The Interstellar Medium

1
The Interstellar Medium

• Physical Astronomy
• Professor Lee Carkner
• Lecture 13

2
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

3
Jeans Criterion

• From the virial theorem the condition for
  collapse is U gt 2K
• We can write R in terms of the density
• R (3M/4pr)1/3
• If the clouds mass is greater than the Jeans
  mass MJ it will collapse
• MJ (5kT/GmmH)3/2 (3/(4pr))1/2

4
Free-Fall Time

• We can also write in terms of the Jeans length
• RJ ((15kT)/(4pGmmHr))1/2
• isothermal, optically thin, radiates
  gravitational energy quickly
• tff (3p/32Gr)1/2

5
Other Factors

• We assume that gravity and thermal pressure are
  only factors
• Other include
• The perturbation that starts collapse
• Rotation
• Magnetic fields

6
Magnetic Pressure

• Clouds have magnetic fields of 1-100 nT
• Can write an expression for the minimum mass
  necessary for collapse in the presence of a
  magnetic field
• MB cBpR2B/G1/2
• Can also write in terms of Msun, nT and pc
• MB 70 Msun (B/1nT)(R/1pc)2

7
Parts of the ISM

• Coronal gas
• T 106 K, n lt104 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

8
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 bands dimmed more than red

9
Extinction Curve

• Cant 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

10
Hydrogen

• Hydrogen near sources of radiation (like hot
  stars) can become ionized (H II)
• Most hydrogen is un-ionized (H I)
• In dark cores of clouds, molecular H2 can be
  produced
• Need dust grains to shield from UV light and
  provide nucleation sites

11
Stromgren Radius

• The radius of an H II region is given by
• rS (3N/4pa)1/3 nH-2/3
• Photons with wavelength less than 91.2 nm
• nH is number density of hydrogen (108 m-3)

12
Chemistry

• Cloud density is low, so it is easier for atoms
  to combine if they are both on a small dust grain
• Some are very complicated, e.g., amino acids and
  organic molecules
• Many molecules have strong mm transitions and are
  used to map molecular clouds

13
Next Time

• Read 12.3
• Homework 12.2, 12,15, 12.17