Complex organic molecules in prestellar cores
Contribution of large bandwidths
Aurore Bacmann (IPAG)
coll. Enrique García García, Alexandre Faure (IPAG)
Complex organic molecule
observations in hot cores
•
first detections: 1970s in Sgr B2, Orion
(CH3CHO: Gottlieb et al. 1973, CH3OCHO: Brown et al. 1975, C2H: Tucker
et al. 1974)
•
spectral surveys in massive star forming regions (Sgr B2, Orion): Cummins
et al. (1986), Blake et al. (1987)
•
also in low-mass star forming regions: hot corinos (Cazaux+ 2003, Bottinelli
+ 2004)
→ similar molecular inventory
shows chemical diversity of hot cores
methyl formate
emission from hot
corino
Sutton et al. (1985)
Freq (GHz)
PdBI
Bottinelli et al. (2004)
Orion A
IRAS16293-2422
Grain surface COM formation
Previously favoured scenario (Garrod
et al. 2006)
•
simple molecule accretion during
prestellar phase (10 K),
hydrogenation
•
protostar induced warm-up
(> 30 K): diffusion of reactive
species
•
complex organic molecules formation,
desorption at higher temperatures
•
10K
30K
100K
grain
grain
grain
observations in the gas
Need for warm temperatures to account for COM formation
Search for complex organic molecules in
prestellar cores
observations IRAM
30m telescope
CH3OCH3
physical conditions: no embedded source,
temperature 10K, Av > 10
→ no external UV nor thermal energy
depletion of most molecules on grain surfaces
Bacmann et al. (2012)
CH3OCHO
Detection of saturated (terrestrial) complex
organic molecules in the gas phase of a
prestellar core (10K)
Bacmann et al. (2012)
Cernicharo et al. (2012)
CH3CHO
CH3OCH3
CH3CN
CH3OCHO
HCCCHO
CH3SH
No radical mobility at 10 K. Formation mechanism?
Relevance of large receiver bandwidths for
complex molecule observation
from CDMS
from CDMS
CH3OCH3
1700 lines at 10 K
CH3OCH3
7000 lines at 150 K
Organic molecules: complex rotational structure
emitted energy spread over many transitions covering a large range
of the millimetre spectrum
Relevance of large receiver bandwidths for
complex molecule observation
2011
2012
SO
CH3CHO
CH3OCH3
90 MHz
1 CH3OCH3 transition
1800 MHz (x4)
4 CH3OCH3 transitions
3 CH3CHO transitions
13 CH3OCHO transitions
Gain in badnwidth coverage: factor 80 → spectral survey of 3mm band
(Δν ~ 20 GHz) with rms ~ 2-3 mK at resolution 50 kHz in ~ 30 hours
enormous gain in integration time (weak lines)
Consequences of large bandwidths
• with only one transition: assumption on Tex to estimate the abundance,
assumption of a homogeneous source, transition-dependent
example of prestellar core structure
adapted from Nielbock et al. (2012)
non homogeneous physical structure = variation in excitation condition
• Simultaneous observation of a large number of transitions
→ better estimate of the excitation conditions
• multitransition maps + source model → abundance profile
→ change in the type of science
• collisional rate coefficients can be useful
Models
•
Chemistry induced by cosmic rays or secondary photons
(Reboussin et al. 2014)
•
Diffusion of species heavier than H (Minissale et al. 2013,
Reboussin et al. 2014)
•
Reactive desorption + radiative associations (Vasyunin &
Herbst 2013, Balucani et al 2015)
•
Eley-Rideal type mechanism (Ruaud et al. 2015)
Many unknowns
reaction rates/barriers, adsorption energies, grain surfaces, etc.
Ruaud et al. (2015)
COM formation in the cold gas
Need to characterise in details:
- abundances of complex organic molecules and of their
supposed precursors
- spatial distribution of COMs
- physical properties of (prestellar) sources in which these
species are observed
⇒ IRAM 30m observations:
- spectral survey of a sample of prestellar sources
- multi transition maps of prestellar cores in various COMs
Observations of other cold sources
•
detection of COMs in prestellar cores in L1689B (Bacmann et al. 2012),
L1544 (Vastel et al. 2014), in the cold cloud B1-b (Cernicharo et al. 2012,
Öberg et al. 2010)
•
survey of 7 additional prestellar sources
→ more COM detections
CH3CHO in 5 sources (out of 7)
CH3OCHO in 2 sources
CH3OCH3 in 2 sources
General presence of these species in the cold gas
sensitivity limit
CH3OCH3
•
Rather constant CH3CHO/CH3OH ~ 10% in
the various sources
•
Similar to abundances in low mass
protostellar envelopes (Öberg et al. 2014)
higher than in hot corinos (~ 1%: Bisschop
et al. 2008, Öberg et al. 2011)
•
COM radical precursors
•
Crucial role played by radicals in complex organic molecule synthesis
•
important intermediate species in CH3OH formation
•
successive hydrogenations: CO → HCO → H2CO → H3CO → CH3OH
(Charnley et al. 1992, Watanabe & Kouchi 2002)
•
•
observations of a sample of 8 prestellar sources
HCO
CH3O
detection rate: 100%
detection rate: 50%
HCO and CH3O widely detected in the gas phase of prestellar cores
Abundance ratios
•
Similar abundance ratios in the sources of the sample, despite different
absolute abundances (~ 1 order of magnitude range)
HCO : H2CO : CH3O : CH3OH
10 : 100 :
Vasyunin & Herbst (2013)
1
: 100
•
too much CH3O and CH3OH formed in
the model at steady state
•
away from steady state, ~ agreement
between model and observations for
CH3O and CH3OH but the model
predicts 10 times too much H2CO
Abundance ratios
obs: X(HCO) ~ 10-10
obs: X(CH3O) ~ 10-11
Steady-state gas-phase approach
HCO from H2CO
CH3O from CH3OH
neutral neutral path
H2CO + OH → HCO + H2O
HCO + HCO+ → H2CO+ + CO
⇒ X(HCO) ~ 10-11
⇒ X(CH3O) ~ 10-11
ion molecule path + dissociative recombination
H2CO + HCO+ → H2COH+ + CO
H2COH+ + e → HCO + H2
⇒ X(HCO) ~ 10-10
HCO + HCO+ → H2CO+ + CO
⇒ X(CH3O) ~ 4 10-11
branching ratios are uncertain / unknown
⇒ identification of reactions which should be measured in the laboratory at very low
temperatures, or species for which spectroscopic data are lacking (Bacmann & Faure
2015)
COM Mapping
CH3OH
350 μm SPIRE/Herschel map of L1689B
(André et al. 2010)
résolution 19"
extended emission from COMs
CH3OH, CH3CHO
CH3CHO
see also Tafalla et al. (2006), Bizzocchi
et al. (2015)
Summary
•
on-going study of COMs in cold gas
•
COMs are a general feature of cold gas
•
emission is extended
•
gas phase production of radicals?
What we need
•
confusion, blending not a problem (+ no need to detect weak
lines among forest of strong lines)
•
sensitivity is a problem (most COMs have weak lines)
•
most crucial needs: laboratory measurements
-
reaction rates at low T, branching ratios
-
spectroscopic data