some chemistry in assorted star-forming regions
DESCRIPTION
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). - PowerPoint PPT PresentationTRANSCRIPT
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Some Chemistry in Assorted Star-forming Regions
Eric Herbst
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Some Regions Associated with Star-Formation
pre-stellar cores (L1544)
low mass protostars (IRAS 16293) protoplanetary disks Hot cores PDR’s
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A pre-stellar core (cold but with a dense center)
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H2D+ - detected by Caselli et al. (2003)
D/H =
1.5 x 10-5
“H2D+ is the main molecular ion in the central..”
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L1544 – a prestellar coreCCS – gray scale
Dust emission peak
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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
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Fractionation in the gas-phase….
H3+
CO, N2, O
HCO+
N2H+
OH+
H2D+e-
HD
DCO+, HCO+
N2D+, N2H+
OD+, OH+
CO, N2, O
e-
H2
H H HH2 H
H H D
HD H
H2 D
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When species are depleted….
H3+
CO, N2, O
HCO+
N2H+
OH+
H2D+e- HD
DCO+, HCO+
N2D+, N2H+
OD+, OH+
CO, N2, O
e-
H2
H H HH2 H
H H D
HD H
H2 D
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At higher densities….
H3+
CO, N2, O
HCO+
N2H+
OH+
H2D+e- HD
DCO+, HCO+
N2D+, N2H+
OD+, OH+
CO, N2, O
e-
H2
H H HH2 H
H H D
HD H
H2 D
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Accretion model without HD2+
and D3+:
n(H2) 104 (cm-3) 106 (cm-3)
H2D+/H3+ 0.938 27.37
DCO+/HCO+ 0.217 0.492
N2D+/N2H+ 0.215 0.484
D/H 0.075 0.355
NH2D/NH3 0.313 1.208
HDCO/H2CO 0.133 0.381
Times of peak D/H ratios: 10(6) yr and 2 x 10(4) yr
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Deuterium fractionation:
H3+ + HD H2D+ + H2
H2D+ + CO HCO+ + HD 2/3
DCO+ + H2 1/3
• Maximum DCO+/HCO+ ratio is 0.5
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Deuterium fractionation:
H2D+ + HD HD2+ + H2
HD2+ + CO HCO+ + D2 1/3
DCO+ + HD 2/3
• DCO+/HCO+ ratio reflects the total degree of deuteration of H3
+
HD2+ + HD D3
+ + H2
D3+ + CO DCO+ + D2 1
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Fractional abundances:
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Molecular D/H ratios:
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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!
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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!
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Accretion and Diffusion
DUST
HD
CO
Surface reactions produce
the following molecules:
H2CO, HDCO, D2COCH3OH, CH3OD
CH2DOH, CHD2OH
CH2DOD, CHD2OD
CD3OH, CD3OD
O
H2O, HDO, D2O, CO2, H2, HD, D2
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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.
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Dust continuum – IRAS 16293
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Methanol fractionation from a grain surface chemistry model:
Abundance CH3OH 1 x 10-7
Fractionation CH3OD 0.22
CH2DOH 0.8
CH2DOD 0.16
CHD2OH 0.2
CHD2OD 0.048
CD3OH 0.02
CD3OD 0.004Accreting D/H ratio = 0.4 (Stantcheva & Herbst 2003)
IRAS
0.04
0.9
0.2
0.03
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Methanol fractionation from a protostellar model. T=50 K; n(H2)=106cm-3
What happens as the evaporated material ages?
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After methanol desorbs from the grains:
CH3OD
CH2DOH
CH3ODH+
CH2DOHH+
CH3OD
CH3OH
CH2DOHH3
+e-
e-
H3+
e-
Osamura et al. 2004
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Compared with the observations:
Observations of IRAS 16293-2422 from Parise et al. 2002; 2003
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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.
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OMC: KL
HOT CORES
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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:
HHCOOCHeOCHHHCO
HOCHHHCOCOHOHCH
33
23223
)(
)(
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Ab Initio Calculations
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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!!!!!