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?
Molecules Identified in PNe CO CN
CCH
N2H+
HCN CH H2
CH+
HNC OH
HCO+
H2O
H2CO
C2H2
CO+ CS
C2H
C3H2
SiS
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
Multiple Velocity Components
Young et al. 1999

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 The Map 1000″ x 800″ region
ARO 12m on Kitt Peak
HCO+ (J = 1-0)
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) (390, -30) (130, -180) (-15, 270) (-240, -100)
(-120, 240)
(-372, 0)
(-300, -200)
HCO+
J = 1-0

11. Helix Nebula (NGC 7293) HCO+ J = 1 → 0 CO J = 1 → 0 Young et al. 1999
~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

13. 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

14. 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