The results of a systematic investigation of polycyclic aromatic hydrocarbon (PAH) emission in H II regions, planetary nebulae (PN), and proto-planetary nebulae (PNN), are reported. Data is obtained from the low resolution spectra (LRS) of IRAS. The results show that: PAHs are formed in carbon rich objects; and PAH emission is ubiquitous in general interstellar medium and requires the presence of ultraviolet photons, in planetary and proto-planetary nebulae, PAH emission is seen only where an ionizing flux is present and in carbon rich objects

Cox, P.

Demuizon, M.

Lequeux, J.

English

NASA Technical Reports Server

N91-1492 ,
A SURVEY FOR "PAH" EMISSION IN H II REGIONS, PLANETARY AND PROTO-
PLANETARY NEBULAE
M. de Muizon,* P. Cox,** and J. Lequeux***
*Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden NL and
Observatoire de Paris
**MPI fur Radioastronomie, Auf dem Hdgel 69, D 5300 Bonn, FRG
***Radioastronomie, Ecole Normale Sup_rieure, 75231 Paris CEDEX
05 F and Observatoire de Paris
SUMMARY
We report on preliminary results of a systematic investiga-
tion of PAH emission in H II regions, planetary nebulae (PN) and proto-
planetary nebulae (PPN). This emission occurs in a vast majority of
H II regions showing the ubiquity of the carriers. With the remarkable
exception of NGC 6302, PAH emission is only seen in carbon-rich PNs
and PPNs and is quite common in this case. This shows that PAHs are
formed at least in part in carbon-rich evolved objects.
INTRODUCTION
We are engaged in a systematic search for infrared emission
features in the IRAS/LRS (Low Resolution Spectra) data base. This data
base contains a total of about 170 000 individual spectra of various
qualities concerning about 50 000 IRAS sources. These spectra cover
the spectral ranges 7.8 to 13.5Mm and 10.5 to 22.5_m. A selection has
been published in the LRS Atlas_IRAS Science Team, _986) but many gooa
quality spectra are still in the data base. We have systematically
examined the individual spectra of known H II regions, planetary nebu-
lae (PN) and proto-planetary nebulae (PPN) as well as all spectra with
emission lines or with a flat or rising continuum in the LRS Atlas.
The resulting selection constitutes a new data base which, although
being neither complete nor homogeneous, contains a very large amount of
information. An extensive literature search has added to this base a
number of objects with published emission lines in the near IR (up to
about 30 _m)
In the course of this work we experienced a number of diffi-
culties with the Low Resolution Spectra:
I) Many individual spectra are affected by spurious "spikes" which so-
metimes superimpose over a real emission line. This problem is discus-
sed by de Muizon et al. (1988) and necessitates examination of indivi-
dual spectra. A revised version of the LRS Atlas where correction has
Ilia| _ 9NTENTIO_AI,.LY O{JLNS
95
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MRYELENGTH (all)
la) BD+30°3639 PN
101550. -284q31. 5 OOl I AVE F5FEB88
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51
_ m ¸ t ¸ --
_b) HR 4049 PPN
140505. -6)05628. 5 015 I RVE HI INOCRT
.'m • i i i i i . "
Ic) G 312.112 H II
155643. -523618. 5 103 I AVE HIICRTI
oo _mm m'.m _.m i?m.m l_a,_ m'r.u _k.m _ ue _.ma _.u
ld) G 329. 353 H II
Figure I" IRAS Low-Resolution
Spectra of representative objects
been made for spikes is in prepa-
ration.
2) The automatic classification
g_ven in the LRS Atlas can be mis-
leading: a confusion is possible
for example between silicate abs-
orption centered at 9.7mp and PAH
emission at 7.7, 8.6 (generally
unresolved) and 11.3pm.
3) The LRS instrument being a
slitless prism spectrograph, wave-
length resolution varies with wa-
velength and with the angular ex-
tent of the source and the wave-
length scale may be somewhat dls-
placed depending on the brightness
distribution over the source.
RESULTS
IR emission bands form a
spectrum of features at 3.3 + 3.4,
6.2, 7.7, 8.6 and 11.3_m which
appear together in a variety of
objects (see e.g. Cohen et al.
1986 and references herein). These
features are generally attributed
to a mixture of Polycyclic Aroma-
tic Hydrocarbons (PAH): L6ger and
Puget (1984). Usually the observa-
tions with a single instrument do
not cover the whole spectrum and
one must combine observations made
in various ways, resulting in a
variable signal-to-noise ratio for
a single source. The LRS include
the 7.7pm feature (on the edge of
the spectra), the 8.6_m one (of-
ten unresolved from the previous
one) and the 11.3_m band. Exam-
ples are shown on Fig. I for one
PN (BD+30°3639, Fig. la), one PPN
(HR 4049, Fig.lb, see Waelkens et
al. 1987) and two H II regions:
G312.112 (Fig. Ic), not in the LRS
Atlas, is a typical spectrum with
PAH emission while G329.353 (Fig.
Id) shows strong silicate absor-
ption and apparently no PAH emis-
sion. The interpretation of this
sort of objects at the shortest
96
wavelengths may be ambiguousbut there is no feature at 11.3_m. Either
there are no PAHsor their emission is very absorbed. There are only 2
certain H II regions without PAHemission and without silicate absorp-
tion. Both are of high excitation and deserve further attention.
We have added to our sample objects with PAH emission detected
by other observers (the list of references is too lengthy to be given
here). It should be remarked that higher-resolution spectra such as
those of Roche ad Aitken (1986 and references herein) are more sensiti-
ve than LRS, but much less objects have been observed.
Provisional results are given in the following Table I. Infor-
mation on C/O ratios and exciting stars of PNs is mainly from Zuckerman
and Aller (1986).
TABLE 1
STATISTICS FOR H II REGIONS
Certain H II regions with certain PAH emission .................. 83
with possible PAH emission ................. 31
without PAH but strong silicate absorption. 14
without PAH, without silicate absorption... 2
Total...
Objects of unknown nature (many being probably H II regions)
with certain PAH emission .................. 34
with possible PAH emission ................. 13
without PAH but strong silicate absorption. 15
without PAH, without silicate absorption... 3
Total...
Note: strong silicate absorption may mimic or hide PAHs on LRS
STATISTICS FOR PNs AND PPNs
Carbon-rich, or ionized by a WC star
with certain PAH emission .................. 15
with possible PAH emission ................. 5
C/0>I, no detected PAHs (NGC 6543 and 6884) 2
Total... _-2
Oxygen-rich certain PAH emission (NGC 6302) ............ I
possible PAH emission ....................... 3
Total... 4
Notes to Table 1
CRL 618 = AFGL 618, in a transition stage from PPN to PN, is
a carbon-rich object (from chemistry) showing rather surprisingly sili-
cate emission. We class it as an object with possible PAH emission as
a marginal emission at 3.3 and 6.21_m can be seen on Fig. I of Russell
et al. (1978). The good LRS shows do PAH feature. Radio continuum and
recombination-line emission have been detected and there are fast in-
creases in the continuum flux (Martin-Pntado et al. 1988). It may be
97
that PAHs, if really absent, have not had time to form or to be excited
by UV photons. Further observations of this fascinating object are
clearly needed.
NGC 6302 is a type-I PN which has been consistently classed
as carbon-rich (C/0>I): see Zuckerman and Aller (1986). However as
discussed by Roche and Aitken (1986) the value of C/O may be suspect
in view of the very high range of ionizations which goes from Ne II to
Ne VI. On the other hand it shows OH maser emission (Payne et al. 1988)
which tends to confirm a low C/O ratio. This object clearly deserves
further studies.
The 3 oxygen-rich objects with possible PAH emission are NGC
2440, 3242 and Hb 12. The only evidence for PAHs is the 3.3Mm emission
reported by Martin (1987). There is no visible accompanying 3.4Mm
emission and we suspect that one may be dealing with Pf6 rather than
PAH emission.
CONCLUSIONS
From Table I and the previous discussion we can reach conclu-
sions which although not really new (see e.g. Barlow, 1983, Cohen et
al. 1986, Roche and Aitken, 1986, Zuckerman and Aller, 1986) are based
upon a substantially increased data base, especially for the 7.7_m
feature which cannot be observed from the ground.
I) "PAH" emission is ubiquitous. It obviously requires the
presence of UV photons and is seen near essentially all H II regions
(presumably at the interface with neutral gas). This confirms the
ubiquity of PAHs in the general interstellar medium.
2) In PNs and PPNs, "PAH" emission is seen only where an
ionizing flux is present (for PPNs) and in carbon-rich objects, with
the remarkable possible exception of NGC 6302. Most objects with C/0>I
show PAH emission: there are only 2 clear exceptions. Clearly carbon-
rich evolved stars are sites for the formation of PAHs.
3) We confirm the existence of important variations in the
7.7/11.3_m band intensity ratio first discussed by Cohen et al. (1986).
REFERENCES
Barlow, M.J.:1983, IAU Symposium 103 "Planetary Nebulae", ed. D. Flower
Reidel, Dordrecht, p. 105
Cohen, M., Allamandola, L., Tielens, A.G.G.M., Bregman, J., Simpson, J.
P., Wittebord, F.C., Wooden, D., Rank, D.:1986, Astrophys. J.
302, 737
IRAS Science Team:t986, Astron. Astrophys. Suppl. 65, 607
L_ger, A., Puget, J.L.:1984, Astron. Astrophys. 137-[ L5
Martin, W.:1987, Astron. Astrophys. 182, 290
Martin-Pintado, J., Bujarrabal, V., _hiller, R., Gomez-Gonzalez, J.,
Pianesas, P.:1988, Astron. Astrophys. 197, L15
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de Muizon, M., Cox, P., Lequeux, J.'1988, Astron. Astrophys. in press
Payne, H.E., Phillips, J.A., Terzian, Y.'1988, Astrophys. J. 326, 368
Roche, P.F., Aitken, D.K.'1986, Month. Not. Roy. Astron. Soc._1, 63
Russell, R.W., Soifer, B.T., Willner, S.P "1978, Astrophys. J._O, 568
Waelkens, C., Lamers, H., Waters, R.'1987, The Messenger (ESO) 4T, 29
Zuckerman, B., Aller, LoH.'1986, Astrophys. J. 301, 772
99



A survey for PAH emission in H II regions, planetary and proto-planetary nebulae

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