Computational Laboratory Astrophysics for Star Formation: getting the microphysics right

Transcription

1 Computational Laboratory Astrophysics for Star Formation: getting the microphysics right Phillip C. Stancil Department of Physics and Astronomy and the Center for Simulational Physics University of Georgia

2 Introduction: Astro-photos to Astro-physics Input data Model Intensity Wavelength Output

3 Introduction: Astro-photos to Astro-physics Input Garbage data Model Intensity Wavelength Output??

4 Outline Introduction Making Molecules: sticking on grains Exciting Molecules: molecular collisions Breaking Molecules: photodissociation Summary

5 Collaborators and Funding Junko Takahashi Benhui Yang Vijay Veeraghattam Samantha Fonseca dos Santos N. Balakrishnan Katie Manrodt Bob Forrey Joel Bowman Steve Lewis

6 Sticking Coefficients on Ice Mantles: H K amorphous ice Sticking coefficient Ω 2 = 2 Ω 2 = K 3 K Hollenbach & Salpeter (1970) Leitch-Devlin & Williams (1985) Current Work 10 K ice LAMMPS Molecular Dynamics code TIP4P Water-Water Potential H2-H2O potential of Zhang et al. (1992) 0.2 H2(v=0,J=0) H 2 Kinetic Energy (K) S 1 Veeraghattam et al. 2013a, in prep.

7 Sticking Coefficients on Ice Mantles: benchmarking experiment Sticking coefficient Current Work Matar et. al (2010) 10 K amorphous ice Experiment: H2 beam at 62 o from surface normal H 2 Kinetic Energy (K) Veeraghattam et al. 2013a, in prep.

8 Sticking Coefficients on Ice Mantles: atomic H Early days Sticking coefficient MD: Buch & Zhang 1991 QM: Hollenbach & Salpeter 1970 (Ω 2 =1) QM: Hollenbach & Salpeter 1970 (Ω 2 =2) 10 K amorphous ice Hollenbach & Salpeter used in most models MD = molecular dynamics Buch & Zhang considered small cluster H atom kinetic energy (K)

9 Sticking Coefficients on Ice Mantles: atomic H 1990s-2000s Sticking coefficient MD: Al-Halabi & van Dishoeck 2007 MD: Masuda et al MD: Buch & Zhang 1991 QM: Hollenbach & Salpeter 1970 (Ω 2 =1) QM: Hollenbach & Salpeter 1970 (Ω 2 =2) Exp: Manico et al K amorphous ice More MD simulations H2 recombination experiment H atom kinetic energy (K)

10 Sticking Coefficients on Ice Mantles: atomic H Today Sticking coefficient MD: Al-Halabi & van Dishoeck 2007 MD: Masuda et al MD: Buch & Zhang 1991 QM: Hollenbach & Salpeter 1970 (Ω 2 =1) QM: Hollenbach & Salpeter 1970 (Ω 2 =2) MD: Current Work Exp: Manico et al Exp: Watanabe et al K amorphous ice TIP4P water-water potential. H-H2O potential of Zhang et al. (1991) MD code of Masuda et al. (1998) S=1, below 100 K H atom kinetic energy (K) Veeraghattam et al. 2013b, to be submitted

11 Non-LTE Molecular Spectra: Role of Collisional Excitation H2(J1) + CO(J2) Critical density (cm -3 ) CO(v=0,J2) J 2 =1 J 2 = Temperature (K) Density is below critical density in most environments Upper levels not thermalized Level populations depend on collisional rates Yang et al. 2010, ApJ, 718, 1062

12 J =10 Pure Rotational De-excitation: H2(v1=0,J1=1) + CO(v2=0,J2=20) Rate coefficient (cm 3 /s) Symbols: Flower J2 =10 J2 =19 Rigid-rotor approximation Close-coupling calculations with new surface (JS04) J2=1-40, paraand ortho-h Temperature (K) K Yang et al. 2010, ApJ, 718, 1062

13 Pure Rotational De-excitation: sensitivity to potential energy surfaces H + CO(v=0,J=1) Rigid-rotor Rate coefficient (cm 3 s -1 ) >0 1975, Chu & Dalgarno Close-coupling calculations Semi-empirical potential surface Temperature (K)

14 Pure Rotational De-excitation: sensitivity to potential energy surfaces H + CO(v=0,J=1) Rigid-rotor Rate coefficient (cm 3 s -1 ) >0 1975, Chu & Dalgarno 1976, Green & Thaddeus Close-coupling calculations Alternate semiempirical potential surface Temperature (K)

15 Pure Rotational De-excitation: sensitivity to potential energy surfaces H + CO(v=0,J=1) 2002, Balakrishnan et al. Rigid-rotor Rate coefficient (cm 3 s -1 ) >0 1975, Chu & Dalgarno 1976, Green & Thaddeus Close-coupling calculations New ab initio potential surface (WKS) Temperature (K)

16 Pure Rotational Excitation: sensitivity to potential energy surfaces H + CO(v=0,J=0) Cross section (10-16 cm 2 ) cm -1 PES: J=0 J MRCI CCSD(T) BBH WKS Homonuclear: forbidden odd ΔJ transitions Near-homonuclear: odd ΔJ transitions suppressed (Lee & Bowman 1987) New H-CO results violated this trend J' Shepler et al. 2007, A&A, 475, L15

17 Pure Rotational De-excitation: sensitivity to potential energy surfaces H + CO(v=0,J=1) Rate coefficient (cm 3 s -1 ) CCSD(T) MRCI 2002, Balakrishnan et al. 1975, Chu & Dalgarno , Green & Thaddeus 1->0 Rigid-rotor Close-coupling calculations J= Temperature (K) T= K Yang et al. 2013a, ApJ, submitted

18 Pure Rotational De-excitation: He + HF(v=0,J=6) Full 3D Rate coefficient (cm 3 s -1 ) J'= Temperature (K) Close-coupling calculations v=0-1, range of J T= K Yang et al. 2013b, in prep.

19 Pure Rotational De-excitation: effect of vibrational state He + HF(v=0,1;J=6) Full 3D Rate coefficient (cm 3 s -1 ) J'= v=0 v= Temperature (K) Close-coupling calculations v=0-1, range of J T= K Yang et al. 2013b, in prep.

20 Pure Rotational De-excitation: effect of vibrational state He + HF(v=0,1;J=6) Rate coefficient ratio (v=1 / v=0) J'= Scaling via exponential energy gap law Temperature (K) Yang et al. 2013b, in prep.

21 Rovibrational Transitions for Diatom-Diatom: Collisions in 6D H2(v1J1) + H2(v2J2) Rate Coefficients (cm 3 s -1 ) ,10 -> 20,00 10,01 -> 00,11 20,00 -> 10,10 6D scattering calculation Not rigidrotor 6D potential energy surface (Hinde 2008) Temperature (K) First for v=2 Fonseca dos Santos et al. 2013, J. Chem. Phys., 138,

22 Summary Computational approaches can provide reliable data for astrophysical modeling Methods use state-of-the-art quantum techniques However, an on-going effort, with challenges... Many other processes need explicit investigation: Atom/ion fine-structure excitation by H, H2 Other grain-catalyzed reaction parameters (diffusion barriers, activation energies, etc.) Gas-phase neutral-neutral reactions, etc.

23 Pure Rotational Excitation: benchmarking experiment He + H2O(v=0,Jk-1,k+1) 10 Exp. (C. Yang et al. 2010) Cross section (10-16 cm 2 ) SAPT-H SAPT-P VB SAPT-H (C. Yang et al. 2010) Final state Yang et al. 2013, ApJ, 765, 77