4 He Abundance from HII Regions

1. 4He Abundance from HII Regions

2. Overview • Theory and theoretical predictions of Yp • Observational Technique. • Relative strengths of He I and H emission lines. • Physical parameters and their effects. • Difficulties • What do observations tell us?

3. Early Universe, before BBN • (n/p) decreases with temp. exponentially • n/p freezes out at 1/6 ( ~0.8 MeV) • at n/p ~ 1/7 4He is produced (~80 keV) • Produced with an efficiency of 99.99% because of tight binding and lack of mass-5.

4. What does Yp dependent on? • Because of high efficiency 4He is insensitive to the baryon abundance. • 1 1010, .22 Yp.25 • Sensitive to expansion rate • If H were bigger bigger n/pYp increases

5. Why HII Regions? Stars burn H to He. There will be a net increase in He in galaxies over time. Need to deduce Yp from observed Y. There will be less evolutionary uncertainty in low metallicity HII Regions (~1/50 solar metallicity)

6. Relative emission line strengths. • The goal is to get a He/H abundance from relative strengths of their emission lines. • From theoretical understanding of the relative strengths and observations of “standard stars” one can deduce a first approximation. • The actual He/H will now depend on the physical parameters of the region in space.

7. Determining physical parameters and their effects. • Reddening • Known: Theoretical understanding of H I recombination line emissivities and relative intensities. • Derived: • , where C(H) is a logarithmic reddening correction.

8. Underlying Stellar absorption • Found in a similar manner. • Comparing the observed and theoretical ratios for the Balmer lines and reducing them. • Effects of stellar absorption and reddening will reduce the intensities of emission lines. • Many early type stars in H II regions have strong He I absorption lines which reduces the energy at that level.

9. Determinations of electron density and temperature • Temperature is derived from the relative intensities of two sets of collisionally excited emission lines in [O III] . • Photoionization modeling gives accurate temperatures for these ratios. • Density is determined from collisionally excited line ratios in [S II] • Where one of the lines is considered a “high density regime” and the other a “low density regime”

10. Effects of density and temperature • There is a collisional excitation from the metastable 2S level. • This effect depends exponentially on temp. • and linearly with density.

11. Red spectrum of H II Galaxy

12. From Y to Yp • Remember that observed abundance of 4He is the upper limit to the primordial abundance. • Yp can be derived by a linear regression with another element with an atomic number Z

13. Difficulties • Although uncertainties can be derived from Monte Carlo simulations, most of the uncertainty arrives from systematic errors. • IT98 .244 +/- 0.002 • PSTE .228 +/- 0.005 • PPR 2000 .2345 +/- 0.0026

14. Other Difficulties • As mentioned before physical properties such as temperature, density, and absorption will have drastic effects on measurements of abundance. • Also dY/dZ may not be always linear and may vary as a function of Z or randomly, depending on chemical content.