International Ultraviolet Exploration observations of NGC 3918 and IC 2448 are presented. Combining these observations with data in the optical range and computed model structure, the chemical composition for these objects is derived. For NGC 3918 log C = -3.02, log N = -3.61 and log 0 = -3.22; while for IC 2448 log C = -3.44, log N = -81 and log 0 = 3.54

Daltabuit, E.

Pena, M.

Torres-Peimbert, S.

English

NASA Technical Reports Server

THE HIGH'EXCITATION PLANETARY NEBULAE
NGC 3918 AND IC 2448
S. Torres-Peimbert, M. Pefia and E. Daltabuit
Instituto de Astronom_a
Universidad Nacional Aut6noma de Mgxico
ABSTRACT
We present IUE observations of NGC 3918 and IC 2448. Combining these ob-
servations with data in the optical range and computed model structures we
derive the chemical composition for these objects. For NGC 3918 we obtain
log C= -3.02, log N= -3.61 and log O= -3.22; while for IC 2448 we obtain
log C= -3.44, log N= -3.81 and log O= -3.54.
8
INTRODUCTION
These planetary nebulae are ideally suited for observations with IUE since
they _ave high surface brightness and small angular size.
They have been observed in the visual range by Torres-Peimbert and Peim-
bert (ref. 1). They derive for NGC 3918 N(He)/N(H)= .ll2, log N= -3.72,
log O= -3.10 and log Ne= -3.84 and for IC 2448 N(He)/N(H)= .lll, log N= -4.49,
log O= "3.39 and log Ne= -4.08 under the assumption of t2= 0.035.
OBSERVATIONS
For NGC 3918 we have the following large aperture exposures: SWP 1906
(7 min), SWP 3191 (20 min), SWP 3192 (i0 min), LWR 1767 (12 min), LWR 2753
(20 min), LWR 2754 (i0 min), LWR 2809 (6 min) and the following small aperture
exposures: SWP 1906 (20 min), SWP 3216 (15 min), LWR 2809 (12 min). For
IC 2448 we only have available the large aperture exposures:SWP 3194 (30 min)
and LWR 2756 (30 min).
We have applied the mean calibration correction (ref. 2) to the observed
fluxes to derive absolute fluxes. We present in Figurel composite large
aperture spectra of these objects.
In Table II we present the derived intrinsic fluxes for all emission
lines, I(k), the observed fluxes F(1641) and F(H_) and the reddening correction
at Hfl, C(HA). To obtain the intrinsic fluxes we have used the relation log
I(k)/I(HS)= log F(k)/F(HS)+C(HS)f(k). We have derived the reddening correction
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by assumir_ that: i) the fluxes relative to H8 in the visual range are the
same for the observed region as for the entire nebula, 2) the reddening law_
f(_), behaves as the expression given by Seaton (ref. 3) and 3) the intensity
ratio of the Hell recombination lines I(1641)/1(4686) is 6.6 (ref. h).
The internal errors in the measurements, as estimated from the differe,tt
exposures, are 415% for NGC 3918, and presumably similar for IC 2448. For
IC 2448 the listed flux for _1549 is only an upper limit, since the line wai_
saturated.
MODEL IONIZATION STRUCTURES
We have computed model ionization structures that will be described in
detailelsewhere. The models predict line intensities forC, N, 0, Ne, Mg,
Si and S. For comparison we present a few representative models in Table III.
To try to fit NGC 3918 we computed models for an exciting star of T,=
150,000 (from ref. 5). We have tried unsuccessfully to adjust both I(5007)
and I(1909) simultaneously by varying density distribution and chemical abu=-
dances. The resonant lines of CIV, NV and MgIl appear systematically too
bright in our models. This effect has been noted by'other authors (refs. 6
and 7) who explain it by dust absorption. In order to adjust the lines of
low ionization, it is necessary to assume that the nebula is density bounded.
For our models of IC 2448 we have adopted an exciting star of T,=125a000.
We encounter the same problems in our attempt to adjust 1(5007) andl(1909)
and have found it necessary to assume a density bound nebula to improve the
fit for ions of low stages of ionization. For this object wedo not have
observations of the faint lines and thus have less restrictions for our model.
IONIC ABUNDANCES
For NGC 3918 from the ratiol(1666)/l(5007) of 0++ we obtain the same
temperature as from I(4363)/I(5007). We also are able to derive a temperature
of 13800 °K from the I(1602)/I(2423) ratio of Ne+1; however none of our models
predict the temperature of the highly ionized region to be drastically differ-
ent for the 0++ region.
To derive ionic abundances we adopted an empirical scheme of dividing the
nebulae in three regions: thoseof low, medium and high stages of ionization.
For elements in lowstages of ionization we have assumed the observed T(NII)
to be applicable. Our models do not predict such a sharp decrease in temper-
ature in the outer shells and thus the derived abundances are probably over-
estimated, but these ions are not dominant in either nebula. For the
elements in the intermediate stages we have used T(OIII) as derived from ob-
servations (ref. i); and for the higher stages of ionization we have used tem-
peratures 600 K higher'than T(OIII), since our models predict this tempera- _
ture variation. We have assumed t2= 0.0 for our derivation of the ionic abun-
642
dances. The results are listed in Table II. For both objects, the 0++ abun-
dances derived from A5007 and A1666 are in excellent agreement and give us confi-
dence in the adopted reddening.
All ionic abundances derived frc_ultraviolet lines are highly depend-
ent on temperatures, and we typically expect errors of _0.10 dex in the ionic
determinations for errors of ±300 °K.
DISCUSSION
We have derived total abundances from the ionic ones. In the case of
NGC 3918, for oxygen and nitrogen we have added all the ionic abundances in a
straightforward way. For carbon we have taken into account only I(1909) and
from comparison with our models we have adopted a ratio of relative volumes of
carbon to oxygen x(C++)/x(0++)= 0.74. In the case of IC 2448 we have added
0+ and 0++, and have corrected by a factor icf= He/(He+), which is in agree-
ment with our models. For nitrogen we have adopted an intermediate value be-
tween those derived from the comparison of our models and N+_ and N+_ . For
carbon we have assumed that the resonance line I(1549) has been depleted by a
factor of 2 due to dust and have added all other ions. Comparison with our
models shows that the carbon abundance is probably underestimated. The total
abundances relative to Ikydrogenin NGC 3918 are log O= -3.22, log N= -3.61
and 10g C= -3.02; in IC 2448 are log O= -3.54, log N= -3.81 and log C= -3.44.
We are grateful to M. Peimbert for fruitful discussions.
REFERENCES
l, Torres-Peimbert, S. and Peimbert, M.: Photoelectric Photometry and Physical
Conditions of Planetary Nebulae. Rev. Mexicana Astron. Astrof. Vol. 2,
No. 3, Aug. 1977, pp. 181-207.
2. Bohlin, R.C. and SniJders, M.A.J.: Photometric Calibration of the IUE.
NASA Newsletter No. 2, Nov. 1978.
3. Seaton, M.J.: Interstellar Extinction in the UV. M.N.R.A.S., Vol. 187,
No. 3, June 1979, PP. 73p-76p.
_. Seaton, M.J.: Calculated Intensities of HeII Recombination Lines in the
Ultraviolet. M.N.R.A.S., Vol. 185, No. l, Oct. 1978, pp. 5p-Sp.
5. Hummer, D.G. and Mihalas, D.: Surface Fluxes for Model Atmospheres for the
Central Stars of Planetary Nebulae. JILAReport No. I01, June 1970.
6. Pequinot, D., Aldrovandi, S.M.V., and Stasinska, G.: Charge Transfer Reac-
tions. A Consistent Model of theP!anetary Nebula NGC 7027. Astron. and
Astroph., Vol. 63, 1978, pp. 313-324.
643
7. Bohlin, R.C., Marionni, P.A., _dStecher, T.P.: The Rocket Ultraviolet .
Spectrum of the Planetary Nebula NGC 7027. Ap. J., Vol. 202, 1975, pp.415-_20,
644
TABLE I .- UNREDDENED.FLUXES
RELATIVE TO H8a)
Ion I(A)/I(H_)
NGC 3918 IC 2448
1215 HI <-0.51 -0.41
1240 _-V -0.54 -0.25
1401 OI_ +SIV+SilV -0.46 ....
1488 NIV] -0.43 _l. O0
1549 CIV +0.64 +0.37
1575 [_eV] .... -i.01
1602 [_elV] -1.17
1641 HeII +0.43 +0140
1666 OIII_ -0.55 -0.63
1749 NII_ -0.63 4-0.82
1909 CllI_ +0.69 _+0.58
2298 ........ -o.75
2326 Cil]+_in) -o.38 -o.99
2386 NeII -1.57
2423 +O.ll
2_71 [bill -0.98 ....
2512 Hell -1.33 ....
2734 Hell -1.03 -1.12
2800 Mgll <-1.34 <-i.i0
2837 0111 -0.98 -1.17
3023+47 0111 -0.73 -1.06
3133 OIII -0.04 -0.26
3204 HelI -0.66 -0.83
,i
l--ogF(164i) .... -10.05 -10.69
log F(HS) .... -10.03 -10.84
c(as) .... + 0.40 + o.22
a) LogI(X)II(H_).
645
TABLE II.- TEMPERATURE, D_NSITY
AND IONIC ABUNDANCESaj
NGC 3918 IC 2448
T(NII) 8900 9400b)
T(OIII) 12100 12500.,
T(NelV) 13800 13100°)
Ne(SII) 9000 12000
Ne(rms) 4600 1700
C+ 2326 -3.84 -4.62
C++ 1909 -3.52 9-3.72
C+3 1549 -3.82 -4.20
N+ 6584 -4.68 -6.17
N++ 1749 -4.11 64.40
N+3 1488 -4.00 54.71
N+_ 1240 -4.21 -4.05
0+ 3727 -4.06 -5.08
0++ 5007 -3.54 -3.71
0++ 1666 -3.55 -3.72
0+3 1401 -3.46:
Ne++ 3869 -4.28 -4[38
Ne+3 2_23 -4.09 -4.90
S+ 6717+31 -6.30 -7.51:
S++ 6311 -4.99 ....
S+3 1393 -4.50:
Ar ++ 7136 -5.97 -6:51:
a)
Given in log N(x+m)/N(H+).
b) Adopted.
646
TABLE III.-COMPARISON OF MODELS WITH OBSERVATIONS.
LINE INTENSITIES,TEMPERATURES AND RELATIVE VOLUMES. a)
NGC 3918 IG 2448
Obs. N1 N2 N3 Obs. I1 I2
HeI 4471 3,8 3.1 3.0 3.1 4.0 3.3 3.0
Hell 4686 44.0 41.8 44.0 43.1 38.0 34.7 41.2
CI_ 2326 41.0 122.0 34.7 133.0 i0.0 9.1 12.0
CII_ 1909 490.0 1070.0 567.0 1170.0 3380.0 411.0 460.0
CIV 1549 450.0 2540.0 1950.0 1970.0 230.0 i010.0 885.0
[_I] 5200 0.5 0.0 0.0 0.1 ..... 0.0 0.0
I_. 6584 66.0 45.0 24.9 60.7 2.5 10.5 14.2
NIIIJ 1749 23.0 56.6 60.1 59.4 _ 15.0 51.5 57.3
NIV] 1488 37.0 71.0 ll2.0 54.0 . 65.7 55.2
NV 1240 29.0 174.0 296.0 104.0 56[0 8.3 6.1
[09_ 6300 3.8 13.0 4.7 15.2 . 0.0 0.i
_II]. 3727 61.0 277.0 222.0 349.0 "613 28.3 39.9
DIII] 5007 1570.0 1590.0 2210.0 1630.0 1060.0 1400.0 1290.0
OIVJ 1401 <35.0 18.8 39.9 16.4 ..... 40.5 14.2
T(C .) .... 12600 12830 11890 .... 13240 13260
T(C++) .... 12680 12940 12050 .... 13340 13330
T(C +_) 13530 14180 12930 .... 13540 13480
T(N +) 8900 12590 12820 11900 .... 13250 13270
T(O+) .... 12620 12830 11840 .... 13110 13130
T(O ++) 12100 12700 12970 12100 12500 13250 13260
x(C +) .... 0.035 0.017 0.041 .... 0.008 0.010
x(C ++) .... 0.363 0.329 0.441 .... 0.383 0.432
x(C e3) .... 0.386 0.415 0.340 .... 0.574 0.525
x(N +) .... 0.015 O.OO8 0.018 .... O.OO5 O.OO7
x(N++) .... 0.438 0.395 0.520 .... 0.481 0.541
x(N +3) .... 0.295 0.318 0.255 .... 0.477 0.420
x(O +) .155 0.iii 0.066 0.137 .030 0.018 0.020
x(O ++) .513 0.447 0.461 0.421 .676 0.653 0.602
x(O +3) .... 0.195 0.213 0.228 .... 0.300 0.349
N H (cm-3) 4600 5000 5000 5000 1700 4000 2000
e h) .26 1.0 1.0 0.5 .02 1.0 1.0
log O/H -3.22 -3.30 -3.20 -3.20 -3.54 -3.54 -3.54
log C/O +0.20 +0.40 0.00 +0.40 +0.i0 +0.i0 +0.i0
log N/O -0.39 -0.20 -0.30 -0.20 -0.27 -0.16 -0.16
Ri (pc) c) ..... 001 .001 .001 ..... 001 .107
R e (pc) .05 .068 .067 .084 .13 .o74 .ll6
R. (R@) d) .... 0.10 O.10 O.10 .... 0.10 0.i0
T, (103°K) .... 150 150 150 .... 125 125
a) Relative to H6, where I(H6) = 100, b) e is the filling factor.
T(x+m)E fNeN(X+m)dV/fNeN(X) dV. c) Ri, Ro are inner and outer radii.
x(X+m)_fNeN(X+m)dV/fNeN(X) dV. d) R,, T, are central stars parameters.
647


The high-excitation planetary nebulae: NGC 3918 and IC 2448

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