Astrochemistry KEM376, 5 Credits Master’S Programme in Chemistry and Molecular Sciences Spring 2019, Department of Chemistry

Home , Photodissociation region

Astrochemistry KEM376, 5 credits Master’s programme in Chemistry and Molecular Sciences Spring 2019, Department of Chemistry

Lecture 2: Conditions and processes in interstellar clouds 23.01.2019 Gas composition of the Milky Way galaxy

11 I Total baryonic mass of the Galaxy is ∼ 10 M , of that ∼ 7% is gas, the rest is stars 12 (the mass of the dark matter halo is ∼ 2 − 4 × 10 M ) 6 I Ionized component: Very diffuse ionized gas at 10 K, denser component at 104 K Hot ionized component ﬁlls about 1/2 of the volume, but contains only 0.25% of the mass I Phases of neutral gas −3 -Warm neutral gas: nH ∼ 0.6 cm , T ∼ 6000 K, 1/4 of the mass −3 -Cool neutral gas: nH ∼ 30 cm , T ∼ 100 K −3 -Diffuse gas: nH ∼ 100 cm , T ∼ 80 K 3 7 −3 -Molecular gas: nH ∼ 10 − 10 cm , T ∼ 10 − 300 K The three densest components contain 3/4 of the mass but occupy only about 1% of the interstellar volume Pressure conﬁnement of interstellar clouds From Yamamoto: Introduction to Astrochemistry (Springer 2017)

Interstellar clouds lie along the line p/k = nT = 103 K cm−3 Exceptions: gravitationally bound dense clouds, hot cores, HII regions Types of molecular regions According to Williams et al.: Dynamical Astrochemistry (Cambridge 2018)

−3 Type nH (cm ) T (K) Form of H Characteristic molecule 2 Diffuse 10 30 − 100 H, H2 H2 (UV) 3 Translucent 10 15 − 20 H2, H CO ∗ 4 Dense PDR 10 50 − 500 H2, HH2 (IR) 4 Cold dense 10 10 − 20 H2 CO 5 Pre-stellar core 10 8 − 15 H2 NH3 Protostellar envelope 4 7 -Cold outer 10 − 10 8 − 100 H2 H2CO 7 9 -Warm inner 10 − 10 100 H2 HCOOCH3 4 5 Strong shock 10 − 10 1000 H2, H SiO Protoplanetary disc 6 10 -Outer 10 − 10 10 − 500 H2 H2CO 9 15 + -Inner 10 − 10 100 − 3000 H2 N2H Evolved CSE∗ 8 -Outer 10 10 − 100 H2 HC3N 10 13 -Inner 10 − 10 100 − 2000 H2 HCN ∗PDR = photodissociation region, CSE = circumstellar envelope Galactic interstellar medium (ISM)

-Ionized and neutral gas (mainly hydrogen), dust (∼ 1% of the mass) 10 -Fraction of the baryonic mass of the Milky Way ∼ 7% (≤ 10 M ) -The different components can be distinguished in images, e.g. Hα, radio continuum, HI (21-cm), mid- and far-infrared Interstellar clouds (1)

I The ISM is highly turbulent and has structures at various scales I The relatively dense matter can be roughly divided into three cloud categories: -HII regions (H+), -diffuse or neutral hydrogen clouds (H) -molecular clouds (H2)

HII regions are ionized by UV radiation from O- ja B-type stars. 4 Masses up to 10 M , electron 5 −3 density 1 < ne < 10 cm Red glow caused by the Hα line HII regions attenuate EUV photons with E > 13.6 eV (λ > 912 Å, Lyman continuum)

M83, Hubble WFC3 Hydrogen spectrum Diffuse clouds

I Diffuse molecular clouds: H neutral, UV ﬁeld attenuated so that H2 can survive −3 I T ∼ 30 − 100 K, nH ∼ 100 − 500 cm I Atoms with ionization potentials lower than 13.6 eV mostly singly ionized: C+,S+,P+, whereas N and O neutral − + − −4 I High ionization fraction n(e ) ∼ n(C ), n(e )/nH ∼ 10 , ions destroyed by electron recombination Photodissociation of H2 (I) + I Ionization energy of H2 (to from H2 ) is 15.4 eV, and it is therefore not ionized in regions shielded by neutral hydrogen (with Ei = 13.6 eV)

I Because H2 has no electric dipole moment, neither radiative ∗ association (H + H → H2 → H2 + γ) nor direct photodissociation ∗ (H2 + γ → H2 → H + H) do not work. Photodissociation of H2 (II)

I Dissociation of H2 can occur through excited electronic states, followed by decay to unbound vibrational states of the ground 1 + electronic state X Σg . In most cases, however, the molecule decays to bound states. 1 + 1 + Lyman band: X Σg → B Σu (Emin = 11.2 eV) 1 + 1 Werner band: X Σg → C Πu (Emin = 12.3 eV) Dissociation reaction:

1 + 1 + 1 H2(X Σg ) + γ → H2(B Σu or C Πu) → 2H(1s1/2) + γ The de-excitation and dissociation will emit UV radiation. Decay to vibrationally excited states produces also infrared radiation (IR ﬂuorescence).

I H2 becomes self-shielding at column densities 14 −2 N(H2) > 10 cm , where Lyman and Werner band photons are already absorbed. Molecular clouds

I Molecular clouds: Composed almost entirely ofH 2 3 −3 I T ∼ 10 − 20 K, nH2 ≥ 10 cm . Contain gravitationally bound condensations, “dense cores”, and young stars formed or forming in these. I Carbon neutral or molecular: C → CO − −7 I Low ionization n(e )/n(H) ≤ 10 , ionization dominated by cosmic rays

The ρ Oph cloud, Spitzer infrared camera, blue 3.6 µm, green 8µm, red 24 µm Photodissociation regions

Orion Bar (Goicoechea et al. 2016, Nature, VLT/MUSE) - red: HCO+ (extended molecular cloud), green: S+ (hot ionized gas), blue: O (ionization front) Photodissociation region (PDR, photon dominated region) is a transition layer between atomic and molecular gas PDR: at the edge of a molecular cloud (1)

-Hydrogen becomes completely molecular at visual extinctions of 21 −2 AV ∼ 1 − 2 (NH ∼ 2 − 4 × 10 cm ) -The self-shielding of H2 and the interstellar dust contribute to the sharp transition H → H2 (dissociation is possible only via certain absorption lines, formation of H2 on grain surfaces) PDR: at the edge of a molecular cloud (2)

Variation of chemical abundances at the edge of a diffuse cloud model radiated by the average interstellar radiation ﬁeld (ISRF), Gerin, Neufeld & Goicoechea 2016 The model has a constant pressure p/k = 4 × 103 K cm−3 Main carbon and nitrogen hydrides are shown Note on extinction

I The extinction of light in the interstellar space is caused by dust particles. Gas and dust are assumed to be well mixed. I Extinction in magnitudes at wavelength λ is deﬁned by

Iλ Aλ = −2.5 lg , Iλ,0

where Iλ is the observed brightness, Iλ,0 is the intrinsic −Aλ/2.5 brightness, lg = log10. In other words, Iλ = Iλ,0 10 I Optical thickness τλ is deﬁned by

−τλ Iλ = Iλ,0 e .

I Relationship between Aλ and τλ:

Aλ = 2.5 lg e τλ ≈ 1.086 τλ

I Usually the following relationship between AV (visual extinction) and the total hydrogen column density NH is assumed: N H ≈ 1.8 × 1021 cm−2 mag−1 AV Heating

I Photoelectric effect: Removal of electrons from dust grains (including PAHs) by radiation; important at cloud edges mag with AV < 2

I Photoionization of C( Ei = 11.2 eV), minor effect at cloud edges

I Collisional de-excitation of H2: Molecules excited by Lyman and Werner band photons can decay to vibrational levels. The kinetic energy is transferred to gas through collisions; cloud edges

I Cosmic rays - high energy protons, atomic nuclei, electrons, positrons, photons (from stars, supernovae, shocks, AGN) produce ions and excited molecules that interact with the rest of the gas; main source of heating in the interiors of molecular clouds Cooling

I Cooling converts kinetic energy (thermal motion) to radiation which escapes the system I Fine structure lines of abundant ions and neutral atoms, such as C+, Si+, C, O, are important near cloud edges. − The lines are excited in collisions with e , H, or H2. (Fine structure: spin-orbit coupling splits an electronic level into a triplet) Species Transition ∆E/k (K) λ (µm) 3 3 C P0 → P1 24 609 3 3 C P1 → P2 39 370 + 2 2 C P3/2 → P1/2 92 158 3 3 O P0 → P1 99 146 3 3 O P1 → P2 228 63.2 + 2 2 Si P3/2 → P1/2 413 34.8 I Rotational lines of molecules, especially CO dominate cooling in molecular clouds. Minor coolants: OH, CH, HD I Thermal dust emission becomes important in dense clouds 5 −3 (n(H2) ≥ 10 cm ) where CO and other common species accrete onto grains Thermal balance

Heating, Γ Cooling, Λ

HII region photoionization - electronsO +,O++ “forbidden” lines HI cloud photoelectric effect - electronsC +, O, Si+ ﬁne structure lines + + + H2 cloud edges photoelectrons,C , O, Si , Fe ,C UV-excited H2, warm dust H2 cloud cosmic particles, CO, secondary electrons thermal dust emission

In thermal equilibrium Γ = Λ - the cloud settles to a kinetic temperature determined by the density, composition, and external radiation ﬁeld

HI clouds can have a wide range of densities and temperatures, so that the pressure is roughly constant, nT ∼ 1000 cm−3 K. The efﬁciency of the photoelectric heating is Γ ∝ n, and the efﬁciency of line cooling is Λ ∝ n2, so the temperature decreases with density. Cosmic ray ionization (I)

I Cosmic ray protons( E ∼ 100 MeV) penetrate easily through 24 −2 −2 thick clouds (NH . 10 cm , Σ . 2 g cm ). I Collisions of cosmic rays with H2 or He are the principal source of ionization in molecular clouds:

+ − H2 + c.r. → H2 + e + c.r. (86%) + − H + H + e + c.r. (4%) H + H + c.r. (10%)

He + c.r. → He+ + e− + c.r.

I The ionization rate of H2 by cosmic rays is parametrized by a −3 −1 −17 −16 −1 coefﬁcient: ζn(H2) cm s , where ζ ∼ 10 − 10 s I The degree of ionization is typically − − −8 −7 X(e ) = [e ]/[H2] ∼ 10 − 10 in molecular clouds. Cosmic ray ionization (II)

I The products of cosmic ray ionization initiate interstellar chemistry + + + I H2 reacts immediately: H2 + H2 → H3 + H + (H3 is a universal proton donor) + I He important for dissociative ionization, e.g., He+ + CO → C+ + O + (destruction of He in collisions with H2 inhibited)

Cosmic-ray induced chemistry according to Marco Padovani, INAF, Firenze The scene of intestellar chemistry (I)

I The gas is almost pure hydrogen H, Helium (He) constitutes 28% of the mass, or 1 He atom per 10 H atoms I The mass fraction of heavier elements is less than one percent (number densities < 1/10000 times that of H) The most common elements in the gas phase: H, He, O, C, Ne, N, S I Solid or amorphous particles (“dust”) contain about 1% of the mass, typical size ∼ 0.1µm Dust particles bind almost all Si, Mg, and Fe, and serve as catalytic surfaces for chemical reactions The scene of intestellar chemistry (II)

I The temperatures and densities in neutral and molecular interstellar clouds are low -relative abundances of molecules are far from thermodynamic equilibrium, determined by reaction kinetics -three-body reactions unlikely -reactions like A + B → AB may only occur on grain surfaces or with photonic stabilization − −8 −7 I The degree of ionization is low, X(e ) ∼ 10 − 10 , sustained by cosmic ray ionization Main ions are atomic species (diffuse clouds) and protonated species (molecular clouds) I Low temperatures favour exothermic reactions with no or small activation energies: radical reactions, ion/electron chemistry