high-population, complete, and unbiased samples. An interpretation of the model within current /\CDM cosmological scenarios is shortly outlined.

Age vs. oxygen abundance and oxygen abundance distribution in nearby stars: the scatter due to stochastic star formation

CAIMMI, ROBERTO
2012

Abstract

A large dispersion observed in the age-metallicity relation inferred from different populations within the solar neighbourhood, is interpreted as largely due to intrinsic scatter and compared to predictions from inhomogeneous simple models of chemical evolution with stochastic star formation (Caimmi 2000). More specifically, the interstellar medium is idealized as substructured into a discrete number of n identical active or quiescent regions where star formation can or cannot randomly occur, respectively. The evolution is idealized as a series of identical time steps where stellar nucleosynthesis takes place within active regions, and subdivided into an assembling stage where the volume undergoes substantial changes, and a subsequent relaxed stage where the volume remains more or less unaltered. At the end of any step, a mass fraction (including newly synthesised elements) from short-lived stars is returned to the interstellar medium, instantaneously mixed with remaining gas from active and quiescent regions, and a new set of identical n regions is made for the next step. The theoretica age-oxygen abundance relation (TAOR) and the theoretical differential oxygen abundance distribution (TDOD) are computed for three different runs where the state of a region (active or quiescent) is decided by different sequences of random numbers. For each Galactic environment, different cases involving low (n=10) and high (n=100) number of regions are considered together with the case of expected evolution (infinite n) exploited in an earlier attempt (Caimmi and Milanese 2009). Finally, the TAOR and the TDOD are compared with the empirical age-oxygen abundance relation (EAOR) and the empirical differential oxygen abundance distribution (EDOD), respectively, inferred from subsamples made of halo, bulge, thick disk, thin disk stars and, in addition, thick + thin disk stars calculated for selected thick to thin disk mass ratios. The main results are (1) the TAOR fits to an acceptable extent (for at least one computer run) to the EAOR for both the halo, the thick disk, and the thick + thin disk, regardless of the number of regions, while the same holds for the thin disk only if the number of regions is low i.e. via stochastic effects, and (2) the TDOD fits to an acceptable extent (for at least one computer run) to the EDOD for all the above mentioned environments, regardless of the number of regions, where the fit is further improved for the thick + thin disk if the number of regions is low i.e. via stochastic effects. The results need to be confirmed using high-population, complete, and unbiased samples. An interpretation of the model within current /\CDM cosmological scenarios is shortly outlined.
2012
Galaxies: evolution, morphology and dyanmics
9781620811856
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/2578444
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