the primordial 4 he abundance: the astrophysical perspective
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The primordial The primordial 44He abundance:He abundance:the astrophysical perspectivethe astrophysical perspective
Valentina LuridianaValentina Luridiana
Instituto de Astrofísica de Andalucía (CSIC)Instituto de Astrofísica de Andalucía (CSIC)
GranadaGranada
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
4He is the easiest to measure
4He 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 YP 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,
YP = Y (Z=0)(Peimbert & Torres-Peimbert 1974, ApJ 193, 327)
YP is found by extrapolation of the dY / dZ 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 Te
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 Te
- recombination lines depend weakly on Te
Once Te has been obtained, the ionic abundances are derived from the line intensities
The ionic abundances are summed to obtain the elemental intensities
)(I)(IT
)(N)(N
H4686
HHe 2
)H()He(
)H()He(
)H()He(
NN
NN
NN
)(I)(IT
)(N)(N
H5876
HHe 1
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
parameterstemperature 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
)(I)(IT
)(N)(N
HH
He
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
)H()He(
)H()He(
)H()He(
NN
NN
NN
If H II regions were density-bounded in all directions, the problem would not exist
1. applying selection criteria 2. building tailored photoionization models3. 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 models3. 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 models3. 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 Te fits all?
Problem n. 3: Temperature fluctuations inside H II regions can bias the abundance values
Recombination lines weigh smoothly the Te
structure, collisional lines are enhanced in Te peaks
Hairy problem! The temperature used to find the ionic abundances must be determined with care, otherwise the abundances will be over / underestimated
)H()He(
)()5876(
NNT
HII
)H()O(/
)()5007(
NNKTEeT
HII
recombination line
collisional line
Problem n. 4: A minor contribution to the Balmer lines comes from collisional excitations
)(I)(IT
)(N)(N
HH
He
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
The collisional contribution enhances H more than H, mimicking the effect of reddening
KTEeRjCj /)3,1(
)H(
)H(
KTEekRjCj /)4,1(
)H(
)H(
recII
obsII
)H()H(
)H()H(
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 0335-052, Z=1/40 Zo
Our models of SBS 0335-052 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
)(C)(f)(F)(F
)(I)(I
HH10
HH
HH
The observed reddening sets an upper limit to the collisional contribution!
int)β(Cgal)β(Ccol)β(Cobs)β(C HHHH
Our strategy is based on a personalized treatment of the H II regions
problem solution
physics atomic parameters cross fingers
stellar
parameters
stellar absorption exclusion of star; stellar libraries
ionization structure tailored models
nebular
parameterstemperature structure self-consistent solution
H I collisions upper 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 YP
Izotov et al. 1999 0.2452 0.0015 0.0070
Peimbert et al. 2002 0.2391 0.0020 0.0010
Future (2006) 0.2??? 0.0020 0.0005
Questions?
YP 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 Ne
The electronic density (Ne) is inferred from suitable line ratios
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