The underlying stellar absorption contribution to the primordial helium abundance determination

Abstract

We carried out an exploratory analysis of the contribution of the underlying
stellar absorption to the total uncertainty of the abundance of primordial
helium using simple stellar populations models and observational data from the
Sloan Digital Sky Survey. Results indicate that our analysis yields a lower
limit to the error on the helium abundance determination if the stellar
absorption is neglected.

Full text

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arXiv:1104.3845v1 [astro-ph.CO] 19 Apr 2011
Resolved Stellar Populations
ASP Conference Series, Vol. TBA, 2005
D. Valls–Gabaud & M. Chavez (eds)
The underlying stellar absorption contribution to the
primordial helium abundance determination
F. F. Rosales-Ortega, R. Terlevich, E. Bertone and M. Chavez
Instituto Nacional de Astrof´ ısica,´Optica y Electr´ onica
Apartado postal 51 y 216, CP 72000, Puebla, Pue. Mexico
Abstract.
the underlying stellar absorption to the total uncertainty of the abundance of
primordial helium using simple stellar populations models and observational data
from the Sloan Digital Sky Survey. Results indicate that our analysis yields a
lower limit to the error on the helium abundance determination if the stellar
absorption is neglected.
We carried out an exploratory analysis of the contribution of
1.Introduction
The determination of Yp, the mass fraction of primordial helium, is of fundamen-
tal importance due to its important cosmological implications. (see Olive & Skillman
(2004) for a discussion). The discrepancy among different measurements of Yp
has been reduced to a 1-2% level, nevertheless this uncertainty still imply a
wide range of possible values of the barion-to-photon ratio η, an important
parameter of the Standard Big Bang Nucleosynthesis, usually obtained from
observed primordial abundances. The classical method for the determination
of Yp(Peimbert & Torres-Peimbert 1974) is based on the abundance analysis of
extra-galactic H II regions through their emission lines and the extrapolation to
zero metallicity.
Several systematic errors can affect a precise determination of the Yp. We
focus here on the uncertainty produced by an underlying stellar absorption,
which causes a systematic decrement of the intensity of helium nebular emission
lines. An accurate assessment of this factor is important if one requires high
precision results.
Theoretical single stellar population (SSP) models appear as an effective
tool to perform a thorough investigation of the absorption contribution for the
lines commonly used to determine Yp such as He I λ4471, λ5876, λ6678 and
He II λ4686, which are nebular lines of particularly small equivalent width. We
present here the results of an exploratory analysis on this issue.
2.He stellar absorption
We measured the equivalent width of the four He lines of interest (He I λ4471,
λ5876, λ6678, and He II λ4686) using the pure absorption high resolution
SSP models (Bressan 2005, private communication) based on the theoretical
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Rosales-Ortega et al.
Figure 1.
with variable age of the young component and a fixed age of 1 Gyr of the old
component, at metallicities of Z⊙/50 (left panel) and Z⊙/5 (right panel).
Spectral energy distributions around the He I λ4471 line of SSPs
libraries of stellar spectral called BLUERED and UVBLUE (Bertone et al.
2004; Rodriguez-Merino et al. 2005) respectively.
In order to account for the possible presence of mixed population we cal-
culated 24 combinations of SSPs of different ages and chemical composition.
For these combinations we considered two metallicities (Z⊙/50 and Z⊙/5) and
a set of young (10-500 Myr) and old (1-2 Gyr) populations. The SED mixed
populations (Fij) were calculated such that Fij= aFyoungi+bFoldjwhere a and
b were 0.8 and 0.2 respectively. The EW of He lines for each flux combination
were measured after the SEDs had been degraded to the spectral resolution of
the Sloan Digital Sky Survey (R = λ/∆λ = 1800). Some examples of SEDs (at
a fixed age of 1 Gyr of the old component) in the region around the He I λ4471
line are shown in Figure 1.
The intensity of the line falls abruptly with increasing age of the young
population and its absorption becomes almost negligible for ages older than 200
Myr. This same behavior is common to all the lines that we have included in
our analysis.
3. Comparison to observations
In order to quantitatively asses the importance of the stellar absorption, we
measured the variation of the He abundance on the metal poor extragalactic H
II region SDSS J094401.86-003832.1, which we selected from the SDSS catalog
based on the quality observed spectrum. Figure 2 shows the observed spectrum
around the He I λ4471 emission line together with the theoretical SSP flux with
ages of 10 Myr and 1 Gyr for the young and old components, respectively. This
combination presents the largest stellar absorption and we can notice that, even

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Underlying stellar absorption contribution to primordial He determination
3
Wavelength
Flux?+?constant
Figure 2.
with Z⊙/50, fyoung= 10 Myr and fold= 1.5 x 109yr showing the influence
of the absorption feature for He I λ4471.
SDSS J094401.86-003832.1 spectrum and theoretical SSP model
though it is small compared with the emission feature, its effect on the EW is
not negligible. Table 1 reports the equivalent widths of the three He I lines
in emission (measured on the observed spectrum) and in absorption (measured
on the SSP SED partially shown in Figure 2 .) as well as the variations in
the He ionic abundance y+with and without taking into account the effect
of underlying stellar component. The He ionic abundance has been computed
following Pagel et al. (1992). We found the maximum He abundance variation
to be about 6% for the HeI λ4471 line.
4.Conclusions
In the exploratory analysis presented here, we found (based on Table 1) that the
difference between the He abundance determined with and without taking into
account the underlying stellar absorption component is on average 3%. Despite
that this is a small value, the effect is not negligible, as such a variation over-
estimate systematically the helium abundance of individual regions which are
then used to fit a linear regression in order to get a value of YP. Greater values

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Rosales-Ortega et al.
Table 1. The equivalent width of the HeI lines and the He ionic abundance.
Equivalent width (˚ A)Abundance y+(x 103)
He I
λ4471
λ5876
λ6678
Object
9.04 ± 0.04
51.20 ± 0.3
18.4 ± 0.5
SSP model
0.51 ± 0.02
0.36 ± 0.02
0.23 ± 0.01
without correction with correction
∆y+
87.02 ± 1.51
86.85 ± 1.45
81.78 ± 1.62
92.03 ± 1.50
87.40 ± 1.42
82.94 ± 1.65
5.1 (5.86%)
0.55 (0.60%)
1.16 (1.42%)
of y+would yield an overestimated primordial helium abundance, therefore it is
important to obtain an accurate correction for this effect.
However, we stress that the grid of SSP models that we used had a lower
age limit of 10 Myr; younger stellar populations, with ages characteristic of O-B
stars, the typical ionizing sources of HII regions, should be taken into account
to better map their contribution to the EW of He lines. Therefore our result
represents a lower limit to the error on the helium abundance determination if
the stellar absorption is neglected.
Acknowledgments.
Department and specially Miguel Chavez Dagostino for his kindly support which
made him possible attend such a successful conference.
FFRO would like to thank the INAOE Astrophysics
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