The IUE observations were used to determine the composition of the ejecta (especially C and Si abundances) and to test models for the ionization and excitation of the ejecta of two oxygen-rich supernova remnants (N132D in the Large Magellanic Cloud and 1E 0102-7219 in the Small Magellanic Cloud). Time-dependent photoionization by the EUV and X-ray radiation from 1E 0102-7219 can qualitatively explain its UV and optical line emission, but the density and ionization structures are complex and prevent a unique model from being specified. Many model parameters are poorly constrained, including the time dependence and shape of the ionizing spectrum. Moreover, the models presented are not self-consistent in that the volumes and densities of the optically emitting gas imply optical depths of order unity in the EUV, but absorption of the ionizing radiation was ignored. It is possible that these shortcomings reflect a more fundamental limitation of the model assumptions. It is assumed that the electron velocity distribution is Maxwellian and that the energy deposited by photoionization heats the electrons directly. The 500 eV electrons produced by the Auger process may excite or ionize other ions before they slow down enough to share their energy with other electrons. Many of the excitations would produce photons that could ionize lower ionization stages

Blair, W. P.

Danziger, J.

Matteucci, F.

Raymond, J. C.

English

NASA Technical Reports Server

N89-10699
117
IUE OBSERVATIONS OF OXYGEN-RICH SUPERNOVA REMNANTS
W.I >. lJlah '1 , .l.C. 17aynion(I 7. J. Danziger 3. and F. Matteu(:ci :1
I'l'lw, Johns l lopkins University
21 larvard-Sniithsouian Center for Astroilhy<_les
3l_llropean CJonthorn Observatory
Oxygen-rich supernova, l'emnants present all
opportunil,y to observe material ejected hi type II
snpernowt explosions. |LIE obsorvatious hell) to dcl,er-
inine i,lie eoinposition of the ejecta (especially C alid Si
abundances) and {o test models for the ionization and
excitation of tile ejecta. We present UV observations
of two oxygen-rich supernova remnants (N132D hi The
Largo Magellanic Cloud and 1E 0102-7219 in the
Small Magollanic Cloud) and discuss the shuilarities
and difl_'rences betv,'een thenl.
[<eywords: nebuhle:snpernova remnants - nlleleosyn-
thesis- abun,:lances
1. Introduction
A few supernova, remnants (SNRs) contain
relnarkabie high velocity knots of material enth'e]y
devoid of iiydrogen and helhim. At optical
wavelengths these knots emit [O11] and [bill] lines
most strongly, with weaker lines of neon, sulrer, cal-
alum and argon in some cases (Refs. 6 and 13) and
sometiines neutral oxygen recombination lines (Ref.
18).
"l'he elemental composition of this high velocity
material, undihited since its ejection by the supernova
exl)losion, provides a look at the products of
nucleosynthesis inside the progenitor star before and
during the explosion. It therefore provides a direet
test of stellar evolution and nueleosynthesis calcula-
tions. Ultraviolet observations are ext, remely impor-
tant for such tests, because all the strong emission
lines of earl)on, magnesium and silicon are located in
the lUg wavelength range. In addition, ultraviolet
observations can provide tests of models for the excita-
tion and ionization of the ejcted material by shock
waves (Refs. 6 and 11) or by photoionization (Ref. 3).
Of the seven rcuinants known to possess
hydrogen-free knots, three are hi our galaxy (Puppis
A, C_ A and G292.0+1.8) and are unfortunately too
highly reddened for IUB study. The SNR in NGC 4 big
is extremely luminous intrinsically, but is so distant
that only a weak detection of [O IIl] X1663 was possi-
ble with 1UE (Rof. 2). The l,arge Magellanic Cloud
remnant 05'10-69.3 is not only oxygen-rich, but also
shows many similarities to the Crab Nebula; IUE
ohservations of this ohject have been attempted twice,
hut only a faint conthmum from the object has been
detected (see 17el. 1). Only the snpernova remnants
Nla2D in The Large Magellanic Cloud and 1E 0102-
7219 in The Small Magellanie Cloud are sufficiently
bright and unreddened for detailed study with lug.
Below we discuss our lUg observations of these two
objects and the conclusions that can be drawn from
combined UV/optical data.
2. IUE Observations
Both N132D and 1E 0102-7219 show optical emis-
sion knots that are bright in the forbidden lines of
oxygen and neon, with expansion velocities of 2250
and 6500 km s -I and diameters of 32 and 6.9 pc,
respectively (Refs. 7, 14 and 17). Although the exact
relative line intensities (lifter, by-in-large the optical
spectra arc quite similar. This is in contrast to what
we have found in the UV.
We have combined ESA and NASA shifts to
obtain long SWP and LWP exposures on bright por-
tions of O-rich material in each of these objects. The
[UE spectra have been reduced from the extended
line-by-line files at the RDAF at NASA/GSFC.
Reseau marks, lilts and hot camera pixels were
removed from the line-by-line data before re-extracting
the spectra. Figures la and lb show the resulting
S\VP and LWI' spectra of N132D, while Figure 2a and
2h show the corresponding spectra of 1E 0102-7219.
Table 1 presents thixes from these observations, which
A Decade of UV Astronomy with IUE, Prec. Celebrawry Symposium, GSFC, Greenbelt, USA, 12-15 April 1988, ESA SP-281, Vol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001328 2020-03-20T05:58:12+00:00Z
118 W P BLAIR ET AL
correspond to the northeastern edge of the N132D ring
and the southeastern side of 1E 0102-7219. The line
intensities have not been corrected for reddening, but
with E(B-V) = .08 -- .09 (Refs. 3 and 14), the relative
intensities of the UV lines are only marginally affected.
Comparison of the spectra and the line intensities
shown in Table 1 indicates both similarities and
differences in the emission from the two objects. Many
of the same lines are seen although the relative intensi-
ties vary. No Si III} ),1982 emission is detected, so the
),1400 feature can be attributed to O IV] ill both
objects. Comparison of the observed line strengths
indicates that the gas in 1E 0102-7219 has larger C,
Ne and Mg abundances relative to O than in N132D
by a factor of roughly two.
One of the most striking differences between the
two spectra is the O I ),1356 line, which is at least
twice as strong as O Ill] )`1663 in 1E 0102-7219, but
entirely absent in N132D. This line is formed by
recombination (Ref. 3), along with the optical )`7774
multiplet (Refs. 12 and 18). This identification is
confirmed by optical spectra, which show the )`7774
line present in 1E 0102-7219 (Ref. 3), but not in
N132D (unpublished spectrum). For the SMC rem-
nant, the relative intensities of X1356 and )`7774 can
be used to scale the UV and optical line intensities,
since these lines come from the same recombination
cascade.
Another difference in the spectra is the presence or"
strong C 1I])`2325 emission in N132D while none was
detected in 1E 0102-7219. The presence of C Ill, C III]
and C IV emission lines at comparable intensities in
the UV spectrum of N132D (and the absence of the O
I recombination lines) is a strong indicator that shock
heating is responsible for exciting the emission in this
object. The absence of C Ill in the spectrum of the
SMC remnant even though the carbon abundance is
higher calls into question whether shocks are the main
cause of the observed emission.
A related difference is apparent when the UV and
optical line intensities are compared. In the SMC rem-
nant, tile UV lines are weaker than expected from
most shock models with respect to the optical lines.
The cleanest line ratio that demonstrates this is O Ill
I(1663)/|(5007_, which depends only on the ten]pcra-
turein tile O "+ region. This ratio in 1E 0102-7219 is
only 0.08 after reddening correction (Ref. 3). Shock
wave models predict I(1663)/I(5007) in the range 0.3 --
0.9 (Refs. 6 and 12), which is difficult to avoid since
the ratio is only weakly temperature dependent at
electron temperatures high enough to collisionally ion-
ize O + to 02+ . For N132D we do not yet have an
accurate UV to optical normalization, but from a
crude comparison with the surface brightness estimate
in Ref. 14, the ratio is between 0.1.5 and 0.60. roughly
in accord with the expectations of shock heating.
3. Models and Interpretation
The differences mentioned above may well be
related to differences in the mechanism exciting the
emission in each object. The N132D observations were
only made recently so we have not yet calculated
detailed models For this object. Comparison of our
observed UV line intensities to published models of a
given shock velocity or set of assumed abundances
(Refs. 6 and 12) do not provide a good match. How-
ever, the large IUE aperture no doubt samples
material with a range of densities, shock velocities and
compositions. The qualitative indicators discussed
above provide convincing evidence that shock heating
is the dominant excitation source in N132D.
Since 1E 0102-7219 is the strongest extended soft
X-ray source in the SMC (Ref. 16), we consider fiX-ray
photoionization models for this object. The
1 5 X 1037 ergs s -I X-ray luminosity of 1E 0102.2-7219
(Ref. 9) implies an X-ray flux flux to density ratio far
higher than in the other known oxygen-rich SNRs, and
photoionization should be correspondingly more
important. Detailed measurements of the X-ray spec-
trum are not available, so we have used the X-ray
emission code described in Ref. 1.5 with updated
atomic data summarized in Ref. 5 to generate model
spectra covering the 5-1000 eV energy range at 2.1 eV
resolution. Models of young SNRs generally show very
strong emission fi'om the helium-like ions of the ele-
ments present (e.g. Refs. 8 and 10) so we have added
the emission of carbon in a 106 K plasma, oxygen at
1 6 x l0 s K, and neon at 3 Xl06 I( to give a spectrum
resembling those predicted by models. Several vari-
ants on this X-ray spectrum were used to examine the
sensitivity of the results to the assumed X-ray spec-
trum. In particular, we tried much stronger carbon
line emission, strong EUV emission in lines such as O
V X630, and attenuated EUV emission to simulate the
effects of absorption by the optically bright gas.
These alternatives changed the equilibrium tempera-
tures of the photoionized gas by < 30% and some of
the line ratios by factors of two, but they do not alter
the general conclusions below.
The X-ray spectrum was used to compute the
equilibrium ionization state and heating rate of gas at
various densities, and the temperature was iterated
until radiative cooling balanced the heating. The
abundances assumed were appropriate for tile ejecta
fi'om a Type 11 SN explosion. Some typical results of
these models are as Follows. The gas in the SMC rem-
nant must span a wide range in density. Regions
denser than about 10 cm -3 are needed to produce the
O I recombination lines, but they are too cold to pro-
duce any line excitation except in tile far infrared.
Regions near 1 cm -3 are x_ai'm enough to produce the
observed ultraviolet lines, but the temperatures are
low enough that the UV to optical line ratios arc mod-
est. A fraction of order 10% of t_l_e apparent, volume
OXYGEN-RICH SUPERNOVA REMNANTS 119
I '1
I -I,I
I
L-14
It 132 I_), lll_e'a 51-ql, <:;111,_
I I I
o lvl c Iv
1 2
t] 14 li
E I 1,1
It
E-15
G
E-15
4x
-I r-i,_ r-
I 1212(Rj
I_WP 12(;71, I_lne_q45-61
[ I I
c Ill
[Ne IV]
a.rI,o,,, "<" 1
_
__. I I J
2400 2600 2800 3OO0
WAVELENG III (A)
I"igUle I: IUE spectra o[' NI32D hi the Large Magellanic Cloud. a) SWI > 31578, 685 la]niil,0 (,\lie)-
sure, b) I,\'VP 12671, 730 inhiube exposllre, l-Iii, s and reseau marks have }Jeen ronio\ed ali(I i, he (hd, a
]l:i\'l' ])cell smoothed over three pixels.
C_
&
\
7
E
o
-7
T
b
12
r t
IE 0102-7_:
o i
I
__r __ i
1.300 1400 15011
, ,SWP 27926, 865 MIN
iv
o llll c iiii
t 1- I ------ L---.
1600 1700 1800 19110
WAVELENGIll ( A )
2,
E
o
x
_m
dJ
i • i. L t __L__
23(}0 2400 2500 ?600 2700 2800 2900
W^Vr_LEUGm( St )
l"igurc 2: IUE spectra of IE 0102-7219 in the Small Mngellanic Cloud. n) S\'VP 27926, 865 nlinul.e
(>kllosqr0. I)) IAVI' 78 t5, 810 lllinllte expo<qlre, lilts alld l'escall iIl$tl'kS h_iv(, },eel/ l'(.qllOV(.([ _llld tile
,h,l, ri )i',/ve beeil sluoothed over Liiroe I ixels.
Table I
Ultraviolet Emission Line Intensities for O-rich "?NRs
Relath'e _o O III] X1663 = 100
Ion \Vavelengt_h 1E 0102 --7219 N132D
C 1I X1335 <,I0 <8
0 I >,1356 20-t < 13
0 I\;], Si IV kl-lO0 328 152
C IV X1550 600 179
He H ),1640 <27 <13
0 Ill] 5,1663 I00 I00
N III 1 X1750 <27 < 13
Si Ill] XiSg2 <13 <10
C III] X1909 133 101
C II 5`2325 <67 152:
[Ne IX,'} 5`2125 191 73
[0 II] X2-t70 62 58
Mg II 5,2800 7,t ,1-t
F(O IlI] 5,160a)t
(in ergs cm-" s- )
2 6 E -I-t 7.7 E --11
of a torus 3.5 pc in radius with a minor radhls one
tenth _s hu'ge eontainhig g_m at a density n o _ 1 cm -a
can account for the observed lO I11]optical luminosity,
and shnilar volunles at higher densities can produce
the oxygen reeonlbination radiation and the UV lines.
There. are two major ditt]eultles with the equili-
brium photoionization models. First, the
[0 I1]X3727/[O I11]X5007 ratio is predicted to be less
than about 0.3, while the obse, rved ratio varies
between 1.3 and 2. This reflects the fact that oxygen
is mostly doubly ionized in models warm enough to
effectively excite the X3727 transition. Second, the
IO II1] emission comes mostly from 10,000 - 1.5,000 K
gas in the model, but the I(4363)/I(5007) ratio sug-
gests a teniperature of 2,5,000 K.
Both difficulties may be related to an unwar-
ranted as<_uniption of ionization equilibrhlm. While
the heating and cooling timeseales are a few years, so
that therilial balance is a valid approxhnation, the
ionization timeseales are _ 2000 .,,'ears, whieh is dose
120 W P BLAIR ET AL
to the expansion age of 1E 0102-7219. Hence, time
dependent models should give a lower ionization state
than predicted by the equilibrium model and also be
somewhat warmer. We have run a series of time-
dependent photoionization models where gas having
the same abundances as above was allowed to expand
freely at 3900 km s -1 for 2400 years while exposed to
the X-ray flux deseribed above. The gas was initially
taken to be eold and neutral, but the initial eonditions
turned out to be irrelevant since the gas always cooled
quickly, then slowly warmed and ionized late in the
evolution.
Again, these models indicate that a mixture of
densities is needed to produce both the high ionization
lines and the O I reeombination lines. These particu-
lar models improve the [O II]/[O III] ratio, but still fall
short of the observed value. They also predict a larger
C III]/C IV ratio than observed and far too much C II]
),2325. These problems could be alleviated by increas-
ing the carbon line intensities and decreasing the oxy-
gen lines in the ionizing X-ray spectrum. However,
considering the large number of free parameters, we
are loath to adjust the model parameters to try to
"fit" the observed speetrum.
In summary, time-dependent photoionization by
the EUV and X-ray radiation from 1E 0102-7219 can
qualitatively explain its UV and optical line emission,
but the density and ionization structures are complex
and prevent a unique model from being specified.
Many model parameters are poorly constrained,
including the time dependence and shape of the ioniz-
ing spectrum. Moreover, the models presented here
are not self-consistent in that the volumes and densi-
ties of the optically emitting gas imply optical depths
of order unity in the EUV, but absorption of the ioniz-
ing radiation has been ignored.
It is possible that these shortcomings reflect a
more fundamental limitation of the model assump-
tions. We have assumed throughout that the electron
velocity distribution is Maxwellian and that the energy
deposited by photoionization heats the electrons
directly. In fact, the _ 500 eV electrons produced by
the Auger process may excite or ionize other ions
before they slow down enough to share their energy
with other electrons. Many of the excitations would
produce photons that could ionize lower ionization
stages.
Our inability to define a unique model for either
of these objects at present may also be related to the
complexity of the objects themselves. It may be that
shocks and photoionization are needed to explain the
emission from the SMC remnant. Spatial variations in
composition or physical conditions such as are seen in
Cas A (Ref. 4) would tend to be averaged together in
our IUE large aperture spectra, which incorporate a
fair fraction of the emission from each object; this
would make a unique interpretation very difficult.
Also, differences in aperture positions and sizes
between UV and optical spectra ean cause confusion if
spatial variation- are present. Higher spatial resolu-
tion observation- and combined UV/optieal coverage
uith the same aperture sizes will be strengths of the
Faint Object Spectrograph on the Hubble Space Tele-
scope, and we plan to continue these studies with that
instrument when it, becomes available.
It is a pleasure to thank the dedicated staff of the
IUE Observatory on both sides of the Atlantic for
their part in helping procure the ultraviolet data.
This project has been supported by the following
grants: NAG 5-701 and NAG 5-988 to The Johns
IIopkins University, and NAG 5-87 to the Smithsonian
Astrophysical Observatory.
4. References
1. Blair W P & Panagia N 1987, in Exploring the
Unwerse w, tb the IUE SateIhte, Y Kondo (Ed),
Dordrecht, D. Reidel Publ. Co., 549.
2. Blair W P et al 1984, .4p J. 279,708.
3. Blair W P et al 1988, Ap J. Submitted.
4. Chevalier R A & Kirshner R P 1978, Ap J. 219,
931.
5. Cox D P Sz Raymond .J C 1985, Ap J. 298, 651.
6. Dopita M Aet al 1984, Ap J. 282, 142.
7. Dopita M A & Tuohy I R 1984, Ap J. 282, 135.
8. Hamilton A J S et al 1983, Ap J Suppl. 51, 115.
9. Hughes J P 1987, Ap J. 314, 103.
10. Hughes J P & Helfand D J 1985, Ap J, 291, 544.
11. Itoh H 1981, PubA S g. 33, 1.
12. Itoh tt 1986, Pub A S J. 38, 717.
13. Kirshner R P & Chevalier R A 1977, Ap J. 218,
142.
14. Lasker B 1980, Ap J. 237,765.
15. Raymond J C & Smith B W 1977, Ap J Suppl.
35, 419.
16. Seward F D & Mitchell M 1981, Ap J. 243, 736.
17. Tuohy I R & Dopita M A 1983, Ap J Letters.
268, L11.
18. Winkler P F & Kirshner R P 198.5, Ap J. 299,
981.


IUE observations of oxygen-rich supernova remnants

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