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Investigating the Chemical Complexity of Planetary
Nebulae
Emily Tenenbaum, Stefanie Milam, Lindsay Zack, Kiriaki Xilouris, Nick Woolf and Lucy
Ziurys
University of Arizona
Outline
● What is a planetary nebula?● Evolutionary chemistry of planetary
nebulae● Diffuse cloud connection & molecular
cycle of the ISM● Observations of c-C3H2, C2H, H2CO in
evolved PNe● Future directions
What is a PNe?
from D. Prialnik “An Introduction to the Theory of Stellar Structure and Evolution”, 2000
•Final stage of 1-8 M stellar evolution•Lifetime ~10,000 yr•Pulsation & radiation pressure cause mass loss via wind•Strong UV flux
Hot, small, central star.Teff=50,000-200,000 K
Detached CSE
1017-1018 cm
Chemical Evolution of PNe
Young PNe Evolved PNe~12,000 yr
NGC 7027, from HST & NICMOSNGC 7027, from HST & NICMOS Helix PNe, from HST & Kitt Peak WIYN 0.9 m
HCO+ H2 C2H
CO+ CO c-C3H2
N2H+ CN H2CO
CH+ CS Large aromatic molecules
OH HCN
H2O HNC
HCO+ HCN
CO HNC
H2 Large aromatic moleculesCN
The Diffuse Cloud Connection
• PNe gas disperses into diffuse clouds
• Liszt et al. show rich diffuse cloud chemistry
• Chemical rxn network models do not explain observed molecular abundances
• Are molecules in diffuse clouds PNe remnants?
• Do similar chemical processes occur in PNe and diffuse clouds? Liszt, Lucas & Pety Astronomy & Astrophysics 2006
Diffuse Cloud•Tkin=80
•50 particles/cc•Exposed to the interstellar UV field
Chemical Recycling in the ISM
• recycling/reprocessing of molecules in the ISM
• unique photo- dissociation region (PDR) chemistry
• aromatic infrared band (AIB) and gas phase chemistry connection
Diffuse cloud
Molecular cloud
Protostar
Main sequence star
Red-giant star
AGB star
Planetary Nebula
The Detection of C2H
C2H
X 2Σ
IH= ½
=0.8 D
KP 12m
Milam et al. in preparation
1/20
N=13/2
J=1/2F=01
2
1
1
0
• Detected in 3 evolved PNe• Ntot ~ 1012-1014 cm-2
• The molecule is continually formed throughout PNe lifetime, or it survives
• Observations disagree with most models with the exception of the Howe et al. 1994 model Molecules survive in self-shielding
clumps• C2H is present in diffuse clouds
Age (yr)
12,000
8,000
10,000
261.84 262.04 262.24
0.00
0.01 M4-9
Frequency (GHz)
TA*
(K)
CCH: N=3 2J=7/2 5/2
J=5/2 3/2
Helix
The Detection of c-C3H2 in Helix
Milam et al. in preparation
c-C3H2 1A’ ground state=3.4 D
Thaddeus, Vrtilek, Gottlieb The Astrophysical Journal Letters, 1985
•Fuente et al. (2000) suggest c-C3H2 is photodissociation product of PAH’s
The Helix does not show AIB, but it does have c-C3H2
•c-C3H2 is observed in diffuse clouds
Helix
The Detection of H2CO in the Helix
H2CO 1A ground state
=2.3 D
•Ntot=1 x 1013 cm-2
•Tex=8 K
•Helix C/O = 0.81
•Observed in diffuse clouds
How do Molecules Survive in PNe?
CO H2 CO H2
H2 C2H COH2 CO
UV light
mm-wave rotational emission
•Howe, Hartquist & Williams (1994) predicted the existence of self-shielding clumps of dust & H2 in PNe
•Molecules are shielded from UV light by dust, H2, CO
•Tkin, clump= 20 K
•Density ~106 particles/cc
CO J=2-1
Future Research
• Search for H2S and SO in evolved PNeBoth molecules are observed in diffuse
clouds
• Search for H2CO in PNe with C/O > 1
• Search for C4H in PNe with and without AIBPDR studies show C4H correlates with
AIB emission
Acknowledgements
• Prof. Lucy Ziurys• Stefanie Milam• Prof. Neville Woolf• Dr. Aldo Apponi• Dr. Kiriaki Xilouris• Lindsay Zack• Dr. DeWayne Halfen, Mike Flory, Robin
Pulliam, Ming Sun• NSF Graduate Research Fellowship• NASA Astrobiology
Lifecycle of Intermediate Mass StarsDense molecular cloud fragments Protostar
•0.1-10 Myr
Main Sequence Star•H He in core•10-7000 Myr
Red Giant Star•He C in core•H He in shell•1-6 Myr
White Dwarf•Inert C/O core•Dim radiation produced by thermal motion of nuclei•1 Byr
AGB Star•Inert C/O core•H He and HeC in shells•Molecule-rich envelope•~1 Myr
PNe•Tiny, dense central star with HeC in shell, inert C/O core•Huge surrounding nebula of gas and dust •10,000 yr
Diffuse cloud•50 particles/cc•Tkin~ 100
Molecular Cloud•1,000 particles/cc•Tkin~10 K•1 Byr
The Helix Nebula
CO J=2-1
580-700 nm 15.5 m [NeIII]
6.9m H2
P. Cox et al. The Physics and Chemistry of the Interstellar Meduium, 1999
•Strong UV flux from the central star (Tstar is 110,000 K) can photodissociate molecules
•Presence of molecules (CO, H2) was surprising
red=IR 4.5 & 8 mgreen=Hα 660 nm blue=OIII 500nm
Connection to Aromatic Molecules?
• Infrared emission corresponding to vibrations of aromatic molecules is detected in PNe
• Studies of UV exposed molecular cloud regions show a correlation between AIB emission and c-C3H2, C2H, and C4H emission. These small C-chain molecules may be photodissociation products of large aromatic hydrocarbons. (Pety et al. Astronomy & Astrophysics 2005)
Aromatic Infrared Bands (AIBs) in a spectrum of the PNe NGC 7027 (from S. Kwok Nature 2004)
Possible structure corresponding to AIBs (from S. Kwok Nature 2004)
IR image of the Helix showing dust emission.(from the Spitzer Space Telescope website)