Arcturus Exposed: Non-LTE Analysis of Carbon and Oxygen Abundances in Arcturus

1. Arcturus Exposed: Non-LTE Analysis of Carbon and Oxygen Abundances in Arcturus By: Jayme Derrah Supervisor: Dr. Ian Short AUPAC 2007

2. Chemical Abundances of Stars • Why do we care? • Important for understanding stellar evolution • When stars die, elements are released into space that form other stars and planets

3. Carbon, Oxygen and the Sun • Why Carbon and Oxygen? • Oxygen is the 3rd most common element in the universe • CNO cycle in stars produces C and O • Recent studies by Asplund et al. revealed that revisions need to be made in the C and O abundances in the sun • Used a realistic 3-D code that had never been done before

4. Red Giants • Stage in stellar evolution after leaving the main sequence • Stars’ outer layers expand and cool • Are big, bright objects that can be seen at large distances • This makes it easier to take the spectrum

8. General Information: Spectral Type: K1.5III Radius: ~15.9 Rsol Teff: 4300 K Surface Gravity: ~1.5 Metallicity: -0.3 to -0.7 (20-50% of the sun) Arcturus: Standard Red Giant

9. Arcturus: Observed Spectrum • Observations done at Kitt Peak Observatory, using the Coude Feed telescope. • Hinkle, Wallace, Valenti, Harmer: Visible and Near Infrared Atlas of the Arcturus Spectrum 3727-9300 A; 2003, ASP

10. Arcturus: Observed Spectrum-2

11. Stellar Spectroscopy • Two methods used: • Line Profile Fitting • Qualitative measure of line strength • Equivalent Width Measurements • Quantitative measure of line strength • Width of a rectangle centred on a spectral line that has the same area as the line

12. PHOENIX • Developed By: Hauschildt, P.H.; Baron, E. ; Allard, F. • PHOENIX is a state of the art stellar atmosphere code • Can calculate the spectra for all types of stars • Simultaneously treats most elements in non-LTE • Many more in LTE

13. LTE vs. Non-LTE: LTE • Local Thermodynamic Equilibrium (LTE): • Only valid in a perfect blackbody environment • Boltzmann and Saha relations can be used • In reality, radiation leaks into space, so LTE is invalid. • Crude approximation to reality • So why use LTE?

14. LTE vs. Non-LTE: LTE • Local Thermodynamic Equilibrium (LTE): • Only valid in a perfect blackbody environment • Boltzmann and Saha relations can be used • In reality, radiation leaks into space, so LTE is invalid. • Crude approximation to reality • So why use LTE? • It’s easy! 

15. LTE vs. Non-LTE: Non-LTE • Non- Local Thermodynamic Equilibrium (Non-LTE): • Must solve coupled 1st order rate equations for the atomic transitions to get level populations • Much more realistic approximation

16. LTE vs. NLTE: Spectra

17. a- Enhanced Model • Scaled Solar models scaled down for elements where Z>2. Why? – Easy • However, all elements are made at different rates throughout galactic chemical evolution • a-capture makes even numbered elements • Old metal poor stars have more even numbered elements than odd, relative to the Sun • More realistic model for Arcturus

19. Results: Equivalent Widths (mA)

20. Results: Equivalent Widths 2 • Legend • x = Observed values • y = LTE values • z = NLTE values • v = NLTE- alpha enhanced values

21. Problems • Only one Carbon line • Telluric absorption in IR • Line blending in IR • Two Oxygen lines could not be used • Too weak • Equivalent width could not be determined (blending in observed spectrum)

22. Conclusions • Are these models accurate for Arcturus?

23. Conclusions • Are these models accurate for Arcturus? • Nope 

24. Conclusions • Are these models accurate for Arcturus? • Nope  • Further revisions need to be made to current models to accurately model Arcturus • C and O abundances need to be revised • Oxygen abundances depend on which line- shouldn’t be true • Models unrealistic in other ways (i.e. 3-D effects, starspots, etc.)

25. Acknowledgements Dr. Ian Short Joel Tanner Questions ?