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 =B2/20, 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

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 = 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

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