Observations of the ultraviolet spectra of two locations in the H II region NGC 346 and of the entire H II region IC 1644 in the Small Magellanic Cloud (SMC) were made using the International Ultraviolet Explorer (IUE) satellite. The abundance of carbon in the nebulae was derived using theoretical model analysis combined with ground-based spectrophotometry of other emission lines. The abundance of C relative to H in the SMC was found to be lower by -0.9 dex compared with the Sun and lower by -0.8 dex compared with the Orion Nebula. This C deficiency is similar to that of O, Ne, S, and Ar in the SMC, but not as great as found for N. The sites and history of C nucleosynthesis in galaxies is similar to that of O, Ne, S, and Ar, in contrast to that of N, which appears to be more complex, perhaps because of a mixture of secondary primary sources or a significant contribution from intermediate-mass long-lived stars

Dufour, R. J.

Shields, G. A.

Talbot, R. J., Jr.

English

NASA Technical Reports Server

THE CARBONABUNDANCEIN TWOH II
REGIONSOF THE SMALLMAGELLANICCLOUD
Reginald J. Dufour and RaymondJ. Talbot, Jr.
Rice University
Gregory A. Shields
University of Texas at Austin
ABSTRACT
Observations of the ultraviolet spectra of two locations in the H II
region NGC346 and of the entire H II region IC 1644 in the Small Magellanic
Cloud (SMC) were made using the International Ultraviolet Explorer (IUE)
satellite. From measurements of the C III] L1909 lines, the abundance of
carbon in the nebulae was derived using theoretical model analysis combined
with ground-based spectrophotometry of other emission lines. The abundance of
C relative to H in the SMCwas found to be lower by -0.9 dex compared with the
Sun and lower by -0.8 dex compared with the Orion Nebula. This C deficiency
is similar to that of O, Ne, S, and Ar in the SMC, but not as great as found
for N (_ - 1.2 dex). Therefore, it is concluded that the sites and history
of C nucleosynthesis in galaxies is similar to that of O, Ne, S, and Ar, in
contrast to that of N, which appears to be more complex, perhaps because of a
mixture of secondary primary sources or a significant contribution from inter-
mediate-mass long-lived stars.
INTRODUCTION
The Large and Small Magellanic Clouds (LMC and SMC) provide a unique
opportunity to study in detail the structure and composition of two galaxies
representing an earlier epoch in the chemical evolution of galaxies compared
with our own. Recent ground-based spectroscopic studies of H II regions in
the Clouds by the Peimberts (refs., 1,2), Aller et al. (refs. 3,4), Pagel et
al. (ref. 5), and Dufour (refs, 6.7), show that [O/H] is -0.5 in the LMCand
-0.9 in the SMC(where the bracketed ratio represents the difference of the
logarithm of the abundance ratio between the galaxy and the sun). Nitrogen
shows the greatest deficiency of all elements studied (He, N, O, Ne, S, and
At) with [N/H] _ -I.I in the LMCand [N/H] _ -1.4 in the SMC. Also, helium is
slightly deficient in H II regions in the Clouds compared with similar nebulae
in the Galaxy.
While spectrophotometric studies of H II regions in the Magellanic Clouds
with ground-based telescopes have resulted in the accurate determination of
the abundances of a number of elements of astrophysical importance, others,
such as carbon, can only be readily observed in the ultraviolet. Therefore,
we have initiated observations of the ultraviolet spectra in the
_I135-3255 A range of several H II regions in the LMCand SMCwith the Inter-
national Ultraviolet Explorer (IUE) satellite. In this note we report the
first results of observations of two H II regions in the SMC.
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OBSERVATIONSAND RESULTS
The spectra of two H II regions in the SMC, NGC346 and IC 1644, were
obtained in May 1979 with the Short Wavelength Prime (SWP) and Long Wavelength
Redundant (LWR) cameras of the IUE at low dispersion. The SWPobservations
covered the XL1135-2085 region with a spectral purity of 30 X. The I0 x 20 arc
second elliptical aperture was used in all of the observations. Emphasis w_s
placed on the SWPobservations in order to measure the C III] k1909 and C I_
k1549 lines necessary for determining the carbon abundance.
The nebula NGC346 is the most prominent H II region in the SMClocated
in the northeast area of the main body of stars. It is listed as N66 in
Henize's (ref. 8) catalog of LMCand SMCemission nebulae. Since the nebula
has a significant apparent size, 490 x 580 arc seconds, two locations were
observed: N66A--a bright knot located in the southern part of the nebula, and
N66NW--a smooth star-free area located in the northwest lobe of the nebula.
These two locations were chosen primarily because there exists published
ground-based photoelectric spectrophotometry of them (refs. 2, 7).
The nebula IC 1644 is a small ( ~ I0 arc second diameter) H II sphere of
high surface brightness located southeast of the SMCbar in the Shapley's
"wing" feature. It is listed as N81 in Henize's catalog. Dufour et al.
(refs. 7, 9) have published photoelectric and photographic spectrophotomet_,/
for all significant emission lines in the _3727-7136 spectral "region of NSI
from ground-based observations.
Generally two or more integrations of each location were obtained with
the SWPand LWRcameras. A log of the observations is given in Table I. The
observations of NGC346/N66A and NGC346/N66NWregions were obtained during
the USI shift and have low particle background. The IC 1644/N81 observations
were obtained during the US2 shift and have very high particle noise back-
grounds. In addition, the underlying stellar continuum is considerable for
NGC346/N66A and particularly so for IC 1644/N81. Because of the high
particle background and stellar continuum encountered in the IC 1644/N81.
observations, only two lO-minute SWPintegrations were usable for quanti-
tative measurements of the five obtained.
The photometrically and geometrically corrected spectra on the GO tapes
prepared by NASA/GSFCwere analyzed using interactive picture processing
systems at Rice University and at NASA/JSCin Houston. SWPspectra processed
with the incorrect Intensity Transfer Function were corrected with the three-
agency 4th file method. The resulting 55 line x 602 word spectra background
arrays where then analyzed using a FORTHinteractive spectral analysis system
originally developed by F.H. Schiffer, III at NASA/JSC. This permitted us to
use a variety of smoothing, background subtraction, and spectral extraction
techniques optimized for each observation. The only prominent lines (other
than Ly _ predominantly from the geocorona) detected in the nebulae were the
C lllJ _1909 pair unresolved on our own spectra; as was Mg II
LL2796,2804, which might have been expected to be detectable on some of the
LWRspectra.
Measurements of the C III] k1909 strengths were made using an interactive
Tektronixs cursor routine to set continuum levels and integrate areas.
Several measurements of each line were made in an attempt to evaluate the
magnitude of possible errors due to the prejudice of the operator in setting
the continuum level, estimating the line width, etc. Comparison of the line
strengths measured on two different spectra of a given object provided an
672
estimate of the instrumentalerrors. The areas in instrumentalunits (FN)
were convertedto the observedflux in ergs sec"_ cm"_ using the large aper-
ture SWP calibrationfor low dispersionderived by Bohlin et al. (ref. 10).
These were correctedfor interstellarextinction(which is rather small for
the SMC) using the UV extinctioncurve of Savage and Mathis (ref. 11) along
with publishedextinctionsfor the nebulaefromground-based observationsof
the H_/H# ratios. The 1909 reddening-correctedfluxeswere then scaled to
the HB fluxes observedfor each regionby previousinvestigations. For the
two regionsin NGC 346 we used the ratio of the areas of the IUE and ground-
based entrance apertures;while for the smallernebula IC 1644 we assumedthat
it was entirelywithin both the IUE and ground-basedapertures.
The line intensitiesfor C Ill] L1909 derived from our observationsalong
with those of other importantdiagnosticlines observedfrom the ground are
presentedin Table II. We also presentestimatedupper limitsto the
C IV _1549 and Mg II _2800 lines derived from the IUE spectra. We give esti-
mated errors for the _1909 intensitiesbased primarilyon the uncertaintiesin
the measurementsof the line profiles. The actual errors for _1909 may be two
or three times larger,due to the uncertaintiesin scalingthe IUE _1909 fluxes
to the ground-basedH# fluxes as well as uncertaintiesin the absolute
calibrationof the IUE SWP spectra. The ground-basedobservationswere made
with photoelectricscannersand have probableerrors of the order of 10 percent.
Other relevantdata about the nebulaeare presentedin the tables also.
ANALYSISANDDISCUSSION
Elementalabundancesfor H, He, C, N, O, Ne, S, and Ar in the nebulae
were derivedby Shields using a nebula modelingcode similarto that used in
previousstudiesof H II regionsin M101 (ref. 12) and M83 (ref. 13). Since
all three nebulae have rather similarspectra,we used a singlemodel
differentialanalysisapproach. A "standardmodel" was calculatedto match
the spectrumof NGC 346/N66A (T*eff = 45,000°K)and used to derive the temper-
ature fluctuationsfor variousions comparedwith T(O++) obtainedvia the [0
Ill] L_5007/4363ratio. The computedfluctuationswere used with the observed
[0 Ill] temperaturesand line strengthsto calculatevariousionic abundances
for those ions with observedlines. Elementalabundanceswere then inferred
from these ionic abundancesusing ionizationcorrectionsfor ions without
observed lines from the standardmodel. The final resultsare presentedin
Table Ill.
Using the estimatedupper limits to the strengthsof the C IV _1649 lines
for N_C 346/N66A in the model,.wefound that about 92 percentof the carbon in
th_±H zone was in the form of C++. Consequently,the correctionsfor C+ and
CTTT in the SMC nebulaeare small, so the fact that we did not observeC IV
should not significantlyaffect the accuracyof the C+_bundancesderived.
Based on model and observationaluncertaintiesin T(O ) alone,we estimate
that the uncertaintyin the C abundancefor the variousnebulae is about 0.1
dex. The good, but possiblyfortuitous,agreementof the C abundancesbetween
the three nebulae suggeststhat the accuracy by which the C abundanceis now
known in the SMC is better than ± 0.2 in the logarithm. Since this model is
also the first theoreticalmodel analysisof an H II region in the SMC, it is
gratifyingto note that the other abundancescalculatedagree very well with
those of previousinvestigationsfor which the analyseswere based on standard
nebulardiagnosticformulae.
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On the right side of table 3 we give the logarithmicdifferencesbetween_
the SMC mean abundancesand those determinedfor the Sun (ref. 14 for C,N, ai_d.....
O; ref..15for He, Ne, Mg, S, and Ar) and for the Orion Nebula (ref. 16). I't
is clear from inspectionof the table that the C/O ratio in the SMC is essen..
tiall_ identicalto that found for the Sun and the Orion Nebu'la,The
abundancedeficiencyof C relativetoH in'the SMC 'com'pa'redwith the Sun and
Orion is clearlymore similarto that of the elementsO, Ne, S, and Ar than to
that of N, Which is substantiallymore deficientthan the others. Therefore,
we concludethat the nucleosynthesisof C foll@wscloseIxthat of O_ Net St
and Ar as predictedby theoreticalnucleosynthesismodels(refs.17,18) . The
large variationof N/O while C/O remainsconstant impliesthat the stellar
sourcesof N do not contributea large fractionof the C.
It is of interestto comparethis result with the recent study of abun-
dances in severalhigh excitationplanetarynebulaewith the IUE by Aller and
Keyes (ref. 19). Severalof the nebulaestudiedwere overabundantin N by
factorsof 5-10compared to solar and Orion values. None of these N-rich
nebulaeshowed significantenhancementsin the C abundance. These resultsand
ours suggestthat the site(s)of C nucleosynthesisis the same as that of O,
Ne, S, and At. The processes,,presumablyfrom intermediatemass stars (ref.
20), which subsequentlyenrich galaxieswith N later in their evolutionappar-
ently do not significantlyaffect the relative abundancesof C, O, Ne, S, and
Ar.
We also note in ending that the Mg abundancefound for the two locations
in NGC 346 using our estimatedupper limit is at least 1.4 dex lower than the
solar value. Unlessthe nucleosynthesisof Mg is similarto that of N in the
SMC, which is unlikely,the low abundanceof Mg is apparentlydue to its
depletionby dust in the H II regions. The magnitudeof this depletionis
then rather surprising,when one notes that the dust contentof the SMC is
very low comparedwlth the LMC and our galaxy.
REFERENCES
1. Peimbert,M. and Torres-Peimbert,S., 1974. Astrophys.J., 193, 327.
2. Peimbert,M. and Torres-Peimbert,S., 1976. Astrophys.J., IFO-3-.581.
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9. Dufour, R.J., and Killen, R.M., 1977. Astr_phys.J., 211, 68.
10. Bohlin, R,C., Holm, A.V., Savage, B.D.:,Snijde'rs,M.A.-_'T.,and Sparks,
W.M., 1980. Astron. and Astrpphys.,in press.
11. Savage,B.D., 'andMathis,J.S., 1979. A.nn.R.ev..Astron..... andAs.trophys.,
17, 73.
12. S_i'elds,G.A., and Searle,L., 1978. Astroph_s.J., 222, 821.
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Astrophys.J., 236, 119.
14. Lambert, D.L., I_. Mon. Not. R. Astr. Soc., 182, 249.
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15. CamePon,A.G.W., 1973, in Explosive Nucleosynthes!s, eds., D.N. Schramm
and W.D. Arnett, Untv. of Texas Press: Austin, p.3
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18. Arnett, N.D._, 1978. Astrophys. J., 219, 100_. '_
19. A11er, L.H., and Keyes, c.D., i979.-O'CLA Astron. and Astrophys. Preprint
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675
TABLE I -- IUE OBSERVATIONSLOG
Nebula SWPExposures LWRExposures
(min) (min)
NGC346/N66A 30,120 130
NGC346/N66NW 120,180 30,30
IC 1644/N81 10,10,15" 20,20,20
20*, 60* 30*
*Not usable due to saturated pixels + high background
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TABLE II - LINE INTENSITIES FOR SMCH II REGIONS
_(A) ION NGC346 NGC 346 IC 1644N66A N66NW N81
1548,1551 C IV <20 <15 <30
1907,1909 C III] 123.±19.* 120.±10.* 162.±32.*
2796,2804 Mg II <20: <20. -
3726,3729 [0 II] 95.5 108. 128.
-3869 [Ne III] 44.7 35.0 37.8
4340 Hy 46.8 46.6 45.5
4363 [0 llI] 7.08 6.07 6.44
4471 He I --- 3.33 3.50
4861 H_ I00. !00. lOO.
4959 [0 llI] --- 172. 177.
5007 [0 Ill] 513. 501. 525.
5876 He I 10.2 11.0 11.9
6312 [S Ill] 2.6: 1.8 ---
6563 Ha 282. 286. 286.
6584 IN II] 4.37 6.38 6.30
6678 He I 2.95 3.51 3.08
6716 [S II] 6.76 _ 14.4 6.0
6730 [S Ill 4.47 J 5.4
7136 [Ar III] 9.77 -- 6.7:
7320,7330 [0 II] 2.34
C(HB) 0.17 0.07 0.04
log I(HB) -11.41 - 11.12 -11.02
reference (optical) 1 7 7, 9
*Errors quoted for _1909 are based on comparative statistics between two
observations; actual errors due to absolute calibration of IUE and zero point
transfers may be two or three times larger.
677
TABLEIll-- MODELABUNDANCERESULTS
(1) (2) {3) (4) (5) (6) (7) ,',)
rlr,C 346 NGC 346 IC 1644 MEAN SUN ORION
_ _ SM'---C"(_'efs.14; 15) (-_ (4)- (5) C;!- (';)
lle 10.91 10.92 10.94 10.92 --- 11.00 .... f)_:']-
c 7.64 /.76 7.85 7.75 8.67 8.52 -0.92 -F).77
c_ N 6.49 6.69 6.68 6.62 7.99 7.76 -1.37 -I.l,i
"J 0 8.00 8.06 8.06 8.04 8.92 8.15 -0.88 _,71
r_e 7.23 7.20 7.20 7.21 8.03 7.90 -0.82 -F).69
Mg <6.1 <6.1 --- <6.1 7.52 --- <1.4 ---
S 6.4: 6.6: 6.6: 6.6: 7.20 7.41 -0.6 -!.8
Ar 5.80 --- 5.66 5.73 6.57 6.7: °0.84 -I.0
C/N 1.15 1.07 1.17 1.13 0.68 0.76 0.45 r}.37
C/O -0.36 -0.30 -0.21 -0.29 -0.25 -0.23 -0.04 -(].r16


The carbon abundance in two h 2 regions of the small Megallanic Cloud

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