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

Outline why how tools method uncertainties my work how:

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

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

The determinations of Y P are progressively converging, but significant scatter remains

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

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

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

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

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

for example, the line ratio [O III ] 4363 / 4959,5007 is sensitive to the electronic temperature T e The electronic temperature is inferred from suitable line ratios

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

The ionic abundances are summed to obtain the elemental intensities

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

The sources of uncertainty in the determination of Y can be grouped into three broad categories physics atomic parameters stellar parameters underlying stellar absorption ionization structure nebular parameters temperature structure H I collisional enhancement

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

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

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

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

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

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

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

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

The collisional contribution enhances H  more than H , mimicking the effect of reddening

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

Our models of SBS take into account the slit bias

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

An upper limit to the collisional contribution is set by the observed H  / H  ratio The observed reddening sets an upper limit to the collisional contribution!

Our strategy is based on a personalized treatment of the H II regions problemsolution physics atomic parameterscross fingers stellar parameters stellar absorptionexclusion of star; stellar libraries ionization structuretailored models nebular parameters temperature structureself-consistent solution H I collisionsupper limits; exclude low Z

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

... still much work to be done before the last word can be said! Source YPYP Izotov et al Peimbert et al Future (2006)0.2??? Questions?

Y P through time: references

Energy-level diagram of He I

Collisional cooling

Heating by photoionization

Ionization thresholds for common ions

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