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(1) Interstellar listed according to the number of Ehrenfreund & Charnley (2000)

Planets and (2015-2016) G. Vladilo Molecules that contain atoms with low cosmic abundance have a low number of atoms

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Interstellar molecules Molecules with few atoms

• About two hundreds gas-phase molecular species have been detected so far • Detected in diffuse clouds • Besides simple molecules with a few atoms, also complex molecules with a • Complex molecules are absent in relatively large number of atoms have been detected diffuse clouds because of: – physical conditions http://www.astro.uni-koeln.de/cdms/molecules diffuse clouds are less protected from interstellar – Observational bias: radiation field than dense clouds Different types of molecules are observed in different types of – observational limitations interstellar or circumstellar regions diffuse clouds have Some of them are only observed in dense molecular clouds relatively low column densities and this prevents Symmetric molecules are harder to detect: they could be more to detect complex species abundant than what observed which typically have low abundance

2 4 Interstellar Important example of interstellar organic • Aromatic ring – Stable electronic structure that results from the superposition of atomic orbitals; the electrons are delocalized and shared by all atoms • Plays an important role in the ISM – Starting point for the formation of complex aromatic compounds PAHs=Polycyclic Aromatic

Detected multiple rotational transitions in the sub-millimetric spectral range in molecular clouds at different locations in the Galaxy

Adande et al. (2013) 5 7

Complex interstellar molecules Saturation of interstellar organic molecules

• Complex organic molecules are Examples of interstellar • Saturated hydrocarbons found in: – atoms are held by single bonds and are saturated with – star-forming regions • Interstellar organic molecules are characterized by a low degree of saturation – circumstellar envelopes of evolved, late-type stars – Examples: Asymptotic giant branch (AGB) Benzene, C6H6, unsaturated – dense clouds in the direction of detected in the ISM, as well as other hydrocarbons with even lower the Galactic center degree of saturation Herbst & van Dishoeck (2009) , C6H12, saturated not detected in the ISM • Interstellar molecules with a large number of atoms are organic – i.e., based on carbon

Cyclohexane

6 8 Complex organic Complex organic molecules in the ISM: molecules in the ISM tentative evidence for

• Glycine (NH2CH2COOH) – Several emission lines attributed to interstellar glycine have been reported • (CH2OHCHO) Kuan et al. (2003) – First example of interstellar detected in the millimetric band towards Sagittarius B2(N), a source in the direction of the Galactic center (Hollis et al. 2000) – The identification is not confirmed by a subsequent analysis performed by testing a larger number of lines expected for glycine Snyder et al. (2005)

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Complex organic molecules in the interstellar medim Astrobiological interest of glycine

• Glycolaldehyde (C H O ) 2 4 2 Generic for – First example of interstellar sugar • Glycine is the simplest aminoacid found in biological proteins C (H O) n 2 n (NH2CH2COOH) – versus structure – Its existence in the interstellar space would demonstrate the existence of chemical pathways potentially able to synthesise basic ingredients of molecules in the interstellar space – The “lateral group” R is simply a hydrogen

Conventional depiction of aminoacids

10 12 Which is the maximum complexity of Importance of interstellar dust interstellar organic molecules in the gas phase?

• Effects on astronomical observations As molecular complexity increases, the identification of the – Reddening and extinction of astronomical sources molecule tends to become uncertain – Depletion of chemical abundances in the interstellar gas

Gas-phase molecules with a high number of atoms could be present in the , even though it is difficult to • Physical effects in the interstellar medium prove their existence – Transformation of UV photons into IR photons – Cooling of the ISM by means of thermal emission – Catalyist for the formation of interstellar molecules

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Interstellar dust Interstellar dust The solid phase component of the ISM

Atomic nuclei ⇓ Observational evidence Atoms ⇓ Sky maps in the optical band Molecules Reflection nebulae Infrared Interstellar abundances Dust Extinction curves

Hard to make a sharp distinction between long molecules and small dust grains

14 16 The Horsehead Nebula Sky maps in the optical band Reflection nebulae as a diagnostic tool of dust properties

• Dark regions – Absence of stars in large field images – Dust grains absorb the optical/UV light of • Dust grains in reflection nebulae scatter the background stars stellar photons

• A detailed study of reflection nebulae provides information on some physical properties of the grains – Albedo Ratio between scattering and extinction cross-sections of dust grains – Phase function Angular distribution of scattered light

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Dark clouds Galactic infrared emission

In some cases, dark clouds are associated with pre-stellar cores • Evidence for thermal emission from interstellar dust – Dust is heated by interstellar radiation – The infrared emission cools the gas • Galactic infrared emission maps the distribution of interstellar dust Dark cloud B68 ESO-VLT Alves et al. (2001) 1983, IRAS satellite – All sky map in the bands at12, 25, 60 e 100 µm – The emission is concentrated in the Galactic plane – IR clouds (cirrus) found outside the Galacic plane

Interstellar dust is an ingredient of the nebula from which star and planetary system form

Interstellar dust is heavily reprocessed during the epochs of stellar Composite mid-and far-infrared intensity observed in the 12, 60, and 100 µm wavelength bands. and planetary formation Mosaic of IRAS Sky Survey Atlas images. Emission from interplanetary dust in the solar system, the "zodiacal emission,” was modeled and subtracted.

18 20 Elemental abundances and depletions Infrared absorptions in the interstellar medium

The interstellar (resonance) ISO SWS spectrum in the mid- • Observations of background sources with transitions of the main ionization strong IR emission along lines of sight IR (2.4 to 45 µm) towards the stages of the most abundant interstecting dust-rich regions young stellar cluster NGC7538 astrophysical elements are found in – Vibrational bands of solid compounds are IRS9 embedded in a molecular detected cloud (Whittet et al. 1996) the UV range • Ice and organic compounds

– H2O, CO, CO2, CH3OH … • The measurements of interstellar • Silicates abundances with high resolution UV – 9.7 µm e 18 µm spectroscopy indicate that: Stretching vibration modes of SiO bonds – For most elements the interstellar and bending vibration modes OSiO abundances X/H, measured in the modes, respectively gas phase, are lower than the Examples of silicates corresponding solar abundances Pyroxenes: Mg Fe SiO – This deficiency is known as x (1-x) 3 “interstellar depletion” Olivines: Mg2yFe2(1-y)SiO4 21 23

Interstellar depletions

– Interpretation Vibration modes of a fraction of the atoms is incorporated in dust grains and, as a result, is interstellar solids: not counted in the gas-phase column density measurements ice and dust – Galactic interstellar depletions are calculated assuming that the total abundance of the interstellar medium (gas plus dust) is solar

δX = log10 (NX/NH ) log10 (X/H)sun A high spectral resolution is required to discriminate This expression is equal to the definition of [X/H], but the physical between different types of meaning is completely different silicates • Interstellar depletions vary Pyroxenes: MgxFe(1-x)SiO3 – between different elements

Olivines: Mg2yFe2(1-y)SiO4 – in different types of interstellar regions

22 24 Element-to-element variations of interstellar depletions High resolution X-ray absorption spectroscopy

• Refractory elements • High-resolution X-ray absorption spectroscopy of bright Galactic – Strong depletions sources provides us with a powerful technique for constraining the e.g., Ti, Ni, Fe, Cr, Mn, .. properties of the ISM • Volatile elements • Unlike UV absorption lines, one can measure the metal abundances – Weak depletions in both the gas and the dust phase from the X-ray edge depth, e.g., S, Zn leading to a direct determination of the ISM abundances (gas plus dust) based on simple physics • Correlation between depletion and • Example of application: if most of the iron is depleted, as we deduce and condensation temperature from gas-phase observations in the UV, we should find most of iron in dust. The total abundance, measured in the X-ray, should be – Empirical evidence that Condensation temperature approximately solar, testing the validity of the assumption supports the interpretation of Temperature at which the 50% of an element depletions in terms of condenses to a solid compound in a cooling gas • In future observations at high spectral resolution, X-ray absorption incorporation of a fraction of of solar fine structure can be used as a diagnostic of the chemical state of an element elements in dust form See Lodders K, 2003, ApJ, 591, 1220

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Variations of depletions in different • Cold and dense clouds types of interstellar regions – Strong depletions • Warm and hot gas – Weak depletions

• Further evidence that depletions are due to the incorporation of atoms in dust form – Dust grains survive (or grow by accretion) in cold and dense clouds – Dust grains tend to be destroyed in hot, low density regions

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