Introduction to Astrochemistry
Paola CaselliCenter for astrochemical studies,
Max-Planck-Institute for extraterrestrial Physics
(and our astrochemical origins)
McGuire 2018
Interstellar molecules
(total known: 204)
AminoacetonitrileinSgrB2(N)
(Bellocheetal.2008)
C
C
O
O
N
H
H H
H H
Glycine-thesimplestaminoacid
L-Alanine L-Aspar-cAcid L-Glutamine Glycine
>200aminoacids(plusfa;yacidsandnucleobases
guanine,adenine,uracil)havebeeniden-fiedin
meteorites;20aminoacidsareusedinlife.
ComplexOrganicMoleculesinourSolarSystemComplex Organic Molecules
at the dawn of our Solar System
OUTLOOK
Dark Clouds and Pre-stellar CoresBasic Astrochemistry
D-fractionation “Complex” organic molecules (COMs)
The dawn of protoplanetary disks
~100,000lightyears
Dame,Hartmann&Thaddeus2001
CO(1-0)
~ 1 light year
Our Milky Way and its Dark Clouds
2.6 mm
Dark Cloud as seen in dust continuum emission 500 µm
Credit:ESA/Herschel/SPIRE
Interstellar dust grains: amorphous silicates and carbonaceous material, with sizes from ~10 Å to ~ 1 𝜇m
Credit: ESA/Herschel/SPIRE
outeredge
Caselli 2011
Dark Cloud as seen in dust continuum emission 500 µm
Pre-stellar core
Central densities: ≥ 106 H2 per cc
Crapsi, Caselli, Walmsley, Tafalla 2007
In pre-stellar cores, the gas temperature drops to ~6 K→ molecular freeze-out and D-fractionation.
Karssemejer et al. 2012, PCCP
Dynamical&ChemicalTimescales
t free− fall =3π
32Gρ
≈ 4 ×107 / nH yr
t freeze− out =1
αndπad2vt
≈ 109/nH yr
tambipolar ≈ 2.5 ×1013x(e) yr
≈ 4.5 ×108 / nH yr
Walmsley 1991
from van Dishoeck et al. 2003
Pontoppidan+2007; Chiar+2011; Boogert+2015
Spitzer
Evidence of molecular freeze-out: ice features
Freeze-out and Deuterium fractionation
D-fraction increases towards
the core center to ~ 0.2
(Caselli et al. 2002; Crapsi et
al. 2004, 2005)
N2D+(2-1)
Dust emission in the pre-stellar core
L1544 (Ward-Thompson et al. 1999)
1.3mm dust continuum emissionN2H+(1-0)
In our Galaxy, H2 formation happens on the surface of dust grains,
with a rate (cm-3 s-1) given by (Gould & Salpeter 1963; Hollenbach & Salpeter
1970; Jura 1974; Pirronello et al. 1999; Cazaux & Tielens 2002; Bergin et al. 2004; Cuppen
& Herbst 2005; Cazaux et al. 2008):
nHº gas number density
vHº H atoms speed in gas-phase
A º grain cross sectional area
ngº dust grain number density
SHº sticking probability
g º surface reaction probability
The formation of H2
Once H2 is formed, the fun starts…
H2 is the key to the whole of interstellar chemistry. Some important species that
might react with H2 are C, C+, O, N… To decide whether a certain reaction is
chemically favored, we need to examine internal energy changes.
H2 4.48
CH 3.47
OH 4.39
CH+ 4.09
OH+ 5.10
Dissociation energy (eV)Molecule
C + H2 ® CH + H ??
C+ + H2 ® CH+ + H ??
O + H2 ® OH + H ??
O+ + H2 ® OH+ + H ??
Question: Can the following reactions proceed in the cold
interstellar medium?
Once H2 is formed, the fun starts…
Dissociation
energy or bond
strength
C + H2 ® CH + H ??
4.48 eV 3.47 eV
The bond strength of H2 is larger
than that of CH èthe reaction is
not energetically favorable.
The reaction is endothermic (by
4.48-3.47 = 1.01 eV) and cannot
proceed in cold clouds, where
kb T < 0.01 eV !
Once H2 is formed, the fun starts…
(endothermic by 1.01 eV)
(endothermic by 0.39 eV)
(endothermic by 0.09 eV)
C + H2 ® CH + H
C+ + H2 ® CH+ + H
O+ H2 ® OH + H
XXX
O+ + H2 ® OH+ + H (exothermic by 0.62 eV!)
H2 4.48
CH 3.47
OH 4.39
CH+ 4.09
OH+ 5.10
Dissociation energy (eV)Molecule
Rate coefficients and activation energies
The rate coefficient k (cm3 s-1) of a generic reaction A + B -> C + D is
given by:
s º total cross section of the reactants
v º relative velocity
<The average is performed over the thermal distribution>
Most reactions possess activation energies Ea (~0.1-1 eV) even if
exothermic and k is given by the Arrhenius formula (Herbst 1990):
Ion-Neutral reactions
A+ + B ® C+ + D
Exothermic ion-molecule reactions do not possess activation energy
because of the strong long-range attractive force (Herbst &
Klemperer 1973; Anicich & Huntress 1986):
V(R) = - a e2/2R4
R
kLANGEVIN = 2 pe(a/µ)1/2
~ 10-9 cm3 s-1
independent of T
A + BC ® AB + C
1 eV for endothermic reactions
E ~0.1-1 eV for exothermic reactions
kb T < 0.01 eV
in molecular clouds
Energy to break
the bond of the
reactant BC.
Energy released by
the formation of
the new bond AB.
Example: O + H2 ® OH + H
(does not proceed in cold clouds)
Duley & Williams 1984,
Interstellar Chemistry;
Bettens et al. 1995, ApJ
Neutral-Neutral reactions
X
OH + CH3OH èCH3O + H2O
Neutral-Neutral reactions
Accelerated
chemistry at
low interstellar
temperatures,
facilitated by
tunneling.
Shannon et al. 2013, Nature Chemistry
The formation of H3
+
H2 + c.r. ® H2+ + e- + c.r.
After the formation of H2, Galactic cosmic rays (relativistic particles
accelerated by supernovae) ionize H2 initiating fast routes towards the
formation of complex molecules in dark clouds:
Once H2+ is formed (97% of the times a c.r. hits H2), it very quickly
reacts with the abundant H2 molecules to form H3+, the most
important molecular ion in interstellar chemistry.
H2+ + H2 ® H3
+ + H H H
H
106 sites
Tielens & Hagen (1982); Tielens & Allamandola (1987); Hasegawa et al. (1992);
Tielens 1993; Cazaux & Tielens (2002); Cuppen & Herbst (2005); Cazaux et al. (2008);
Garrod (2008)
Surface Chemistry
quantum tunneling
thermal hopping
Surface chemistry: fast H (and D) diffusion
REACTANTS: MAINLY MOBILE H, H2 AND (at T > 20-30K) HEAVIER ATOMS AND RADICALS
A + B ® AB association reaction
H + H ® H2
H + X ® XH (X = O, C, N, CO, etc.)
WHICH CONVERTS
O ® OH ® H2O
C ® CH ® CH2 ® CH3 ® CH4
N ® NH ® NH2 ® NH3
CO ® HCO ® H2CO ® H3CO ® CH3OH
e.g. Watson & Salpeter 1972; Tielens & Hagen 1982; Hasegawa et al. 1992; Caselli et al. 1993;
Cuppen & Herbst 2005; Garrod et al. 2008; Cazaux et al. 2010
Accretion
Diffusion+Reaction
µ10/[Tk1/2 n(H2)/
(104 cm-3)] days
tqt(H) ~10-5-10-3 s
Interstellar dust particles and their icy mantles
Burke & Brown 2010
Interstellar dust particles and their icy mantles
H3+ + HD Þ H2D+ + H2 + 230 K
H2D+ / H3+ (and D/H) increases if the
abundance of gas phase neutral species
decreases (Dalgarno & Lepp 1984).
Roberts, Millar & Herbst (2003)
Deuterium fractionation in cold clouds
D/H ~ 0.3 !
CO/H2 ê
N2 ® N2D+ + H2
H2D+ + CO ® DCO+ + H2
(Watson 1974)
ortho-H2 can slow down /
suppress the deuterium
fractionation
H3+ + HD H2D+ + H2 + 230 K
Pagani+1992
Gerlich+2002
Hugo+2009
Sipilä+2013
Kong+2015
Bovino+2017
H3+ + HD è H2D+ + p-H2
H3+ + HD ç H2D+ + o-H2
Deuteration toward young stellar objects
Large abundances of multiply deuterated species in
protostellar envelopes (Ceccarelli et al. 1998; Parise et al. 2002, 2004,
2006; van der Tak et al. 2002; Vastel et al. 2003)
The youngest protostars show very large
deuterations, especially of organic molecules
H2O:
Coutens+ 2012,2013
Persson et al. 2012
Taquet+ 2012,2013
Butner et al. 2007
Vastel et al. 2010
H2S:
Vastel et al. 2003
NH3:
Loinard et al. 2001
van der Tak et al. 2002
H2CS:
Marcelino et al. 2005
H2CO:
Ceccarelli et al. 1998
Parise et al. 2006
CH3OH:
Parise et al. 2002
Parise et al. 2004
Parise et al. 2006
Credit: ESA/Herschel/SPIRE
Molecular cloud Dense core outskirt Dense core center
AV ≤ 3 mag 3 ≤ AV ≤ 10 mag AV ≥ 10 mag
D/H in carbonaceous chondrites and IDPs
Hydrated silicates and hydrous carbon:
D/H ~ 1.2-2.2×10-4 (Robert 2003), similar
to terrestrial oceans.
http://www.psrd.hawaii.edu/May06meteoriteOrganics.html
Micrometer-sized “hot spots” in organic
matter within chondrites and IDPs:
D/H up to 0.01 (e.g. Alexander et al. 2007;
Remusat et al. 2009).
• dust heating, X-rays nearby protostars
(mantle processing and evaporation)
• dust (mantles and cores) sputtering +
vaporization along protostellar outflows
COMs toward young stellar objects
• Dust heating + energetic particles nearby protostars (icy mantle processing and evaporation; e.g. Garrod+2008)
• Dust (icy mantles and cores) sputtering + vaporisation along outflows (e.g. Caselli+1997)
HCN HNCO
CH3OH c-C3H2
Organics in Pre-stellar core:chemical differentiation
Spezzano+2017(see also Bizzocchi+2014; Wirström+2014; Spezzano+2016)
CN, N2H+, NH3 CH3CCH
HCO, OCS, SO, SO2
C3H, C4H, HC3N, HCCNC, CH3CN, H2CCO, CS, HCS+,CCS, H2CS
42000 AU
Silvia Spezzano
Jiménez-Serraetal.2016
ComplexOrganicMolecules
DustPeak MethanolPeak
seealsoÖbergetal.2010;Bacmannetal.2013;Vasteletal.2014;Bacmann&Faure2016
CH3OCHO CH3OCH3
HCCCHO c-C3H2O
(c-C3H2)CH2 CH3NC
CH2CHCN HCCNC
CH3OCHO CH3OCH3
HCCCHO c-C3H2O
(c-C3H2)CH2 CH3NC
CH2CHCN HCCNC
Methanol Peak
seealsoMarcelino+2007;Öberg+2010;Bacmann+2013;Vastel+2014;Bacmann&Faure2016
Complex Organics in Pre-stellar core
HCN Peak
Vasyuninetal.2017
Gas+grain
chemistry
inL1544
Abundancew.r.t.totalH
Radius(pc)
• physicalstructure
• gas-grainchemistry
• reacBvedesorpBon
• photodesorpBon
• neutral-neutralreacBons(Shannon+2008;Balucani+2015;
Barone+2015;Skouteris+2017)
Radius (pc)
Anton Vasyunin
IRAM30mantenna
Biver et al. 2015 (see also Altwegg et al. 2016)
Similar COM abundances in comets and star forming regions
ThedawnofprotoplanetarydisksCaselli&Ceccarelli2012
Boley2009
Ileeetal.2011,Evansetal.2015
B fieldRotating and contracting magnetised cloud
Protostellar disk formation enabled by removal of very small dust grains (VSGs)
VSGs (10-100 Å) are highly conductive and well coupled with the magnetic field, which slows down rotation during contraction.
Removal of VSGs (e.g. via adsorption onto larger dust particles) reduces magnetic flux in the inner region, enabling disk to form.
without VSGs
with VSGs
Zhao, Caselli et al. 2016, 2018
Bo Zhao
ORGANIC MATTER
Insoluble Organic Matter (IOM)(Remusat et al. 2005; Cody & Alexander 2005)
Solu
ble
Org
anic
Mat
ter
(SO
M)
(Piz
zare
llo e
t al
. 2006)
ORGANIC MATTER IN PRIMITIVE METEORITES
Henning & Semenov 2013
http://www.spacetelescope.org/images/opo9545c/
Protoplanetary disks
http://www.spacetelescope.org/images/opo9545c/
Protoplanetary disks
http://www.eso.org/public/news/eso1436/
ALMA
Deuterium fractionation in protoplanetary disks with ALMA
Mathews+2013DCO+/HCO+ ~ 0.3, as in pre-stellar cores
Complexcyanidesandthecomet-like
composi4onofaprotoplanetarydisk
HCN HC3N CH
3CN
Öbergetal.2015
ALMA
see also Walsh+2016, Loomis+2018
Pre-stellar cores: nc ≥ 106 cm-3, Tc = 6-7 K, large CO freeze-out (>90%) & D-fraction (>10%) –First steps toward pre-biotic chemistry.
Large D-fraction at all phases of star and planet formation (including Solar System), with D/H in organics > D/H in water —Storage of pre-stellar ice.
COMs abundances in comets similar to those in star & planet forming regions —Solar System chemistry not unique and similar at different times of evolution.
Depletion of very small grains enables disk (and planet!) formation. — Importance of microphysics for dynamics + rich chemistry.
SUMMARY
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