Cairo University
Faculty of Science
Chem. 1
6-9 March 2000
1st Conference on Chemistry
organized by
the Department of Chemistry
_____________________________________________
Chemical Models for NGC 7293 “Helix Nebula”
A. Ali1, O. M. Shalabiea1,2 , and M. S. El-Nawawy1
1
2
Department of Astronomy, Faculty of Science, Cairo University, Egypt.
Astronomy program, SEES, Seoul National University, Seoul 151-742, Korea
Abstract
We have studied the chemistry in dense (104-105 cm-3), and cool (T = 25 K) gas clumps,
such as observed in the Helix nebula (NGC 7293). Two gas-phase chemical evolutionary
models have been constructed for the evolved planetary nebulae NGC 7293. The nebula
is modeled as carbon-rich progenitor evolved from asymptotic giant branch to late
planetary nebula phase. The clumpy neutral envelope assumed to be subjected to
ultraviolet radiation from the central star and reasonable amount of X-rays that enhances
the rate of ionization in the clumps. Our results are in good agreement compared with the
observed abundances that have been detected in the molecular clumps of NGC 7293 such
as, CN, HCN, HNC, HCO+, CS, HC3 N, SiO and SiC2.
1.
Introduction
Planetary Nebulae (PNe) represent a short transition stage in the evolution of the
low and intermediate mass stars (0.8-8 Msun). Planetary nebulae link between Asymptotic
Giant Branch (AGB) phase and the white dwarf phase. The presence of neutral matter in
planetary nebulae is now well established. The observations confirm that the chemical
composition of the molecular gas in PNe is radically different from that in interstellar
clouds and circumstellar envelopes of AGB stars (Bachiller et al. 1997).
The molecular envelope has been formed from highly fragmented, spheroid shells
or rings around the ionized nebulae. The CO and H2 observations of Helix nebula imply
the existence of a substantial quantity of cool, molecular gas, perhaps in the form of
dense, shielded clumps (Bachiller et al 1989ab, 1993, Sahai et al. 1991, and Kastner et al.
1996).
The clumps have a cometary’s shape with tail pointing away from the central star.
Meaburn et al. (1992) estimated the mean gas number density (nH ), in these clumps, to be
about or equals 6.2x105 cm-3. Meaburn & Lopez (1993), have observed also dusty
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globules of gas in the Dumbbell nebula (M27) with lower number density (nH ~ 5.0x104
cm-3) than those observed in the Helix nebula. Bachiller et al. (1997) reported that those
clumps, which form the envelopes of the evolved PNe, have also high densities (few x
105 cm-3) and low temperatures (~25K) depending on the multi-line observations of CN,
CO, and HCO+. The clumping of the gas in the neutral envelopes of PNe is an essential
aspect for both of their 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 effect of shocks.
Balick (1978) has modeled the transition zones of the ionized PNe. He predicted
the presence of simple molecules, like H2, H2+, HeH+, OH and CH+. Howe et al. (1992)
modeled the chemical evolution of clumps, such as observed in Helix nebula, during
transition from the red giant phase to PN phase. They found that the reformation of
molecules in the shocked gas leads to the synthesis of small molecules and ions such as
CH, CH2, and CH3+. On 1994, Howe et al. constructed another steady state chemical
model. Although, their estimated result for CN/HCN ratio was comparable to the
observed ratio, the HNC/HCN ratio predicted by their model was factor 20 lower than the
observed. Also one can see that their HCO+ abundance was about 3 orders of magnitude
lower than that observed. Recently Hasegawa et al.(1999), and Yan et al.(1998) have
modeled the neutral envelope of the young nebula NGC 7027 after the extensive
observations from Infrared Space Observatory (ISO).
Therefore, we have been motivated to deeply investigate this PN object
based on such recent observations and the updated set of the chemical reaction
network (Millar et. al. 1997). The goal of this research is to construct gas-phase
models to follow the chemical evolution of Helix nebula. In Sec. 2, Initial
parameters and the chemical network are given. Our results and discussion are
presented in Sec 3.
2. Initial parameters and the chemical network
We have constructed two models M1, and M2 for the Helix nebula. The initial
parameters of the two models are summarized in Table 1. The density, temperature, and
ultraviolet (UV) radiation in the two models are roughly as that observed in the clumps of
evolved PNe.
The ionization rate by cosmic ray is taken as that used for average in interstellar
medium (ISM) value (1.3x10-17 s-1) in M2. While for M1, the cosmic ray ionization rate
is considered to be 1.3x10-13 s-1 which is 104 times than that used in M2 .
We have used the chemical reaction network and rate coefficients given by Millar
et al. (1997). The chemical network in our models has 3893 reactions and 297 species.
The initial elemental abundances relative to n (where n is the total number density
of hydrogen) are given in Table 2.
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Table 1. Initial parameters for the two models
Parameters
M1
M2
UV-radiation in units of I.S.M
~ 100
Ionization rate by cosmic ray
1.3x10-13 s-1
1.3x10-17 s-1
Total no. density
6.0x104 cm-3
6x104 cm-3
25K
0.5 mag.
25K
0.8 mag.
Temperature
Visual extinction (Av)
~ 1000
The observed molecular species in the envelopes around AGB stars (Olofsson,
1997) are taken as initial molecular abundances for our chemical models. The exact value
for the carbon abundance in Helix nebula is unknown. Therefore we have assumed its
relative abundance to n is 5x10-4, same as in ISM. The clumps are treated as carbon rich
(i.e; nC/nO > 1) clumps, so we have used nC/nO = 1.5. The adopted value for He relative
abundance is 0.13, Clegg (1987). The observed nitrogen to oxygen (N/O) ratio in the
Helix nebula is to be 0.4.
Table 2. Initial elemental abundances relative to the total hydrogen.
Elements
Abundances Elements
Abundances
H
H2
He
C
O
N
0.1
0.45
0.13
5.0x10-4
3.3x10-4
1.3x10-4
Fe
Mg
NA
S
Si
Cl
3.0x10-6
8.0x10-6
2x10-6
1.0x10-6
1.0x10-6
4.0x10-6
3- Results and discussion
First of all, The effect of shocks are excluded in the two models as a matter of
simplicity, however it might play a role in the early evolutionary stages especially the
proto-PN phase when stellar winds compress and heat the inner surface of the molecular
envelope. The first model M1 is characterized by high ionization rate in which, we
assumed a value of 104 times that of interstellar medium. The high ionization rate can be
attributed to a high cosmic ray ionization rate plus soft X-rays from the hot central star.
X-ray observation for several PNe has been reported earlier by Apparao and Tarafdar;
1989; Tarafdar and Apparao, 1989. Their observations strengthened the conclusion that
the X-ray emanated from the central star and not from the nebula. The main reason
beyond our assumption for that high ionization rate in M1, is the highly observed
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abundance of HCO+ in the evolved PNe.This has been found by Bachiller et al. 1997,
based on the work by Deguchi et al. (1990).
On the other hand, the second model M2 is characterized by intermediate UV
radiation from the central stars. The quantity of the incident UV radiation from the central
star on the PN envelope depends on the evolutionary stage of PN. It should be much
higher than the normal interstellar medium (~ 105) at early phases where the PN shell is
still near to the central star. While it is very low (~10-100) at late phases where the PN
shell is swept away from the central star. In model M1 we have assumed UV radiation
value of 100 in units of ISM, and 1000 in model M2. The high UV radiation and density
creates the so-called photon-dominated regions (PDRs) in the neutral gas, such as
observed at the edge of the H II regions (Jansen & Van Dishoeck 1995), in reflection
nebula (Jansen et al. 1995), and inside clumpy molecular clouds in star-forming regions
(Meixner et al. 1994).
The abundances are calculated after an evolutionary time of 12,000 yr that
represents the dynamical age of Helix nebula. That age is calculated using the linear
radius and expansion velocity of the object. The dynamical age for PNe depend on the
distance estimation, which is more uncertain in most objects.
The calculated abundances of the two models are shown in Table 3. HCN, HNC,
,HCO+, and HC3N molecular abundances are in good agreement with the observed
values. The calculated values of CN molecular are overabundant in the two models, while
SiC2 fits well in model M1 and less abundant in model M2. CS, and SiO are less
abundant than the observed limits.
The HNC/HCN, and HCO+/HCN ratios in model M1 are ~ 0.4, and ~ 0.5
respectively, while in model M2 are ~ 0.7, and ~ 0.6 respectively. The two ratios are
compared well with the observed averaged values (0.5, and 0.5) given by Bachiller et al.
(1997). The CN/HCN ratio is ~ 18 in M1, and ~ 28 in M2. The two values are in the
range 2-3 times that mentioned by Bachiller et al. (1997). Comparing our results with that
of Howe et al. (1994), they predicted HCO+ abundance is about 3 order of magnitude
lower than that observed, CN abundance is about 10 times its observed value, and
HCN/HCN ratio is factor 20 lower than the observed value.
Table 3. Gas-phase relative abundances in NGC 7293
Species
12
CO
CN
HCN
HNC
HCO+
CS
HC2N
SiO
SiC2
*
NGC 7293
Abundances with respect to 13CO
Observation*
M1
M2
9.3
9.3
9.3
-2
-2
3.3x10
8.4x10
7.4x10-2
-3
-3
5.0x10
4.7x10
2.6x10-3
-3
-3
3.0x10
1.8x10
2.0x10-3
-3
-3
2.0x10
2.3x10
1.7x10-3
-4
-3
> 3.0x10
5.5x10
3.0x10-2
-4
-9
> 7.0x10
7.7x10
1.9x10-8
-4
-4
> 3.0x10
9.1x10
4.9x10-4
-4
-5
> 9.0x10
3.1x10
7.4x10-3
Bachiller et al. (1997)
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Our models predicted high abundances for some other unobserved species such
as, CH, CH2, CH3, OH, and H2O. The presence of OH and H2O molecules in the carbonrich environment confirm the detection of these two molecules by ISO in NGC 7027 (Liu
et al. 1997).
Yet our understanding of the chemistry of PNe is far from complete. Further
research work is needed for this NGC 7293 and other PNe at different evolutionary
stages. Several factors such as a full treatment of the radiation transport within such
clumpy structure PNe shells, and gas-grain interaction may play a crucial role in the
studies of chemistry of PNe. On the time being gas-phase chemical models of another
three PNe (NGC 6720, M-4-9, NGC 6781) will be ready by Ali, et al. (2000).
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