Chemical model for NGC 7027
A. Ali1 , M.Y. Amin2, and M.S. El-Nawawy3
1,3
2
Department of Astronomy, Faculty of Science, Cairo University, Egypt.
Department of Astronomy, Faculty of Science, King Saud University, Saudia Arabia.
Abstract
We have studied the chemistry of the molecular envelope of the young carbon-rich
Planetary Nebulae NGC 7027. Steady state chemical model has been constructed for the nebulae
including the effect of the shock wave. The density and temperature of post shock region are 1x105
cm3, and 25 K respectively, these values are consistent with the values observed in the clumps of
neutral envelopes of most observed planetary nebulae. Our steady state results showed good
agreement between the calculated molecular abundances and the observations. Our model confirm
the importance of including the shock effect during the chemical studies of the early evolutionary
stage in planetary nebulae.
1.Introduction
The young carbon rich planetary nebula NGC 7027 represents the best source for the study
of photochemistry of planetary nebulae. It is one of few celestial objects that radiate through the
electromagnetic spectrum from radio to X-ray. Observations with the Advanced CCD Imaging
Spectrometer (ACIS) aboard the Chandra X-Ray Observatory show that the X-ray emission from
NGC 7027 is extended and is bipolar in morphology.
NGC 7027 is very bright and has rich optical spectrum originates from the ionized
nebula. The ionized nebula is surrounded by an expanding molecular envelope which was first
discovered by Mufson et al. (1975) through the detection of CO rotational lines in the millimeter
waveband. Extensive observations with ISO Showed that NGC 7027 is also very rich in far-infrared
emission lines. Liu et al. (1996), Cenicharo et al.(7991), Liu (1997), and Bachiller et al. (1997)
reported detections of many molecular species such as OH, CH, CH+, CN, HCN, HNC, and HC3N.
Bachiller et al. (7991) reported that the clumps which form the envelopes of the
evolved PNe maintain high densities (few x 105 cm-3) and low temperatures (25 K), from multiline observations of CN, CO, and HCO+. Their observations, of a sample of seven objects at
different stages of development, showed that there are clear trends in the chemical
evolution of the molecular envelopes. As a star evolves beyond the AGB, through the proto-PNe
and PNe phases, the abundances of SiO, SiC2 , CS, and HC3N decrease, such that they are not
detected in the PNe, while the abundances of CN, HCN, HNC, and HCO increase
dramatically.The clumping of the gas in the neutral envelopes of PNe is an essential aspect of both
physical and chemical evolution. It represents the environment needed for the survival of the
molecular gas, especially during the transition from AGB stage to early phase of PNe. This
transition stage is dominated by high temperature due to the effects of shocks.
Based on recent extensive observations by the Infrared Space Observatory (ISO),
Volk and Kwok (1997) presented a model of neutral envelope of NGC 1201 depending on ISO
results of CO, H2O lines as well as previous radio and infrared observations. Yan et al.(1999)
presented a thermal-chemical model of the neutral envelope of NGC 1201. Also very recently,
Hasegawa et al. (2000) have developed a spherically symmetric, steady-state chemical model for
NGC 7027.
Their predicted CN/HCN ratio is about 30 throughout the wind region
and 1-10 in the dense shell, comparable to the observed values in PNe.
The aim of the present paper is to investigate the chemistry of NGC 7027 taking into
consideration the effect of the shock during transition from AGB stage to the early stage of
planetary nebulae formation. In Section 2, we present the initial model parameters and the chemical
network. Section 3 shows the results including discussion of our model. Conclusions are given in
the final section.
2. The model
2.1 Initial parameters for the model
We have constructed a model for NGC 7027. The cosmic ray ionization rate is assumed
to be three order of magnitude higher than that of interstellar value (1.3x10-17). This enhancement
factor stimulates the large ionization rates that can be produced by X-rays (see, Ali, et al. 2001). We
assumed ultraviolet (UV) radiation to be 100 times the averaged UV interstellar field. The UV
radiation field incident on the inner surface of the molecular gas rises rapidly to very high value
(about 105) and then steadily decreases because of the expansion of the envelope and the decrease in
luminosity of the central star to about 10-100 (Bachiller et al.1997).
Table 1: Initial elemental abundances relative to total hydrogen
Elements
Abundance
Element
Abundance
H
0.1
N
0.47 x n(O)/n(H2)
H2
0.45
S
1.6x10-8
He
0.13
Si
1.5x10-9
C
1x10-4
Mg
1.5x10-9
-5
O
1x10
Na
1.5x10-8
2.2 Chemical network
The chemical scheme used in the present work is based on UMIST rate file, RATE99 (Le
Teuff et al. 2000). The chemical network in our model has 4046 chemical reactions for 392 species.
In this study we did not include the possible effects of gas-grain interaction. The species in the gas
phase are assumed to be in chemical equilibrium amongst themselves, prior to the arrival of the
shock. For chemical calculations in pre-shock region, we assumed the initial fractional abundances
given in table (1).
The steady-state results of pre-shock region are not used as initial abundances in the postshock region, because these values are greater than that observed after the shock. We stopped the
calculations at density equals 105 cm-3, and post shock temperature of 25 K, which are consistent
with that observed in the clumps of planetary nebulae.
The initial abundance of atomic hydrogen is assumed to be 10% of the total hydrogen
density. Table 1 gives the initial elemental abundances relative to total hydrogen nuclei, which are
fixed in this study except the nitrogen abundance. The nitrogen abundance is assumed to be nN/nO =
0.47 (Bachiller et al. 1997). The initial abundance of CO parent molecule is taken to be 5x10-5. The
nebula originated from carbon rich star, so we assumed that the carbon abundance is larger than
that of oxygen. In our model, we assumed the C/O ratio to be 2.5.
2.3 Shock model
2
We have used the one dimensional computer code described in Amin (2001). The
conservation of mass and density () between the pre-shock and post-shock are given by (Shull and
draine 1987):
 v   0 v0
(1)
 v 2  p   0 v0 2  p0
(2)
where p is the pressure and the subscript o refers to the values of pre-shock variables. The density
() as a function of time behind the shock front (Shull and draine 1987) is
n 2 Q(T )
d

dt A1  1  A3   2
(3)
where Q(T) is the net results of cooling and heating, (n) is the number density, A1 = 5/2 (P1+ ρ1v12),
and A3 = 4 ρ12 v12 .
In order to calculate the energy loss, the following processes of cooling and heating have
been considered (Amin et al. 1991 and Howe et al. 1992):
1- Rotational and vibrational cooling by H2, CH, CO, OH, and H2O.
2- Atomic cooling by O, C, and C+: The fine structure excitation of atoms or ions with
non-zero spin by spin change collision with atomic hydrogen, is followed by
subsequent radiative de-excitation.
3- Cooling by gas-grain inelastic collision
4- Cooling by ionic and electronic collisional excitation.
5- Chemical heating: occurs when an exothermic reaction converts some of its energy
into kinetic energy of the gas.
6- Cosmic ray heating.
In order to calculate the time-dependent chemistry behind the shock, the following steps are
carried out:
1- Using GEAR program we have followed the chemical evolution for non-dissociative
shock of speed 10 km/s, propagating into molecular gas with a density n=3.1x105
cm-3, The initial pre-shock temperature is assumed to be 25 K.
2- The chemical structure of the post-shock region is studied at time t = ti (an initial
value of time).
3- The output of step 2 is introduced to the subroutines of cooling , heating , and
dynamics in order to calculate a new step of shock dynamics.
4- A new step of chemistry is followed at time t = ti + Δt with new density and new
temperature. Steps 1-4 are repeated until a time at which the post-shock temperature
reaches to its pre-shock value.
3. Results and discussion
Figure 1. represents the relation between density and time in the post shock region. It is clear
that the density increases gradually with the time from 4x103 cm-3 to 2.0x105 cm-3 during average
time of 880 years.
Figure 1: The relation between the density and time in post shock
Figure 2. represents the relation between temperature and time in the post shock region. There is
gradual decline in temperature with increasing time in post shock region. The temperature decreases
from 3000 K to 25 K in average time of 880 years.
The calculated abundances of some selected molecules are plotted against time and the
results are shown in figure 3. This figure confirm the growth of CN, HNC, HCN, CS, and SiO
molecular abundances during the transition from AGB stage to PN stage. The calculated
abundances relative to 13CO in our model are shown in Table 2. Most the molecules are in good
agreement with the observed values. HCO+ calculated abundance is half the observed value. HCN
abundance fits the upper limit observed value. Also the abundances of CS, HC3N, SiO, and SiC2
are fitting their lower limit observed values.
The abundances of model also predicts that OH (4.4x10-2), and H2O (5.8x10-1) are
considerably high even in these carbon-rich environments. These results are consistent with the
detection of these two species in NGC 7027 by ISO (Liu et al. 1996). The ratio of N(CO)/N(H2) =
1.2x10-4. This value is consistent with the observed value 2x10-4 (Omont 1993, and Hasegawa et al.
2000).
Figure 2: The relation between the temperature and time in the post shock
Figure 3: The calculated abundances of CN, HNC, HCN, CS, and SiO
molecules against time
Table 2: Observed and calculated abundances relative to 13CO in NGC 7027.
Abundances with respect to 13CO
Observation*
Our model
> 25
30
CN
3.3x10-2
2.6x10-2
HCN
> 5.0x10-3
4.1x10-3
HNC
3.0x10-4
4.9x10-3
HCO+
7.0x10-3
3.5x10-3
CS
< 1.2x10-4
2.4x10-4
HC3N
< 2.1x10-4
1.1x10-6
SiO
< 2.5x10-5
2.0x10-5
SiC2
< 2.0x10-4
3.3x10-4
Species
12
CO
*
Bachiller et al. (1997)
4. Conclusions
In this work, the effect of shock waves on the chemical evolution of NGC 7027 is studied.
The main …… can be summarized as follows:
1. The high abundance of HCO+ in PNe can be explained in terms of a high ionization rate,
enhanced by a factor of about 103 above the ISM value, most likely because of X-ray
emission from the hot central star.
2. The model shows that molecular species such as OH, and H2O are likely to be abundant
in our object.
3. The abundances of CO, CN, HCN, and HC3N are in good agreement with the
observations, while HCO+, CS, SiC2, and SiO are in rough agreement.
4. The abundance of HNC is one order of magnitude less than the observed value.
5. The chemistry of shocked regions have an essential role in the treatment of the
chemistry in young planetary nebulae.
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