The impact of knowledge of H II regions, planetary nebulae and supernova remnants due to International Ultraviolet Explorer is examined. The more relevant aspects related to the physical conditions of gaseous nebulae are reviewed. The following properties of gaseous nebulae are discussed: (1) density and temperature distribution; (2) ionization structure; (3) chemical composition; (4) internal dust; and (5) shock velocity for supernova remnants. The CNO abundances of planetary nebulae are compared with stellar evolution models

Peimbert, M.

English

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NEW INSIGHTS INTO THE PHYSICAL STATE OF GASEOUS NEBULAE
Manuel Peimbert
Instituto de Astronom_a
Universidad Nacional Aut6noma de M_xico
ABSTRACT
The impact of our knowledge of H II regions, planetary nebulae and super-
nova remnants due to IUE is briefly examined. Some of the more relevant as-
pects related to the physical conditions of gaseous nebulae are reviewed. The
analysis of IUE data is under process and already significant results have
been obtained on the following properties of gaseous nebulae: a) density and
temperature distribution, b) ionization structure, c) chemical composition, d)
internal dust, and e) shock velocity for supernova remnants. The CNO abundan-
ces of planetary nebulae are compared with stellar evolution models.
INTRODUCTION
To understand the nature of gaseous nebulae it is necessary to study
their physical properties; the most important are: density and temperature
distributions, ionization structure, chemical composition, dust presence and
mass motions. From a profound knowledge of gaseous nebulae, it is possible to
derive accurate chemical compositions to study such important problems as stel-
lar and galactic chemical evolution. The three types of gaseous nebulae that
will be considered in this review are: H II regions, planetary nebulae (PN)
and supernova remnants (SNR).
There are at least four areas in which ultraviolet observations arepara-
mount for our understanding of gaseous nebulae: a) the determination of the
ionization structure from the detection of ions without strong optical emis-
sion lines such as CII, CIII, CIV, NIII, NIV, NV, OIV, OV, SiII, SiIII, SiIV
and MgII, b) the determination of the density structure from the ratio of
emission lines such as 1907/1909 of CIII and 2422/2425 of NeIV, c) the de-
termination of the temperature structure by comparing the line intensity of
the same ion originating from three different energy levels like _1663, 4363
and 5007 of OIII which for an electron density Ne<10 _ cm-3 permits us to de-
termine the electron temperature, Te, and the mean square temperature variation
over the observed volume, t2, d) the presence of internal dust from the op-
tical thickness of resonance lines like 1548 and 1551 of CIV and from the
gaseous abundances of certain elements that might be partially locked up in
grains like Mg, Si and C.
557
https://ntrs.nasa.gov/search.jsp?R=19810017417 2020-03-21T12:07:01+00:00Z
Excellent review papers on the possibilities for UV research of gaseous
nebulae and on PN results derived from IUE data are presented elsewhere (ref.
1,2).
H II REGIONS
There have been three determinations of abundances of the Orion nebula
based on IUE data (ref. 3,4,5). The three groups find a C/O ratio similar to
the solar one (see table I) implying that the amount of carbon locked up in
grains inside the Orion nebula is not substantial.
Perinotto and Patriarchi (ref. 3) obtain log Mg+/H + _ -6.7 for the Orion
nebula; to estimate Mg/H they make use of the value Mg+/Mg= 0.15 derived from
a model of IC 418 by Harrington et al. (ref. 8) and conclude that their
observed upper limit corresponds to a Mg/H value an order of magnitude smaller
than the solar value implying that most of the Mg is locked up in grains. The
Mg ionization correction factor should be verified due to its very large value
and to the higher ionization degree of the Orion nebula relative to that of
IC 418; furthermore from the models by K_ppen (ref. 9) for objects like IC 418
and the Orion nebula with+33000_ T_40000 °K and l< (R_/R®)(Ne_/100)< l0 it
is obtained that 0.008QMg /Mg _0.04. The possibility of resonance line scat-
tering reducing the Mg 2800 line intensity should also be considered.
By comparing the C++ 1907+1909 intensity with that of C+ 4267 it is found
that t2= 0.016 for the Orion nebula (ref. 4).
Rosa (ref. 10) has observed two extragalactic H II regions, NGC 604 and
NGC 5471, which show a continuum energy distribution with stellar absorption
and emission lines. In addition NGC 5471 shows strong C++ 1907+1909 emission
of nebular origin.
From the work of Bohlin et al. (ref. 5), Perinotto and Patriarchi (ref.ll)
and others, it is found that most of the continuum observed in galactic H II
regions is due to dust-scattered light from the ionizing 0 and B stars. From
these data the dust-scattering efficiency and albedo as a function of wave-
length have been estimated. The results depend on the model adopted for the
spatial distribution of the dust and stars, therefore more observations and
better models are needed to decide among several possibilities.
The relatively low temperatures and high dust content of H II regions with
solar abundances makes the detection of UV emission lines very difficult and
consequently the determination of their chemical abundances. Alternatively,
extragalactic metal-poor H II regions have higher electron temperatures which
produce emission lines relatively brighter than the stellar dust-scattered
continua. Metal-poor H II regions that can be observed without their ionizing
stars, like those in the Magellanic Clouds, are particularly good for the
study of emission lines.
558
PLANETARY NEBULAE
The determination of the electron density, Ne, from UV lines is particu-
larly important for the study of the regions of high degree of ionization.
From the ratio 1907 [CIII] to 1909 CIIIS and the computations by Nussbaumer
and Schild (ref. 12) the Ne values of several PN have been obtained (ref. 13,
14). From the KNe IV] 2422/2425 ratio the Ne value has been obtained for
NGC 7662 (ref. 15).
There is no good method to determine directly from visual data Te for the
region where helium is twice ionized. This can be done by combining IUE data
with visual observations or by using only IUE data. From the _elVS 1602/
(2422+2425) and 2422/2425 ratios it is possible to obtain Ne and Te. The
_elV] 1602/(2422+2425) ratio has already been measured in NGC 3918 (ref. 16).
By comparing _4267 of CII and _1907+1909 of CIII it has been found that t2=
0.02 for IC 418 (ref. 4); while from _1663,4363 and 5007 of 0111 it has been
found that t2= 0.00 for the regions where the O111 lines originate.
The most important result derived so far on PN from IUE data is a set of
CN0 abundances that are presented in table II. The 0/H ratios are very similar
to those derived from visual data. The UV carbon abundances are mainly based
on C++ and C+ ionic concentrations and ionization structure models; for CIV
1548, 1551 the dust-free models predict higher intensities than observed
(ref. 2, 16), the difference is most likely due to the presence of internal
dust that absorbs the resonance line radiation. The UV carbon determinations
are of higher accuracy than those based on the _4267 CII lines (ref. 18) due
to the larger number of observed ions and to the faintness of the _4267 lines
which typically have been overestimated by factors ranging from 1.5 to 2.5
(ref. 4). The UV nitrogen abundances in Table II are based on N++, N+++ and
N++++ ionic concentrations while the visual determinations are based only on
N+, which for many PN is only a trace ion; therefore the UV results are more
reliable. It can be seen from Table II that the difference between the iUE
and visual N/O determinations increases with degree of ionization which
implies that the visual method breaks down by underestimating the N/O ratio
in objects of high ionization degree.
The C abundances in Table II include only the gaseous component in the
PN shell, and since part of the C might be locked up in grains inside the neb-
ula, correspond to lower limits to the total values. For NGC 7027 it has been
estimated that the amount of carbon locked up in grains is similar to that
listed in Table II (ref. 24). In Table II the abundances of the sun and the
Orion nebula are presented for comparison. It is clear from this table that,
even without considering the amount of C in the form of dust, C and N have
been substantially enriched during the evolution of their central stars.
Renzini and Voli (ref. 25), based on the work by Iben's group, have computed
the evolution of the surface abundances of He, C, N and 0 in intermediate mass
stars from the main sequence phase up to the ejection of the PN. In figures 1
and 2 we compare the models with the observations of Table II. For C/O the
models agree reasonably well with the observations. Alternatively for N/O not
only PN of Type I (ref. 26) with high He/H values present large N/O values but
also other PN of Type II like NGC 7027, NGC 6886 and IC 2448, which originated
559
from starswlth M/M@_ 2.4 (ref. 27), present N/O values larger than predicted,
possibly indicating that the stellar evolution models should include turbulent
diffusion and meridional circulation (ref. 25).
The intensity ratio of the NV, CIV and Mgll resonance doublets (kk1239/
1243, 1548/1551, 2796/2803) is expected to be equal to 2, the ratio of the
respective collision strengths in dust-free nebulae. In all the reported cases
the ratios are smaller than 2 (ref. 13,14) indicating dust presence. Moreovelc
the ionization structure models without dust predict a higher intensity than
observed for the ClV doublet (see above) possibly indicating that the doublet
has been attenuated by internal dust. Pequignot and Stasinska (ref. 28) have
found that the observed Mgll doublet in NGC 7027 is a factor of i0 smaller than
the value predicted by their dust-free models, they have estimated that a
fraction of this difference is due to internal dust attenuation and that the
rest is due to a substantial amount of Mg embedded in dust grains. From the
Mg doublet ratio and a model containing dust it will be possible to quantita"
tlvely evaluate these two effects.
SUPERNOVA REMNANTS
Most of the SNR lines in the UV originate in regions where 4Xl0_<Te <
4x10s °K this temperature range corresponds to the gap left by the visual and
X-ray observations. The UV is particularly well suited to derive accurate
shock velocities, vs, for cases where Vs _ i00 km s-I. From the value of Vs,
the emission line intensities and models available, it is possible to deter-
minethe chemical composition of the SNR.
From IUE observations of the Cygnus Loop vs values in the 100-130 km s-I
range have been obtained (ref. 29,30,31). By combining these observations with
shock models and neglecting radiative transfer models due to the presence of
dust it is found that k_1548+1551 CIV, 1907+1909 _II_ + CIII] and 2326 CII]
are too weak by factors of i0, 2.5 and 1.5 (ref. 29,30) and by i0, 1.6 and 1.4
(ref. 31) respectively; it has been suggested that these observations can be
explained by progressive destruction of graphite grains. Benvenuti et al.
(ref. 30) find that _1335 CII is too weak by about an order of magnitude while
Raymond et al. (ref. 31), from the 1335/2326 ratio, find that the optical depth
in the resonance lines due to dust reduces the _1335 flux by a factor of ten.
As mentioned before, resonance line scattering reduces the CIV line intensities
in PN and it is highly probable that in SNR a similar situation prevails which
would make the C underabundance in the Cygnus Loop marginal. Considering that
C is depleted by about an order of magnitude in the interstellar medium this
result would imply that substantial destruction of graphitegrains occurs in,
shocks with Vs_ 130 km s,I. Raymond et al.(ref. 31) present some evidence in
favor of a depletion of _3 in Si.
The Crab nebula is the only young SNR observable with the IUE; this SBrR
has not been significantly contaminated by the interstellar medium, therefore
its chemical composition still corresponds to that of the SN. Davidson et al.
(ref. 32) from preliminary IUE results obtain for the Crab nebula that N(C)/
N(O)<3. By considering that in this object 0/0o_l/3 (ref. 33) the C/0 upper
560
limit implies that, contrary to expectations, at least this SN did not produce
much carbon.
REFERENCES
1. 0sterbrock, D.E.: Physical Characteristics of Ionized Gaseous Nebulae, in
Scientific Research with the Space Telescope, NASA CP-2111, 1979, Pp. 99-
121.
2. Nussbaumer, H.: Observations of Planetary Nebulae. 2nd European Meeting on
IUE, Tubingen, in press.
3. Perinotto, M.$ and Patriarchi, P.: The Abundance of C and Mg in the Orion
Nebula. Ap. J., Vol. 235, 1980, pp. L13-L16.
4. Torres-Peimbert, S._ Peimbert, M.; and Daltabuit, E.: IUE and Visual Ob-
servations of the Orion Nebula and IC 418: The C Abundance. Ap. J., _980,
in press.
5. Bohlin, R.C.; Hill, J.K._ Stecher, T.P.; and Witt, A.N.: Long-Slit Spectro-
scopy in the Rocket UV of the Orion Nebula. Ap. J., 1980, in press.
6. Lambert, D.L.: The Abundances of the Elements in the Solar Photosphere-VIII:
Revised Abundances of C,N and 0. M.N.R.A.S., Vol. 182, 1978, pp. 249-272.
7. Lambert, D.L._and Luck, R.E.: The Abundances of the Elements in the Solar
Photosphere-IX: Na to Ca. M.N.R.A.S., Vol. 183, 1978, pp. 79-100.
8. Harrington, J.P.; Lutz, J.H.; Seaton, M.J.; and Stickland, D.J.: UV spectra
of PN. I. The Abundance of C in IC 418. M.N.R.A.S., 1980, in press.
9. KSppen, J.: Photoionization Models for Gaseous Nebulae. III. Third Period
Elements. Astron. Astrophys. Suppl. Ser., Vol. 39, 1980, pp. 77-81.
10. Rosa, M.: IUE UV Spectra of Giant Extragalactic H II Regions. Astron.
Astrophys. Letters, 1980, in press.
ll. Perinotto, M._and Patriarchi, P.: IUE Observations of the Continuous Spec-
trum of the Orion Nebula. Ap. J., 1980, in press.
12. Nussbaumer, H.;and Schild, H.: CIII Observable with IUE. Astron. Astrophys.
Vol. 75, 1979, pp.L17-L19.
13. Flower, D.R.; Nussbaumer, H.; and Schild, H.: The EUV Spectra of Young PN.
Astrono Astrophys., Vol. 72, 1979, PP. L1-L3.
14. KSppen, J.;and Wehrse,R.: High Dispersion EUV Observations of PN. Astron.
Astrophys.,1980, in press.
561
15. Lutz, J.H.;and Seaton, M.J.: The NeIV 2D-aS Lines in the PN NGC 7662.
M.N.R.A.S., Vol. 187, 1979, PP. lp-7p.
16. Torres-Peimbert, S.$ Pefia,M.; and Daltabuit, E.: The High-Excitation PN
NGC 3918 and IC 2448, 1980, this Symposium.
17. Aller, L.H._and Keyes, C.D.: IUE Observations of Seven High-Excitation PN
Proc. Natl. Acad. Sci. USA, 1980, in press.
18. Torres-Peimbert, S.$and Peimbert, M.: Photoelectric Photometry and Physical
Conditions of PN. Rev. Mexicana Astron. Astrof., Vol. 2, 1977, pp. 181-
207.
19. Kaler, J.B.: The Enrichment of N and He in PN. Ap. J., Vol. 228, 1979, pp.
163-178.
20. Perinotto, M.; Panagia, N.; and Benvenuti, P.: Physical Conditions and
Abundances of CNO Elements in NGC 7027. Astron. Astrophys., 1980, in
press. °
21. Harrington, J.P._ Lutz, H.J._ and Seaton, M.J.: in The First Year of IUE,
ed. A.J. Willis, University College London, 1979, P. 199.
22. Flower, D.R.: UV Spectra of PN II. The Young PN IC 4997. M.N.R.A.S., 1980_
in preparation.
23. Peimbert, M.; an_ Torres-Peimbert, S.: Chemical Composition of the Orion
Nebula. M.N.R.A.S., Vol. 179, 1977, pp. 217-234.
24. Panagia, N.; Bussoletti, E.$ and Blanco, A.: in CNO Isotopes in Astrophy-
sics. ed. J. Audouze, D. Reidel Publ.,1977, p. 45.
25. Renzini, A._and Voli, M.: AdvancedEvolutionary Stages of Intermediate-Mass
Stars I. Evolution of Surface Compositions. Astron. Astrophys., 1980, in
press.
26. Peimbert, M.: in PN Observations and Theory , ed. Y. Terzlan, D. Reidel
Publ., 1978, pp. 215-224.
27. Peimbert, M.;and Serrano, A.: On the H and N Enrichment of the Interstellar
Medium by PN. Rev. Mexicana Astron. Astrof., 1980, in press.
28. P_quignot, D._nd Stasi_ska, G.: An Abundance Gradient for Gaseous Mg in the
PN NGC 7027. Astron. Astrophys., Vol. 81, 1980, pp. 121-127.
29. Benvenuti, P.; DIOdorico, S.; and Dopita, M.A.: UV Spectrum of SNR Reveals
C Depletion of the Interstellar Medium. Nature, Vol. 277, 1979, pp. 99-
102.
30. Benvenuti, P.$ Dopita, M._ and DV0dorico, S.: Far UV Spectrophotometry of
SNR Observations and Astrophysical Interpretation. Ap. J., 1980, in pre_s.
562 ,
31. Raymond, J.C.; Black, J.H._ Dupree, A.K.; Hartmann, L.; and Wolff, R.S.:
UV Observations of the Cygnus Loop. Ap. J., 1980, in press.
B2. Davidson, K._ Gull, T.; Stecher, T._ Maran, S._ Kafatos, M.; and Trimble,
V.: IUE Observations of the Crab Nebula. 1980, in preparation.
B3. Davidson, K.: Emission-Line Spectra of Condensations in the Crab Nebula.
Ap. J., Vol. 228, 1979, pp. 179-190.
563
TABLE I.- ORION NEBULA - CHEMICAL ABUNDANCES a
ObJect O/H C/H Mg+/H+ Mg/H t2 Ref.
(optical) (UV) (UV)
Orion 8.40 -<5.3 .... 0.00 3
" 8.52 8.35 ........ 0.00 4
" 8.52 8.48 ........ o.oo 5
Sun 8.92 8.76 .... 7.62 .... 6,7
a
Given in 12+log N(X)/N(H).
TABLE II.- PLANETARY NEBULAE - CHEMICAL ABUNDANCES
Object 0/Ha C/0b N/0b (C+N)/0b N/Ob He/Hc Ref.
(uv) (uv) (uv) (uv)(visual)(visual)
NGC 2392 8.74 -0.39 -0.33 -0.06 -0.29 0.091 17,18,19
NGC 2440 8.72 -0.20 +0.08 +0.26 +0.33 0.135 17,18,19
NGC 2867 8.65 +0.19 -0.55 +0.26 -0.52 O.112 17,19
NGC 3918 8.78 +0.20 -0.39 +0.30 -0.60 0.113 16,18
NGC 6302 8.79 +0.43 +0.25 +0.65 -0.15 0.191 17,19
NGC 6741 8.84 +0.26: -0.14: +0.41 -0.13 0.123 17,19
NGC 6886 8.72 +0.18: +0.08: +0.43 -0.53 0.102 17,19
NGC 7027 8.62 +0.49 -0.i0 +0.59 -0.37 0.113 20,18,19
NGC 7662 8.58 -0.07 -0.54 +0.06 -0.88 0.094 21,18
IC 418 8.60 +0.26 ................ 8
IC 418 8.70 +0.33 ............ 4
_c2448 8.46 +O.lO -o.27:+0.25 -i_8 o.111 16,18
IC 4997 8.04 -0.40 ................ 22
Me 2-1 8.86 +0.05 -0.74 +0.12 ........ 17
PNd 8.70 +0.14 -0.24 +0.30 -0.42 0.119
Orion 8.65 -0.08 .... -0.03 -0.97 0.i00 23
Sun 8.92 -0.25 -0.93 -0.17 -0.93 .... 6
a Given in 12+log N(O)/N(H). bGiven in log N(X)/N(Y). c Given in N(He)/N(H).
d
Average of this table without IC 4997 •
564
i i ! i
,_\5... X Inifiol Volues"4 • IUE obs.
,/ 20.6 Modelsa, 0• --- Modelsa, 2
"3 I\ ' O"0.3 \ /4\
I't_ "5 • ,..i. L , l
0.0 • _5 Me
'2 I
• \
-0.3 X L _6 MeI \_
\
* , I I I I ,I
O.lO 0.12 0.14 0.16 0.18
He/H
Figure i. The model values (Renzini and Voll, ref. 25) correspond to the sur-
face abundances at the time of the PN ejection. Along each sequence the
values of the initial stellar mass are reported. The parameter _ is the
ratio of the mixing length to the pressure scale height, and _= 0 simply
means that hot-bottom nuclear burning has been omitted.
1 I I 1 I I
I
0,3 i
I •
.L 3.5 M,
I
0.0 i
J
I
I•
I
I
_-0.3,_z • • fal 6_Mw 8 Me
--°_ • 2_e3 I 4 ,,,_1 _ 5 i ,,
-0.6 I't'• X Initial Volues
• IUE obs.
X --Models cz=O
- 0.9 -- Models n =2
* , I I , I
0.10 0.12 0.14 0.16
He/H
Figure 2. Same as Figure i.
565


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