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6 Origin of organic : Interstellar medium

The interstellar medium (ISM) plays a vital role in the evolution of : The most important aspect of Galactic ecology is probably the cycle of matter from the ISM to and back to the ISM. Therefore, over many generations of stars the chemical composition of the ISM is enriched with heavy elements. The ISM is very heterogeneous with huge differences in chemical and physical properties, spanning many orders of magnitudes in particle and temperatures from 10K in cool molecular clouds to million degree hot bubbles. In the context of the properties of the ISM are of high significance because it represents such an important factor in the evolution of chemical elements and because it sets the stage for and formation. Of particular interest for is the presence of large organic which can form in the ISM and which can survive the often harsh conditions present in the ISM.

In this chapter we look at the following aspects:  What is the general composition of the ISM?  What are the microscopic processes responsible for cooling and heating of the ISM?  What is the main chemistry going on in the ISM, which creates reaction networks and establishes atomic and molecular abundances?  How did the first molecules (in the early ) form?  How did molecules influence the formation of the first generation of stars?  How can the ISM be traced observationally?

6.1 Introduction

The interstellar medium accounts for 10−15% of the total of the Galactic disk. It tends to concentrate near the and along the spiral arms, while being very inhomogeneously distributed at small scales. It consists mainly of (99% by mass) plus a small fraction of dust (1% by mass).

Interstellar gas

 99% of the interstellar medium is composed of interstellar gas (the rest is dust), out of which o 70.4% (by mass) is (either molecular or atomic) o 28.1% is o traces of other elements, mainly C, N, O  neutral and molecules, and  average : 1 particle cm-3 (varies from 10-2 to 106 cm-3 ). In comparison: air on ~1019 molecules cm-3

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-1 S.V. Berdyugina, University of Freiburg

Even though the interstellar gas is very dilute, the amount of matter adds up over the vast distances between the stars.

The extreme heterogeneity causes a large range of chemical and physical conditions to exist in the ISM. In first approximation three different phases are distinguished:

1. Cold clouds of neutral atomic or molecular hydrogen  T=10–100K  H II regions concentrated around hot stars (ionization by UV radiation

2. Warm medium  T=10,000K  H II regions concentrated around hot stars (ionization by UV radiation)  This phase has also a diffuse, low density component with a volume filling factor of 20–50%

3. Hot gas  T=1,000,000K  Shock heated by supernovae and from early type stars.  Very tenuous and pervasive. Fills large parts of the .

A more detailed distinction into different phases is given in the Table:

Densities Fractional Mass Phase Temperature 9 (cm–3) volume (10 M) Hot intercloud 1,000,000 K 10–2 30–70% —

H II regions 10,000 K 102–104 < 1% 0.05

Warm ionized 10,000 K 0.2–0.5 20–50% 1.0

Warm neutral 10,000 K 0.2–0.5 10–20% 2.8

Cold neutral 50–100 K 20–50 1–5% 2.2

Molecular clouds 10–20 K 104 < 1% 0.05

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-2 S.V. Berdyugina, University of Freiburg

The cold clouds of neutral or molecular hydrogen are the birthplace of new stars if they become gravitationally unstable and collapse. The neutral and molecular forms emit radiation in radio .

The 21 cm emission line of the neutral hydrogen is used to trace the distribution of HI regions. It is due to :

1. changing the alignment of the spin relative to the nuclear spin from  to  by collision (excitation) 2. emitting the at 21 cm when changing the spin from  to  (de- excitation), rate = 1 transition per106 yr!

It is found that hydrogen is concentrated to the galactic plane.

Interstellar dust

1% of the mass of ISM is dust grains. They become apparent because of  , i.e. continuum absorption and scattering of starlight  scattering starlight while producing diffuse light in the  depletion of (Ti, Fe, Mg, Cr, Ni) by factors of 10 to 1000  solid state spectral lines

Size: ~0.25-0.5 micron Shape: irregular Composition: Si (flakes or needles), C (graphite), H2O (), Fe Density: 1 grain per cubic football field (500,000 m3)

In the , the interstellar attenuation of visible light along the line-of-sight is m –1 on average about AV  1.8 kpc . Because blue light is more strongly scattered the presence of dust leads also to a reddening of background stellar light.

Although representing only 1% (by mass) of the ISM, dust plays a very important and crucial role for the chemical and physical properties of the ISM as we will see later (e.g. catalyst for formation and efficient coolant).

Because the shape of dust grains is often elongated (e.g. needles) and because the galactic can orient these grains, passing radiation is subject to dichroism, i.e. selective absorption of only one linear polarization direction. The perpendicular linear polarization direction is much less absorbed, thus the transmitted radiation field becomes linearly polarized. Measuring this linear polarization allows us to diagnose the galactic magnetic field.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-3 S.V. Berdyugina, University of Freiburg

Figure: galactic magnetic field inferred from linear polarization measurements.

Interstellar clouds

The distribution of the ISM is clumpy:

Diffuse clouds:  do not completely obscure the light from bright background stars  electronic transitions of atoms and molecules can be superposed on the stellar spectra  visible and UV wavelengths  enriched by stars at late stages of the evolution

Dark clouds:  dense clouds with rich chemistry  rotational emission of molecules  mm, submm and radio wavelengths  places of star births

Mass: 10-106 M Radius: 1-1000 pc Temperatures: 10-50 K Density: 106- 105 molecules cm-3 Number: > 5000 in the Galaxy

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-4 S.V. Berdyugina, University of Freiburg

Interstellar nebulae

Dark nebulae: (Horsehead, ): complete blocking of starlight by dust

Emission nebulae: hydrogen is ionized by hot stars and emits visible (red) light when recombine with electrons

Reflection nebulae (NGC 1999, Orion): Is a region of dusty gas surrounding a star where the dust reflects the starlight

V380 Ori, T=10,000 K, M=3.5M

Bok Globule: Is a cold cloud of gas, molecules, and , which is so dense that it blocks all of the light behind it AV~10 T~10K

M=1-1000 M R~1 pc

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-5 S.V. Berdyugina, University of Freiburg

Interstellar molecules

First interstellar molecules CH, CH+ and CN were identified between 1937 and 1941. Over the last 60 years, many interstellar molecules containing up to 13 atoms have been identified. As of 2008, there are more than 140 molecules listed as detected in the interstellar medium or circumstellar shells.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-6 S.V. Berdyugina, University of Freiburg

6.2 Microscopic processes

Cooling of the interstellar gas

Interstellar clouds cool by emitting radiation. The radiation mechanism is initiated by a collisional excitation to an excited state, so that or molecule gains the from the of the colliding particle. The subsequently radiated photon can escape from the cloud. Thus, the gas loses kinetic energy, so it cools. We summarize the process as follows:

A + B  A + B* Collision Emission B*  B + h

Cooling processes are efficient if  Collisions are frequent  fair density and abundance (hydrogen)  Excitation energy is comparable or less than the thermal kinetic energy  A high probability of de-excitation after the collision  allowed transition  Photon is emitted before the second collision occurs  density is not too high  The emitted photon is not reabsorbed in the cloud  gas is optically thin

Important atomic cooling transions in interstellar clouds for T ~ 100K: Atom/ Transition Colliding partners E/k

+ 2 2 – C P1/2 – P3/2 H, e , H2 92 K + 2 2 – Si P1/2 – P3/2 e 413 K 3 3 – O P2 – P1,0 H, e 228 K, 326 K

Important molecular cooling transitions in interstellar clouds for T ~ 100K: E/k Molecule Transition Colliding partners (lowest transition)

1 H2 X : J  J–2 H, H2 510 K 1 HD X : J  J–1 H, H2 130 K CO X1: J  J–1 H, H2 5.5 K

CO is the next most abundant interstellar molecule after H2. In dark dense clouds typically

4 3 n(H2)  10 cm

5 n(CO)  10 n(H2).

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-7 S.V. Berdyugina, University of Freiburg

CO is the most important coolant in dense clouds because it possesses a dipole moment so its rotational transitions are permitted. The CO molecule relaxes therefore to its ground state very quickly, cooling the gas and being ready for another collisional excitation. However, it can also become an efficient absorber of its own , so the cloud can become optically thick for CO photons. When such radiation trapping occurs, the efficiency of CO cooling is reduced and other less abundant molecules, for example OH and H2O, may contribute significantly to the cooling.

The cooling rate for molecular transitions depends on number of excited molecules n(J’), the energy difference between levels EJ’J’’ and the transition probability (Einstein coefficient) AJ’J’’:

1 3 rnJEAcool (') J 'JJJ '' ' '' [erg s cm ] J ' where g nne J ' EJ ' / kT . J ' Q

Heating of the interstellar gas

There are several sources of energy for the interstellar gas  Starlight  Cosmic rays  X-rays  Stellar winds  Novae  Supernovae

Photoionization by starlight

A + h  A+ + e

The electron possesses the energy of (h  Eion). It interacts with the gas and shares its energy through elastic collisions with atoms and molecules, thus providing a heat source.

Some of the energy is lost due to inelastic collisions when electrons excite transitions in atoms or molecules which is subsequently radiated.

In cool clouds the heating by photoionization of C, Si and Fe occurs.

Ionization by cosmic rays and X-rays

Cosmic ray particles consist primarily of high-energy and electrons with of a few MeV.

Soft X-rays occur with a range of photon energies but their peak intensity is at 0.1 keV.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-8 S.V. Berdyugina, University of Freiburg

Cosmic ray protons and X-rays can both ionize H atoms:

(p, X) + H  (p’, X’) + H+ + e.

For a 2 MeV , the electrons arising due to ionization have a wide distribution in energy, with a mean energy of 30 eV. In a mainly neutral medium, these electrons may cause excitations or further ionizations. Collisions with other electrons will slowly share the energy with the gas. When the electron energy is reduced below 13.6 eV, hydrogen ionization can no longer occur. When it is below 10.2 eV, excitation of H atoms is not possible. Calculations show that about 3.4 eV of kinetic energy is injected per electron produced by the ionization.

X-ray ionization is more important for He atoms than for H (because of the cross section to absorb X-rays). A 50 eV X-ray photon colliding with a he atom produces and electron of 25 eV, but only 6 eV is deposited as heat.

+ Ionization of H2 by cosmic rays or X-rays (ionization potential 15.4 eV) produces H2 :

+  (p, X) + H2  (p’, X’) + H2 + e

+ H2 undergoes a series of reaction:

+ + H2 + H2  H3 + H

+  H3 + e  H2 + H

+  H3 + e  H + H + H

The last two reactions are exothermic (actually also the first one) and the total excess of energy in the products is about 11 eV. Two-thirds of it is used for heating.

The secondary electron produced in ionization can take part in reactions with H2:

  e + H2  e + H2 ellastic collision

  e + H2  e + H2* inellastic collision (H2 is excited to upper states including the electronic B1+ state)

  +  e + H2  e + H2 + e ionization

  e + H2  e + H + H dissociation

Relaxation of electronically excited H2* molecules produces an UV radiation which can be important deep inside dark clouds where starlight cannot penetrate but cosmic ray protons can.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-9 S.V. Berdyugina, University of Freiburg

6.3 : introduction

Synthesis

A + B  AB + h radiative association

A + B:g  AB + g grain surface formation

A + B  AB + e associative detachment (less important in clouds)

Destruction

AB + h  A + B

AB+ + e  A + B Dissociative recombination (low T)

AB + M  A + B + M Collisional dissociation (high T)

Rearrangement

A+ + BC  AB+ + C Ion-molecule exchange (very rapid)

A+ + BC  A + BC+ Charge transfer

A + BC  AB + C Neutral-neutral exchange (radical- radical reactions are rapid)

Reaction networks

Exploring the reaction networks we hope to describe the chemistry of the interstellar medium and calculate abundances of molecules formed in the chemistry.

Let us consider the formation of molecule M in the following chemical reaction:

A + B  M + N,

1 3 which has a reaction rate coefficient kf [s cm per molecule].

1 The molecule M can be destroyed by photodissociation with rate kph [s ] and by the reaction

M + X  Y + Z,

1 3 with the rate kd [s cm per molecule].

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-10 S.V. Berdyugina, University of Freiburg

Then, we can write an equation for number density of M changing with time:

d nM() knAnB ()() k ()() M nM  knMnX ()() dt fphd

Since the chemistry proceeds rapidly, we can calculate equilibrium abundances assuming

d nM()0 . dt

To find n(M), we need to know abundances of all other species, A, B, X, etc, so that we have to write down a set of equations as above for each atom, molecule and ion involved in the chemistry. Such sets of equations are usually solved numerically. Sometimes useful estimates can be done from simple calculations (exercises).

6.4 Primordial chemistry and the first molecules

Molecular began in the recombination era. As soon as electrons and nuclei combined to form atoms, these atoms began to form molecular ions and molecules. The appearance of the first molecules marked the dawn of chemistry and set the stage for the subsequent evolution of the Universe.

First molecules

A study of the early Universe is severely limited by the lack of observational data. To predict the behaviour of the primordial gas, we must rely upon our knowledge of non- equilibrium chemistry, physical cosmology, hydrodynamics, and with constraints provided by the of the present Universe and few objects from the

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-11 S.V. Berdyugina, University of Freiburg observable distant past, such as quasars and Lyman  absorption systems. Future satellite missions offer the hope of observing the first generation of cosmological objects.

Recombination

  3 to 4 105 years, T  4500 to 3000 K, E  1/4 eV, z  2500

The recombination era began at a of about z  2500 when the temperature of the Universe became cool enough for nuclei and electrons to combine to form atoms. Before this time, the primordial gas was hot, dense, and completely ionized due to the cosmic background radiation (CBR). The matter and radiation were strongly coupled.

As the Universe expanded, the total particle density fell as nz(1 ) 3 , and the temperature of the CBR decreased as

Tzr 2.725(1 ) K .

The chemistry of this era was relatively simple. The only nuclei that were present were

Element First ionization energy, eV Second ionization energy, V H 13.6 - D - 4He 24.6 54.4 3He 7Li 5.4 75.6 Be 9.3 18.2 B 8.3 25.2

Recombination took place only at temperatures corresponding to 1/4 eV, which is much smaller than the ionization energies given above. The reason is the very small ratio of the baryon to photon number densities, which leads to a sufficient amount of high energy photons in the tail of the black body distribution that can ionize atoms even when the temperature has dropped clearly below the ionization energy (cf. Chapter 1: the same reason also caused the bottleneck).

At lower temperature, electrons and nuclei combined for the first time and formed first atomic ions and neutral atoms through radiative recombination:

He++ + e-  He+ + h Ei=54.4eV z > 2500  < 105 years

He+ + e-  He + h Ei=24.6eV z ~ 2500  ~ 105 years

H+ + e-  H + h Ei=13.6eV z ~ 1300  ~ 3105 years

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-12 S.V. Berdyugina, University of Freiburg

Since the hydrogen nuclei account for ~76% of the mass of the baryonic-gas, its formation at z ~ 1300 resulted in a phase transition to a mostly neutral Universe and decoupling of matter from radiation.

(Primordial) molecule formation processes

In the early Universe, formation of molecules occurred in a very different environment compared to the present interstellar medium. The important catalyst of chemical reactions – interstellar dust – was yet to form in later stages of the Universe evolution.

Molecule formation in a dust-poor environment often takes place through two-body association reactions, as densities are usually too low to allow for the more common three-body association reactions.

In order to conserve momentum in the formation of a molecule from two colliding species, either a photon or an electron must also be given off. The following two-body reactions are important in a dust-poor environment.

Radiative association

A + B  AB*  AB + h

The collision complex AB* is stabilized after the photon is emitted. The photon can be emitted spontaneously or its emission may be stimulated by the ambient radiation field. In the simplest case A and B are atoms, but they can also be neutral molecules. Usually, A and B are considered to be in their respective ground electronic states. However, if one collision partner is in an excited state due to some sort of optical or collisional pumping mechanism, the rate for radiative association may be considerably enhanced.

The reaction rate is also generally larger for more complex reactants. Therefore, radiative associations are thought to play a significant role in the formation of large molecules in dense molecular clouds. The opposite process is called photodissociation.

Figure: Illustration of the direct radiative association process. The two atoms A and B approach in the vibrational continuum of an excited electronic state and then emit a photon, thereby forming the molecule AB in a vibration- rotational state.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-13 S.V. Berdyugina, University of Freiburg

For a collision energy E and when creating a molecule AB in the lower v”J” vibrational- rotational state, the emitted photon has an energy h = E + Ev”J”

For the gas temperature T characterized by a Maxwellian distribution, the spectral signature is an emission continuum.

The total cross section (T) for radiative association is obtained by summing cross sections over all initial electronic states. The cross sections scale as the third power of the emitted photon energy:

3 (T)  (E + Ev”J”)

In the Table below radiative cross sections for some molecules are listed.

Molecule T (K) (T) (cm3 s1) + H2 10 1.51020 100 7.91020 1000 5.31018 4000 6.21017 + He2 100 7.21021 1000 5.81020 10000 2.61017 HD 100 0.81026 1000 1.01026 10000 0.41026 LiH 100 3.21020 1000 2.11020 5000 4.01021

Some typical features:

 radiative association can also occur within a single electronic state, but such a mechanism is very slow  energetic transitions are highly favored  in collisions of ground state atoms, cross sections are larger for formation of molecules with larger binding energies  radiative associations of electronically excited atoms may be particularly favored but a significant excitation mechanism should exist  spontaneous emission of a photon in the association process can be enhanced by the stimulated emission if a strong background radiation field present  there are competing processes which can occur for more energetic radiative association reactions: o for molecular ions, the radiative association mechanism competes with radiative and non-radiative charge transfer o for excited atoms, associative ionization can overwhelm radiative association

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-14 S.V. Berdyugina, University of Freiburg

Associative detachment

A + B  AB + e

This process is important when formation of stable negative ions is possible. Light elements such as H, Li, B, C, F and O have positive electron affinities and can form stable negative ions. This process is very important for formation of H2 in dustless environment.

Associative ionization

A* + B  AB+ + e

This process involves an electronically excited collision partner A*. It can in some cases compete with radiative association. The opposite process, dissociative recombination, is a major destruction process for molecular ions in a variety of astronomical environments.

Very few rate coefficients (cross sections) for the two-body association reactions have been measured reliably in the laboratory. At normal laboratory densities, measurements of these processes are usually swamped by three-body association reactions. Most of the information on rates of molecule formation comes from detailed theoretical calculations.

First molecules: Helium chemistry

The first element to become neutral was He, with an ionization potential of 24.6 eV. + + + + Thus, the first molecules to be formed were He2 , HeH , HeD and LiHe from the radiative associations of ions with neutral and ionized He

+ + 5 He + He  He2 + h z ~ 2500  ~ 10 years

H+ + He  HeH+ + h

Li+ + He  LiHe+ + h

He+ + D  HeD+ + h

These molecular ions would have been rapidly destroyed by dissociative recombination (collisions with electrons). They never reached significant amounts.

Primordial Hydrogen chemistry

By T ~ 3000 K, the recombination of protons and electrons was accomplished and the Universe had become almost completely neutral. Radiative associations of ions with neutral hydrogen formed other important molecules

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-15 S.V. Berdyugina, University of Freiburg

+ + H + H  H2 + h

H+ + D  HD+ + h

H + D  HD + h

However, the H2 molecule could NOT be formed through the process of radiative association

H + H  H2 + h

The association of two hydrogen atoms in the ground electronic state, H(1s) and H(1s), can only lead to one of two electronic states of the H2 molecule:

3 + 1 + b u (spin S=1) or X g (spin S=0)

Formation along the triplet state potential energy curve does not occur, since this state is repulsive and would thus require a highly forbidden triplet-singlet radiative transition (selection rule S=0) to form the stable singlet state..

Formation along the singlet state potential energy curve is not possible because the dipole moment of H2 is zero.

Formation along the excited singlet states is possible but requires higher energies and densities.

H  H+ (ionization)

H2 dissociation

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-16 S.V. Berdyugina, University of Freiburg

Two main mechanisms for H2 formation in the early Universe have been generally accepted. They were important at different epochs:

100 < z < 500

+ + H2 + H  H2 + H

This is a charge transfer reaction.

z < 100

H + e  H + h

  H + H  H2 + e

This is the so-called H sequence. The first reaction is the radiative recombination and the second one is the associative detachment. This process becomes important at lower because the negative hydrogen ion can not be instantly destroyed by the cooled CBR (ionization energy of H is ~1.6eV).

The HD molecule can be formed by radiative association, because its dipole moment is not zero. It is however very small and, in fact, the reactions analogous to the H2 formation are more efficient.

Other important reactions producing and involving molecular hydrogen in the early Universe include  radiative association of H(n=1) and H(n=2) (beginning of the recombination era) + +  formation of H2 through proton exchange of HeH with H (z>500) + +  formation of H3 through association of H2 and H2

The molecular abundances settled to a constant value by z ~ 100, since their timescales for formation became larger than the age of the Universe due to the decrease in density.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-17 S.V. Berdyugina, University of Freiburg

Figure: The fractional abundances of the first molecules in the recombination era as a function of the redshift. Neutral species (solid lines), positive ions (dotted lines), negative ions (dashed lines), and triatomics (dot-dashed lines).

6 The most abundant molecule was H2. Although its abundance was only 10 of the atomic hydrogen, it played an important role in the formation of the first astrophysical objects.

6.5 Structure formation: The role of the H2 molecule

Molecules were not formed homogeneously in the space. Numerical simulations show that the molecular abundance fluctuations for H2, HD and LiH are up to several times larger than the baryonic fluctuations.

z = 1425 z = 1200 z = 600 z = 10

Figure: H2 number density in slices cut through the simulation box (size 8.85 Mpc) at four redshifts. The scale of the pixel values are indicated at right (from −100010−6 to 100010−6).

These results indicate that pronounced inhomogeneous chemical abundances were present already during the dark age. This has direct implications for the spectrum of the first bound objects since gas cooling depends mainly on the molecules H2 and HD. The imprint of the chemical fluctuations could be in principle searched in CBR fluctuations.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-18 S.V. Berdyugina, University of Freiburg

Thus, the recombination era provided initial conditions for the next stage in the evolution of the Universe – the epoch of structure formation.

Collapse of the First Objects

The gas would have remained in the same state unless some type of perturbation was introduced. Perturbations may have resulted from the . Collapse is expected to have begun near z ~ 100 to 20, and the first stars (Population III) could have formed by z ~ 20 to 10.

Due to the homogeneity of globular clusters observed in the Milky Way and other galaxies, it was suggested that globular clusters were the first objects to form. From the 5 mass of globular clusters M ~ 10 M we estimate that the minimum mass of a primordial gas cloud being unstable to collapse should be of the same order of magnitude.

Gravitational collapse can only be initiated and it can only continue if the gravitational energy can be removed (cf. Chapter 2). In primordial gas energy removal is primarily provided by radiative cooling from the H2 molecule via rotational and vibrational line emission.

Line emission of H2 J Since the dipole moment of the H2 molecule is zero, electric dipole 12  1 + transitions do not occur in the ground electronic state X g . 4

Only magnetic quadrupole transitions will have non-zero probability. The selection rule for them: 17  J 2 3

These transitions are form the so called S-branch (analogous to R-, 28  P- and Q-branches). Do not confuse this with the spin! 2

Notation: S(0), S(1), S(2), … for J”=0, 1, 2, … 1 0 Because of relatively small energy differences between rotational levels (as compared to the levels of atomic hydrogen), the molecular hydrogen is an efficient cooling agent for

T ~ 100 to 10 000 K.

With H2 cooling, a proto- can continue to collapse and even fragment into objects which later will become stars. The of fragments are predicted to be as large as 100 to 1000 M. Density of the fragments also increases allowing for more efficient three-body reactions for the H2 formation

H + H + H  H2 + H

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-19 S.V. Berdyugina, University of Freiburg

The role of the third particle is to carry away the excess of the energy.

Other primordial molecules such as HD and LiH can also effectively cool the collapsing gas. Since their electric dipole moment is not zero, they are expected to be important for cooling at temperatures below which H2 is an efficient coolant.

The abundance of LiH, being quite low in the recombination era, can be increased in protostellar clouds by three-body reactions

Li + H + H  LiH + H

Observations of the first molecules

The earliest gas is observed in quasars at z < 7, which corresponds to a galaxy where the first generation stars have already significantly evolved.

It is expected that recombination era primordial gas cannot be directly observed. The flux calculated for vibration-rotational transitions of H2, HD and LiH integrated over redshifts z=1000 to z=0 is found to be 108 times smaller than the measured microwave background radiation.

It is suggested though that high redshift primordial material can be observed during 47 protostellar formation. For example, at z = 50 about 10 erg/M of gravitational energy must be radiated away for a protocloud to collapse to a . This energy can only leave the cloud in form of line emission due to molecular cooling! One of the candidates for searching the primordial molecular emission is LiH as it contributes at later stages of the collapse, meaning smaller redshifts.

Molecules in the first ejecta

Most Population III stars are expected to be massive and finish their with a supernova explosion.

Core of a massive star at the end of Silicon Burning:

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-20 S.V. Berdyugina, University of Freiburg

Because of the layered structure of the core, the ejecta are also layered, with successive shells consisting of Fe, Si, O, C, He and H. Molecules in the expanding envelope will be preferentially made from the constituents within a shell or in an adjacent shell.

In addition to the He and H chemistry, C, O and Si are candidates to produce new molecules, in particular CO and SiO.

CO

C+ + O  CO+ + h radiative association

CO+ + O  CO + O+ charge transfer

SiO

Si+ + O  SiO+ + h

SiO+ + O  SiO + O+

The supernova ejecta created the interstellar medium!

6.6 “Modern” astrochemistry

Modern Hydrogen chemistry

Reactions taking place in the early Universe to produce H2 are not sufficient to explain the observed abundances of H2 in interstellar clouds because of low concentrations of +  H and H . The only plausible formation route of H2 is therefore on the surfaces of grains.

The grain surface plays a role of the third body  prolongs the contact of atoms to combine into a molecule  stabilizes the molecule by taking the excess of the energy

The atom sticks to the grain’s surface and can move across the surface in searching of the second atom. Clearly, it should be bound to the surface long enough for a second atom to arrive. This seems to be satisfied.

Other molecules can also be formed on the grain surface as binding energies for other atoms are larger than for hydrogen.

For instance, before H2 is detached from the surface, O can arrive and form H2O, which in cool clouds will be deposited as ice on the grain surface.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-21 S.V. Berdyugina, University of Freiburg

Carbon chemistry

Hydrocarbons are important constituents of molecular clouds.

In diffuse clouds, atomic is mostly in the form of C+, because its ionization potential is less than 13.6 eV. Here the carbon chemistry is most probably initiated by the radiative association reaction

+ + C + H2  CH2 + h, followed by

+ + CH2 + H2  CH3 + H

+  CH3 + e  CH2 + H

The build-up of polyatomic hydrocarbons is limited by rapid photodissociation by UV radiation. In denser parts of clouds, neutral hydrocarbons CH, CH2 and CH3 react with O and form CO and H2CO. Once the carbon is locked up in the very stable CO molecule, formation of more complex hydrocarbons ceases.

In dense clouds, shielded from the UV radiation, the amount of C+ is low. The carbon chemistry is initiated by

+ + C + H3  CH + H2

+  CH2 + H

+  CH3 + h

Complex hydrocarbons are formed mainly by carbon insertion reactions, e.g.

C + C2H2  C3H + H

This gas chemistry produces strongly unsaturated hydrocarbons, in agreement with observations.

Oxygen chemistry

Oxygen is mostly neutral even in diffuse clouds. The gas-phase oxygen chemistry is initiated by charge exchange reaction and forming OH+:

O + H+  O+ + H

+ + O + H2  OH + H

OH+ is highly reactive and lead to formation of neutral oxygen-bearing molecules like OH and H2O.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-22 S.V. Berdyugina, University of Freiburg

In cold dense clouds, the reaction

OH + O  O2 + H rapidly transform a significant part of the oxygen into O2.

Nitrogen chemistry

In contrast to the carbon and oxygen chemistries, the reactions that initiate the chemistry are still not well understood. Most probably it stars with

+ + N + H2  NH + H

+ + which leads further to NH2 and NH4 and then, via dissociative recombination, to neutral NH, NH2 and NH3.

The N+ ions are mainly produces by cosmic ray ionization.

Neutral-neutral reactions play an important role in formation of other species. Reactions of atomic N with CH, OH and NO lead to CN, NO and N2, respectively.

Reactions of CN with C2H2 are probably a major source of complex molecules as HC2nCN.

Other element chemistries

These involve sulfur, chlorine, metals, large molecules such as PAH (polycyclic aromatic hydrocarbons), carbon chain molecules and grain surface chemistry.

6.7 Observations

Studying molecules in the ISM is important, not only because it allows us to improve our understanding of the chemical and physical properties of the ISM itself, but also because of the close relation with stars and the galactic evolution and because of possible implications for the development of life on Earth and maybe on other .

Understanding physical and chemical processes in ISM:  Detection and identification of complex molecules in different environment (e.g., cold clouds, PDRs, diffuse clouds, etc.) provides critical information for understanding the possible formation (and destruction) pathways of these molecules.  Constraining chemical models.  Can be used to study evolution of clouds and the process.  Long carbon chain molecules (e.g., PAH’s) play a major role in ISM physics and chemistry.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-23 S.V. Berdyugina, University of Freiburg

Implications for life on Earth:  Some of the complex molecules found in the ISM are large organic molecules, thought to be important to life.  It is possible that chemical processes in the interstellar medium provided essential material that allowed the emergence of life.

Discovery of interstellar spectral lines

1904 Ca II line at 3900 Å 1920s Na I 5890 Å 1937 CH 1940 CN 1941 CH+ 1969 H2O (maser) 1970 CO …

+ H3

Mc Call et al. (2003)

+ The H3 molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. For example, the reaction + of H3 with atomic O leads, eventually, to the production of , while the reaction with HD leads to the production of a wide variety of deuterated isotopomers.

Molecules in the ISM and molecular lines are identified by comparing observed spectra with laboratory data and/or simulated spectra. Once identified, we can start to employ the lines for diagnostics. For example, with the above observations combined with + + theoretical considerations of the chemical network of H3 (remember e.g. that H3 is produced via cosmic-ray ionization of H2) it was possible to determine the cosmic-ray flux in a diffuse cloud (observed towards  Persei), which was found to be 40 times larger than previously believed.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-24 S.V. Berdyugina, University of Freiburg

H2O

González-Alfonso et al. (1998)

H2O lines between 5.3 and 7.2 m observed with the Short Spectrometer (SWS) on board the Space Observatory toward the Orion BN/KL complex, a deeply imbedded star forming region in the Orion . Surprisingly, the H2O rovibrational lines with  < 6.3 m  the R-branch  are observed in absorption, while those  > 6.3 m  the P-branch  are observed in emission. This behavior is a consequence of H2O absorption and reemission of continuum photons (look at the paper for details, though it involves some molecular physics). The main point however is that such measurements can be used to determine the abundances of water vapour in the observed .

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-25 S.V. Berdyugina, University of Freiburg

H2O

Cernicharo et al. (1997)

Sgr B2 is a molecular cloud complex near the galaxy center. The infrared spectrum is dominated by a distribution caused by dust at a temperature of 30 K. On top of the continuum a series of molecular lines are apparent, in particular water vapor lines in absorption. The fact that H2O is seen here in absorption, rather than emission, suggests that in Sgr B2, where the continuum emission by dust in the far infrared is optically thick, the H2O lines arise in a region where the excitation temperatures are smaller than the dust temperature (30 K). It is concluded that the H2O absorption lines probably arise in the tenuous and extended envelope of Sgr B2 where collisional excitation is negligible and the excitation is mainly due to absorption of photons emitted by the dust. This illustrates a nice example of how we can infer information about the physical and chemical properties of the ISM, in particular also its structure.

Carbohydrides, etc.

Cernicharo et al.

Many organic molecules have been identified in the ISM, such as (C2H2) or (C6H6).

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-26 S.V. Berdyugina, University of Freiburg

Acetone = (CH3)2CO

–11 n ~ 510 n(H2) T ~ 20 K

Combes et al. (1987)

The figure shows the first detection of in interstellar space (in Sgr B2). Acetone is one of the complex molecules, clearly identified with three transitions.

PAH (?)

C5H12 ?

C6H6 ?

Leger & Puget (1984)

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-27 S.V. Berdyugina, University of Freiburg

Polycyclic Aromatic Hydrocarbons (PAHs) are an important and ubiquitous component of the organic materials. The infrared bands are observed at 3.29, 6.2, 7.7, 8.7, 11.3, and 12.7 μm and are often accompanied by minor, weaker, bands and underlying broad structures in the 3.1 - 3.7, 6.0 - 6.9, and 11 - 15 μm ranges. In the model dealing with the interstellar spectral features, PAHs are present as a mixture of radicals, ions, and neutral species. The proposed excitation (pumping) mechanism of the IR bands is a one- photon mechanism that leads to the transient heating of the PAH molecules and ions by stellar (UV), visible, and/or NIR photons. The IR bands are associated with the molecular vibrations of PAH structures present either as free molecules and ions. It should be mentioned though that the presence of PAH is in principle only suspected but not really robustly proven. However, the PAH model has considerably evolved over the years thanks in large part to the extensive laboratory and theoretical efforts that have been devoted to this issue over the years. There is a wide consensus now that PAHs are the best candidates to account for the IR emission bands.

PAHs actually represent a big current challenge to ISM chemists. The infrared emission bands associated with PAHs arise from vibrational transitions (such as C-C or C-H stretching or bending modes), which are relatively similar for most PAH molecules. However, the electronic spectra are unique; therefore, if the electronic spectra of PAHs or their cations were known, astronomers could search for specific molecules. Laboratory studies of reaction rates with appropriate modeling can identify PAHs for spectroscopic study, which in turn can enable identification of PAHs in the ISM.

Solid state spectral lines

Solid state spectral lines: we see vibrational bands without rotational structure (broad absorption). This means that molecules vibrate and absorb while being in dust conglomerates (solid-state vibrations).

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-28 S.V. Berdyugina, University of Freiburg

Sgr A

Figure: (a–c) Spectra of the stretching mode of CO. (d–f ) The bending mode of water vapor in the direction of Sgr A*. Panels (a) and (d) show the observations and the continuum (including ); (b) and (e) show the contribution of the ices and of the war CO; (c) and ( f ) show the normalized spectrum and model spectrum (shifted for clarity); and (g) shows the details of the ortho- and para-H2O transitions. Moneti et al. (2001).

Here we see solid state CO and H2O lines (broad absorptions) plus a superimposed rotational spectrum due to gas phase molecules. Comparison to the model gives an idea about physical properties such as temperature (requires strong cold, but also some weak warm gas component).

The CO data indicate that the absorbing gas is extremely cold, T  10 K, suggesting that it is located in the dark clouds of the different spiral arms that K intersect the lines of sight. It is found that nearly all the CO is in the gas phase, while the H2O is almost entirely frozen onto dust grains.

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-29 S.V. Berdyugina, University of Freiburg

6.8 Molecular clouds in other galaxies

1970s CO detection in other galaxies soon after the detection in ISM

CO traces molecular clouds and star formation regions (CO specially used as tracer for H2, which we cannot see), here shown for the examples of Andromeda and NGC 1068.

M31, Andromeda, in the optical (on the left) and CO (on the left)

NGC1068

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-30 S.V. Berdyugina, University of Freiburg

NGC253 SiO

H13CO+ 3mm-cont

Observations in molecular lines can reveal much more information than just continuum observations. We see structure of the inner buldge of NGC 253 revealed by different molecules (different conditions for chemistry).

QSO J1148 +5251

One of the most distant quasars

z ~ 6.42 d ~ 13109 lyr t ~ 870106 yr

7 CO ~ 10 M

CO (3-2) dust at 1.2 mm

At z~6, we see already the molecule CO, which means that the first SNe already exploded and formed ISM, which is remarkable taken the age of the universe of ~800 Myr only at this redshift!

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-31 S.V. Berdyugina, University of Freiburg

IC 342 (nearby galaxy)

A nearby galaxy: another example, showing various sites of chemistry "factories". Meier & Turner (2005).

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-32 S.V. Berdyugina, University of Freiburg

Advantage of molecular lines

140 K

40 K

20 K

Advantage of molecular lines: measuring different transitions (i.e. different excitation energies) one can get the energy distribution and find the temperature of the medium very precisely (much more precisely than with atoms).

Astrobiology: 6 Origin of organic matter: Interstellar medium 6-33 S.V. Berdyugina, University of Freiburg