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Published byAdriel Atkinson Modified over 2 years ago

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AS 4002 Star Formation & Plasma Astrophysics MOLECULAR CLOUDS Giant molecular clouds – CO emission –several tens of pc across –mass range 10 5 to 3x10 6 M o –clumpy substructure –lifetimes ~ 10 7 y ~ crossing time of clumps Temperatures too cold for H 2 and He to emit Trace molecules like CO, H 2 O, HCN, NH 3 excited by collisions with H 2 Several thousand radiative transitions in range 0.7 GHz (43 cm) to 3800 GHz (77 m). CO 10 4 times less abundant than H 2 Others rarer still. Some density diagnostics: –excitation of CO requires n(H 2 ) ≥ 10 8 m -3 –excitation of NH 3 requires n(H 2 ) > 10 10 m -3

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AS 4002 Star Formation & Plasma Astrophysics Substructures (clumps) – CO emission –M cl ~ 10 3 to 10 4 M o –R ~ 2 to 5 pc –n(H 2 ) ~ 10 8.5 m -3 –T ~ 10 K –cf. Taurus - Auriga complex. Cores – NH 3, H 2 CO, HC 3 N, CS emission –M core ~ 1 M o ; massive envelope ~ 10 2 M o –R ~ 10 -1 pc –n(H 2 ) > 10 10 m -3 –T ~ 10 K

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AS 4002 Star Formation & Plasma Astrophysics Scalar virial theorem Start with a set of particles. Stationary wrt an inertial frame at time t 0. Typical particle: mass m, position r(t) acted on by force P has eq. of motion:

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AS 4002 Star Formation & Plasma Astrophysics Sum over all particles Moment of inertia I “Virial” Twice total thermal KE of system: Total mass of particles Temperature, assumed uniform Mass of H atom Mean mol. wt. Isothermal sound speed

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AS 4002 Star Formation & Plasma Astrophysics The virial Forces P contributing to virial at points of application r: –collisions with other particles in system –self-gravitation due to other particles in system –collisions with external material Equal & opposite pairs, so no net contribution Produce pressure P at external boundary S, contributing Grav. force per unit mass at r Mass of particles in vol. element dv at r. Inward normal (For p uniform over S) Grav. binding energy

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AS 4002 Star Formation & Plasma Astrophysics For a body of gas released from rest at time t 0 under given external pressure p: Virial equilibrium If the initial state is also an equilibrium state we must also have: (necessary but not sufficient!) > 0 gives expansion < 0 gives contraction

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AS 4002 Star Formation & Plasma Astrophysics Self gravitation & thermal pressure Scalar virial equilibrium – pure thermal support: Thermal pressure of warm surrounding ISM Volume: for sphere, Virial coefficient A=(3/5) for uniform sphere, increasing with central condensation

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AS 4002 Star Formation & Plasma Astrophysics Spherical cloud Equilibrium condition becomes:

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AS 4002 Star Formation & Plasma Astrophysics Maximum equilibrium pressure For fixed M, c s, get p-R relation defined by equilibrium condition: 2 equilibria: one stable, other unstable. No equilibrium possible for pressures greater than point on relation where dp 0 /dR = 0: 0.1 1 10 100 0.1110 R/R 1 p 0 /p 1 unstable stable p/p 1 ~

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AS 4002 Star Formation & Plasma Astrophysics Jeans mass & length Express critical R, M in terms of mean density:

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AS 4002 Star Formation & Plasma Astrophysics Non-thermal support Using these expressions, for average clump n=10 8.5 m -3 and T=10K: –M J ~ few M o, J ~ 1 pc But clumps have masses 10 3 to 10 4 M o and are NOT collapsing on a free-fall timescale. Need some other means of support. Two main observational clues: –High CO linewidths u imply very supersonic fluid motion –Polarization maps indicate ordered magnetic fields Empirical power-law relation:

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