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

RNA polymers may have formed more than 4.17 billion years ago (Pearce et al. 2018, PNAS)

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

Evidence of molecular freeze-out: CO “holes”

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)

Basic astrochemistry

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

The start of astrochem

istry

Hydrocarbons and CO

Water and O2

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

Deuterium fractionation

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

Ceccarelli, Caselli et al. 2014

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

Cazaux et al. 2011; Taquet et al. 2012

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).

Interstellar Complex Organic Molecules

• 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