The role of H 3 + in extremely metal deficient stars.

1. The role of H3+ in extremely metal deficient stars. Gregory J. Harris University College London, UK.

2. Introduction. • As we have seen, H3+ observed in many astrophysical environments. • Planets and the interstellar medium. • Not yet observed in stars. • Heavy elements (metals) ionise at low T. • H3+ destroyed by electrons. • H3+ does not form in solar metallicity stars. • Extensive surveys leading to the discovery of many extremely metal poor stars [Fe/H] < -3. • Current record Frebel et al (2005) [Fe/H] ~ -5.4 • [C/H] ~ -2.0 • Ages of these stars approach the age of the Universe. • Kinematics of Halo objects. • May be first generation (population III) stars. • Or second generation (population II.5). • Could H3+ be important in stars with extremely low metallicities? HE 0107-5240 Second most iron poor star [Fe/H] ~ -5.2 Christlieb et al. (2002) The physics, chemistry and astronomy of H3+, London, UK.

3. The chemical elements in the early Universe. • According to big bang theory • Universe consisted of H & He with trace quantities of Li and Be. • Heavy elements produced by stars. • Li/H = (4.15 +/- 0.5)x10-10 (Coc et al. 2004). • WMAP and theory • Li/H = (1.2 +/- 0.5)x10-10 (Ryan et al. 2000). • observation of halo stars. The physics, chemistry and astronomy of H3+, London, UK.

4. Population III equation of state. • Accounts for: • e-, H2, H, H+, H-, H2+, H2-, H3+, He, He+, HeH+, Li, Li+, LiH, LiH+. • Based upon the Saha equation. • Thermodynamic equilibrium • Convenient to use ratios relative to neutral atomic species. The physics, chemistry and astronomy of H3+, London, UK.

5. Dominant positive ions for Population III gas. N (cm-3) 10-7 g cm-3 H3+: Lenzuni et al. (1991) Li: Mayer & Duschl (2005) 10-5 g cm-3 The physics, chemistry and astronomy of H3+, London, UK.

6. 15 element EoS. The physics, chemistry and astronomy of H3+, London, UK.

7. Abundance of metals and H3+ The physics, chemistry and astronomy of H3+, London, UK.

8. Monochromatic Opacity. • Low metallicity opacity subroutine. • Uses number densities of each species calculated by EoS. • Continuous absorption: • Bound-free: H, H-, He, He+, H2+ • Free-free H, H-, He, He-, He+, H2-, H2+ • Collision induced: H2-He, H2-H2, H-He • Scattering: • Rayleigh scattering: H2, H, He. • Thomson scattering: e- • Line opacity: • H, H3+, H2O, HCN, CO, CN, CO, TiO, OH, NH, FeH. The physics, chemistry and astronomy of H3+, London, UK.

9. Population III continuous opacities The physics, chemistry and astronomy of H3+, London, UK.

10. H3+ line opacity. The physics, chemistry and astronomy of H3+, London, UK.

11. Model atmospheres. • Up to 3 fold increase in opacity, due to H3+. • Will affect model atmospheres. • Saumon et al. (1994) • Updated MARCS (Gustafsson et al. 1975) • A 1D plane parallel, cool stellar atmosphere code. • Uses mixing length theory of convection. • Updated opacity and equation of state subroutines. The physics, chemistry and astronomy of H3+, London, UK.

12. Atmospheric structure, as a function of metallicity. The physics, chemistry and astronomy of H3+, London, UK.

13. Synthetic stellar spectra. H3+ lines might be detectable. • 2.0-2.3 microns. • Requires the right target star! • Dwarf stars: H3+ lines obscured by CIA. Tef =3750 K Log g = 1.0 [Z/H]=-3 The physics, chemistry and astronomy of H3+, London, UK.

14. Stellar evolution. • As hydrogen (and other elements) fuses and depletes the structure and observable properties of stars change. • Virtually all energy generated by the star escapes through its atmosphere (photosphere). • Does H3+ affect stellar low metallicity stellar evolution? • CESAM, (P. Morel 1997). • A 1 D, low and intermediate mass stellar evolution code.. • Grey model atmosphere. • Opacity independent of frequency. • Rosseland mean opacity. • Quick to solve. • Updated to include new EoS and Opacity. The physics, chemistry and astronomy of H3+, London, UK.

15. Low mass population III evolution. H3+ important in the evolution of extremely low mass stars (0.4 Msol) The physics, chemistry and astronomy of H3+, London, UK.

16. Non-grey stellar evolution. • Opacity is non-grey, a function of frequency! • Saumon et al. (1994), Baraffe et al. (1995;1997;1998) & Chabrier et al. (1997) • Conclude grey approximation invalid for Tef < 5000 K. • About 0.8 – 0.6 Msol depending upon metallicity. • Non-grey models have lower Tef and luminosity. • Lower temperature, more H3+! • NG-ELMS. • Combination of MARCS non-grey model atmosphere and CESAM stellar evolution codes. • Unique: computes model atmosphere at run-time. The physics, chemistry and astronomy of H3+, London, UK.

17. Grey and non-grey evolutionary tracks (Z=10-5). 0.9 Msol 0.7 Msol The physics, chemistry and astronomy of H3+, London, UK.

18. Future work. • Computing a grid of low to zero metallicity evolution models. • These models will all us to re-assess the impact of H3+ on stellar evolution. • Grid of models will be used to fit and determine more accurate age of low metallicity stars (globular clusters). • New evolutionary timescales for the Galaxy? • New lower stellar age limit to the Universe? The physics, chemistry and astronomy of H3+, London, UK.

19. Conclusion. • H3+ is a vital electron donor in cool gasses of Z < 10-5. • Li important at lowest temperatures. • H3+ lines might be observable in giant stars with Z < 3x10-5, Tef < 3750 K. • Candidate star? • H3+ has a strong affect on the evolution of extremely low mass stars (<0.4 Msol). • Reducing Tef for and luminosity. • Grey approximation insufficient to model low mass, low metallicity stars of M< 0.9 Msol. The physics, chemistry and astronomy of H3+, London, UK.

20. Acknowledgements. • Jonathan Tennyson (UCL). • Tony Lynas-Gray (Oxford). • Steve Miller (UCL). • B. Gustafsson. & P. Morel. • UK Particle Physics & Astronomy Research Council (PPARC). • You! The physics, chemistry and astronomy of H3+, London, UK.