MOLECULAR COLLISIONS IN THE INTERSTELLAR MEDIUM

By SARAH BROWN

CONTENTS INTERSTELLAR MOLECULES INTRODUCTION CHEMISTRY IN INTERSTELLAR CLOUDS CHEMICAL BISTABILITY IN DENSE CLOUDS INTERSTELLAR SHOCKS AND CHEMISTRY THE MHD CONSERVATION EQUATIONS THE STRUCTURE OF INTERSTELLAR SHOCK WAVES SHOCK WAVES IN DARK CLOUDS SHOCK WAVES IN DIFFUSE CLOUDS THE PRIMORDIAL GAS THE GOVERNING EQUATIONS THE ROLE OF MOLECULES CHEMISTRY GRAVITATIONAL COLLAPSE THE ROTATIONAL EXCITATION OF MOLECULES THE BORN–OPPENHEIMER APPROXIMATION THE SCATTERING OF AN ATOM BY A RIGID ROTATOR THE ROTATIONAL EXCITATION OF NON-LINEAR MOLECULES THE VIBRATIONAL EXCITATION OF LINEAR MOLECULES THE SCATTERING OF AN ATOM BY A VIBRATING ROTOR EXCITATION OF H2 AND HD IN COLLISIONS WITH H2 MOLECULES COOLING FUNCTIONS THE EXCITATION OF ?NE STRUCTURE TRANSITIONS THEORY OF ?NE STRUCTURE EXCITATION PROCESSES RADIATIVE TRANSFER IN MOLECULAR LINES THE RADIATIVE TRANSFER EQUATION THE OH RADICAL PRODUCING POPULATION INVERSION CHARGE TRANSFER PROCESSES THE LANDAU–ZENER MODEL THE ‘ORBITING’ MODEL THE QUANTUM MECHANICAL MODEL SELECTIVE POPULATION OF EXCITED STATES ELECTRON COLLISIONS SELECTION RULES AND LS-COUPLING ELECTRON COLLISIONAL EXCITATION RESONANCES FORBIDDEN LINE EMISSION FROM HERBIG–HARO OBJECTS 10 PHOTON COLLISIONS 10 1 INTRODUCTION 10 2 THE OSCILLATOR STRENGTH 10 3 THE TRANSITION PROBABILITY 10 4 PHOTOIONIZATION AND RADIATIVE RECOMBINATION 10 5 RADIATIVE TRANSITIONS IN MOLECULES REFERENCES INTRODUCTION OUR PERCEPTION AND UNDERSTANDING OF THE INTERSTELLAR MEDIUM HAVE BEEN TRANSFORMED OVER THE LAST APPROXIMATELY 50 YEARS BY THE 1950S, OPTICAL AND 21 CM RADIO MEASUREMENTS HAD DEMONSTRATED THE PRESENCE OF GAS CONTAINING PREDOMINANTLY ATOMIC HYDROGEN, TOGETHER WITH A FEW ATOMS AND IONS (NA, K, CA+, TI+, FE) THAT HAD ABSORPTION LINES IN THE VISIBLE PART OF THE SPECTRUM THE GAS WAS COLD (T ?100 K) AND HAD A NUMBER DENSITY N(H) ?1 CM?3 ‘MOLECULES’WERE REPRESENTED BY THE RADICALS CH, CH+ AND CN, WHICH HAVE ABSORPTION BANDS IN THE VISIBLE A DUST COMPONENT WAS KNOWN TO BE PRESENT, MOST DIRECTLY FROM ITS OBSCURATION OF STARLIGHT OVER THE LAST HALF-CENTURY, THE CONSTRUCTION OF NEW TELESCOPES, BOTH GROUND-BASED AND ON BOARD SATELLITES, AND RAPID DEVELOPMENTS IN RECEIVER TECHNOLOGY HAVE LED TO OVER MOLECULAR SPECIES BEING IDENTI?ED IN THE INTERSTELLAR MEDIUM (SEE TABLE 1 1) FOREMOST AMONG THE MOLECULAR SPECIES IS THE MOST ABUNDANT MOLECULE IN THE UNIVERSE, H2 MOLECULAR HYDROGEN WAS ?RST DETECTED IN ABSORPTION IN THE ULTRAVIOLET PART OF THE SPECTRUM, THROUGH ITS X1+ G – B1+ U LYMAN ELECTRONIC BANDS, BY MEANS OF A SOUNDING ROCKET [1] IT HASSINCEBEENOBSERVEDATMUCHHIGHERSPECTRALRESOLUTIONBYTHECOPERNICUS[2] ANDTHEFUSE [3] SATELLITES H2 HAS ALSO BEEN OBSERVED IN THE FAR INFRARED BY THE INFRARED SPACE OBSERVATORY (ISO) SATELLITE [4], AND THROUGH ATMOSPHERIC WINDOWS IN THE NEAR INFRARED THUS, ELECTRONIC, ROTATIONALANDROVIBRATIONALTRANSITIONSOFH2 HAVEALLBEENOBSERVEDINTHEINTERSTELLARMEDIUM COLD H2, AT T ?< 30 K, MAPS THE MOLECULAR CLOUDS IN GALAXIES BUT REMAINS DIF?CULT TO OBSERVE, BEING DETECTABLE ONLY BY MEANS OF ULTRAVIOLET ELECTRONIC ABSORPTION LINES IN THE LYMAN AND WERNER (X1+ G – C1U) BANDS FROM ABOVE THE EARTH’S ATMOSPHERE FURTHERMORE, A BACKGROUND CONTINUUM ULTRAVIOLET SOURCE, SUCH AS A HOT STAR, IS NECESSARY FOR SUCH OBSERVATIONS AS SCATTERING AND ABSORPTION BY DUST ARE PRONOUNCED AT SHORT WAVELENGTHS, SUCH OBSERVATIONS ARE LIMITED BY THE EXTINCTION ALONG THE LINE OF SIGHT THROUGH THE MOLECULAR CLOUD AND TOWARDS THE STAR CONSEQUENTLY, CO, WHICH IS THE NEXT MOST ABUNDANT INTERSTELLAR MOLECULE AFTER H2 [N(CO)/N(H2) ?10?4], HAS BEEN USED EXTENSIVELY AS A TRACER OF MOLECULAR MATERIAL CO IS READILY OBSERVABLE FROM THE GROUND AT MILLIMETRE WAVELENGTHS; ITS LONGEST WAVELENGTH ROTATIONAL TRANSITION, J = 1 ?0, FALLS AT 2 6 MM INDEED, RADIATIVE TRANSITIONS BETWEEN THE LOW-LYING ROTATIONAL LEVELS OF CO ARE MAJOR CONTRIBUTORS TO THE COOLING OF MOLECULAR CLOUDS IN THIS INITIAL CHAPTER, MY AIM IS TO SUMMARIZE KEY ASPECTS OF OUR CURRENT UNDERSTANDING OF THE CHEMISTRY OF THE INTERSTELLAR MEDIUM WE SHALL CONSIDER ‘DENSE’ (OR ‘DARK’) MOLECULAR CLOUDS, WHOSE INTERIORS ARE SHIELDED FROM THE GALACTIC ULTRAVIOLET RADIATION ?ELD BY DUST ABSORPTION IN THE OUTER LAYERS THIS BACKGROUND RADIATION ?ELD OWES ITS EXISTENCE TO HOT (E G O AND B) STARS THAT EMIT IN THE ULTRAVIOLET ALTHOUGH THOSE PHOTONS BEYOND THE H I CS H+ HC7N CH2OHCHO C8H CH2CHCHO LYMAN LIMIT AT 91 2 NM ARE ABSORBED, BY ATOMIC HYDROGEN, IN THE IMMEDIATE VICINITIES OF THE HOTSTARS,PHOTONSOFLONGERWAVELENGTHSPROPAGATEINTOTHEGENERALINTERSTELLARMEDIUM,WHERE THEY ARE ULTIMATELY ABSORBED BY EITHER THE DUST OR THE GAS ACCORDINGLY, WE SHALL ALSO CONSIDER ‘DIFFUSE’ (OR ‘TRANSLUCENT’) CLOUDS, WHICH ARE TRAVERSED BY THIS RADIATION ?ELD THE DISCUSSION WILL CONCENTRATE ON REACTIONS IN THE GAS PHASE, ALTHOUGH IT IS CLEAR THAT GRAIN-SURFACE REACTIONS ARE SIGNI?CANT IN THE PRODUCTION OF SOME SPECIES, NOTABLY H2 INDEED, REACTIONS BETWEEN H ATOMS ON THE SURFACES OF GRAINS ARE THE ONLY EFFECTIVE MEANS OF FORMING MOLECULAR HYDROGEN IN THE INTERSTELLAR MEDIUM 1 2 CHEMISTRY IN INTERSTELLAR CLOUDS