The Orion nebula, a naked-eye H II region

Transcription

1 The Orion nebula, a naked-eye H II region HII regions are gas clouds with ongoing star formation. The newlyformed young stars (mostly of O & B- type) emit copious amounts of UV photons which exite and ionize the surrounding gas The Orion nebula is about 10 pc across and lies at a distance of 450 pc Typical gas-kinetic temperatures are of the order of kk and densities are atoms/cm 3 Mike Bessell, MSSSO

2 Case study II: primordial helium Terlevich, Skillman & Terlevich (1995) Until March 15st (next lecture): read the overview article get an idea what these people have done be able to report the main result of their study write up questions you have / problems you see Take a first look now

3 The Orion nebula R: [S II] G: [O III] B: Hα

4 a schematic view of an H II region stars supply UV photons The stellar UV photons ionize the surrounding gas. Usually a single kinetic temperature and density is assigned to the whole H II region. There are also two-zone models which account for different ionization levels and cooling agents. Depending on whether or not the H II region can be resolved, one can observe multiple lines of sight. This is, e.g., possible in the Galaxy and in the nearby (50 kpc) Magellanic Clouds.

5 a controversy develops from the mid-1990s to the mid-2000s two teams measured significantly different values for Y p Y p = ± (Peimbert et al. 2000) vs. Y p = ± (Izotov & Thuan 1998) ± (Izotov & Thuan 2004)

6 potential biases high resolution (spectral, spatial) linear detectors atomic physics atomic data fluorecent and collisional excitation n ν τ n η BBN observations dust, winds, shocks stratification, extension (the H II region is not uniform) contamination by stars (underlying absorption, Wolf-Rayet stars, SNe) extrapolation to A(O) = 0 (all H II regions have non-zero metallicity) modelling ionization corrections H II region physics

7 the from-hindsight picture with the measurement of η 10 by WMAP, Y p can be constraint to Y p = ± ± (syst.) Spergel et al. (2007) The primordial He abundance is therefore in better agreement with the measurements by Izotov & Thuan (2004).

8 the solution? Fukugita & Kawasaki (2006) claim that the underlying stellar absorption has not been properly treated by Izotov & Thuan (2004). The corrected value for the same ovservational data is Y p = ± in perfect agreement with WMAP. dy/dz 1! dy/dz 4 Fukugita & Kawasaki 2006, ApJ 646,691

9 What about Peimbert et al. (2000)? Olive & Skillman perform a nonparametric re-analysis of the Peimbert et al. study: "The significant difference is due primarily to assumptions made in analyzing the observations. In other words, an independent analysis, using reasonable assumptions, produces a value of He + /H + = ± which is over 4% (or 6σ) higher than the PPR00 value. They now, from hindsight,obtain Y p = ± Olive & Skillman 2004, ApJ 617, 29

10 Lesson learned? Like in the case of D (lecture March 1st), we do not understand (and most often underestimate) the errors, both the random ones and the systematic ones. In this sense, the from hindsight view allows to turn the argumentation around and address some of the systematic issued.

11 The Solar spectrum Li I 6707

12 primordial lithium a way to potentially measure log ε (Li) p was discovered in the early 1980s: log N Li = 2.05 a certain subclass of ancient halo stars (unevolved, warm) was found to have a uniform lithium abundance Spite & Spite 1982, Nature 297, 483 Spite & Spite 1982, A&A 115, 357

13 consequences for η In this first study, η 10 could be constrained to η 10 = 3 ± 2 giving clear evidence that the universe does not contain enough baryons to fall back on itself.

14 refining the picture By studying stars in a narrow T eff range, Ryan et al. could show that there is a trend of A(Li) with [Fe/H] of d A(Li) / d [Fe/H] 0.12 This trend can be explained by Galactic production of lithium through cosmic-ray spallation processes in the ISM. This trend has nonetheless been challenged recently by revised T eff calibrations. Ryan et al. 1999, ApJ 523, , ApJ 530, L57

15 Case study III: primordial lithium Ryan et al. (2000) Until March 22st (next lecture): read this article get an idea what these people have done be able to report the main results write up questions you have / problems you see Take a first look now