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6 Origin of Organic Matter: Interstellar Medium 6 Origin of organic matter: Interstellar medium The interstellar medium (ISM) plays a vital role in the evolution of galaxies: The most important aspect of Galactic ecology is probably the cycle of matter from the ISM to stars 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 densities and temperatures from 10K in cool molecular clouds to million degree hot bubbles. In the context of Astrobiology 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 star and planet formation. Of particular interest for life is the presence of large organic molecules 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 Universe) 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 mass of the Galactic disk. It tends to concentrate near the Galactic plane and along the spiral arms, while being very inhomogeneously distributed at small scales. It consists mainly of gas (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 hydrogen (either molecular or atomic) o 28.1% is helium o traces of other elements, mainly C, N, O neutral atoms and molecules, ions and electrons average density: 1 particle cm-3 (varies from 10-2 to 106 cm-3 ). In comparison: air on Earth ~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 winds from early type stars. Very tenuous and pervasive. Fills large parts of the galactic halo. 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 wavelengths. The 21 cm emission line of the neutral hydrogen is used to trace the distribution of HI regions. It is due to hyperfine structure: 1. changing the alignment of the electron spin relative to the nuclear spin from to by collision (excitation) 2. emitting the photon 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 extinction, i.e. continuum absorption and scattering of starlight scattering starlight while producing diffuse light in the Galaxy depletion of metals (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 (ice), Fe Density: 1 grain per cubic football field (500,000 m3) In the Milky Way, 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 molecule formation and efficient coolant). Because the shape of dust grains is often elongated (e.g. needles) and because the galactic magnetic field 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, Orion): 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 cosmic dust, 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 atom or molecule gains the energy from the kinetic energy 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/ion 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 photons, so the cloud can become optically thick for CO photons.
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