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Mapping the Helix Nebula: HCO+ Chemistry and Dense Gas Clumps

This study investigates the chemistry of the Helix Nebula (NGC 7293), focusing on HCO+ emissions to understand dense gas clumps in this evolved planetary nebula. Using ARO observations, we aim to create a detailed map of HCO+ (J=1-0) and identify chemically interesting regions influenced by strong UV radiation from the central star. Our findings reveal that the chemistry in evolved planetary nebulae is more complex than previously thought, with indications of dense gas preserving molecules and the active chemical processes occurring within.

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Mapping the Helix Nebula: HCO+ Chemistry and Dense Gas Clumps

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  1. HCO+ in the Helix Nebula Lindsay N. Zack Lucy M. Ziurys Department of Chemistry Department of Astronomy Steward Observatory Arizona Radio Observatory University of Arizona

  2. Planetary Nebulae • Glowing shell of gas and plasma formed by low to intermediate mass stars in their final stage of evolution • Strong UV radiation field from central star • Shapes and sizes vary

  3. Chemistry in Planetary Nebulae • Strong UV field should destroy molecules in PNe • Several molecules have been detected in young PNe • Primarily ions and radicals • Survival in clumps of gas and dust? Tenenbaum et al., in preparation

  4. The Helix Nebula Age: ~12,000 years Distance: ~200 pc Angular Size: ~1000” • Very old • Lots of dust and gas • Atomic gas : Ha, N II, O I, C I • Molecular gas: CO and vibrationally excited H2 • Interesting structure • Cometary globules

  5. CO (J = 2-1) Map of the Helix Young et al. 1999 Multiple Velocity Components

  6. Why HCO+ ? • m = 3.89 D • High critical density (ncr ~ 105 cm-3)indicates that HCO+ emission is present in dense gas around the Helix • CO: m = 0.11 D; ncr ~ 103 cm-3 • Dense gas is shielding and can preserve molecules

  7. Mapping the Helix in HCO+ Goals… • Complete a fully sampled map in HCO+ (J = 1-0) • Identify “new” clumps of dense gas that may be chemically interesting • Examine the kinematic structure of the Helix • Determine density and temperature distributions • Model HCO+ densities with LVG analysis • Examine chemistry of old PN in detail

  8. HCO+ Observations • ARO 12m on Kitt Peak • HCO+ (J = 1-0) 89.18853 GHz • Optimal project for new ALMA-type Band 3 receiver • Tsys < 200 K KP 12m • The Map • 1000″ x 800″ region • 35″ spacing (half beam-size) • 775 positions total • 500 kHz resolution filterbanks • 3s rms noise level < 20 mK

  9. Further Observations • ARO SMT on Mt. Graham • HCO+ (J = 3-2) 267.5576 GHz • ALMA-type Band 6 receiver SMT Examine select positions in the Helix and compare to J = 1-0 transition

  10. HCO+ J = 1-0 (125, 185) (-15, 270) (-120, 240) (390, -30) (-372, 0) (130, -180) (-240, -100) (-300, -200)

  11. Helix Nebula (NGC 7293) HCO+ J = 1 → 0 • ~16% complete • 125 positions finished • 3s rms noise level < 20 mK Beam Size (70″) Beam Size (70″) CO J = 1 → 0 Young et al. 1999

  12. Summary • Chemistry in evolved planetary nebulae is more active and complex than originally thought • Presence of HCO+ (J = 3-2) indicates that very dense gas clumps exist in the Helix • HCO+ (J = 1-0) is widespread across the Helix and can be used to identify more chemically interesting areas

  13. Acknowledgements • Dr. Lucy Ziurys • Dr. DeWayne Halfen • Ziurys Group: Robin Pulliam, Emmy Tenenbaum, Ming Sun, Gilles Adande, Jessica Dodd, Jie Min, Matthew Bucchino, Brent Harris • ARO operators, engineers, and staff • Funding: NASA and NSF

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