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Planetary Nebulae as a Testground of Interstellar
Molecular Chemistry
Tatsuhiko Hasegawa
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1. Chemical Models – Interstellar Clouds
2. Elemental Abundances
3. Molecules in Planetary Nebulae
4. Chemical Models – Planetary Nebulae
5. 12C/13C from molecular line observation
6. Summary
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- Studies of molecules in space (Astrochemistry).
1. What (and how much) molecules are present in space ?2. How are these molecules created and destroyed ? What are the expected molecular abundances ?3. Better understanding of chemical reactions rates and molecular constants – collaborations with chemists and molecular spectroscopists, or looking into chemistry journals.
A chemical model = a simulation of chemistry with a chemical code
Types of chemistry1. Gas-phase reactions (gas-phase model)2. Grain chemistry – reactions between adsorbed molecules
– reactions between molecules and grain
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1. How many (and what) elements to consider ? H, C, N, O, S, Si, ,,, [then, Elemental Abundances ]
2. How many (and what) molecular species to consider ? => 50 to 350 species.
3. Building a data base of chemical reactions between the molecules under consideration.
=> 500 to 3500 reactions.4. Generating and coding chemical rate equations for the set of molecules under consideration.
dn(i)/dt = -n(i) n(j) R(i+j->k+l) – n(i) R(i+hv->k+l) + n(l) n(k) R(l+k->i+j) + n(k) R(k+hv->i+j)
5. Application to your astronomical project/object. => Steady state or time dependent ? Cloud structure or only one point ? => Input: Tk, n (total), and radiation field (or AV )
3. Chemical Models - Simulation in astrochemistry (gas-phase)
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Elemental Abundances
C/H N/H O/HSolar 4.6 (-4) 1.0 (-4) 8.3 (-4)Orion 3.4 (-4) 0.7 (-4) 4.0 (-4)HII region average 2.9 (-4) 0.4 (-4) 5.0 (-4)-Oph [HI cloud] 0.7 (-4) 0.2 (-4) 1.8 (-4)
NGC 7027 [C-rich PN] 13. (-4) 1.9 (-4) 5.5 (-4)NGC 6302 [Type I PN] 1.0 (-4) 8.3 (-4) 5.0 (-4)
- Chemistry simulations have been done with O > C > N [the solar abundance set; -Oph set; scaled variants].
- Chemistry simulations are possible with N > O > C or C > O > N. => No way to test the models in galactic molecular clouds.
- Molecular regions in PNe are suitable for observational tests of a chemical code.
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NGC 7027 HCO+, HCN, HNC, CN, [CCH], Bachiller et al. (1997)
C3H2, OH, CH, CH+, CO+, N2H+ Josselin&Bachiller (2003)
NGC 6720 HCO+, HCN, HNC, CN Bachiller et al. (1997)
NGC 7293 HCO+, HCN, HNC, CN
NGC 6781 HCO+, HCN, HNC, CN
M 4-9 HCO+, HCN, HNC, CN
NGC 2346 HCO+, HCN, HNC, [CCH] Bachiller et al. (1989)
NGC 6072 HCO+, HCN, HNC, CN Cox et al. (1992)
IC 4406 HCO+, HCN, HNC, CN
NGC 6302 HCO+, HCN, [SO, CCH, CN] Sahai et al. (1992)
Molecular Detections in Planetary Nebulae(in addition to CO detections in 50 planetary nebulae)
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IRAS21282 HCO+, HCN, CN, [C2H] Likkel et al. (1988)
BV 5-1 HCO+, HCN, CN Josselin&Bachiller (2003)
K 3-94 HCO+, HCN, HNC, CN
M 1-13 HCO+, HCN, HNC, CN
M 1-17 HCO+, HCN, HNC, CN
K 3-34 HCO+, HCN, CN
IC 5117 HCO+, HCN, HNC, CN
KjPn 8 CN Huggins et al. (1997)
CPD-56 HCO+ Sahai et al. (1992)
Molecular Detections in Planetary Nebulae – continued
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NGC 7027JCMTHasegawa & Kwok 2001 ApJ, 562, 824
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NGC 7027JCMTHasegawa & Kwok 2001 ApJ, 562, 824
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NGC 6302JCMTHasegawa & Kwok
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NGC 6302JCMTHasegawa & Kwok
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Molecular Abundances and Densities from (sub-)mm lines
NGC 7027 NGC 6302Element abundance C > O > N N > O > CTk (assumed) (K) 800 800
nH
(cm-3) [obs] 1.3 - 5.0 (+5) 1.3 - 4.1 (+6)
X(HCO+) 1.5 (-9) 2.2 (-10)X(H13CO+) 4.0 (-11) 9.3 (-11)X(HCN) 1.2 (-9) 4.1 (-10)X(H13CN) 1.1 (-10)X(CN) 1.7 (-8) 5.5 (-9)X(CCH) 1.1 (-8)X(CO+) 4.3 (-10)
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Chemical Model of NGC 7027 and Elemental Abundance Effects
• Steady state, gas-phase chemistry
• 9 elements, 106 species, 1500 reactions
• Radiative transfer for UV–optical photons with dust opacity.
• IUV(inner boundary) = 3.4 x 104 x DISRF (determined with CLOUDY ionization code).
• Density and temperature are given. No dynamics.
• 1-D spherical geometry (91 grid points)
• 3 models with different sets of elemental abundances :
1. NGC7027 (C-rich elemental abundances).
2. NGC6302 (extreme Type I elemental abundances).
3. Solar elemental abundances.
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Model molecular abundances. Heavy elements are mostly atomic or ionized.
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Model molecular abundances. Notice high abundances of OH, CH, HCO+.
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PNe – 12C/13C measurements in the (sub-)mm band
RGB stars – 12C/13C = 5 – 20 in 10 – 50 % of samples. Optical-IR obs stimulated theoretical work. [Mid 1970s ~]
AGB stars – 12C/13C = 3 – 10 in 5 – 20 % of ~ 60 carbon stars. Optical-IR obs + elaborate atmosphere-models. [Mid 1980s ~](Sub-)mm 13CO & CO obs + advanced radiative transfer models.(Sub-)mm obs of various 13C-molecular species.=> Modelers are responding with replications and predictions.
PNe – 12C/13C = 10 – 30 ( 2-3 in a few PNe. > 60 in a few PNe).(Sub-)mm 13CO (& CO) line obs. ~20 detections. [Late 1980s ~]
Palla, Bachiller, Stanghellini, Tosi, Galli (2000) [Italy]Balser, McMullin, Wilson (2002) [NRAO]Bachiller, Forveille, Huggins, Cox (1997) [France]
1. Better S/N (re-observing) are needed.2. More PNe should be observed in 13CO.3. Confirmation obs in H13CO+, H13CN, 13CN lines are needed.
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12C/13C estimates in PNe – examples
NGC7027 NGC6302 IRAS21282I (CO) / I (13CO) 24 1.5 87I (HCO+) / I (H13CO+) 38 9.I (HCN) / I (H13CN) 4.7
testing
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Summary
• Diversity in elemental abundance makes PNe a suitable testground of astrochemistry.
• Molecular abundance studies in 12 PNe in the (sub-)mm band are possible at modest costs.
• Isotopic ratios (12C/13C) in 40 PNe can be estimated through molecular line observations in the (sub-)mm band at modest costs.
• Your supports will be appreciated.