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The ISO-SWS spectra of two bright reflection nebulae, NGC 7023 and NGC 2023, are presented. We discuss the emission of molecular hydrogen from these photodissociated interfaces. Details of the aromatic infrared band profiles as well as the continuum emission are also analysed. The ISO-LWS spectrum of NGC 7023 is also presented, at two positions in the nebula. The dust temperature at the brightest far-infrared position of NGC 7023 is estimated to be 45 K.
Key words: ISO; reflection nebula; dust emission bands; molecular hydrogen
emission.
ISO is providing us with high-resolution spectra of the
Aromatic Infrared Bands between 3 and 13
µm (hereafter AIBs) in
a wide variety of galactic environments. At the spectral resolution of
ISOCAM-CVF
(
= 40), the AIBs have been shown to be
similar in interstellar regions with effective radiation fields
ranging from 1 to 104 times the
interstellar radiation field (Boulanger et al. 1998 and references therein).
Using SWS data at higher spectral resolution, we show here that there are
differences between the AIB spectra of two reflection nebulae, NGC 2023
and NGC 7023. The physical conditions of the gas, associated with
the AIBs, are discussed through the analysis of the molecular hydrogen
emission.
= 1000 (speed 4).
A restricted part of this spectrum has been observed at higher spectral
resolution
(Moutou et al. 1999).
The SWS spectrum of NGC 2023 was observed at a
spectral resolution of
= 500 (AOT01, speed 3)
in 1998.
Data reduction has been done within the SWS-Interactive Analysis environment.
Our observations have higher spectral resolution than previously
published
mid-IR spectra of NGC 7023 and NGC 2023
from the Kuiper Airborne Obsevatory (Sellgren et al. 1985)
and of NGC 7023 with ISOPHOT-S and ISOCAM
(Laureijs et al. 1996; Cesarsky et al. 1996).
NGC 7023 and NGC 2023 are two bright reflection nebulae irradiated by hot stars. The SWS aperture for NGC 7023 was centered 27'' W 34'' N (position 1) of HD 200775 (Teff=17,000 K). For NGC 2023, the SWS aperture was 60'' S of HD 37903 (Teff=22,000 K).
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Figure 1 shows the mid-IR spectra of the two nebulae, with an offset added to NGC 7023 for clarity (see caption of Fig.1). The relative contribution of continuous emission and AIBs to the total emitted energy is comparable in both objects. More than 60% of the 3 - 20µm energy is emitted in the AIBs. Reflection nebulae have a high feature-to-continuum ratio: this makes the study of faint details in the AIB profiles much easier than in more strongly irradiated sources where the AIBs are drowned by a strong mid-IR continuum (e.g., planetary nebula, H II region interfaces).
by a single rotational
temperature of 411 K if one adopts an ortho-to-para ratio (hereafter Rop)
of 1. These results,
although derived from observations with a much larger beamsize, are consistent
with the findings of
Lemaire et al. (1996). Indeed, the warm gas emission we detect is probably
dominated by the
filaments shown in the Lemaire et al. map.
For NGC 2023, using the S(1) at 17.03µm through to the S(3) at
9.66µm lines, we find
a rotational temperature of 333 K also with Rop=1.
While determining the rotational temperature, we have left aside higher levels
(Tu > 5000 K) because their populations are strongly affected by the UV
pumping (and subsequent radiative
decay) of the H2 molecule. Densities of about 105 cm-3 have been
inferred for both nebulae
(Lemaire et al. 1996; Field et al. 1998; Martini et al. 1997). At these high
densities, the low rotational
transitions (from upper levels with Tu < 5000 K) we are discussing here are
collisionally thermalized and
the rotational
temperature is equal to the gas temperature, Tgas.
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Our low ortho-to-para ratio confirms earlier work (Chrysostomou et al. 1993
and references therein) on photodissociated interfaces. The H2 molecule forms
on the surface of dust
grains and is expected to be rejected in the gas with a high vibration-rotation
energy content and
a Rop value close to 3.
After formation, Rop can be changed by gas-phase spin exchange reactions of
H2 with atomic hydrogen
and protons for Tgas>300 K or by H2-grain collisions for Tgas<300
K.
A value of Rop=1
corresponds to an equilibrium temperature
Teq 
80 K
(Burton et al. 1992). The fact that the dust temperature
( 40 K, see §3) and the gas temperature (300 - 400 K) are both very
different from Teq points at the importance of out-of-equilibrium effects;
this point is
also highlighted by the high Rop values (between 2 and 3) predicted by
recent stationary
models (Draine & Bertoldi 1996).
We note that H2 rotational populations in equilibrium at 40 K and 300 - 400 K
correspond to
Rop=0.15 and 3 respectively.
Two scenarios have been proposed by Chrysostomou et al. to explain the observed low Rop values. First, if the newly formed molecular hydrogen resides long enough on the surface of the dust grain, Rop will be between 3 and the equilibrium value of 0.15 set by the grain temperature Tdust. To get Rop=1 , the residence time of H2 on the dust grain should be approximately half the time required for the H2 rotational populations to reach equilibrium at Tdust after formation (see Fig.9 in Chrysostomou et al.).
Alternatively, in cold gas, Rop is fixed at a low value by gas-grain interactions (0.15 for Tdust=40 K). As the photodissociation front propagates into the molecular cloud at velocities of the order of 1 km/s, cold gas is advected through the hot (Tgas of a few 100 K) interface. In the hot gas, spin exchange reactions can drive the low Rop to the observed value.
The residence time of H2 on the grain goes as exp (450K/Tdust) (Tielens & Allamandola 1987) while the rate for spin exchange reactions goes as exp(-3200K/Tgas). From photodissociation region models, variations of Tgas are expected to be much larger than those of Tdust: Rop should thus vary much more rapidly in the second scenario (cold gas advection) than in the former (modified H2 formation). High spatial resolution observations yielding Rop profiles across photodissociated interfaces should help discriminate the 2 scenarios.
We compare here qualitatively the AIB profiles of both nebulae. More quantitative results will be presented in a forthcoming paper (Sellgren et al. 1999). We remark that the AIB spectrum of NGC 2023 multiplied by 2.2 superimposes nicely to that of NGC 7023. As the AIB flux is proportional to the radiation field intensity (Sellgren et al. 1985), this suggests that the radiation field in NGC 7023 is twice as strong as that of NGC 2023 (at the positions given in §1). The two AIB spectra look very similar (width, position of the AIBs) as expected in view of the similar physical conditions (density, radiation field) derived for NGC 7023 (Lemaire et al. 1996) and NGC 2023 (Steinman-Cameron et al. 1997; Field et al. 1998). There are, however, significant spectral differences in the AIB profiles which we detail below. These differences reflect changes in the physico-chemical state of the AIB carriers.
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Figure 3a shows that the 3.3 and the 3.4µm AIBs have identical profiles in both nebulae. The 3.4/3.3µm band ratio remains thus the same while the radiation field intensity is multiplied by a factor 2.
Figure 3b shows that the 6.2 µm AIB is asymmetrical and vary slightly between the two nebulae. Both profiles show a pronounced wing towards long wavelengths, which is interpreted as the consequence of anharmonic couplings during the cooling of the molecule (Barker et al. 1987; Joblin et al. 1995). In NGC 2023 the local continuum is more important, with respect to the 6.2µm band, and has a steeper rise than in NGC 7023. This suggests that the underlying continuum has a different origin from the AIBs.
Figure 3c demonstrates that the
7 - 9µm range
is the most complex. The main AIBs are at 7.6, 7.8, and 8.6µm. The
intensity of the
7.6µm-component is stronger in NGC 7023 than in NGC 2023. Furthermore, the
blue shoulder at 7.45µm
observed in NGC 7023 (see also Moutou et al. 1998)
is not seen in NGC 2023.
Roelfsema et al. and Verstraete et al. (1996) have shown that the profile
of the ``7.7''µm
AIB and the distribution of energy between the 7.6, 7.8
and 8.6µm
AIBs varies with the radiation field.
In the PAH model, the ``7.7''µm
AIB falls in the
spectral range where the effects of ionisation are the most dramatic (Pauzat et
al. 1997;
Langhoff 1996). Such a prominent 7.6µm
feature as well as the 7.45µm blue
wing are unusual and have only been seen towards compact H II regions
(Roelfsema et al. 1996)
and towards the post-AGB star HR 4049 (Molster et al. 1996). These latter
authors attributed the
7.45 and 7.6µm
new features to small ionized PAHs. Ionized PAHs have a
strong 7.7/11.3µm
band ratio (Langhoff 1996). Since the AIB spectrum in NGC 2023 is scalable to
that of NGC 7023 (see Fig.3), the 7.7/11.3µm
band ratio is the same for both
nebula implying that the fraction of ionized PAHs is the same in both cases.
>From the absence of the 7.45µm-band
and the weaker 7.6µm feature in
NGC 2023, we conclude
that the carriers of these bands are produced by other processes than
photoionization (photochemical
evolution, fragmentation...).
Figure 3d shows that the
11.3µm AIB has a
profile similar to the 6.2µm band. A red asymmetry
is observed, which again suggests strong anharmonic effects. We note that the
red wing of the
11.3µm
band is more pronounced in NGC 2023.
The 11.3µm
band also shows some weak sub-features
within its profile. In particular, the 11.0µm
band (previously
detected by Witteborn et al. 1989 and Roche et al. 1991) is clearly seen in both
nebulae.
The 11.0µm
band also appears in many objects with a similar ratio to the
11.3µm band
(Molster et al. 1996; Roelfsema et al., Verstraete et al. 1996) while the
7.7/11.3µm band ratio presents large variations
(compare for instance the AIB spectrum of the M17-SW interface and that of NGC
7023).
This again rules out photoionization as the cause of the
11.0µm-band in
moderatly excited sources.
Finally, we show the complete LWS spectrum (AOT01 observing mode) of NGC 7023, at two nebular positions labeled 1 and 2. Unfortunately, we did not obtain any LWS spectrum of NGC 2023.
The LWS position 1 in NGC 7023 is the same as for SWS (see §1) and is close to the far-infrared peak of Whitcomb et al. (1981). Position 2 is located 100''N of the star. The LWS beam is 80'' in diameter, so the fields slightly overlap. Position 1 has been observed in the guaranteed time program of J.P. Baluteau and Position 2 was part of our open-time program on reflection nebulae. Both spectra are shown in Figure 4. The data reduction was done with the LWS-Interactive Analysis and ISAP softwares.
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The LWS continuum emission is due to big dust grains in thermal equilibrium with
the
radiation field. We estimate the dust temperature by fitting a modified
blackbody emission
curve to the spectra with a dust emissivity law proportional to the frequency
.
The fits are shown on Figure 4.
For Position 1, we also used the SWS spectrum of NGC 7023 (not shown here) to
further
constrain the fit. The effective temperature of dust grains at Position 1 is
Tdust = 45 ± 2K, while
at Position 2, the temperature drops to Tdust
= 30 ± 2K.
These temperatures are compatible with the temperature map obtained by
Whitcomb et al. (1981). Poorer fits to the LWS spectra were obtained with a dust
emissivity
proportional to
2
(in this latter case, the above temperatures would drop
by
approximately 6 K). For a dust emissivity ~
, the radiation field goes
as T5
and so the change in Tdust implies that the radiation field is stronger at
Position
1 by a factor of 8 than at Position 2.
Atomic emission lines accounting for the cooling of the gas are visible on both spectra, namely CII (158µm), OI (63µm) and NI (145µm). They will be discussed elsewhere.
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upper limits. The
rotational temperatures
have been estimated with Rop=1.