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The Interstellar Medium Physical Astronomy Professor Lee Carkner Lecture 12 Questions The corona has a number density of ~1011 m-3 and a temperature of ~1 million K. What is the gas pressure in the corona? P=nkT=(1011)(1.38X10-23)(106) = 1.38X10-6 Pa What would the magnetic field have to be to equal that pressure (in Gauss)? Pm =B2/2m0, B = (2m0Pm)½ = [(2)(4pX107)(1.38X10-6)]½ = 1.86X10-6 T = 0.02 G Star Formation The gas is mostly hydrogen and fills most of the mass and volume The dust is mostly silicates (some ices) and accounts for most of the extinction This protostar is powered by gravitational energy Jeans Criterion Assume the cloud has a balance between the outward thermal kinetic energy (K) and the inward gravitational energy (U) U = (3/5)(GM2/R) K = (3/2)NkT R = (3M/4pr)1/3 If the cloud’s mass is greater than the Jeans mass MJ it will collapse MJ = (5kT/GmmH)3/2 (3/(4pr))1/2 Free-Fall Time RJ = ((15kT)/(4pGmmHr))1/2 We can find an estimate for the time it takes the cloud to collapse if we make some assumptions Free-fall time depends only on density tff = (3p/32Gr)1/2 Other Factors Other include The perturbation that starts collapse Rotation Magnetic fields Magnetic Pressure Field “freezes in” to the cloud and get compressed and thus stronger as the cloud collapses MB = cBpR2B/G1/2 Can also write in terms of Msun, nT and pc MB ~ 70 Msun (B/1nT)(R/1pc)2 Parts of the ISM Coronal gas T = 106 K, n <104 m-3 Intercloud Medium T ~ 104 K, n ~ 105 m-3 H II regions T ~ 104 K, n ~ 103 m-3 H I regions T ~ 100 K, n ~ 107 m-3 Molecular clouds Very cool, dense clouds T ~ 10 K, n ~ 109 m-3 Extinction Can parameterize as an additive factor in magnitude, the extinction, Al ml = Ml + 5logd + Al Dust grains preferentially scatter short wavelengths, causing reddening Blue light dimmed more than red Extinction Curve Can’t fit curve with just normal dust grains Might be due to carbon molecules such as graphite and polycyclic aromatic hydrocarbons (PAH) Sizes from microns to less than a nanometer Hydrogen Hydrogen near sources of radiation (like hot stars) can become ionized (H II) Might produce pink emission nebula Most hydrogen is un-ionized (H I) Might produce blue reflection nebula In dark cores of clouds, molecular H2 can be produced Might produce black dark nebula Stromgren Radius rS ~ (3N/4pa)1/3 nH-2/3 N is the number on ionizing photons per second produced by source star a is the recombination coefficient (~ 3.1 X 10-19 m3/s) nH is number density of hydrogen (~108 m-3) Chemistry Molecules can form both on dust grains and in the gas phase Molecules mostly composed of CHON elements Many molecules have strong mm transitions and are used to map molecular clouds Next Time Test #2 Same format as Test #1 Covers lectures 7-12 For first class after break (Jan 10) Read 12.3 Homework: 12.2, 12,15, 12.17