This document discusses research on measuring reaction rate coefficients for reactions of the CN radical with various hydrocarbon molecules like propane, propylene, and butadiene at high temperatures representative of circumstellar envelopes of asymptotic giant branch stars. The research uses a high enthalpy source to generate a high temperature flow containing CN radicals, which are then reacted with the hydrocarbon species. Laser-induced fluorescence is used to monitor the CN radicals and determine reaction rate coefficients as a function of temperature from 1000-2200 K. The research aims to provide important kinetic data to understand chemical processes in the envelopes of evolved stars.

1. A. GARDEZ, L. BIENNIER, G. SAIDANI, E. HUGO, R. GEORGES and B. R. ROWE MFHT4 Conference | March 30-31 and April 1st, 2011 | Orléans, France

2. The classification and evolution of a star The Hertzsprung–Russell diagram shows the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. THE ASYMPTOTIC GIANT BRANCH (AGB) STAR 1

3. Structure of a ABG star Degenerate C/O Core Convective Stellar envelope Pulsations Radiative zone He- and H-burning shells pseudo photosphère H 2 , CO, H 2 O, TiO, SiO,… CN, C 2 , C n H m , SiC, CH, HCN, … 10 30 10 16 10 9 - 10 6 10 5 - 10 2 10 8 3000 1000 100 r(R*) 10 3 5 1 10 -5 T(K) n(cm -3 ) Dust particules Extended circumstellar envelope Dynamical atmosphere Shock waves Wind acceleration Dynamical atmosphere of ABG stars is the theater of rich physical and chemical processes. 2

4. Molecules detected in the circumstellar envelope of AGB stars Olofsson, H. and coll (2005) Σ = The (rough) number of source detected in each species. The CN + C n H m reactions studied play an important role in the chemistry of these hot environments. 3

5. The available laboratory kinetics data CN + C 2n H 2 -> products National Institute of Standards and Technology (NIST) Reactivity ? 4 Vinyl acetylene propylene propadiene 1,3 butadiene

6. High enthalpy source (HES) Carbon electrode Water cooled shield Carbon electrode Tantalum Jacket Porous carbon rod Thermal shield 5 Hot gas 10 cm Cold gas

7. Experimental setup Fluorescence photons Photolysis pulse (CN are generated coaxially to the flow tube) Excitation pulse (CN decays were monitored) 7 Cold reservoir Tungsten grid Precursor injector Buffer/reactant injector Roots pump Hot flow Flow reactor

8. Flow temperature map 30% transparent grid Flow tube Nozzle exit Supersonic expansion Subsonic flow Due to the grid, the flow slows down and its temperature increases. T 0 = 2200 K, T wall = 500 K, Argon buffer gas (20 slm). FLUENT 6.3 software 8

9. High temperature flows 9 non heated tube Radial distance from the nozzle Axial distance from the nozzle Velocity profile (Ar buffer gas) Boundary layer (Here viscous diffusion rate ~ thermal diffusion rate)

10. CN radical as temperature probe of the flow Top : experimental LIF spectrum of the B²Σ + ←X²Σ + transition of the CN radical in a Ar flow (15.8 slm). Bottom : simulated LIF spectrum of the CN radical at T rot = 745 K and T vib = 1300 K using the LIFBASE program. Boltzmann plot of the rotational distribution obtained from the LIF spectrum of the CN radical. 10

11. Rate constants measurements: Propane + CN as a test reaction [CN] t = [CN] 0 exp(- k 1st t ) k 1st = k 2nd [C n H m ] 11

12. Experimental results CN + C 3 H 8 -> products Rate coefficients for the reaction of CN (X²Σ+) with propane, C 3 H 8 , as a function of temperature. 12

13. Experimental results CN + C 3 H 6 -> products Rate coefficients for the reaction of CN (X²Σ+) with propylene, C 3 H 6 , as a function of temperature. 13

14. Experimental results CN + C 3 H 4 -> products Rate coefficients for the reaction of CN (X²Σ+) with propadiene, C 3 H 4 , as a function of temperature. 14

15. Experimental results CN + 1,3-C 4 H 6 -> products Rate coefficients for the reaction of CN (X²Σ+) with 1,3-butadiene, 1,3-C 4 H 6 , as a function of temperature. 15

16. Experimental results CN + 1-C 4 H 6 -> products Rate coefficients for the reaction of CN (X²Σ+) with 1-butyne, 1-C 4 H 6 , as a function of temperature. 16

17. Experimental results CN + NH 3 -> NH 2 + HCN (ou CNH) Rate coefficients for the reaction of CN (X²Σ+) with ammoniac, NH 3 , as a function of temperature. 17