Photodissociation and ionisation of molecules due to stellar and cosmic-ray-induced radiation

Transcription

1 Photodissociation and ionisation of molecules due to stellar and cosmic-ray-induced radiation A. N. Heays, A. D. Bosman, and E. F. van Dishoeck Leiden Observatory, The Netherlands

2 Objective Update and extend the Leiden database Astrochemically relevant molecules, ions, and radicals Interstellar photodissociation/ionisation rates Circumstellar photodissociation/ionisation rates Rates in the cosmic ray ionisation field Depth-dependent shielding One less thing for astrochemical modellers to worry about

3 Astrochemistry data sources The current Leiden database Rates, depth-dependence Detailed CO and N 2 shielding Lee 1984, van Dishoeck 1988, van Dishoeck 2006, van Hemert and van Dishoeck 2008 Diatomic molecules: H 2 e.g., Abgrall et al., Sternberg 2014 CO e.g., Visser 2009 N 2 e.g., Lewis 2005, Li 2013, Heays 2014 PHIDRATES Huebner 2015, 1992 Rates and product branching Solar and planetary focus Cosmic ray photodissociation Gredel 1987, 1989 Subsidiary databases UMIST / UDFA / RATE2012 KIDA (+OSU) VAMDC virtual database

4 Cross section data sources MPI-Mainz UV/VIS spectral atlas Comprehensive measured cross sections Leiden database PHIDRATES MOLAT Paris Observatory Harvard CfA molecular database The literature

5 Cross sections H 2 O Photoabsorption cross section (cm 2 ) Electron energy loss, Chan et al Wavelength (nm) Often broadband low-resolution measurements.

6 Cross sections H 2 O Photoabsorption cross section (cm 2 ) Electron energy loss, Chan et al Photoabs., Mota et al Photoabs., Fillion et al. 2003/2004 Photoabs., Yoshino et al. 1996/ Wavelength (nm) Often broadband low-resolution measurements. Complemented by higher-resolutions.

7 Example cross sections Cross section (shifted) photoionisation. photodissociation CH 3 NH 2 CH 3 CHO CH 3 OH CH 4 NH 3 H 2 O N H Wavelength (nm) Widely varying thresholds and peak ranges

8 Radiation fields Intensity (photons s -1 cm -2 nm -1 ) K H ionisation limit Lyman-α TW-Hya Solar K Draine (1978) Wavelength (nm) photo rate = intensity cross section dλ

9 Photodissociation rates Photodissociation rate (s 1 ) Radiation field: Draine (1978) C 3 CH 3 CN CHONH 2 C 2 H 2 Current van Dishoeck (2006), van Hemert (2008) CH 3 CHO HCl CH 4 HCN NH 3 CH 3 OH CO 2 H2 O CH 3 NH 2 NO ISRF photodissociation rates not much changed CO N 2 H2

10 Photodissociation rates Photodissociation rate (s 1 ) C 3 CH 3 CN CHONH 2 C 2 H 2 Draine (1978) Lyman-α, b = 100 km s 1 TW-Hya CH 3 CHO HCl CH 4 HCN NH 3 CH 3 OH CO 2 H2 O CH 3 NH 2 NO Significant dependence on radiation field CO N 2 H2

11 Radiation shielding Intensity Depth Considering: Dust, H 2, H, self-shielding Simple model: Single-sided illumination

12 Dust grain optical properties Photoabs. (cm 2 H 1 ) Grain albedo Draine et al. Sometimes used values Wavelength (nm) Draine et al. dust model Mixed carbonaceous and silicate grains according to Draine 2003, Weingartner & Draine 1992, Li & Draine 2001 Gas:dust mass ratio of 123:1

13 Shielding by dust 14 molecules Shielding function Photodissociation Photoionisation Shielding by Dust 3 Radiation: Draine (1978) γ = H-nucleus column density (cm 2 ) 1 = unshielded, 0 = no photons γ = A V enhancement,

14 Self shielding in the ISRF 14 molecules Shielding function Shielding by Self 0.4 H 2 CO N H 2 O others Column density (cm 2 ) Radiation: Draine (1978) 1 = unshielded, 0 = no photons Important for small and abundant molecules

15 N 2 model potential-energy curves Potential energy (cm 1 ) X 1 Σ + g C 3 Π u F 3 3 Π u o 3 1 Π u c 4 1 Π u / c 5 1 Σ + u c 3 1 Π u / c 4 1 Σ + u G 3 3 Π u cm Σ + u C 3 Π u 2 D+ 2 D b 1 Π u b 1 Σ + u 4 S+ 2 P 4 S+ 2 D Internuclear distance (Å) 1 Π u and 1 Σ + u states absorb and emit photons 3 Π u and 3 Σ + u states have an open dissociation channel Spin-orbit coupling leads to predissociation of 1 Π u and 1 Σ + u states

16 Modelled N 2 spectrum b 1 Σ + u (v = 20) X 1 Σ + g (v = 0) P(J'') R(J'') Transmittance (arb. units) Transition wavenumber (cm 1 ) Upper: Model spectrum. Lower: Laboratory spectrum (Fourier transform spectroscopy, synchrotron SOLEIL).

17 Modelled N 2 spectrum Photoabsorption cross section from X(v = 0)

18 Self-shielding N N 2 N 15 N 0.8 Transmission Wavelength (nm) Sharply peaked 14 N 2 lines quickly saturate 14 N 15 N is unaffected by a saturated 14 N 2 column

19 Self-shielding N 2 14 N 2 14 N 15 N H 2 Dust 0.8 Transmission Wavelength (nm) Sharply peaked 14 N 2 lines quickly saturate 14 N 15 N is unaffected by a saturated 14 N 2 column Comparable or more important effect than shielding by H 2 and dust

20 Cosmic-ray induced radiation Model by Gredel et al. Radiation field (prob. distr.) Competing cross sections (cm 2 ) Wavelength (nm) Dust H 2 H Weak continuum continues to 300 nm. Rate = intensity cross section competing absorption dλ

21 Cosmic-ray induced radiation Intensity, linear scale (normalised by photon, nm) Cosmic-ray induced Draine (1978) Wavelength (nm) K blackbody

22 Photodissociation and ionisation due to cosmic rays Rate (s 1, assuming ζ = s 1 ) Cosmic-ray induced photodestruction C 3 C 2 H 2 CHONH 2 CH 3 CN CH 3 CHO HCN CH 3 OH HCl CH 4 H2 O NH 3 CO 2 Photodissociation Photoionisation ζ = ionisation rate of H 2 due to cosmic rays. Rates are 10 4 of those in the standard ISRF. CH 3 NH 2 NO N 2 CO

23 Photodissociation and ionisation due to cosmic rays Rate (s 1, assuming ζ = s 1 ) Cosmic-ray induced photodestruction C 3 HC 3 N C 2 H 4 CS2 C 2 H 2 Current Gredel et al. (1989), dust albedo = 0.5 H 2 S CH 3 CN CH 3 OH SO2 N2 O H 2 CO C 2 H HCN CH HC 3 H CH 4O2 H 2 O CO 2 HCO OH NH 2 NH 3 CH2NO CH 3CN C 2 CO N 2 Well worth the update

24 Cosmic-ray induced photodissociation N 2 H2 emission Transmission Transmission o H 2 :p H 2 = 3: K 100 K 14 N 15 N H 2 P (4) 14 N Wavelength (Å) Only a handful of H 2 emission lines overlap for species with line-like spectra. Resulting sensitivity to ortho/para ratio and temperature.

25 Summary A review of cross sections, rates, and shielding functions for astrochemically-important molecules Full wavelength dependence of cross sections and radiation fields Publication on the internet (soon) home.strw.leidenuniv.nl/~ewine/photo

26 Continuing problems In astrochemistry: Characterisation of the remote radiation fields Variable optical properties of dust grains In chemical physics: Calculation of absolute cross section for radical species Variation of molecular cross sections with temperature and isotopologue Photofragment branching of neutral species

27 Photofragment branching CH 4 Photoproduct cross section (cm 2 ) CH H + CH 2 + CH 4 CH 2 + H 2 CH + H 2 + H Wavelength (nm) Measured ratios CH 2 + H + H CH 3 + H Very few neutral branching ratios measured. Dissociative-ionisation branching not so bad.

28 Less sensitive H 2 O ISRF photodissociation rate Highest lowest resolution data = s 1 Photoabsorption cross section (cm 2 ) Electron energy loss, Chan et al Photoabs., Mota et al Photoabs., Fillion et al. 2003/2004 Photoabs., Yoshino et al. 1996/ Wavelength (nm) Cosmic-ray induced photodissociation rate ( s 1 ) Shielded by... Highest res. Lowest res. Dust Dust, H Dust, H 2, H, self, etc

29 Photoionisation rates Photoionisation rate (s 1 ) C 3 CH 3 CN Draine (1978) Lyman-α, b = 100 km s 1 TW-Hya CHONH 2 C 2 H 2 CH 3 CHO HCl CH 4 HCN NH 3 CH 3 OH CO 2 H2 O CH 3 NH 2 NO Significant dependence on radiation field CO N 2 H2

30 Shielding by H 2-14 molecules Shielding function Shielding by H 2 Radiation: Draine (1978) γ = CO N 2 others Column density (cm 2 ) 1 = unshielded, 0 = no photons