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Molecules in Space Molecules in Space J. Tennyson Volume 3, Part 3, Chapter 14, pp 356–369 in Handbook of Molecular Physics and Quantum Chemistry (ISBN 0 471 62374 1) Edited by Stephen Wilson John Wiley & Sons, Ltd, Chichester, 2003 Chapter 14 Molecules in Space J. Tennyson University College London, London, UK The molecular composition of space is an active research 1 Introduction 1 area. This summary draws heavily on two multi-author (2) 2 Molecular Hydrogen 1 books The Molecular Astrophysics of Stars and Galaxies Molecular Processes in Astrophysics: Probes and Pro- 3 The Interstellar Medium 2 and cesses,(3) which provide much more details than can be 4 The Early Universe 7 accommodated here. 5 Circumstellar Processes 8 6 Atmospheres of Cool Stars 9 7Comets 11 2 MOLECULAR HYDROGEN 8 Summary 11 Notes 12 The Universe is 95% atomic hydrogen. This means that in References 12 nearly all molecular environments, molecular hydrogen is the dominant molecule. Thus, nearly all chemistry occurs in a hydrogen-rich or reducing environment, our own planet 1 INTRODUCTION and our two nearest neighbours being obvious exceptions. The preponderance of molecular hydrogen places a spe- (4) The stars that twinkle brightly in the night sky are com- cial emphasis on both its chemistry and its spectroscopy. posed of ionized, often highly ionized, atoms. For many Indeed, the formation of hydrogen molecules in astronom- years, astronomical spectroscopy concentrated on atoms ical conditions has been a long running puzzle. In the and atomic ions. Much of astrophysics still concerns itself interstellar medium (ISM), which is discussed further in with atomic processes. However, the increased sensitivity the next section, even the so-called dense molecular clouds of telescopes and the opening up of other wavelengths than correspond to what, on Earth, would be considered an ultra- the visible has led to burgeoning of activity in molecular high vacuum. Typically, they contain a few tens of thousand astrophysics. In particular, longer wavelengths such as the particles per cubic centimetre and are cold with tempera- infrared and radio are sensitive to molecular transitions. tures in the range 10 to 100 K. Under these conditions, the Observations at these wavelengths has led to the realiza- chances of three-body gas-phase reactions occurring are so tion that the Universe is substantially molecular. Indeed, small that they can be neglected. This leads to an entirely although the discussion in the following text is largely con- new chemistry. fined to molecules in our own galaxy, the list of molecules Using only two-body reactions, it is difficult to form detected in external galaxies is growing rapidly.(1) molecules directly from atoms: usually, a third body is required to carry off the excess energy. It is possible for this excess energy to be carried off by a photon, result- Handbook of Molecular Physics and Quantum Chemistry, Edited by Stephen Wilson. Volume 3: Molecules in the Physico- ing in a process called radiative association. However, chemical Environment: Spectroscopy, Dynamics and Bulk Proper- radiative association is an extremely inefficient process and ties. 2003 John Wiley & Sons, Ltd. ISBN: 0-471-62374-1. even on astronomical timescales is rarely a major route for 2 Molecules in different environments neutral molecule formation. It is now generally accepted transitions have been observed in the star-forming regions that in dense molecular clouds, molecular hydrogen forms of Orion and elsewhere.(7) In practice, astronomical pure on the surface of grains.(5) There is evidence that micro- rotational spectra probe significantly higher levels of rota- scopic grains, usually referred to as dust, are widespread tional excitation than what has so far been directly observed in cool astronomical environments. It is thought that these in the laboratory.(8) particles are largely silicates, possibly with coatings of ice. Quadrupole transitions are very weak, typically a billion However, the need to invoke dust to initiate molecule for- times weaker than standard dipole transitions. However, the mation process raises the question of how molecules and density of H2 is often many billion times more than other hence grains formed in the first place. The early Universe, species that are routinely detected. Of course, hydrogen is the chemistry of which is discussed in Section 4, contains light, which means that its rotational spectrum cannot be essentially no elements beyond lithium. The heavier ele- seen at radio frequencies. Instead, these low-lying lines lie ments that form into grains in our galaxy have therefore in the far-infrared and higher transitions can be observed been through at least one cycle of stellar processing in using ground-based telescopes in the mid-infrared through which hydrogen is burnt in nuclear reactions and heav- ‘windows’ in the Earth’s atmosphere.(7) ier elements are produced. As molecule formation appears The electronic emission spectra of molecular hydrogen to be a necessary precondition for star formation, how can also be observed from active environments. Typical is were hydrogen molecules formed before there were any the ionosphere of gas giants such as Jupiter where fast elec- grains? This question remains an active area of research. trons, accelerated along magnetic field lines, collide with Figure 1 presents the dominant reactions linking the gas- the atmospheric gas that is largely H2. The resulting auro- phase forms of hydrogen. Included in this scheme are ral emissions can be observed; the Hubble Space Telescope methods of forming molecular hydrogen from hydrogen has been used to record a number of particularly beautiful (9,10) atoms. As can be seen, this involves excited H atoms in and detailed images using these emissions. H2 fluores- their n = 2 state. Note that the 2s state of atomic hydrogen cence can also be observed from shocked(11) and diffuse(12) is metastable or going via the weakly bound negative ion regions of the ISM, both of which are discussed in the next H−.(6) section. Direct detection of extraterrestrial hydrogen molecules relies on spectroscopy. Radio astronomy and more recently infrared telescopes have been the driving forces that have 3 THE INTERSTELLAR MEDIUM characterized the molecular nature of the Universe. As a homonuclear diatomic, the absence of permanent or vibra- The vast regions of space between the stars are largely tional induced dipoles means that standard means of molec- empty. However, in this, near-vacuum matter still aggre- ular detection based, in particular, on strong rotational gates to form clouds that are the major feature of the ISM. transitions should not apply to hydrogen. However, there is so much hydrogen present in the Universe that in fact + HO its weak, quadrupole spectrum is well known. Quadrupole 2 + OH HCO H2 O H2 hν H+ e − H H− H hν e− + + HCO2 H3 CO2 − − H2 e − e H(n = 2) H e H H− hν H2 H2 + OH OH CO + H2 H H e− hν O2 H(n = 2) H = H hν H(n 2) + + H2O H CO + hν e H H H + O H2 + H3 OH H H + 2 C + Figure 1. Key two-body reactions involved in the gas-phase Figure 2. Chemical pathways starting from H3 and involving production of a molecule of molecular hydrogen.(6) oxygen molecules in diffuse interstellar clouds.(14) Molecules in space 3 These clouds are largely molecular.(5) The chemistry of the the following text, the clouds are not entirely neutral. This giant clouds that contain most of the matter in the ISM is because there are some low ionization potential species is now substantially established.(5,13) In discussing this, it present, particularly atomic carbon, and also because very is necessary to distinguish between two proto-typical types energetic γ rays, usually called cosmic rays,dopenetrate. of clouds: the so-called dense clouds that contain 10 000 Ionization of molecular hydrogen by cosmic rays is fol- + or more particles per cubic centimetre and diffuse clouds lowed rapidly by formation of the triangular H3 molecule with typical densities of less than a 1000 particles per cubic centimetre. + + H + H −−−→ H + H (1) Dense molecular clouds are the main molecule-forming 2 2 3 regions, and their chemistry is well documented.(5) These clouds contain sufficient material to become self-shielding This exothermic reaction occurs at essentially every colli- + from ultraviolet radiation. That is, any photons of energy sion. H3 is a strong protonating agent and the starting point greater than the ionization potential of atomic hydro- for much of interstellar chemistry. Figure 2 gives an exam- gen, 13.65 eV, are absorbed. This means that molecules, ple of the reaction networks that can follow the formation + including molecular hydrogen, whose ionization potential of H3 . As can be seen from the large array of molecules is greater than that of H do not get ionized. However, as that have been observed in the ISM, (see Tables 1 and 2), will be discussed when considering reaction schemes in the reaction networks become extensive. Table 1. Neutral molecules identified in the ISM and in circumstellar environments up to the end of year 2000. Molecules marked with an asterisk have only been detected in the circumstellar envelopes of carbon-rich stars. Unconfirmed detections are denoted with a ?. This tabulation is based on one by Wootten [1] which is regularly updated. Molecules with two atoms ∗ ∗ ∗ AlF AlCl C2 CH CN CO CP CS ∗ ∗ ∗ CSi HCl H2 KCl NH NO NS NaCl OH PN SO SiN∗ SiO SiS HF SH SiH Molecules with three atoms C3 C2HC2OC2SCH2 HCN HCO H2O H2S HNC HNO MgCN MgNC N2ONaCNOCS ∗ SO2 c-SiC2 CO2 NH2 SiCN Molecules with four atoms c-C3Hl-C3HC3NC3OC3SC2H2 HCCN HNCO HNCS H2CO H2CN H2CS NH3 SiC3 CH3 Molecules with five atoms ∗ ∗ C5 C4HC4Si l-C3H2 c-C3H2 CH2CN CH4 HC3N ∗ HC2NC HCOOH H2CHN H2C2OH2NCN HNC3 SiH4 Molecules with six atoms ∗ + C5HC5OC2H4 CH3CN CH3NC CH3OH CH3SH HC3NH HC2CHO HCONH2 l-H2C4 C5N Molecules with seven atoms C6HCH2CHCN CH3C2HHC5N HCOCH3 NH2CH3 c-C2H4O Molecules with eight atoms CH3C3N HCOOCH3 CH3COOH? CH2OHCHO H2C6 C7H Molecules with nine atoms CH3C4HCH3CH2CN (CH3)2OCH3CH2OH HC7NC8H Molecules with ten atoms CH3C5N? (CH3)2CO NH2CH2COOH? Molecules with eleven atoms HC9N Molecules with thirteen atoms HC11N 4 Molecules in different environments Table 2.
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