12. The interstellar medium: gas

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12. The interstellar medium gas 12.3 HI
clouds (and IS absorption lines) 12.4
Dense molecular clouds 12.5 Interstellar
masers 12.6 Note on pressures in IS gas
NGC1232
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• HI clouds and interstellar (IS) absorption lines
• Wide distribution throughout galactic disk to R
  20 kpc
• Greatest density of clouds for 4 kpc lt R lt 14
  kpc
• Number along a given lines of sight in glactic
  plane
• 7 or 8/kpc
• Typical size a few pc to a few tens of parsecs
• Typical mass MHI 100 M?
• Temperature T 90 K
• Radio emission ? 21 cm, frequency f 1420.406
  MHz

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21-cm emission in other spiral galaxies This
image shows the HI emission in the face-on
spiral M101 using the Westerbork radio
telescope in Holland. The HI distribution is
easier to determine than in the Milky Way,
because we can observe this galaxy from an
external vantage point.
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HI and CO distribution with radius R in the Galaxy
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The Magellanic Stream and HI high velocity
clouds represent weak sources of 21-cm
emission located well away from the galactic
plane. They are the result of tidal
interation of the Galaxy on the satellite
galaxies, the Magellanic Clouds.
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The 21-cm line The 21-cm line of neutral atomic
hydrogen is known as a hyperfine structure
transition. Metastable upper energy state has e
and p spins parallel, lower energy state,
antiparallel.
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Lifetime of uper energy state 11 million years,
with spontaneous emission of a photon. The upper
energy state is populated by collisions which are
relatively frequent (one such excitation per H
atom occurs about every 400 yr). Three quarters
of all H atoms are on average in the upper state.
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HI line formation
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IS absorption lines
star in galactic plane
observer on Earth
HI cloud
Spectrum of a distant galactic plane star
contains narrow IS absorption lines produced by
heavy elements in IS gas clouds.
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IS lines in a stellar spectrum
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• IS lines due to Na, Ca, Ti, K, Fe and molecules
  
• CN, CH, CH are known in optical region
• IS lines due to C, N, O, Mg, Si, P, S, Cl, Ar,
• Mn, Fe and molecules H2, HD, CO are
• observed in the ultraviolet

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Above narrow IS absorption lines in the
spectrum of a distant galactic plane star
differ markedly from the broader stellar
line. Right multiple components in the IS NaD
line due to clouds at different velocities in
the line of sight.
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• IS line strengths give information on chemical
• composition of IS HI clouds. Some heavy
• elements (e.g. Ca) are greatly depleted in IS
  
• clouds (deficient by a factor 2 10-4),
  while
• others (e.g. C, N, O) are hardly changed
• relative to solar composition.
• Element depletion is by heavy element
• accretion onto dust grains, thereby removing
• some refractory elements from the gas.

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The depletion of heavy elements in HI clouds as
deduced by the strengths of IS absorption lines.
There is no correlation of depletion factor with
atomic weight A, but a good correlation with the
elements condensation temperature Tc.
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Dense molecular clouds The most common molecules
are H2, CO, CN, OH, H2CO. Most molecules (but not
H2) give characteristic radio emission lines,
which allow them to be identified. Over 50 have
been detected. Absorption lines are usually seen
for OH, always for H2CO. Molecule
formula discovery ?
number sources hydroxyl OH
1963 1.8 cm
600 ammonia NH3 1968
1.3 cm 12 water
H2O 1968 1.3
cm 35 formaldehyde H2CO
1969 6.2 cm 150
carbon monoxide CO 1970
2.6 mm 60
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Some interstellar molecules observed in the IS
medium. The first 5 are found as optical/UV IS
absorption lines in stellar spectra the second
set are seen as radio emission lines in dense
molecular clouds, (or as radio absorption
lines when distant sources are seen through
dense molecular clouds).
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Microwave spectrum of emission lines from a
dense molecular cloud
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• Properties of dense molecular clouds
• Temperature T 10 to 30 K
• Number densities n 108 1012 molecules m-3
• mass density ? 10-15 kg.m-3
• Cloud mass may be 103 M?
• Cloud size 10 pc
• Dense molecular clouds are often very dusty

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• Note that dust shields molecules from
• UV radiation from stars, which would
• dissociate most molecules.
• Also dust surfaces provide a site for the
• formation of the H2 molecule. Other
• molecules can form from gas phase
• reactions.

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• Dense molecular clouds are under
• gravitational collapse because there is
• enough mass for self gravity to pull them
• together.
• They are consequently sites of star
• formation

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Above galactic distribution of CO in molecular
clouds Below CO cloud radial velocity vs
galactic longitude
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Some HII nebulae which are
also associated with dense molecular
clouds

• ? Car
• M20
• Trifid nebula
• 3. Orion nebula
• 4. M16 Eagle nebula

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Note on pressures in the IS gas
P n k T
P pressure (Pa) n number density
(m-3) T absolute temp. (K) k Boltzmanns
constant (J.K-1)
Phase n (m-3) T (K)
P (Pa) HII
108 9000 10-11 HI
107 90
10-14 dense molecular 109 to 1012
10 30 10-13 10-10 hot HI
3 105 5000
10-14 coronal gas 103
106 10-14
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• HI clouds are in pressure equilibrium with the
  
• hot HI and coronal gas intercloud medium
• The pressure of HII clouds is much higher
• than the surrounding medium (normally HI)
• and they therefore expand supersonically
• ( 10 km/s) into the surrounding gas.

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• Dense molecular clouds also have much
• higher pressures, but this is the result of
  their
• high masses, causing them to collapse and be
• compressed under their self gravity (they are
  
• the only phase of the ISM where self-gravity
• dominates over gas pressure)
• Note IS gas pressures are always very low.
• On Earth 1 atmosphere 105 Pa, much
• higher than in ISM

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Interstellar masers MASER Microwave
Amplification by Stimulated
Emission of Radiation Observed in OH lines (?
18 cm) and sometimes in lines of H2O (1.35 mm)
and SiO (6.95 mm, 3.47 mm) IR pumping from
thermal IR from dust can cause a population
inversion of OH in gas in a metastable upper
level this is a condition for maser action.
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Stimulated emission can occur, resulting in a
very intense emission line from a small region
of space (generally a few tens of A.U.
across). Maser sources are compact and probably
occur in dusty regions associated with star
formation or in circumstellar dust shells around
M-type stars. There are several OH and H2O maser
sources in the dense molecular cloud associated
with the Orion nebula possibly where new-born
stars are still enshrouded in a coccoon of
circumstellar dust grains.
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END OF LECTURE 9