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UT AST 309L - Cosmic Evolution Part II

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Cosmic Evolution, Part IIHeavy Elements to MoleculesHeavy elements → moleculesFirst a review of terminology:ElementAtomCompoundMolecule↓ElectronsNucleus ↓ElectromagneticElectro-magneticProtons NeutronsNucleusStrong NuclearNeutral atom: # Electrons = # protons ion: =e.g. C+2Carbon nucleus + 4 (6-2) electrons-+H2Molecule++--AttractiveRepulsiveMolecule: Repulsive ~ AttractiveMore delicate than atoms,can be much more complexH atomForces“Bond” is sharing of electronsIs molecule stable?Yes, if EM potential energy less than separate atomsEMPotentialEnergyMoleculeSeparation↑ Optimum Sep.Separate AtomsActivation EnergyActivation energy lower → T ~ 100 - 1000 K(Room Temperature)Questions• Why is room temperature around 300 K?• How commonly is this temperature found inthe Universe?Conventions:H2H – H CO2O = C = O Bond Double BondsMaximum # of Bonds: H 1O 2N 3C 4Carbon very versatile→ Complex chemistryInterstellar MoleculesExist as gas (individual molecules)A few known in 1930’sMany more since 1968 - Radio astronomyRotationVibration+-Radio TelescopeInfraredOptical TelescopeHow we detect Interstellar MoleculesRadio Spectroscopy (Mostly λ ~ 1- 3 mm)+ Precise knowledge of wavelengths for different moleculesLook at Appendix 2Important Examples:Water H2OAmmonia NH3Formaldehyde H2COOH HH HHNHHC = OOthers of Note: CO Most common after H2HCN, HC3N, … HC11N → Carbon chainsCH4(Methane)PAHs (Polycyclic aromatic hydrocarbons)3 Lessons1. Complexity (Up to 13 - atoms) is extraterrestrialMay be more complex (Hard to detect)Glycine ? 1994Polycyclic Aromatic Hydrocarbons (PAHs)(Infrared evidence)2. Dominance of CarbonCarbon Chemistry not peculiar to Earth3. Formation & Destruction Analogous to early EarthDestruction:Ultraviolet light breaks bondsMassive StarsProtection by dust grains: scatter and absorb ultravioletDustStudies of how they scatter and absorb light(Ultraviolet → Visible → Infrared)⇒ Two types, range of sizes up to 10-6 mCarbon SilicatesPAHs → Graphite Si + O + Mg, Fe, …~ SootBoth Produced by old starsFormation of Interstellar Molecules1. H2Must lose the potential energy differencebefore it falls apart (~ 10–14 s)Collisions: OK in lab, too slow in spaceEmit photon: very slow for H2 (107 s)H + H + catalyst = H2 + catalystsurface of dust grainHHH - HH2DustDustDustFormation of Interstellar Molecules2. More complex moleculesProblem is activation energy barrierT ~ 10 K << BarrierUse reactions without activation energiese.g. Molecular ions, like HCO+Cosmic Ray H2 → H2+H2+ + H2 → H3+ + HH3+ + CO → HCO+ + H2XH+ + e– → X + HEnergy + simple mol.→ Reactive mol. ↓More complexIon - Molecule ReactionsNeutral Molecule+IonMolecule or atomElectromagneticPotentialEnergySeparation ofIon and MoleculeNo Barrier!Molecules on Dust GrainsH2ONH3DustCH4Stick on grains“ice”Infrared observations show this: as moleculesVibrate, absorb infrarede.g. H2O absorbs at 3 × 10–6 mCH4absorbs at 8 × 10–6 mIces on Dust GrainsMolecules on Dust GrainsIcy “mantles” contain H, O, C, NFurther reactions possible → more complexmolecules (e.g. Ethanol)→ Building blocks of life ?→ Life ??? Hoyle and WickramasingheNew stars and planets form in same regionsImplications1. Similar (Carbon-Dominated) Chemistry2. Direct Role in Origin of Life?3. Formation + DestructionAnalogous to Early Earth Roles of Dust1. Protection from UV2. H2 Formation3. Depletion → Mantles of IceH2O, NH3, CH4, CO2, HCOOH, …MethaneEstimate of Average Star Formation Rate (R )R* = # of stars in galaxy = N* lifetime of galaxy tgalN* : Count them? NoUse Gravity (Newton’s Laws)Sun orbiting center of galaxy at 250 km s-1 (155 miles per second)Kinetic energy = gravitational potential energy M v2 =*121212G Mg MRgDistance of Sun from center of galaxyRgv2G= MgEstimate 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 = 1.6 × 1011 = 4 × 1011Avg. mass of star 0.4Tgal ~ 1010 yr (studies of old stars)R* ~ 4 × 1011 stars = 40 stars per year (5 - 50)1010Star FormationCurrent Star FormationMolecular 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 MsunA Small Molecular CloudCurrent 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 infraredThe Launch of The Spitzer Space TelescopeSpitzer Space Telescope Launched Aug. 2003, expect a 5 yr life.Visible to Infrared ViewsRCW 49JHK(2MASS)RCW 49HK1RCW 49234Artist’s ConceptionR. Hurt, SSCFeatures:Dusty envelopeRotationDiskBipolar outflowThe 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 starThe DiskThe Star (AUMic) is blocked ina coronograph.Allows you to seedisk. Dust in diskis heated by starand emits ininfrared.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 diskThe 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 cavityThe Bipolar JetRobert Hurt, SSCStudying the DiskPontoppidan et al. 2004/5, ApJ,


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UT AST 309L - Cosmic Evolution Part II

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