Investigating the Chemical Complexity of Planetary Nebulae Emily

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-C 3 H 2, C 2 H, H 2 CO in evolved PNe Future directions

What is a PNe? • 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, 000200, 000 K Detached CSE from D. Prialnik “An Introduction to the Theory of Stellar Structure and Evolution”, 2000 1017 -1018 cm

Chemical Evolution of PNe NGC 7027, from HST & NICMOS Helix PNe, from HST & Kitt Peak WIYN 0. 9 m Evolved PNe ~12, 000 yr Young PNe HCO+ H 2 C 2 H CO+ CO c-C 3 H 2 N 2 H+ CN H 2 CO CH+ CS OH HCN Large aromatic molecules H 2 O HNC HCO+ HCN CO HNC H 2 Large aromatic molecules CN

The Diffuse Cloud Connection Diffuse Cloud • Tkin=80 • 50 particles/cc • Exposed to the interstellar UV field • 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 Liszt, Lucas & Pety Astronomy & Astrophysics 2006

Chemical Recycling in the ISM • recycling/reprocessing of molecules in the ISM • unique photodissociation 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 C 2 H Age (yr) KP 12 m Helix C 2 H X 2Σ 12, 000 IH = ½ =0. 8 D • • • J=1/2 F=0 1 Detected in 3 evolved PNe Ntot ~ 1012 -1014 cm-2 2 N=1 The molecule 3/2 is continually formed 1 throughout PNe lifetime, or it survives Observations disagree with most models with the exception of the Howe et al. 1 1/2 0 0 model 1994 Ø Molecules survive in self-shielding clumps C 2 H is present in diffuse clouds 8, 000 10, 000 Milam et al. in preparation

The Detection of c-C 3 H 2 in Helix c-C 3 H 2 1 A’ ground state =3. 4 D • Fuente et al. (2000) suggest c-C 3 H 2 is photodissociation product of PAH’s Helix ØThe Helix does not show AIB, but it does have c-C 3 H 2 • c-C 3 H 2 is observed in diffuse clouds Thaddeus, Vrtilek, Gottlieb The Astrophysical Journal Letters, 1985 Milam et al. in preparation

The Detection of H 2 CO in the Helix H 2 CO 1 A 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? UV light CO H 2 H 2 C 2 H CO H 2 CO mm-wave rotational emission CO J=2 -1 • Howe, Hartquist & Williams (1994) predicted the existence of self-shielding clumps of dust & H 2 in PNe • Molecules are shielded from UV light by dust, H 2, CO • Tkin, clump= 20 K • Density ~106 particles/cc

Future Research • Search for H 2 S and SO in evolved PNe ØBoth molecules are observed in diffuse clouds • Search for H 2 CO in PNe with C/O > 1 • Search for C 4 H in PNe with and without AIB ØPDR studies show C 4 H correlates with AIB emission

Acknowledgements • • Prof. Lucy Ziurys Stefanie Milam Prof. Neville Woolf Dr. Aldo Apponi Dr. Kiriaki Xilouris Lindsay Zack Dr. De. Wayne Halfen, Mike Flory, Robin Pulliam, Ming Sun • NSF Graduate Research Fellowship • NASA Astrobiology

Lifecycle of Intermediate Mass Stars Molecular Cloud • 1, 000 particles/cc • Tkin~10 K • 1 Byr Dense molecular cloud fragments Protostar • 0. 1 -10 Myr Main Sequence Star • H He in core • 10 -7000 Myr Diffuse cloud • 50 particles/cc • Tkin~ 100 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 PNe • Tiny, dense central star with He C in shell, inert C/O core • Huge surrounding nebula of gas and dust • 10, 000 yr AGB Star • Inert C/O core • H He and He C in shells • Molecule-rich envelope • ~1 Myr

The Helix Nebula • Strong UV flux from the m [Ne. III] central star (T 15. 5 580 -700 nm star is 110, 000 K) can photodissociate molecules • Presence of molecules (CO, H 2) was surprising CO J=2 -1 6. 9 m H 2 red=IR 4. 5 & 8 m green=Hα 660 nm blue=OIII 500 nm P. Cox et al. The Physics and Chemistry of the Interstellar Meduium, 1999

Connection to Aromatic Molecules? 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) • 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 -C 3 H 2, C 2 H, and C 4 H emission. These small Cchain molecules may be photodissociation products of large aromatic hydrocarbons. (Pety et al. Astronomy & Astrophysics 2005) IR image of the Helix showing dust emission. (from the Spitzer Space Telescope website)