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Some Chemistry in Assorted Star-forming Regions

Some Chemistry in Assorted Star-forming Regions. Eric Herbst. Some Regions Associated with Star-Formation. pre-stellar cores (L1544) low mass protostars (IRAS 16293) protoplanetary disks Hot cores PDR’s. A pre-stellar core (cold but with a dense center).

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Some Chemistry in Assorted Star-forming Regions

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  1. Some Chemistry in Assorted Star-forming Regions Eric Herbst

  2. Some Regions Associated with Star-Formation pre-stellar cores (L1544) low mass protostars (IRAS 16293) • protoplanetary disks • Hot cores • PDR’s

  3. A pre-stellar core (cold but with a dense center)

  4. H2D+ - detected by Caselli et al. (2003) D/H = 1.5 x 10-5 “H2D+ is the main molecular ion in the central..”

  5. L1544 – a prestellar core CCS – gray scale Dust emission peak

  6. The model: • multiply deuterated species are now observed in the ISM • observations support the link between high fractionation and CO depletion • we present a pseudo-time-dependent model of deuterium chemistry, including all analogues of H3+, NH3, CH3OH • HD2+ and D3+ may be important even in modeling singly deuterated species

  7. Fractionation in the gas-phase…. H3+ H2 CO, N2, O HD e- H2D+ HCO+ N2H+ OH+ CO, N2, O H H H H2 H e- DCO+, HCO+ N2D+, N2H+ OD+, OH+ H H D HD H H2 D

  8. When species are depleted…. H3+ H2 CO, N2, O HD e- H2D+ HCO+ N2H+ OH+ CO, N2, O H H H H2 H e- DCO+, HCO+ N2D+, N2H+ OD+, OH+ H H D HD H H2 D

  9. At higher densities…. H3+ H2 CO, N2, O HD e- H2D+ HCO+ N2H+ OH+ CO, N2, O H H H H2 H e- DCO+, HCO+ N2D+, N2H+ OD+, OH+ H H D HD H H2 D

  10. Accretion model without HD2+ and D3+: Times of peak D/H ratios: 10(6) yr and 2 x 10(4) yr

  11. Deuterium fractionation: H3+ + HD H2D+ + H2 H2D+ + CO HCO+ + HD 2/3 DCO+ + H21/3 • Maximum DCO+/HCO+ ratio is 0.5

  12. Deuterium fractionation: H2D+ + HD HD2+ + H2 HD2+ + HD D3+ + H2 HD2+ + CO HCO+ + D21/3 DCO+ + HD 2/3 D3+ + CO DCO+ + D2 1 • DCO+/HCO+ ratio reflects the total degree of deuteration of H3+

  13. Fractional abundances:

  14. Molecular D/H ratios:

  15. A comparison of the homogeneous model with observations of CO and D2CO: observations model (Observations from Bacmann et al. 2002; 2003) Heterogeneous shell model does much better!

  16. Fractionation on Grains • One of the strongest predictions of the pre-stellar core model is that the abundance ratio of D to H atoms in the gas becomes quite high (0.1 – 1.0). In reality, these atoms strike dust particles and react to form both normal and deuterated species!! These species stay on the grains until star formation begins to occur and temperatures rise!

  17. Accretion and Diffusion H D Surface reactions produce O the following molecules: DUST H2CO, HDCO, D2CO CH3OH, CH3OD CH2DOH, CHD2OH CH2DOD, CHD2OD CO CD3OH, CD3OD H2O, HDO, D2O, CO2, H2, HD, D2

  18. The Protostar IRAS 16293-2422 • Temperatures have warmed up to near 100 K close to the budding star and 50 K somewhat farther removed. The following methanol isotopomers have been detected: • CH3OH, CH3OD, CH2DOH, CHD2OH, CD3OH in addition to HDCO and D2CO. • The belief is that these species have very recently come off grains.

  19. Dust continuum – IRAS 16293

  20. Methanol fractionation from a grain surface chemistry model: Accreting D/H ratio = 0.4 (Stantcheva & Herbst 2003)

  21. Methanol fractionation from a protostellar model. T=50 K; n(H2)=106cm-3 What happens as the evaporated material ages?

  22. After methanol desorbs from the grains: H3+ e- CH2DOH CH2DOHH+ CH2DOH CH3OD e- H3+ CH3OD CH3ODH+ e- CH3OH Osamura et al. 2004

  23. Compared with the observations: Observations of IRAS 16293-2422 from Parise et al. 2002; 2003

  24. HOT MOLECULAR CORES • Hot cores are regions of warm, quiescent gas near high-mass star-forming regions. Temperatures are 100-300 K and densities are typically 107 K. They are associated with a variety of saturated gas-phase organic molecules: methanol, ethanol, acetaldehyde, methyl formate, acetic acid, glycolaldehyde, ethylene oxide, dimethyl ether, and possibly diethyl ether, glycine, and ethylmethyl ether.

  25. OMC: KL HOT CORES

  26. HOT MOLECULAR CORES II • As in protostellar sources, the chemistry is associated with evaporation from the dust, although the post-evaporation gas-phase chemistry may be crucial in producing larger species from the precursor methanol. • Key reactions in chain to form methyl formate:

  27. Ab Initio Calculations

  28. TWO EXPERIMENTS • 1) SIFT AT HANSCOM AF BASE • dominant product cluster ion (high density) • 2) ICR AT WATERLOO, CANADA • dominant product CH3OCH2+ (low density) • CONCLUSION: no major channel to produce protonated methyl formate • We don’t know how it is made in hot cores. There is work left for you to do!!!!!

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