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)