Transcript Multiply deuterated species in prestellar and protostellar

Some Chemistry in Assorted
Star-forming Regions
Eric Herbst
Some Regions Associated with StarFormation
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)
H2D+ - detected by Caselli et al. (2003)
D/H =
1.5 x 10-5
“H2D+ is the main molecular ion in the central..”
L1544 – a prestellar core
CCS – gray scale
Dust emission peak
The model:
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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
Fractionation in the gas-phase….
H3+
CO, N2, O
e-
HCO+
N2H+
OH+
H2
HD
H2D+
CO, N2, O
HHH
H2 H
e-
HHD
DCO+, HCO+
HD H
N2D+, N2H+
H2 D
OD+, OH+
When species are depleted….
H3+
H2
CO, N2, O
e-
HCO+
N2H+
OH+
HD
H2D+
CO, N2, O
HHH
H2 H
e-
HHD
DCO+, HCO+
HD H
N2D+, N2H+
H2 D
OD+, OH+
At higher densities….
H3+
H2
CO, N2, O
e-
HCO+
N2H+
OH+
HD
H2D+
CO, N2, O
HHH
H2 H
e-
HHD
DCO+, HCO+
HD H
N2D+, N2H+
H2 D
OD+, OH+
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
Deuterium fractionation:
H3+ + HD
H 2D + + H 2
H2D+ + CO
HCO+ + HD
2/
DCO+ + H2
1/
• Maximum DCO+/HCO+ ratio is 0.5
3
3
Deuterium fractionation:
H2D+ + HD
HD2+ + HD
HD2+ + H2
D 3+ + H 2
HD2+ + CO
HCO+ + D2
1/
DCO+ + HD
2/
DCO+ + D2
1
D3+ + CO
3
3
• DCO+/HCO+ ratio reflects the total degree of
deuteration of H3+
Fractional abundances:
Molecular D/H ratios:
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!
Fractionation on Grains
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One of the strongest predictions of the prestellar 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!
Accretion and Diffusion
D
H
DUST
CO
O
Surface reactions produce
the following molecules:
H2CO, HDCO, D2CO
CH3OH, CH3OD
CH2DOH, CHD2OH
CH2DOD, CHD2OD
CD3OH, CD3OD
H2O, HDO, D2O, CO2, H2, HD, D2
The Protostar IRAS 16293-2422
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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.
Dust continuum – IRAS 16293
Methanol fractionation from a grain
surface chemistry model:
Abundance CH3OH
1 x 10-7
IRAS
Fractionation CH3OD
0.22
0.04
CH2DOH
0.8
0.9
CH2DOD
0.16
CHD2OH
0.2
CHD2OD
0.048
CD3OH
0.02
CD3OD
0.004
0.2
0.03
Accreting D/H ratio = 0.4 (Stantcheva & Herbst 2003)
Methanol fractionation from a protostellar
model. T=50 K; n(H2)=106cm-3
What happens as the evaporated material ages?
After methanol desorbs from the grains:
CH2DOH
CH3OD
H3+
H3+
e
DOHH+
CH2
e-
CH3OD
CH3ODH+
e-
Osamura et al. 2004
CH2DOH
CH3OH
Compared with the observations:
Observations of IRAS 16293-2422 from Parise et al. 2002; 2003
HOT MOLECULAR CORES
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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.
OMC: KL
HOT CORES
HOT MOLECULAR CORES II
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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:
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2
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3
CH 3OH  H 2CO  HCO( H )OCH  H 2
HCO( H )OCH3  e  HCOOCH3  H
Ab Initio Calculations
TWO EXPERIMENTS
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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!!!!!