Cosmic Evolution Part II Heavy elements molecules First a review of terminology Compound Element Atom Electromagnetic Electrons Molecule Electromagnetic Nucleus Strong Nuclear Protons Neutral atom ion e g C 2 Neutrons Electrons protons Carbon nucleus 4 6 2 electrons H atom H2 Molecule Attractive Repulsive Molecule Repulsive Attractive More delicate than atoms can be much more complex Bond sharing of electrons Is molecule stable Yes if EM potential energy less than separate atoms Activation Energy EM Potential Energy Separate Atoms Molecule Optimum Sep Separation Activation energy lower T 100 1000 K Room Temperature Questions Why is room temperature around 300 K Conventions H2 H H CO2 O C O Bond Maximum of Bonds Carbon very versatile Complex chemistry Double Bonds H O N C 1 2 3 4 Interstellar Molecules Exist as gas individual molecules A few known in 1930 s Many more since 1968 Radio astronomy Rotation Radio Telescope Vibration Infrared Optical Telescope Important Examples H Water H2O O H H Ammonia NH3 N H H H Formaldehyde H2CO C O H Others of Note CO Most common after H2 HCN HC3N HC11N Carbon chains CH4 Methane PAHs Polycyclic aromatic hydrocarbons Look at Appendix 2 How we detect Interstellar Molecules Radio Spectroscopy Mostly 1 3 mm Precise knowledge of wavelengths for different molecules 3 Lessons 1 Complexity Up to 13 atoms is extraterrestrial May be more complex Hard to detect Glycine 1994 Polycyclic Aromatic Hydrocarbons PAHs Infrared evidence 2 Dominance of Carbon Carbon Chemistry not peculiar to Earth 3 Formation Destruction Destruction Analogous to early Earth Ultraviolet light breaks bonds Massive Stars Protection by dust grains scatter and absorb ultraviolet Dust Studies of how they scatter and absorb light Ultraviolet Visible Infrared Two types range of sizes up to 10 6 m Carbon Silicates PAHs Graphite Si O Mg Fe Soot Both Produced by old stars Formation of Interstellar Molecules 1 H2 Must lose the potential energy difference before it falls apart 10 14 s Collisions OK in lab too slow in space Emit photon very slow for H2 107 s H H catalyst H2 catalyst surface of dust grain H H Dust H H Dust H2 Dust Formation of Interstellar Molecules 2 More complex molecules Problem is activation energy barrier T 10 K Barrier Use reactions without activation energies e g Molecular ions like HCO Energy simple mol Cosmic Ray H2 H2 H 2 H 2 H 3 H H3 CO HCO H2 XH e X H Reactive mol More complex Ion Molecule Reactions Ion Neutral Molecule No Barrier Electromagnetic Potential Energy Separation of Ion and Molecule Molecule or atom Molecules on Dust Grains NH3 H2O Stick on grains ice Dust CH4 Infrared observations show this as molecules Vibrate absorb infrared e g H2O absorbs at 3 10 6 m CH4 absorbs at 8 10 6 m Molecules on Dust Grains Icy mantles contain H O C N Further reactions possible more complex molecules e g Ethanol Building blocks of life Life Hoyle and Wickramasinghe New stars and planets form in same regions Implications 1 Similar Carbon Dominated Chemistry 2 Direct Role in Origin of Life 3 Formation Destruction Analogous to Early Earth Roles of Dust 1 Protection from UV 2 H2 Formation 3 Depletion Mantles of Ice H2O NH3 CH4 CO2 HCOOH Methane Estimate of Average Star Formation Rate R R of stars in galaxy lifetime of galaxy N N tgal Count them No Use Gravity Newton s Laws Sun orbiting center of galaxy at 250 km s 1 Kinetic energy 1 2 M v2 Rgv2 G 1 2 1 2 gravitational potential energy G Mg M Rg Mg 155 miles per second Distance of Sun from center of galaxy Estimate of Average Star Formation Rate R Rg 25 000 ly Mg 1 0 1011 M Add stars outside Sun s orbit Mg 1 6 1011 M N Mg Avg mass of star Tgal 1010 yr 1 6 1011 4 1011 0 4 studies of old stars R 4 1011 stars 40 stars per year 5 50 1010 Star Formation Current Star Formation Molecular Clouds Composition H2 93 He 6 Dust and other molecules 1 CO next most common after H2 He Temperature about 10 K Density particles per cubic cm 100 cm 3 to 106 cm 3 Air has about 1019 cm 3 Water about 3 x 1022 cm 3 Size 1 300 ly Mass 1 to 106 Msun A Small Molecular Cloud Ices on Dust Grains Current Star Formation Occurs in gas with heavy elements Molecules and dust keep gas cool Radiate energy released by collapse Stars of lower mass can form Mass needed for collapse increases with T Star formation is ongoing in our Galaxy Massive stars are short lived Star formation observed in infrared The Launch of The Spitzer Space Telescope Spitzer Space Telescope Launched Aug 2003 expect a 5 yr life Visible to Infrared Views RCW 49 JHK 2MASS RCW 49 HK1 RCW 49 234 A Dark Molecular Cloud L1014 distance 600 ly but somewhat uncertain Red light image dust blocks stars behind and our view of what goes on inside Forming Star Seen in Infrared Three Color Composite Blue 3 6 microns Green 8 0 microns Red 24 microns R band image from DSS at Lower left We see many stars through the cloud not seen in R The central source is NOT a background star L1014 is forming a star C Young et al ApJS 154 396 Artist s Conception Features Dusty envelope Rotation Disk Bipolar outflow R Hurt SSC The Protostar Evolution of the collapsing gas cloud At first collapsing gas stays cool Dust gas emit photons remove energy At n 1011 cm 3 photons trapped Gas heats up dust destroyed pressure rises Core stops collapsing The outer parts still falling in adding mass Core shrinks slowly heats up Fusion begins at T 107 K Protostar becomes a main sequence star The Disk The Star AU Mic is blocked in a coronograph Allows you to see disk Dust in disk is heated by star and emits in infrared Angular Momentum Measure of tendency to rotate J mvr Angular momentum is conserved J constant As gas contracts r smaller v increases Faster rotation resists collapse Gas settles into rotating disk Protostar adds mass through the disk The Wind Accretion from disk will spin up the star Star would break apart if spins too fast Angular momentum must be carried off The star disk interaction creates a wind The wind carries mass to large distances J mvr small amount of m at very large r Allows star to avoid rotating too fast Wind turns into bipolar jet Sweeps out cavity The Bipolar Jet Studying the Disk Robert Hurt SSC Pontoppidan et al 2004 5 ApJ accepted
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