the interstellar medium physical astronomy professor lee carkner lecture 12
Post on 21-Dec-2015
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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 =B2/20, B = (20Pm)½ = [(2)(4X10-7)(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/4)1/3
If the cloud’s mass is greater than the Jeans mass MJ it will collapse
MJ = (5kT/GmH)3/2 (3/(4)1/2
Free-Fall Time
RJ = ((15kT)/(4GmH))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 = (3/32G)1/2
Magnetic Pressure
Field “freezes in” to the cloud and get compressed
and thus stronger as the cloud collapses
MB = cBR2B/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, 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
H2 can be produced
Might produce black dark nebula
Stromgren Radius
rS ~ (3N/4)1/3 nH-2/3
N is the number on ionizing photons per second produced by source star
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
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