The primordial 4 He abundance: the astrophysical perspective

1. The primordial 4He abundance:the astrophysical perspective Valentina Luridiana Instituto de Astrofísica de Andalucía (CSIC) Granada

2. Outline why method how: how tools uncertainties my work

3. The first light nuclides were synthesized in a short time interval following the Big Bang

4. The primordial abundances can be used to determine the baryon-to-photon density h the abundances of the first elements depend on the interplay between the reaction rates and the expansion of the Universe 4He is the easiest to measure 4He is the least sensitive toh (Fiorentini et al. 1998, PhRD 58, 63506)

5. The determinations of YP are progressively converging, but significant scatter remains

6. YP is found by extrapolation of the dY / dZ relation to Z = 0 since the Universe was born with no heavy elements, YP = Y(Z=0) (Fields & Olive 1998, ApJ 506, 177) (Peimbert & Torres-Peimbert 1974, ApJ 193, 327) high-quality measurements of Y and Z are required!

7. H II regions are gas clouds ionized by young, massive stars

8. The chemical composition of an H II region can be determined through the analysis of its spectrum (Izotov, Chaffee, & Green 2001, ApJ 562, 727)

9. Hydrogen and helium show up in the spectrum as series of recombination lines Balmer lines are the most important of the H I spectrum because they are bright and because they fall in the optical range

10. Metals show up in the spectrum as collisionally excited lines the brightest lines arise from levels a few eV above the ground state

11. The electronic temperature is inferred from suitable line ratios for example, the line ratio [O III] l4363 / ll4959,5007 is sensitive to the electronic temperature Te

12. Once Te has been obtained, the ionic abundances are derived from the line intensities the form of the function f depends on the mechanism of line formation: - collisional lines depend strongly on Te - recombination lines depend weakly on Te

13. The ionic abundances are summed to obtain the elemental intensities

14. A different kind of analysis of H II regions can be performed by means of photoionization models photoionization codes predict the structure and emission spectrum of H II regions

15. The sources of uncertainty in the determination of Y can be grouped into three broad categories

16. Problem n. 1: Uncertainty in Yis introduced by the stellar absorption underlying the emission lines solution:good stellar population models

17. Problem n. 2: Uncertainty in Y is introduced by the incomplete knowledge of the ionization structure If the Stromgren radii of H and He do not coincide, the abundance ratio He / H is either underestimated or overestimated

18. If H II regions were density-bounded in all directions, the problem would not exist

19. There are several ways to deal with the uncertainty associated to the ionization structure 1. applying selection criteria 2. building tailored photoionization models 3. using narrow-slit data

20. There are several ways to deal with the uncertainty associated to the ionization structure 1. applying selection criteria 2. building tailored photoionization models 3. using narrow-slit data

21. There are several ways to deal with the uncertainty associated to the ionization structure 1. applying selection criteria 2. building tailored photoionization models 3. using narrow-slit data

22. Problem n. 3: Temperature fluctuations inside H II regions can bias the abundance values One Te fits all? No! Each ion is associated to a typical temperature, and adopting a different one introduces a bias in the derived abundance

23. Recombination lines weigh smoothly the Te structure, collisional lines are enhanced in Te peaks recombination line collisional line Hairy problem! The temperature used to find the ionic abundances must be determined with care, otherwise the abundances will be over / underestimated

24. Problem n. 4: A minor contribution to the Balmer lines comes from collisional excitations

25. The collisional contribution is relevant only in low-metallicity H II regions in high-Te objects, which are the most metal-poor, the collisional contribution is non-negligible and should be factored out

26. The collisional contribution enhances Ha more than Hb, mimicking the effect of reddening

27. To study collisions, we modeled some of the most metal-poor H II regions known (Luridiana et al. 2003, ApJ, 592, 846) SBS 0335-052, Z=1/40 Zo (Thuan et al. 1997, ApJ 477, 661)

28. Our models of SBS 0335-052 take into account the slit bias

29. Several observational constraints are fitted to constrain the spatial structure of the object

30. An upper limit to the collisional contribution is set by the observed Ha / Hb ratio The observedreddening sets an upper limit to the collisional contribution!

31. Our strategy is based on a personalized treatment of the H II regions

32. Our results favor a relatively low primordial helium value, but... L 2003: Luridiana et al. 2003, ApJ, 592, 846 I 1999: Izotov et al. 1999, ApJ, 527, 757 S 1994: Songaila et al. 1994, Nature, 368, 599 K 2003: Kirkman et al. 2003, ApJS, submitted PB 2001: Pettini & Bowen 2001, ApJ 560, 41 TV 2001: Théado & Vauclair 2001, A&A 375, 70 S 2000: Suzuki et al. 2000, ApJ 540, 99

33. ... still much work to be done before the last word can be said! Questions?

34. YP through time: references

35. Energy-level diagram of HeI

36. Collisional cooling

37. Heating by photoionization

38. Ionization thresholds for common ions

39. The electronic density (Ne) is inferred from suitable line ratios the [S II] 6716/6731 ratio is sensitive to the electronic density Ne