The Interstellar Medium Physical Astronomy Professor Lee Carkner Lecture 12

Questions  The corona has a number density of ~10 11 m -3 and a temperature of ~1 million K. What is the gas pressure in the corona?  P=nkT=(10 11 )(1.38X )(10 6 ) = 1.38X10 -6 Pa  What would the magnetic field have to be to equal that pressure (in Gauss)?  P m =B 2 /2  0, B = (2  0 P m ) ½ = [(2)(4  X )(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)(GM 2 /R) K = (3/2)NkT  R = (3M/4  ) 1/3  If the cloud’s mass is greater than the Jeans mass M J it will collapse M J = (5kT/G  m H ) 3/2 (3/(4  ) 1/2

Free-Fall Time  R J = ((15kT)/(4  G  m H  )) 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 t ff = (3  /32G  ) 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  M B = c B  R 2 B/G 1/2   Can also write in terms of M sun, nT and pc M B ~ 70 M sun (B/1nT)(R/1pc) 2

Parts of the ISM  Coronal gas   T = 10 6 K, n <10 4 m -3  Intercloud Medium   T ~ 10 4 K, n ~ 10 5 m -3  H II regions   T ~ 10 4 K, n ~ 10 3 m -3  H I regions   T ~ 100 K, n ~ 10 7 m -3  Molecular clouds  Very cool, dense clouds  T ~ 10 K, n ~ 10 9 m -3

Extinction   Can parameterize as an additive factor in magnitude, the extinction, A m = M + 5logd + A   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 H 2 can be produced   Might produce black dark nebula

Stromgren Radius  r S ~ (3N/4  ) 1/3 n H -2/3  N is the number on ionizing photons per second produced by source star    is the recombination coefficient (~ 3.1 X m 3 /s)  n H is number density of hydrogen (~10 8 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