Koninginnedag 2008Astro 101-8/Astrochemistry1 Astronomy Lecture 8 Astrochemistry Adwin Boogert NASA Herschel Science Center Caltech

Koninginnedag 2008Astro 101-8/Astrochemistry2 Contents –What is Astrochemistry? –Chemical Reactions in Space –How to Observe Molecules –Molecular Evolution: Dense Clouds Young Stars Hot Cores+Disks Stellar Death Diffuse Clouds Astrobiology – Future: Herschel, ALMA, JWST

Koninginnedag 2008Astro 101-8/Astrochemistry3 What is Astrochemistry Astrochemistry studies molecules anywhere in the universe: – how are they formed – destroyed – how complex can they get – how does molecular composition vary from place to place – use them as tracer of physical conditions (temperature, density) – how do molecules in space relate to life as we know it (astrobiology)

Koninginnedag 2008Astro 101-8/Astrochemistry4 Chemical Reactions in Space – Densities atoms and molecules in interstellar medium extremely low: particles/cm 3. Compare: earth atmosphere ultra-high vacuum 10 8 – Therefore chemistry quite unusual to earth standards. Examples common species: HCO + [formyl ion] H 3 + [protonated dihydrogen] Cosmic Abundances H 0.9 H 2 He 0.1 inert O 7e-4 CO C 3e-4 CO N 1e-4 N 2 Ne 8e-5 inert Si 3e-5 dust Mg 3e-5 dust S 2e-5 Fe 4e-6 dust

Koninginnedag 2008Astro 101-8/Astrochemistry5 Chemical Reactions in Space – Some key facts: Abundance H factor 1000 larger than any other (reactive) elements Away from very strong UV fields: H,N,C,O atoms 'locked up' in H 2, N 2, CO. Left over atoms determine chemical environment: –Reducing environment if H>O –Oxidizing environment if H<O – Types of chemistry: Gas phase chemistry Grain surface chemistry (freeze out <100 K) Energetic processing ices Cosmic Abundances H 0.9 H 2 He 0.1 inert O 7e-4 CO C 3e-4 CO N 1e-4 N 2 Ne 8e-5 inert Si 3e-5 dust Mg 3e-5 dust S 2e-5 Fe 4e-6 dust

Koninginnedag 2008Astro 101-8/Astrochemistry6 Chemical Reactions in Space: Gas Phase Despite extreme vacuum conditions, long time scales allow for complex gas phase chemistry. Ion-neutral reactions orders of magnitude faster than neutral-neutral. Species with ionization potential <13.6 eV likely photo-ionized (C  C+) Cosmic rays also important ionization sources

Koninginnedag 2008Astro 101-8/Astrochemistry7 Chemical Reactions in Space: Gas Phase Some key gas phase reactions: H 3 + : (recently discovered, see H 2 + CR  H e - H H 2  H H HCO + : H CO  HCO + + H 2 H 2 O: O + H +  O + + H O + + H 2  OH + + H OH + + H 2  H 2 O + + H H 2 O + + H 2  H 3 O + + H H 3 O + + e -  H 2 O + H

Koninginnedag 2008Astro 101-8/Astrochemistry8 Chemical Reactions in Space: Solid State More realistic grain: Many molecules (H 2, H 2 O) much more easily formed on grain surfaces. Freeze out <100 K.

Koninginnedag 2008Astro 101-8/Astrochemistry9 Chemical processes occurring in space can be simulated in laboratory at low T (>=10 K) and low pressure. Thin films of ice condensed on a surface and absorption or reflection spectrum taken. Temperature and irradiation by UV light or energetic particles of ice sample can be controlled. Astrophysical laboratories: Leiden, Catania, NASA Ames/Goddard, Paris Gerakines et al. A&A 357, 793 (2000) Chemical Reactions in Space: Solid State

Koninginnedag 2008Astro 101-8/Astrochemistry10 Solid 13 CO 2 : Solid 13 CO 2 band profile varies toward different protostars… Chemical Reactions in Space: Solid State

Koninginnedag 2008Astro 101-8/Astrochemistry11 Solid 13 CO 2 : Solid 13 CO 2 band profile varies toward different protostars… …and laboratory simulated spectra show this is due to CO 2 :H 2 O mixture progressively heated by young star Chemical Reactions in Space: Solid State

Koninginnedag 2008Astro 101-8/Astrochemistry12 Chemical Reactions in Space: Inventory 129 molecules currently detected in space (123 listed here)

Koninginnedag 2008Astro 101-8/Astrochemistry13 How to Observe Molecules – Molecules detected (mostly) by vibrational and rotational transitions, at infrared and radio wavelengths. – Electronic transitions occur at X-ray/UV wavelengths  extinction-limited symmetric stretch v1bend v2 asymmetric stretch v1 rotation axis A rotation axis C rotation axis B H 2 O vibration modes H 2 O rotation modes

Koninginnedag 2008Astro 101-8/Astrochemistry14 How to Observe Molecules – Molecules in solid state cannot rotate, just vibrate – Spectra solid and gas phase molecules look very different: Pure rotational lines occur mostly in the far-IR/submm (Herschel!) 115 GHz 807 GHz 576 GHz 922 GHz 691 GHz 461 GHz 231 GHz 346 GHz

Koninginnedag 2008Astro 101-8/Astrochemistry15 Molecules are (Nearly) Everywhere …even on the Sun – T>5000 K, most molecules dissociate – Lower T, molecules quite easily formed, as demonstrated by H 2 O detection in sun spots (T~3000 K) ~13 um

Koninginnedag 2008Astro 101-8/Astrochemistry16 Molecular Evolution Next slides molecular evolution: –Dense Clouds –Young Stars –Hot Cores/Disks –Stellar Death –Diffuse Clouds –Astrobiology Not independent environments. Cycling of matter is key.

Koninginnedag 2008Astro 101-8/Astrochemistry17 Molecular Evolution: Diffuse vs. Dense Medium Hubble telescope image of M51 shows massive young stars (red) 'normal' stars (white) molecular clouds (black) diffuse clouds in between clouds 'processed' by UV photons massive stars very similar to our own Galaxy

Koninginnedag 2008Astro 101-8/Astrochemistry18 Molecular Evolution: Diffuse vs. Dense Medium CO J=1-0 image M51 highlighting giant molecular clouds. [Obtained with CARMA array in Owens Valley by Jin Koda (Caltech)]

Koninginnedag 2008Astro 101-8/Astrochemistry19 Molecular Evolution: Dense Core Molecules in core freeze out at sublimation temperature of molecule. H 2 O T=90 K CO T=16 K Background star H2OH2O H2OH2O NH 4 + silicates extinction Wavelength

Koninginnedag 2008Astro 101-8/Astrochemistry20 Molecular Evolution: Dense Core CO sublimation temperature ~16 K In densest part of core, most CO freezes out N 2 and H 2 lower sublimation temperature (<13 K) cosmic rays penetrate deep in core, ionizing H 2, forming N 2 H + H 2 + CR  H e- H H 2  H H H N 2  N 2 H + + H 2 N 2 H + observable at sub-mm frequencies (e.g. Herschel) better dense cloud tracer than CO

Koninginnedag 2008Astro 101-8/Astrochemistry21 Molecular Evolution: Young Stars Deep ice bands observed toward young stars. As star ages, ices heated: crystallization and sublimation (most volatile species, e.g. CO) first. Actual chemical processing observationally not established, but

Koninginnedag 2008Astro 101-8/Astrochemistry22 Molecular Evolution: Hot Cores , but in immediate vicinity of YSO ices evaporate, and warm gas directly observable at submm/radio wavelengths in rotational transitions. (sub)millimeter-wave gas phase measurements orders of magnitude more sensitive to abundances than IR ice observations Regions called hot cores for massive young stars and corinos for low mass stars. Cazaux et al. 2004

Koninginnedag 2008Astro 101-8/Astrochemistry23 A. Wootten, “Science with ALMA” Madrid SGR B2(N), ALMA Band 6 mixer at SMT Have to be able to separate flowers from the weeds Molecular Evolution: Hot Cores Formic acid Methyl formate Formic acid Dimethyl ether

Koninginnedag 2008Astro 101-8/Astrochemistry24 Resolving Power  up to 10 million, or <0.1 km/s Herschel/HIFI: Ghz ( um) Molecular Evolution: Hot Cores CH 3 OH gas cell measurement using HIFI (Teyssier et al. 2005)

Koninginnedag 2008Astro 101-8/Astrochemistry25 Molecular Evolution: Stellar Death Cas A, Spitzer SN 1987A, HST Stars at end burning phase expel massive shells of matter, enriching ISM with new elements and dust Effect on chemistry strongly depends on stellar mass, and episode of explosion. Some form oxygen-rich dust (silicates), others graphitic dust (and PAHs). supernovae vaporize environment, destroying or modifying dust (graphite  diamond). molecules (CO and SiO) formed in ejecta produce cosmic rays

Koninginnedag 2008Astro 101-8/Astrochemistry26 Molecular Evolution: Diffuse Medium, Mystery 1 Diffuse Interstellar Bands discovered in 1922 in optical spectra of diffuse medium. Over 200 bands detected. Probably a large gas phase species Polycyclic Aromatic Hydrocarbons possible spherical C 60, “Buckminster Fullerenes”, “Buckyballs” problem not solved...: 1 DIB, 1 carrier? PAHs Buckyball

Koninginnedag 2008Astro 101-8/Astrochemistry27  Another enigmatic diffuse medium feature.... the 3.4 um absorption band toward the Galactic Center).  Triple peaks due to hydrocarbons (-CH, -CH 2, - CH 3 ), but what kind of hydrocarbon? Pendleton et al. 1994, Adamson et al. 1998, Chiar et al. 1998, Chiar et al Molecular Evolution: Diffuse Medium, Mystery 2 -CH- -CH 2 - -CH 3 -

Koninginnedag 2008Astro 101-8/Astrochemistry28 Molecular Evolution: Diffuse Medium, Mystery 2  Bacteria? Apples?

Koninginnedag 2008Astro 101-8/Astrochemistry29 Greenberg et al. ApJ 455, L177 (1995): launched processed ice sample in earth orbit exposing directly to solar radiation (EUREKA experiment). Yellow stuff turned brown: highly carbonaceous residue, also including PAH. Molecular Evolution: Diffuse Medium, Mystery 2

Koninginnedag 2008Astro 101-8/Astrochemistry30 Molecular Evolution: Astrobiology Do molecules formed in interstellar medium have anything to do with formation of life? This is topic of astrobiology. Amino acids building blocks of most complex molecules in living organisms...protein. It has been produced in laboratory by heavy processing interstellar ice analog. Also, chirality of amino acids in protein is left-handed. May have been caused by nearby massive star producing polarized light

Koninginnedag 2008Astro 101-8/Astrochemistry31 Future of Astrochemistry is Bright.... Herschel Space Observatory Atacama Large MM Array James Webb Space Telescope ….plus a lot more……