Transcript Observations of deuterated molecules as probes of the

Observations of deuterated
molecules as probes of the
earliest stages of star
formation.
Helen Roberts
University of Manchester
Dark Clouds:
•T
~ 10K
• H2 density ~ 104cm-3
• Contain large, unsaturated
molecules
• Typical D/H ratios are 1-10%
E1 ~ 200 K; E2 ~ 370 K; E3 ~550 K
HD/H2 ~ 10-5
Species
Observed
ratio
NH2D/NH3
0.01
HDCO/H2CO
0.005-0.11
DCN/HCN
0.023
DNC/HNC
0.015
C2D/C2H
0.01
DCO+/HCO+
0.02
N2D+/N2H+
0.08
DC3N/HC3N
0.03-0.1
HDCS/H2CS
0.02
E1 ~ 200 K; E2 ~ 370 K; E3 ~550 K
H3+
HD
H2D+
CO, N2
HCO+, N2H+
Species
Observed
ratio
NH2D/NH3
0.01
HDCO/H2CO
0.005-0.11
DCN/HCN
0.023
DNC/HNC
0.015
C2D/C2H
0.01
DCO+/HCO+
0.02
N2D+/N2H+
0.08
DC3N/HC3N
0.03-0.1
HDCS/H2CS
0.02
DCO+, N2D+
• When comparing observations with results from chemical models,
molecular D/H ratios can be more useful than absolute abundances.
• The predicted abundances are sensitive to the reaction rates used
in the model.
• The molecular D/H ratios are more sensitive to the physical
conditions assumed.
Fractional abundances over time from two gas-phase chemical models.
In each case T=10K and n(H2)=104cm-3. The filled symbols show results
from rate99, the hollow symbols OSU (Roberts et al. 2004).
Predicted abundances relative to H2
The D/H ratios, however, all
agree to within a factor of three.
rate99
OSU
CO
1.5e-4
1.5e-4
Predicted Ratio
rate99
OSU
CS
1.4e-8
1.8e-9
H2D+/H3+
0.1
0.092
N
1.2e-5
1.5e-6
0.05
0.04
H2O
3.4e-6
7.6e-7
N2D+/N2H+
HCN
3.0e-8
4.3e-9
DCO+/HCO+
0.05
0.04
HNC
6.5e-8
6.5e-9
DCS+/HCS+
0.03
0.01
H2CO
3.4e-8
4.7e-9
D/H
0.003
0.006
H2CS
1.8e-10
3.0e-11
NH2D/NH3
0.02
0.03
CH3OH
9.5e-10
2.1e-11
DCN/HCN
0.02
0.02
HC3N
1.0e-10
5.3e-13
DNC/HNC
0.01
0.01
SO
4.2e-8
5.6e-8
HDCO/H2CO
0.04
0.04
C2H
7.7e-10
6.3e-11
HDO/H2O
0.02
0.03
H3+
2.4e-8
2.4e-9
CH3OD/CH3OH
0.03
0.03
HCO+
1.4e-8
4.0e-9
CH2DOH/CH3OH
0.09
0.11
N2H+
8.1e-10
2.0e-10
HDCS/H2CS
0.04
0.05
0.03
0.04
e-
1.5e-7
6.7e-8
HDS/H2S
Several commonly observed species have
abundances differing by more than a factor of five.
• The reason for this, though, is the underlying assumption in the
deuterium chemistry models.
• We assume that deuterated species react with the same rates as
un-deuterated species.
• Where there is uncertainty as to which product the D ends up on,
we assume statistical branching ratios.
H3+ + CO
 HCO+ + H2
k cm3s-1
H2D+ + CO  DCO+ + H2
 HCO+ + HD
1/
3k
cm3s-1
2/
3k
cm3s-1
But some experiments suggest that this may not always be true:
H3+ + e-  H2 + H
H+H+H
H2D+ + e-  HD + H
 H2 + D
1.4e-8 cm3s-1
Experimental rates at
300K (McCall et al. 2003)
5.4e-8 cm3s-1
2/
3
x 1.4e-8
4/
5
x 5.4e-8 cm3s-1
1/
3
x 1.4e-8
1/
5
x 5.4e-8 cm3s-1
H+H+D
Statistical rates
5.4e-8
4.4e-8 cm3s-1
Experimental rates
from Sundström et
al. 1994)
Prestellar cores:
• T ~ 8-10 K
• Central density ~ 106cm-3
• Heavy depletion of species like
CO and CS is observed
Observations from Caselli et al. (1999)
showing CO depletion across L1544
Image of L1544: Contours show N2H+;
colour scale is CCS.
L1544
• Molecular
D/H ratios are
enhanced
(>10%)
H2D+/H2
10-9
NH2D/NH3
0.13
N2D+/N2H+
0.2
DCO+/HCO+
0.12
D2CO/H2CO
0.04
Observations of 5 prestellar
cores by Bacmann et al. (2000)
Prediction
from model
H2,H
e-
H3+
H2
HD
Models which include gasphase reactions and freeze-out
onto grains reproduce the
observational result that D/H
ratios increase with depletion.
H3+ is converted to its
deuterated analogues. Multiply
deuterated H3+ is more efficient
at deuterating other species.
HD,H2 D,H
HD,D2,D,H
D2,D
H2D+
HD2+
D3+
CO,N2,O
HCO+,N2H+,OH+
DCO+,HCO+,N2D+,
N2H+,OD+,OH+
DCO+,N2D+,OD+
H2,H
e-
H3+
H2
HD
HD,H2 D,H
HD,D2,D,H
D2,D
H2D+
HD2+
D3+
• At the very centre of the core, in the last stages before the star
forms, we expect all heavy species to be frozen onto grains.
• At late times the abundance
of H2D+ is similar to HD2+.
• D3+ becomes the most
abundant deuterated
molecule.
• The atomic D/H ratio rises
to ~0.8, which is important for
surface chemistry
Results from an `accretion’ model. T=10K;
n(H2) = 106 cm-3 (Roberts et al. 2003)
Recent Observations of deuterated H3+:
Caselli et al. (2003) detected H2D+
towards L1544. The emission is
strong towards the dust peak, and
much weaker at the off-peak position.
This suggests that H2D+ is most
abundant in the core centre.
Vastel et al. (2004) made the first
detection of HD2+ last year.
The abundance of HD2+ appears
to be similar to that of H2D+.
H2,H
e-
H3+
H2
HD
HD,H2 D,H
HD,D2,D,H
D2,D
H2D+
HD2+
D3+
Once all molecules heavier than H, He and D have frozen out, the relative
abundances of H3+ and its analogues depend on the electron abundance.
As H2D+ and HD2+ can be observed via their rotational spectra, they can be
used to probe the ionisation fraction in the last stages before a star forms.
• These molecules may also be
useful probes of the conditions at
the midplane of proto-planetary
disks, where densities are also
high, and heavy species are
depleted.
• Ceccarelli et al. (2004) recently
detected H2D+ in two disks, and
estimate that the fractional
electron abundance is a few x10-10
Prestellar core: L1544
Protoplanetary disks
Figure from Ceccarelli et al. (2004). Showing a
simple model for H2D+ abundance as a function
of electron abundance.
Protostellar Sources:
• These are complex regions, containing jets and outflows, and having
temperature and density variations.
• Both high and low-mass star protostars are observed to have `hot core’
regions, where the gas has warmed up enough to evaporate grain mantles.
IRAS 16293-2422
DCO+/HCO+
0.009
NH2D/NH3
0.1
HDCO/H2CO
0.15
D2CO/H2CO
0.03-0.16
CH3OD/CH3OH
0.02
CH2DOH/CH3OH
• These hot core regions have enhanced
abundances of saturated molecules (e.g.
H2O, CH3OH, H2S), indicative of surface
chemistry.
• Deuterated molecular
ions tend to have lower
NGC1333 IRAS4A
fractionation (indicates
DCO+/HCO+ 0.01
higher temperature).
NH2D/NH3
0.07
0.3
ND3/NH3
0.001
CHD2OH/CH3OH
0.06
D2CO/H2CO
0.073
CD3OH/CH3OH
0.014
D2S/HDS
0.12
• Surface species can
have very high D/H
ratios. This depends
on the atomic D/H ratio
in the gas when they
formed.
• IRAS 16293 is one of the best studied low-mass star forming regions
and has high deuterium fractionation.
• Parise et al. (2003, 2004) have detected 4 isotopomers of deuterated
methanol there.
• Results for three of
the methanol species
(CH2DOH, CHD2OH and
CD3OH) can be
produced with an
atomic D/H ratio of 0.10.2.
• CH3OD, however, has
a lower fractionation
than we would expect.
Observations from Parise et al. compared with
model results from Stantcheva et al. (2003),
showing methanol fractionation on the grains vs.
the D/H ratio in the accreting gas.
• Could this be due to
reactions in the gasphase after
evaporation?
The models assume that methyl groups which are present on both
products and reactants are unaffected by the reaction (Osamura et al.
2004). So for methanol:
CH2DOH
H3+
CH2DOHH+
e-
CH2DOH
Thus, CH3OD will be
CH3OD
CH3ODH+
converted to CH3OH
eCH3OH in the gas-phase after
evaporation.
The three methanol species which were observed to have high
fractionation in IRAS16293 are those with the deuterium in the methyl
group: CH2DOH, CHD2OH, CD3OH.
H3+
e-
CH3OD
So we make a simple protostellar core model:
• input abundances from the end-point of the prestellar core phase
• PLUS H2O, H2CO,CH3OH and H2S from `surface chemistry’.
We obtain the molecular D/H ratios for these molecules from the model of
Stantcheva et al. (2003) assuming an accreting D/H ratio of 0.3.
• The initial fractional abundance and D/H ratios
assumed for methanol
• As expected, the abundances of those isotopomers
with an –OD group decline faster.
CH3OH
1 x 10-7
CH3OD
0.18
CH2DOH
0.64
CH2DOD
0.11
CHD2OH
0.14
CHD2OD
0.03
CD3OH
0.012
CD3OD
0.0023
If we are confident about the model
parameters (!), then the observed
relative abundances of these
species could tell us the age of the
protostar (a `chemical clock’).
OR…
Results from a `protostellar’ model, assuming
the grain mantles evaporate at t = 0yr. T= 50K;
n(H2) = 106cm-3 (Osamura et al. 2004).
If we know the age of the protostar,
then these observations could
constrain other parameters (e.g.
the cosmic ray ionisation rate).
Conclusions:
Deuterium bearing molecules are extremely useful probes of conditions in
interstellar and protostellar regions.
The chemical models rely on theoretical determinations and laboratory
measurements of rate coefficients both in the gas-phase and on grain
surfaces.
Dark Clouds
Do deuterated species react with the same rates as their analogues?
Prestellar Cores
H2D+ and HD2+ observations may prove useful in determining the ionisation fraction.
Are accretion models sufficient, or is desorption from grains important even at low
temperatures?
Protostellar Cores
Where species have evaporated from grain-surfaces and subsequently reacted,
observations give us clues about the surface chemistry, but more theoretical and
experimental data is required for a coupled gas-grain model.