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? 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. T eff =50, ,000 K Detached CSE cm

Chemical Evolution of PNe Young PNe Evolved PNe ~12,000 yr NGC 7027, from HST & NICMOS Helix PNe, from HST & Kitt Peak WIYN 0.9 m HCO + H2H2 C2HC2H CO + COc-C 3 H 2 N2H+N2H+ CNH 2 CO CH + CSLarge aromatic molecule s OHHCN H2OH2OHNC HCO + HCN COHNC H2H2 Large aromatic molecules CN

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 T kin =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 C 2 H C 2 H X 2 Σ I H = ½  =0.8 D KP 12m Milam et al. in preparation 1/2 0 N=1 3/2 J=1/2 F= Detected in 3 evolved PNe N tot ~ 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 model  Molecules survive in self-shielding clumps C 2 H is present in diffuse clouds Age (yr) 12,000 8,000 10,000 Helix

The Detection of c-C 3 H 2 in Helix Milam et al. in preparation c-C 3 H 2 1 A’ ground state  =3.4 D Thaddeus, Vrtilek, Gottlieb The Astrophysical Journal Letters, 1985 Fuente et al. (2000) suggest c-C 3 H 2 is photodissociation product of PAH’s  The Helix does not show AIB, but it does have c-C 3 H 2 c-C 3 H 2 is observed in diffuse clouds Helix

The Detection of H 2 CO in the Helix H 2 CO 1 A ground state  =2.3 D N tot =1 x cm -2 T ex =8 K Helix C/O = 0.81 Observed in diffuse clouds

How do Molecules Survive in PNe? CO H 2 H 2 C 2 H CO H 2 CO UV light mm-wave rotational emission 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 T kin, clump = 20 K Density ~10 6 particles/cc CO J=2-1

Future Research Search for H 2 S and SO in evolved PNe  Both 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  PDR 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. DeWayne Halfen, Mike Flory, Robin Pulliam, Ming Sun NSF Graduate Research Fellowship NASA Astrobiology

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

The Helix Nebula CO J= nm 15.5  m [NeIII] 6.9  m H 2 P. Cox et al. The Physics and Chemistry of the Interstellar Meduium, 1999 Strong UV flux from the central star (T star is 110,000 K) can photodissociate molecules Presence of molecules (CO, H 2 ) was surprising red=IR 4.5 & 8  m green=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- C 3 H 2, C 2 H, and C 4 H 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)