sign in sign up
<<
Home , Astrochemistry

101 - Lecture 8 Astrochemistry

Adwin Boogert NASA Herschel Science Center Caltech

Koninginnedag 2008 Astro 101-8/Astrochemistry 1 Contents

–What is Astrochemistry? –Chemical Reactions in Space –How to Observe –Molecular Evolution: •Dense Clouds •Young •Hot Cores+Disks •Stellar Death •Diffuse Clouds • –Future: Herschel, ALMA, JWST

Koninginnedag 2008 Astro 101-8/Astrochemistry 2 What is Astrochemistry

Astrochemistry studies molecules anywhere in the :

–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 (, density) –how do molecules in space relate to as we know it (astrobiology)

Koninginnedag 2008 Astro 101-8/Astrochemistry 3 Chemical Reactions in Space

Cosmic Abundances

–Densities and molecules in 5 3. H 0.9 H2 extremely low: 1-10 particles/cm Compare: 19 He 0.1 inert •earth atmosphere 10 •ultra-high vacuum 108 O 7e-4 CO C 3e-4 CO –Therefore quite unusual to earth standards. N 1e-4 N Examples common species: 2 •HCO+ [formyl ] + Ne 8e-5 inert •H3 [protonated dihydrogen] Si 3e-5 Mg 3e-5 dust S 2e-5 Fe 4e-6 dust

Koninginnedag 2008 Astro 101-8/Astrochemistry 4 Chemical Reactions in Space

Cosmic Abundances –Some key facts: •Abundance H factor 1000 larger than any other (reactive) elements H 0.9 H2 •Away from very strong UV fields: H,N,C,O atoms 'locked up' in H , N , CO. Left over He 0.1 inert 2 2 atoms determine chemical environment: O 7e-4 CO –Reducing environment if H>O C 3e-4 CO –Oxidizing environment if H

Koninginnedag 2008 Astro 101-8/Astrochemistry 5 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 2008 Astro 101-8/Astrochemistry 6 Chemical Reactions in Space: Gas Phase

Some key gas phase reactions:

+ H3 : (recently discovered, see http://h3plus.uiuc.edu)

+ - H2 + CR H2 + e + + H2 + H2  H3 + H

HCO+:

+ + H3 + CO  HCO + H2

H2O:

O + H+  O+ + H + + O + H2  OH + H + + OH + H2  H2O + H H O+ + H  H O+ + H Koninginnedag2 2008 2 3 Astro 101-8/Astrochemistry 7 + - H3O + e  H2O + H Chemical Reactions in Space: Solid State

Many molecules (H2, H2O) much more easily formed on grain surfaces. Freeze out <100 K.

More realistic grain:

Koninginnedag 2008 Astro 101-8/Astrochemistry 8 Chemical Reactions in Space: Solid State

Chemical processes occurring in space can be simulated in laboratory at low T (>=10 K) and low pressure. Thin films of condensed on a surface and absorption or reflection

spectrum taken. Temperature and irradiation by UV or energetic particles of ice sample can be controlled. Astrophysical laboratories: Leiden, Gerakines et al. A&A 357, 793 (2000) Catania, NASA Ames/Goddard, Koninginnedag 2008 Astro 101-8/Astrochemistry Paris 9 Chemical Reactions in Space: Solid State

13 Solid CO2:

13 •Solid CO2 band profile varies toward different …

Koninginnedag 2008 Astro 101-8/Astrochemistry 10 Chemical Reactions in Space: Solid State

13 Solid CO2:

13 •Solid CO2 band profile varies toward different protostars… •…and laboratory simulated spectra show

this is due to CO2:H2O mixture progressively heated by young

Koninginnedag 2008 Astro 101-8/Astrochemistry 11 Chemical Reactions in Space: Inventory

129 molecules currently detected in space (123 listed here)

http://www.cv.nrao.edu/~awootten/allmols.html Koninginnedag 2008 Astro 101-8/Astrochemistry 12 How to Observe Molecules

–Molecules detected (mostly) by vibrational and rotational transitions, at and radio wavelengths. –Electronic transitions occur at X-ray/UV wavelengths  extinction-limited

H2O vibration modes

symmetric stretch v1 bend v2 asymmetric stretch v1

H2O rotation modes

rotation axis A rotation axis B rotation axis C

Koninginnedag 2008 Astro 101-8/Astrochemistry 13 How to Observe Molecules

–Molecules in solid state cannot rotate, just vibrate Pure rotational lines occur –Spectra solid and gas phase molecules look mostly in the far-IR/submm very different: (Herschel!)

922 GHz

807 GHz 691 GHz 576 GHz 461 GHz 231 GHz 346 GHz 115 GHz

Koninginnedag 2008 Astro 101-8/Astrochemistry 14 Molecules are (Nearly) Everywhere …even on the –T>5000 K, most molecules dissociate – Lower T, molecules quite easily formed, as demonstrated by H2O detection in sun spots (T~3000 K)

~13 um Koninginnedag 2008 Astro 101-8/Astrochemistry 15 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 2008 Astro 101-8/Astrochemistry 16 Molecular Evolution: Diffuse vs. Dense Medium

Hubble 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 2008 Astro 101-8/Astrochemistry 17 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 2008 Astro 101-8/Astrochemistry 18 Molecular Evolution: Dense Core Background star n o i t c n i

t +

x NH4 e H2O

H2O

Wavelength

•Molecules in core freeze out at sublimation temperature of .

•H2O T=90 K •CO T=16 K Koninginnedag 2008 Astro 101-8/Astrochemistry 19 Molecular Evolution: Dense Core

•CO sublimation temperature ~16 K •In densest part of core, most CO freezes out

•N2 and H2 lower sublimation temperature (<13 K) •cosmic rays penetrate deep in core, + ionizing H2, forming N2H

+ •H2 + CR  H2 + e- + + H2 + H2  H3 + H + + H3 + N2  N2H + H2

+ •N2H observable at sub-mm Koninginnedag 2008 Astro 101-8/Astrochemistryfrequencies (e.g. Herschel) 20 •better dense cloud tracer than CO 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 2008 Astro 101-8/Astrochemistry 21 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 2008 Astro 101-8/Astrochemistry 22 Molecular Evolution: Hot Cores

Have to be able to separate flowers from the weeds

Formic Methyl Dimethyl formate ether

SGR B2(N), ALMA Band 6 mixer at SMT A. Wootten, “Science with ALMA” Madrid 2006.

Koninginnedag 2008 Astro 101-8/Astrochemistry 23 Molecular Evolution: Hot Cores

CH OH gas cell measurement using HIFI Herschel/HIFI: 480-1916 Ghz (625-157 um) 3 Resolving Power ν/δν up to 10 million, (Teyssier et al. 2005) or <0.1 km/s

Koninginnedag 2008 Astro 101-8/Astrochemistry 24 Molecular Evolution: Stellar Death

•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 -rich dust (silicates), others Cas A, Spitzer graphitic dust (and PAHs). SN 1987A, HST •supernovae vaporize environment, destroying or modifying dust (graphitediamond).

•molecules (CO and SiO) formed in ejecta

•produce cosmic rays

Koninginnedag 2008 Astro 101-8/Astrochemistry 25 Molecular Evolution: Diffuse Medium, Mystery 1

PAHs •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 C60, “Buckminster Fullerenes”, “Buckyballs” •problem not solved...: 1 DIB, 1 carrier? Buckyball Koninginnedag 2008 Astro 101-8/Astrochemistry 26 Molecular Evolution: Diffuse Medium, Mystery 2

Another enigmatic diffuse medium feature.... the 3.4 um absorption band toward the Galactic Center).

Triple peaks due to

hydrocarbons (-CH, -CH2, -CH ), but what kind of -CH- 3 hydrocarbon? -CH - 3 -CH2-

Pendleton et al. 1994, Adamson et al. 1998, Chiar et al. 1998, Chiar et al. 2000

Koninginnedag 2008 Astro 101-8/Astrochemistry 27 Molecular Evolution: Diffuse Medium, Mystery 2

Bacteria? Apples?

Koninginnedag 2008 Astro 101-8/Astrochemistry 28 Molecular Evolution: Diffuse Medium, Mystery 2

Greenberg et al. ApJ 455, L177 (1995): launched processed ice sample in earth orbit exposing directly to solar (EUREKA experiment). Yellow stuff turned brown: highly carbonaceous residue, also including PAH.

Koninginnedag 2008 Astro 101-8/Astrochemistry 29 Molecular Evolution: Astrobiology

•Do molecules formed in interstellar medium have anything to do with formation of life? •This is topic of astrobiology. •Amino building blocks of most complex molecules in living organisms.... •It has been produced in laboratory by heavy processing analog. •Also, of amino acids in protein is left-handed. May have been caused by nearby massive star producing polarized light

Koninginnedag 2008 Astro 101-8/Astrochemistry 30 Future of Astrochemistry is Bright....

Atacama Large MM Array

Herschel Space Observatory James Webb Space Telescope ….plus a lot more……

Koninginnedag 2008 Astro 101-8/Astrochemistry 31