The primordial 4He abundance: the astrophysical perspective

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The primordial 4He abundancethe astrophysical
perspective
Valentina Luridiana Instituto de Astrofísica de
Andalucía (CSIC) Granada
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Outline
why
method
how
how
tools
uncertainties
my work
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The first light nuclides were synthesized in a
short time interval following the Big Bang
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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 to h
(Fiorentini et al. 1998, PhRD 58, 63506)
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The determinations of YP are progressively
converging, but significant scatter remains
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YP is found by extrapolation of the dY / dZ
relation to Z 0
since the Universe was born with no heavy
elements, YP Y (Z0)
(Fields Olive 1998, ApJ 506, 177)
(Peimbert Torres-Peimbert 1974, ApJ 193, 327)
high-quality measurements of Y and Z are required!
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H II regions are gas clouds ionized by young,
massive stars
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The chemical composition of an H II region can be
determined through the analysis of its spectrum
(Izotov, Chaffee, Green 2001, ApJ 562, 727)
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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
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Metals show up in the spectrum as collisionally
excited lines
the brightest lines arise from levels a few eV
above the ground state
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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
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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
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The ionic abundances are summed to obtain the
elemental intensities
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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
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The sources of uncertainty in the determination
of Y can be grouped into three broad categories
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Problem n. 1 Uncertainty in Y is introduced by
the stellar absorption underlying the emission
lines
solution good stellar population models
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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
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If H II regions were density-bounded in all
directions, the problem would not exist
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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
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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
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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
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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
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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
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Problem n. 4 A minor contribution to the Balmer
lines comes from collisional excitations
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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
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The collisional contribution enhances Ha more
than Hb, mimicking the effect of reddening
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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, Z1/40 Zo
(Thuan et al. 1997, ApJ 477, 661)
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Our models of SBS 0335-052 take into account the
slit bias
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Several observational constraints are fitted to
constrain the spatial structure of the object
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An upper limit to the collisional contribution is
set by the observed Ha / Hb ratio
The observed reddening sets an upper limit to the
collisional contribution!
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Our strategy is based on a personalized treatment
of the H II regions
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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, AA 375, 70
S 2000 Suzuki et al. 2000, ApJ 540, 99
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... still much work to be done before the last
word can be said!
Questions?
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YP through time references
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Energy-level diagram of He I
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Collisional cooling
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Heating by photoionization
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Ionization thresholds for common ions
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The electronic density (Ne) is inferred from
suitable line ratios
the S II 6716/6731 ratio is sensitive to the
electronic density Ne