Non-LTE abundance analysis: K & Sc Huawei Zhang Department of Astronomy, School of Physics, Peking University

Collaborators: Thomas Gehren (LMU) Thomas Gehren (LMU) Keith Butler (LMU) Keith Butler (LMU) Shi Jianrong (NAOC) Shi Jianrong (NAOC) Zhao Gang (NAOC) Zhao Gang (NAOC)

The history of the Galaxy is written in the evolution of its composition. The history of the Galaxy is written in the evolution of its composition. The low mass unevolved stars have long lifetimes, some of them have comparable to the age of the Galaxy. The low mass unevolved stars have long lifetimes, some of them have comparable to the age of the Galaxy. Their atmospheric compositions have preserved much of their natal interstellar clouds. Their atmospheric compositions have preserved much of their natal interstellar clouds.

The determination of the element abundances in stars of different metallicities is important for understanding the chemical evolution of the Milky way. The determination of the element abundances in stars of different metallicities is important for understanding the chemical evolution of the Milky way.

Abundance analysis Still today, the vast majority of abundance analyses of late-type stars rely on the assumption of local thermodynamic equilibrium (LTE). Still today, the vast majority of abundance analyses of late-type stars rely on the assumption of local thermodynamic equilibrium (LTE). Departures from LTE are common place and often quite important. Departures from LTE are common place and often quite important.

LTE vs. NLTE LTE: The level populations can be directly computed from the local gas temperature by the use of the Boltzmann and Saha distributions. LTE: The level populations can be directly computed from the local gas temperature by the use of the Boltzmann and Saha distributions. NLTE: These rate equations must be solved simultaneously with radiative transfer equation for all relevant frequencies. NLTE: These rate equations must be solved simultaneously with radiative transfer equation for all relevant frequencies.

K (Z=19) and Sc (Z=21) are odd-Z elements. K (Z=19) and Sc (Z=21) are odd-Z elements.

K & Sc NLTE model K Sc

K lines: LTE vs. NLTE KI: 7698

Potassium results: the Sun Zhang et al., 2006, A&A, 453, 723 Average solar potassium abundance: Average solar potassium abundance: log   (K) = 5.12±0.03 log   (K) = 5.12±0.03 Corresponds to the meteoritic value (Grevesse & Sauval, 1998). Corresponds to the meteoritic value (Grevesse & Sauval, 1998).

Potassium results: metal-poor stars Zhang et al., 2006, A&A, 457, metal- poor stars 58 metal- poor stars DSAZ FOCES DSAZ FOCES R ~ R ~ S/N ~ S/N ~

The NLTE corrections for metal- poor stars are negative and the average of dex. The NLTE corrections for metal- poor stars are negative and the average of dex.

Potassium results: metal-poor stars Zhang et al., 2006, A&A, 457, 645 Samland(1998) Goswami & Prantzos (2000) Timmes et al. (1995)

Scandium results: the Sun Zhang et al., 2008, A&A, 481, 489 Sc I 5671 LTE NLTE Sc II 5526 NLTE LTE

LTE result: Sc I: 2.90±0.09 Sc I: 2.90±0.09 Sc II: 3.10±0.05 Sc II: 3.10±0.05 Sc II Sc I

NLTE result: Sc I: 3.08±0.05 Sc II: 3.07±0.04 Sc II Sc I Sc II Sc I Sc II

Scandium results: metal-poor stars Zhang et al., 2008, in preparation Thick disk Thin disk Halo [Sc/Fe] ~ [Fe/H]

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