Introducing Cosmic Chemistry

The chemistry of cosmic diffuse gas as well as its astrophysical significance is discussed from the primordial gas to cold cores where stars can form.


INTRODUCTION
Chemistry develops in many astrophysical environments, from the tenuous primordial gas (containing 10 −3 proton per cubic centimeter 1 ) to the atmospheres of planets (n H ≥10 12 cm −3 ).Diluted or diffuse gas at low to moderate densities (n H ≤10 4 cm −3 ) fills up most of the volume of our Galaxy (the interstellar medium or ISM) and of the young Universe.The following presentation focuses on the chemistry of the diffuse cosmic gas.However, we note that this case shares many features with that of denser gas.The chemistry of denser environments such as protoplanetary disks or planetary atmospheres is discussed elsewhere in this volume (see contributions by Engrand, Guilloteau, Monod and Remusat).
In addition to gas temperature and density, ISM chemistry also depends on the composition of the gas as well as on the abundance and nature of small (1 to 100 nm in radius) solid particules called dust grains.We first give a brief overview of the composition and physical conditions of the ISM.We then discuss the dominant reaction schemes in the ISM and how they are probed with observations.The interesting case of chemistry in the young Universe is also briefly considered.

COMPOSITION AND PHYSICAL CONDITIONS IN THE ISM
In the ISM, evolved stars of mass comparable and up to a few times that of the Sun become red giants and expell a significant fraction of their mass in the form of dense winds (n H ∼10 9 cm −3 ).As the temperature decreases in the stellar wind, dust grains form by condensation and the heavy elements synthesized by the star are thus depleted from the gas phase.In the Solar neighborhood (at distances below 1 kpc 2 ), the mass flow from evolved stars accounts for a significant fraction of the elemental enrichment of the ISM.The elemental composition of the ISM is mostly derived from visible-UV spectroscopy.Observations of main sequence stars provide a census of all the elements in the gas and in the dust available in the ISM to form stars.This census provides the interstellar (IS) abundances.A reference set of abundances is given in [22] for the Solar nebula 3 .Depletions in gas phase abundances due to the presence of grains are estimated from absorption studies of IS clouds and a recent comprehensive review can be found in [17] a e-mail: laurent.verstraete@ias.u-psud.fr 1 The gas density n H is always given in terms of protons because this is the most abundant species.In general one has n H = n(H + ) + n(H) + 2n(H 2 ). 2 1 parsec = 1 pc = 3.1 10 18 cm or 3.3 lightyears. 3In the case of more massive B-stars, different abundances are found ( [26]).This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, which permits unrestricted use, distribution, and reproduction in any noncommercial medium, provided the original work is properly cited.
Article available at http://www.epj-conferences.orgor http://dx.doi.org/10.1051/epjconf/20111801001EPJ Web of Conferences Table 1.Interstellar abundances of elements [22] and depletion factor X for the oph line of sight representative of the cold, diffuse ISM (T ∼100 K and n H ∼100 cm −3 , see [17]).[17] and references therein) as expected from dust formation in stellar winds.Depletion also tends to decrease with the first ionization potential of the element, suggesting that accretion of gas phase elements onto grains occurs in the ISM [29,31].This idea is further supported by the fact that depletion increases with the average density of the gas along the line of sight [17].

Element
The physical conditions (gas density n H and temperature T ) in atomic IS clouds are derived from observations of the hyperfine HI transition (1.4 GHz) and of UV absorption lines.In molecular clouds, the CO rotational transitions (mostly J = 1→0 at 115 GHz) are used to derive physical conditions complemented by other far-infrared to submillimeter emission lines from species such as CS, NH 3 , OH, HCO + , H 2 O, etc... which constrain the chemistry of these clouds.Another important physical quantity is the abundance of electrons or ionized fraction x = n e /n H which is only indirectly constrained by observations of atomic or molecular ion lines (e.g., [6,35]).In the neutral ISM, the main sources of electrons are (i) cosmic ray ionization of H and H 2 and, (ii) ionization by stellar UV photons of heavy elements with first ionization potential below 13.6 eV 4 .Cosmic ray ionization is the dominant source of electrons inside dense (UV-shielded) clouds whereas cloud interfaces are UV ionized (see Le Bourlot this volume).Conditions and mass fractions 5 for the neutral ISM where chemistry can develop are given in Table 2.The WNM and CNM are mostly atomic gas but significant fractions of H 2 may exist within CNM clouds, depending on their UV optical depth [11].The numbers in Table 2 are average properties and it must not be forgotten that the ISM has a complex, scale invariant distribution of structures [24].
The ISM is an open system heated by the radiation of stars, cosmic rays and by local deposition of mechanical energy (stellar winds, shocks or turbulence).The ISM is a unique laboratory because it is highly rarefied (the pressure of the CNM is 10 −15 mbar) and has long excitation timescales (1 UV photon absorbed per year and 1 cosmic ray per 100 Myr).The stellar radiation field is the dominant source of energy (0.5 eV/cm 3 in the Solar neighborhood [23]).On short timescales (less than a few years), a few percents of this energy is transfered to the gas via the photoelectric effect on dust grains.The IS gas is cooled by inelastic collisions of electrons, H (or H 2 ) with ions, atoms or molecules which then radiate  [36].Mass fractions of the total ISM mass are also given. Phase WNM 0.1-1 10 4 − 10 3 ∼0.01 25 CNM 10-100 500-100 away spontaneously the energy stored.In the neutral gas, the typical cooling time 6 is short (<100 yrs) and the gas is often assumed to be in thermal balance on cloud evolution timescales (10 4 yrs or longer for the penetration of UV light or mechanical heating in a shock).Chemical timescales are longer and equilibrium is not generally warranted (see Le Bourlot this volume).

MAIN FEATURES OF INTERSTELLAR CHEMISTRY
In the ISM, chemistry starts with the formation of H 2 and develops in shielded regions where the UV opacity is high enough (corresponding to A V >5).The formation of H 2 is important in the ISM because H 2 absorbs dissociating photons and efficiently cools the gas [19].It has been realized long ago that the main route for H 2 formation is by catalysis at the surface of dust grains.In spite of decades of research in this field, the H 2 formation mechanism and the excitation of the molecule formed are still poorly known, in particular because of our lack of knowledge on the relevant properties of dust grains (see Fillion this volume).
On the other hand, observations of UV absorption lines in IS clouds provide an estimate of the mean H 2 formation rate coefficient per proton R 0 along the line of sight.Early observations with Copernicus [18] and more recent FUSE data indicate that R 0 3 10 −17 cm 3 s −1 in the CNM (the formation rate per unit volume is then n(H) n H R 0 ).Given the dust surface (2 10 −21 cm 2 per H), this number amounts to a formation probability of ∼1/3 for each H atom impinging on a grain.Furthermore, the rate coefficient for H 2 formation is observed to vary from a few tenths to a few times the standard value R 0 [1,10,12,13,32].The rotational emission of H 2 observed in excited interfaces favors formation routes on small carbonaceous grains involving chemisorbed sites for H [3,14] while formation in physisorbed sites may be dominant in the interiors of clouds.
Once H 2 is formed, gas phase chemistry can develop depending on the gas ionization and internal energy.Most important processes are ion-molecule reactions (∼10 −9 cm 3 s −1 , Langevin rate), dissociative recombination (∼10 −7 cm 3 s −1 , an important example is H + 3 + e − → H + H + H), UV photodissociation (5 to 30 times 10 −11 G 0 s −1 per molecule for H 2 and CO respectively 7 ), cosmic ray ionization (∼10 −16 s −1 per proton) and gas-grain interactions.Interstellar chemistry is a complex system involving a few 100 species with a few 1000 reactions (see Le Bourlot this volume) and a recent review of reaction networks can be found in [34].In the ISM, two extreme cases can be distinguished.In dense molecular regions protected from UV radiation or dark clouds (with n H /G 0 ≥10 4 cm −3 ), the primary source of energy is cosmic rays which destroys and ionizes H 2 leading to the formation of the key ion Reactions of H + 3 with C or O then initiate the route to CO or H 2 O. On the other hand, in regions where the gas is heated by stellar UV photons (n H /G 0 <10 2 cm −3 ) or shocks 6 The cooling time is defined as C = 3n H k B T /(2 ) with the gas cooling rate in erg/cm 3 /s. 7G 0 is a scaling factor on the intensity of the standard interstellar 6-to-13.6 eV radiation field, νI ν 4 1.6 10 −3 erg/cm 2 /s.

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(with velocities >20 km/s), H 2 is vibrationally excited allowing reactions with high activation barriers to occur, leading for instance to the formation of CH + and OH.UV heated gas is found at the surface of IS clouds in the so-called photon-dominated regions or PDR.The case of diffuse IS clouds is intermediate in excitation (n H = 10 2 − 10 3 cm −3 and G 0 ∼ 1) but raises specific questions (see the review in [30]).For instance, the unexpected (because of the low H 2 abundance) detection of H + 3 in diffuse molecular clouds suggests a cosmic ray ionization rate much higher than previously thought [20,27], possibly the result of cosmic ray confinement by magnetohydrodynamic (MHD) waves [25].The observed excitation of H 2 in PDRs could also be explained with an enhanced cosmic ray rate [28].Other unexpected species are detected in the diffuse ISM like for instance CH + , OH and HCO + which are formed by reactions with high activation barriers (>1000 K).Since the bulk of the diffuse molecular gas is at temperature below 100 K, this implies a strong and localized gas heating by processes such as MHD turbulence or shocks [5,9].
Another key ingredient of interstellar chemistry are surface reactions and photochemistry occuring on dust grains (see Fillion and Demyk this volume).In addition to H 2 , other molecules may form onto grains such as H 2 O, CO 2 , NH 3 and CH 4 [4].In dark clouds, grains are cold (∼10 to 15 K) and gas-phase species freeze out on their surface forming an ice mantle.On the other hand, cosmic rays and the UV flux they generate within clouds remove molecules off grain surfaces.Recent modeling suggests that these processes may explain the low O 2 and H 2 O abundances observed [16].IS chemistry is in fact sensitive to both surface and gas phase reactions [15].The dust surface chemistry is also important in diffuse clouds where grains are warmer and bare [2].Small grains like polycyclic aromatic hydrocarbons (PAHs) are abundant and easily polarizable: they may thus carry a significant fraction of the available electrons and affect the ion-molecule chemistry [6,33].Charge exchange reactions between grains and gas species may also affect the gas temperature and chemistry.For instance in diffuse clouds PAH anions may induce recombination of C + leading to warmer gas [37] and also quenching reactions with C + .It must be noted however that rates for reactions involving dust grains are poorly known, in particular because the composition and structure of interstellar grains is not known in detail.
The field of interstellar and circumstellar chemistry is undergoing a significant leap forward thanks to the spectroscopic data of the Herschel satellite that covers the far-IR to submm wavelength range.Many interstellar molecules are detected, sometimes for the first time (e.g.H 2 Cl + [21]) challenging our present best chemical schemes and probing the local physical conditions as well as the source of excitation of the gas, by radiation (PDRs) or dynamical motions (shocks, turbulence) (see Gerin this volume).

PRIMORDIAL CHEMISTRY
Chemistry also plays an important role in the young Universe because molecules are very efficient coolants for the gas and will help the collapse of density fluctuations to form stars or galaxies.In the standard cosmological model (also called -CDM, that considers cold dark matter and a recent acceleration of the Universe's expansion), the first structures are formed by the collapse of baryonic matter (∼5% of the mass) in the potential wells of dark matter (∼25% of the mass).Simulations clearly indicate that the mass distribution of the structures thus formed depends on the ability of the baryonic gas to cool down 8 .Specifically, distorsions and anisotropies observed in the cosmic microwave background indicate that the Universe has been reionized at a redhift z∼40 suggesting that stars have formed in primordial gas.At that time the cosmic gas is hot (∼10 4 K) and diffuse (∼7 10 −3 cm −3 ) and only contains the heavy elements synthesized during the big bang, namely He, D and Li (about 6 10 5 , 500 and 10 −4 ppm respectively) in addition to hydrogen.In such a poorly evolved gas, dust grains are absent and chemistry only occurs by gas-phase reactions.The chemical network involves some 20 species (e.g.H 2 , HD, HeH, 8 Simulations disregarding this aspect do not reproduce the mass distribution observed.

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Chemistry in Astrophysical Media, AstrOHP 2010 LiH, H + 3 , H 2 D + and other ions) and about 100 reactions [7].The time dependent reactions set is solved from epochs where H is fully ionized (z ∼ 10 4 ) to present.Molecular hydrogen forms slowly 9 via the following routes: with rate coefficients 1.4 10 −18 × T exp(−T /16, 200) and 10 −9 cm 3 /s respectively or: H + with rate coefficients 10 −17 and 6.4 10 −10 cm 3 /s respectively [7].At z = 40, the H 2 abundance reaches a few ppm but H 2 efficiently cools the gas via excitation of its rotational lines by inelastic collisions with H or e − followed by spontaneous emission.The abundances of certain molecules (HD, HeH + ) is very sensitive to the number of baryons per photon before recombination and may provide a valuable test of some cosmological parameters.
This chemical network can then be used to estimate the gas cooling function.This latter quantity incorporated into numerical simulations provides more realistic predictions for the mass distribution of structures collapsed at z ∼ 40.However, some rates remain uncertain in particular those of H − + H → H 2 + e − and H − + H + → H + H leading to a significant uncertainty on the density of the structures formed [8].

SUMMARY
The diffuse cosmic gas is a unique laboratory for the development of a complex chemistry in conditions of high isolation.In the interstellar medium, reactions involving dust grains are also important.Even when present in small amounts, molecules play an important role in the evolution of the gas because they are efficient coolants (e.g.H 2 , CO, H 2 O): thus they allow the formation and collapse of gravitationally bound structures to form stars or larger objects in the primordial gas.The chemical species observed in the interstellar medium are a powerful probe of the local physical conditions (gas density and temperature) and of excitation mechanisms (UV or X-ray radiation, shocks) in our Galaxy but also in external galaxies, as demonstrated by the scientific analysis of the Herschel data.Reactions involving dust grains remain poorly known in part because of our incomplete knowledge of the detailed nature of dust grains.
In Table1, we present a summary of IS element abundances [X/H ] IS and depletion factors X representative of the cold diffuse ISM.The abundance of element X in dust grains is X [X/H ] IS : one sees that the major dust constituents are C, Si and O as expected from extinction and emission studies of dust indicating the presence of hydrogenated carbon and silicates grains (see Demyk this volume).Depletion of elements into dust grains is known to increase with the condensation temperature of the element (see

Table 2 .
Typical conditions in neutral IS clouds with ionization fraction x < 0.1.CNM and WNM stand for Cold and Warm Neutral medium respectively, MC is for Molecular cloud.Estimates of x are from