In this section we present the results from a number of test cases varying the input physical parameters, the number of He I emission lines used in the minimizations, and assumptions about certain physical parameters. Our philosophy here has been to test for relatively small variations, since the final goal is helium abundances for individual nebulae with accuracies approaching 1%. That is, we are confident that if assumptions are grossly in error that the derived abundances are wrong, but, more importantly, if there is a very subtle effect (e.g., a very small amount of underlying absorption or a small amount of optical depth), we need to understand how that will affect our derived helium abundances.
5.1. Cases with no Systematic Errors
We present here the results of running a few series of test
cases. In all cases, input spectra were synthesized with
the prescriptions and assumptions described above or in the
appendices. We chose a baseline model of T = 18,000 ± 200 K,
EW(H = 100), and He / H =
0.080. We then varied the density, aHeI, and
(3889) to produce different
cases. Errors of 2% were assumed for all of the input emission lines and
equivalent widths, and then each of these models were run
through Monte Carlo realizations. We then analyze the
resulting distributions of the results from a
2
minimization solution for He / H, density, aHeI,
and
(3889).
Figure 5 presents the
results of modeling of 6 synthetic He I
line observations for a single case. The
four panels show the results of a density = 10 cm-3,
aHeI = 0, and
(3889) = 0 model.
The solid lines show the input values (e.g., He / H = 0.080)
for the original calculated spectrum. The solid circles
(with error bars) show the results of the
2 minimization solution
(with calculated errors) for the original synthetic input spectrum.
The small points show the results of Monte Carlo realizations
of the original input spectrum.
The solid squares (with error bars) show the means and dispersions
of the output values for the
2 minimization
solutions of the Monte Carlo realizations.
Figure 5 demonstrates several important
points. First,
our 2 minimization
solution finds the correct input
parameters with errors in He / H of about 1% (less than the
2% errors assumed on the input data, showing the power of
using multiple lines). In this low density
case, the Monte Carlo results are in relatively good agreement with the
input data, with similar sized error bars. There is a small
offset to lower densities and a similar small offset to non-zero
values of underlying absorption. We found this effect throughout
our modeling, that when an input parameter such as underlying
He I absorption or
(3889) is set
to zero, the minimization
models of the Monte Carlo realizations (cases with errors) always
found slightly non-zero values (although consistent with zero)
in minimizing the
2.
Note that in the lower right panel of
Figure 5 that the values of the
2 do not correlate
with the values of y+. The solutions at higher values of
absorption and y+ are equally valid as those at lower
absorption and y+.
Figure 6 presents the results of modeling of 6
synthetic He I line observations for a case identical to that of
Figure 5
with the exception of a higher density of 100 cm-3.
For the n = 100 cm-3 case, there is a systematic trend
for the Monte Carlo realizations to tend toward higher values of
He / H. This is because, again, the inclusion of errors has allowed
minimizations which find lower values of the density and
non-zero values of underlying absorption and optical depth.
However, in this case, there is more "distance" from
the lower bound of n = 0, and thus more parameter space to allow the
effects of the parameter degeneracy to be noticed. Note that the size of
the error bars in He / H have expanded by roughly 50% as a result. We can
conclude from this that simply adding additional lines or physical
parameters in the minimization does not necessarily lead to the correct
results. In order to use the minimization routines effectively, one must
understand the role of the interdependencies of the individual
lines on the different physical parameters. Here we have shown
that trade-offs in underlying absorption and optical depth allow
for good solutions at densities which are too low and resulting
in helium abundance determinations which are too high.
This is one of the central results of this study.
Again, note that there is no trend in the values of
2 with
y+.
![]() |
Figure 6. Similar plot to Figure 5 except that the density = 100 as opposed to 10 as in Figure 5. |
Table 1 summarizes the results of a number of
different test cases like those shown in
Figures 5 and 6.
Table 1 is grouped into six different cases of
input with five
different minimization routines. The first two cases correspond those
shown in Figures 5 and 6.
The other four cases consider non-zero values of underlying
absorption, (3889), or both. The
first two columns show the results of minimizing on 3 lines (both assuming
(3889)
is zero and solving for
(3889)).
The next two columns show the results of minimizing on 5 lines (both
assuming zero underlying absorption and solving for the underlying
absorption). The last column shows the results of the six line
method which was used to produce Figures 5 and
6.
![]() |
The numbers in the table correspond to the average of the Monte
Carlo results and their dispersion. The row labeled He / H gives
the results from averaging the He / H abundances from all of the
lines (3, 5 or 6), while the "He / H (3)" row gives the He / H values
derived from averaging only the three main He I lines (after solving
for the physical parameters).
Note that the straight minimizations of the input data always returned
the input data (except in the cases where an assumption is inconsistent
with the input data). Deviations of the Monte Carlo solutions from
the input values result because of: (1) inconsistencies between the
input data and input assumptions, (2) asymmetries in the Monte Carlo
distributions (e.g., in Figure 5, because the
absorption is not allowed
to go negative, the distribution is truncated on one side, and thus
there is a bias to higher values of y+), (3)
degeneracies between different parameters which result in lower
2 values for
values of the physical parameters quite far from their input values.
For the first two cases, (no underlying absorption and no optical
depth), as expected, the 3 line method constraints
on the density are not strong. However the derived helium abundances
are consistent, within the errors, with the input values.
For the 5 line method, since the first two cases (1 and 2) have no
underlying absorption, the
method with the correct assumption finds a solution much closer to
the correct result (although all solutions are consistent with the
correct result, within errors). Again, it is the degeneracy between
density and underlying absorption which is responsible for
the derived low density and high He abundance.
Note, interestingly, that the three line method did not do
any worse (in fact it did slightly better) than the 5 line methods,
unless we assume a priori the correct answer for underlying
absorption (aHeI = 0). Similarly, assuming
= 0 also improves the result in
this case for obvious reasons.
The six line method, within the errors, gave results
consistent with but not equal to the input parameters. Indeed, there is a
systematic trend to lower density and some underlying absorption even
when there is none. The net result is a higher estimate of the He
abundance. This systematic trend can be traced to the degeneracy in the
trends imposed by the different input parameters.
However, the 6 line method does significantly better than the 5 line
method at constraining the underlying absorption (as
the
4206 line anchors the
values of aHeI).
We learn more about the various methods when we consider the remaining
cases in Table 1. When
(3889)
0 (and
aHeI = 0), the five
and six line methods give very accurate results although, once again,
4206 is needed to pin down
the value of aHeI and break the
degeneracy. When the input value of
aHeI
0, then
only the 5 line method which assumes
aHeI = 0 does badly. The 5 and 6 line methods
which solve for aHeI do quite well.
Figure 7
shows the results of the Monte Carlo when both
and
aHeI
0, and
n = 100 cm-3, i.e., case 6 of
Table 1.
Thus it is encouraging that in perhaps more realistic cases where the input
parameters are non-zero, we are able to derive results very close to their
correct values.
![]() |
Figure 7. Similar plot to
Figure 6 except that the
underlying absorption is 0.1 Å and
|
Indeed, Figure 7 shows many of effects we have
been describing in the
previous cases. The average of Monte Carlo realizations is remarkably
close to the straight minimization for all of the derived parameters
(n, aHeI,
and
y+). However, there is an enormous dispersion in
these results due to the degeneracy in the solutions with respect to the
physical input parameters. This results in error estimates for
parameters which are significantly larger than in the straight
minimization. For example, the uncertainties in both the density and
optical depth are almost a factor of 3 times larger in the Monte Carlo.
When propagated into the uncertainty in the derived value for the He
abundance, we find that the uncertainty in the Monte Carlo result (which
we argue is a better, not merely more conservative, value) is a factor of
2.5 times the uncertainty obtained from a straight minimization using 6
line He lines. This amounts to an approximately 4% uncertainty in the He
abundance, despite the fact that we assumed (in the synthetic data) 2%
uncertainties in the input line strengths. This is an unavoidable
consequence of the method - the Monte Carlo routine explores the
degeneracies of the solutions and reveals the larger errors that
should be associated with the solutions.
From the above, we conclude (1) that adding absorption to the
minimization routines can lead to much larger regions of valid solution
space; (2) the trivial result that assuming no underlying absorption will
lead to incorrect solutions in the presence of underlying absorption,
(3) that adding an accurate
4026 observation to a
minimization
solution will provide strong diagnostic power for underlying stellar
absorption, and (4) that Monte Carlo models are required to determine the
true uncertainties in the minimization results.
5.2. Cases with Systematic Errors in I(3889)
In Section 4, it was shown that
3889 is strongly sensitive to
optical depths effects and is required if
7065 is to
be a good tracer of density. It was also pointed out that,
unfortunately,
3889 is
blended with H8 (
3890).
Thus, in order to derive an accurate
F(
3889) /
F(H
)
ratio, the F(
3890) must be
subtracted off and
underlying stellar H I (and He I) absorption must be corrected for.
In the methodology of IT98, the contribution to He I
3889
from H I emission is subtracted off by assuming the theoretical
value for the H I emission (typically, the He I emission accounts
for almost 50% of the blended line). The total emission has to
be corrected for underlying stellar absorption, which is assumed to
be a constant equivalent width for all of the H I lines. This
assumption is a potentially dangerous one. Spectral studies of
individual stars show that while this may be a good assumption to
first order, the equivalent widths of the higher order Balmer lines
are not strictly identical (see, for example, the spectral atlas
of Galactic B supergiants of
Lennon, Dufton, &
Fitzsimmons 1992).
Secondly, this correction is usually large (corrected He I
3889 emission line
equivalent widths generally lie in
the range of 4 to 10Å compared to the underlying absorption
which is in the range of 0.5 to 3Å). A good test of the
uncertainty in this correction would be a comparison of the
corrected higher order (H9 and H10) H I emission line strengths
compared to their theoretical values. In
Appendix B, we show
a few cases in the literature, where these comparisons reveal
evidence of a problem.
Here we investigate the possible effects of a systematically
low strength of 3889
motivated by the possibility of
oversubtracting the underlying H I absorption. We have run
identical cases and analyses as in Table 1, but
altered the input
synthetic spectra by decreasing the relative flux and equivalent width
of
3889 by 10%. These
results are presented in Table 2.
The first two columns are identical (since they are based on
only three unaffected lines) and are repeated for comparison.
Note that this exercise was motivated, in part, by the
systematically low values of He / H derived from
3889
when compared with the main three He I lines in a subsample of
the highest quality data from IT98.
![]() |
The 10% drop in 3889 has
dramatic effects. From Section 4, and especially
Figure 4, it can be seen, that an
underestimate of I(
3889)
will lead to both artificially high
values of
(3889) and
artificially low values of the density.
This is born out in inspection of Table
2. Beginning with the cases in which the input values of
and aHeI are
0, we see
that the density is grossly underestimated in both input cases with
n = 10 and 100 cm-3. In the low density case, the He /
H solutions based on all available lines are low, while the He / H(3)
solutions are close to correct. This main effect is due to including the
low value of He / H from
3889.
However, in the high density
case, it is the values of He / H(3) which are in error on the high
side. This is due to the underestimate of the density (and thus
the corrections for collisional enhancement are too small).
Note that the solutions for
(3889) have been driven to
large values. For both the higher density cases, the density
has been underestimated, the
(3889) overestimated, and
the He / H(3) overestimated.
In Figure 8, we show the results of the 6 line
method Monte Carlo for case 2 of Table 2. Here
the low density, high
(3889), and bias towards higher
values of He / H are
clear (even higher values would be shown if He / H(3) were plotted).
Note that the lower right panel shows that the
2 values
are all systematically high for this case. It would appear
that the
2 is a
sensitive test to check whether
the I(
3889) values are
systematically biased.
![]() |
Figure 8. Similar plot to
Figure 6 except that
I( |
The low density cases show
generally low values of He / H and satisfactory values of
He / H(3) (although the solutions for
(3889) are all
systematically high). The main result of a systematically
low values of I(
3889)
occurs when the nebula has a
high enough density that the collisional enhancement is
important. Then the underestimate of density results in
systematically higher He / H.
5.3. Cases with Systematic Errors in I(7065)
One of the possible problems of using
7065 is that in
a spectrum where the entire wavelength range from
3727 to
7065 is observed in first
order, there is the potential
for contamination of the red part of the spectrum from blue light
in the second order. In principle, if a blue cutoff
filter is used (e.g., a CG385 order separation filter) then there
should be little contamination blueward of 2 × 3850 Å
or 7700 Å. However, order separation filters are not
perfect cut-on filters at 3850 Å, but rather start to filter
out light above 4000 Å, reach 50% transparency at about
3850 Å and then drop to zero transparency somewhere between
3500 and 3600 Å. Thus, there is potential for some second order
contamination for all wavelengths above 7000 Å. If the
observed target has a significant redshift, then the He I
7065 is even more
susceptible to this problem. The problem
is worse for bluer order separation filters (e.g., CG375).
This effect can be quite subtle and there are two separate problems
to consider. The first is contamination of the standard star spectrum.
If a blue standard star (e.g., a white dwarf) is used for flux
calibration, then the far red part of the spectrum detects
additional second order blue photons, which, when used to
calibrate the spectrograph, results in an overestimate of
the red sensitivity of the spectrograph. The second problem
occurs in the target spectrum. Here the far red continuum will
be contaminated by extra second order blue photons. It is
possible that if the blue spectral shape of the standard star
is similar to the spectral shape of the target, then these
two effects will compensate, giving a rather normal looking
red continuum. However, the overestimate of the red sensitivity
will result in underestimated emission line fluxes and equivalent
widths. Since 7065 lies
right at the border of where
this effect can become important, it is very important to check
for this possibility. This is most easily done by obtaining
spectra of both red and blue standard stars and deriving
instrument sensitivity curves independently for the two stars.
The wavelength at which the two sensitivity curves begin to
deviate indicates the onset of second order contamination.
Here we investigate the possible effects of a systematically
low strength of 7065. We
have run identical cases
and analyses as in Table 1, but altered the input
synthetic spectra by decreasing the relative flux and equivalent width
of
7065 by 5%. These
results are presented in Table 3.
Note that this exercise was motivated, in part, by the
systematically low values of He / H derived from
7065
when compared with the main three He I lines in a subsample of
the highest quality data of IT98.
![]() |
The 5% drop in 7065 has
dramatic effects. Beginning with the cases in which the input values of
and aHeI are
0, we see that the density is grossly underestimated in both input cases
with n = 10 and 100 cm-3. In the low density cases,
this does not have a very strong effect on the derived helium abundances
(because the density dependent collisional enhancement term is already
quite small).
In the high density case with no underlying absorption or optical
depth effects, the five and six line methods give a He abundances
which are significantly higher than the correct value of 0.08. In this
case, the three line method which is not distracted by the errant
7065 line does the best at
finding the solution. We see the effect of the
7065 line driving down the
density and
compensating by allowing for non-zero absorption, which is controlled in
the 6 line method due to
4026.
In Figure 9, we show the results of the six-line
method Monte Carlo for
case 2 of Table 3. Case 2 shows the most
discrepant results in Table 3,
and the six-line method is only better than the five-line method with
absorption allowed (but not constrained by
4026. Here it
can be seen clearly that the solutions all favor lower density, and
thus higher He / H. The trade-off between low density and high values
of underlying absorption is clearly shown. Interestingly, the values
for
2 are relatively
low and quite satisfactory. Clearly the
2 is not a good
diagnostic of an underestimated
I(
7065), as the
degeneracies allow mathematically acceptable solutions.
![]() |
Figure 9. Similar plot to
Figure 6 except that
I( |
Case 6 provides an interesting comparison case to consider when
7065 is low.
Notice here, that the 5 and 6 line methods again over-estimate the He
abundance. While both solutions find the approximate underlying
absorption (the 6 line method is better) they underestimate the density
and the optical depth. Here the 3 line method, even with its lack of
sensitivity, still solves for the correct density (to within 8% when
is not assumed to be zero) and
underlying absorption. The He
abundance is again very accurately determined by this method.
In Figure 10, we show the results of the
six-line method Monte Carlo
for case 6 of Table 3. In this case of a small
amount of optical
depth and a small amount of underlying absorption, the solutions do a
pretty good job of finding the correct range of density, optical depth,
and underlying absorption. The underestimated
7065 has resulted
in a bias toward lower density, which has resulted in a bias toward
larger He / H, but the effect is not very large. The main effect is
the size of the error bars. Here it is clear that Monte Carlo errors
in He / H are about 3 times larger than the errors from a straight
minimization. Note again that the
2 values are generally
small.
![]() |
Figure 10. Similar plot to
Figure 7 except that
I( |
The main result of tests with a systematically low
7065
is that for nebulae with densities which correspond to significant
collisional enhancement corrections, the He / H will be overestimated.
The
2 is not
necessarily a good test of whether
7065
has been systematically underestimated.