From: ethan@emx.cc.utexas.edu (Ethan T. Vishniac)
Newsgroups: sci.physics,sci.astro
Subject: The Primordial Abundance of Helium
Date: 28 Jan 1993 09:31:26 -0600
Organization: The University of Texas at Austin, Austin, Texas
Distribution: world
NNTP-Posting-Host: emx.cc.utexas.edu


Given all the discussion of cosmology and the BB recently
I thought I would contribute something new.  I went
to a talk last Tuesday by Evan Skillman who has been
working for several years on abundances in HII (ionized
hydrogen for you physicists) regions in metal poor
galaxies.  One of the goals of his work has been to
recover the primordial value of the Helium abundance
(that is, before stellar enrichment).  Since HII regions
surround bright and massive stars this quantity is
not directly observable even in the most metal poor
galaxies, but rather one plots the He abundance against
the C, N, or O (the last two usually) and looks for
the zero intercept for the data set.  The significance
of the result is that the standard model of cosmological
nucleosynthesis makes lots of He4, but no C12 (or heavier
atoms).  The exact amount of He4 is tied to the current
temperature of the microwave background (very well determined
now) and the average density of ordinary matter (very poorly
determined except through this argument).  Observations of
abundances of D, He3, and Li7 seem to indicate that
one expects Y_p (the primoridal abundance of He4 by mass fraction)
to be between 0.236 and 0.243.  These limits are difficult
to set firmly, but are probably reasonable. The possibility of
a major revision through the discovery of a systematic error
in previous abundance analyses is always present at some level.

On the other hand, previous observations have suggested that Y_p
lies in the range 0.228\pm 0.005.  These observations suffer
from uncertainties in modeling the physical conditions inside
HII regions.  For example there are usually several exciting
stars in bright HII regions and a lot of inhomogeneity in the
emitting gas.  Values far outside this range have been reported
for some metal poor systems, including some that were much
lower, however such reports have been shown to suffer from numerous
problems, including the presence of spectral features due to
the exciting stars which were misinterpreted as part of the HII
regions emission.  Discarding systems for which the necessary
corrections are large one gets the above range, which is discrepant
with the theoretical prediction at the 2sigma level.  Enough
to cause a fair amount of discomfort but not enough to force 
a major revions of the standard model.

One of the problems with the analysis of the observational
data is that the low metal end of the line has been determined
by just one or two objects while the objects at greater
metal abundance still show a fair amount of scatter, probably
due to problems of interpretation.  Evan and his collaborators
have succeeded in filling in part of the low metal range
with 10 new systems found in extremely low luminosity galaxies.
The new data is somewhat better, of course, because of
technical improvements.  He found that:

1) The very low metal systems show about the same amount of scatter
as the previous data.  This means that the size of the error for
the intercept has not changed, although its position did change
slightly.  Using the same general analysis as previously he
quoted a Y_p about 1 sigma higher.

2) The emissivities used in previous analyses are slightly in error.
The nature of the error is such that it didn't show up at temperatures
of 10^4 degrees, typical of HII regions in our galaxy, but do at
the higher temperatures typical of very metal poor HII regions.
The revised emissivities are due to Smits and lead to
Y_p=0.239\pm 0.005.  The answer does not seem to depend on whether
Oxygen or Nitrogen is used as the indicator of stellar enrichment.
In fact, although many people have suggested that N is due to secondary
enrichment, no evidence for this is seen in the data.


The net effect is that the "observed" value of Y_p now sits in the
middle of the predicted range.  Of course, further movements of
comparable size remain possible.
-- 
"Quis tamen tale studium, quo ad primam omnium rerum causam evehimur,
tamquam inutile aut contemnendum detractare ac deprimere ausit?"-Bridel
Ethan T. Vishniac, Dept. of Astronomy, The University of Texas at Austin
Austin, Texas, 78712                   ethan@astro.as.utexas.edu