Download Lecture 9: The interstellar medium (ISM)

ASTR112 The Galaxy
Lecture 9
12. The interstellar medium: gas
Prof. John Hearnshaw
NGC1232
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
ASTR112 The Galaxy
Lecture 9
• Wide distribution throughout galactic disk to R ~ 20 kpc
• Greatest density of clouds for 4 kpc < R < 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 λ = 21 cm, frequency f = 1420.406 MHz
Prof. John Hearnshaw
HI clouds and interstellar (IS) absorption lines
ASTR112 The Galaxy
Lecture 9
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.
Prof. John Hearnshaw
21-cm emission in other
spiral galaxies
HI and CO distribution with radius R in the Galaxy
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 9
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.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 9
ASTR112 The Galaxy
Lecture 9
The 21-cm line
Metastable upper energy state has e and p
spins parallel, lower energy state, antiparallel.
Prof. John Hearnshaw
The 21-cm line of neutral atomic hydrogen is
known as a hyperfine structure transition.
ASTR112 The Galaxy
Lecture 9
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.
Prof. John Hearnshaw
Lifetime of uper energy state ~ 11 million
years, with spontaneous emission of a
photon.
HI line formation
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 9
ASTR112 The Galaxy
Lecture 9
IS absorption lines
HI cloud
observer
on Earth
Spectrum of a distant galactic plane star contains
narrow IS absorption lines produced by heavy elements
in IS gas clouds.
Prof. John Hearnshaw
star in galactic
plane
IS lines in a stellar spectrum
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 9
ASTR112 The Galaxy
Lecture 9
• IS lines due to Na, Ca, Ti, K, Fe and molecules
CN, CH, CH+ are known in optical region
Prof. John Hearnshaw
• 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
ASTR112 The Galaxy
Lecture 9
Right: multiple components
in the IS NaD line due to
clouds at different velocities
in the line of sight.
Prof. John Hearnshaw
Above: narrow
IS absorption
lines in the
spectrum of a
distant galactic
plane star differ markedly
from the broader stellar line.
ASTR112 The Galaxy
Lecture 9
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.
Prof. John Hearnshaw
• IS line strengths give information on chemical
composition of IS HI clouds. Some heavy
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 element’s condensation temperature Tc.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 9
ASTR112 The Galaxy
Lecture 9
Dense molecular clouds
Molecule
formula
hydroxyl
OH
ammonia
NH3
water
H2O
formaldehyde H2CO
carbon monoxide CO
discovery
1963
1968
1968
1969
1970
λ
1.8 cm
1.3 cm
1.3 cm
6.2 cm
2.6 mm
number sources
~600
12
35
~150
60
Prof. John Hearnshaw
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.
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).
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 9
Microwave spectrum of emission lines
from a dense molecular cloud
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 9
ASTR112 The Galaxy
Lecture 9
• Temperature T ~ 10 to 30 K
• Number densities n ~ 108 – 1012 molecules m-3;
mass density ρ ~ 10-15 kg.m-3
• Cloud mass may be ~ 103 M⊙
• Cloud size ~ 10 pc
• Dense molecular clouds are often very dusty
Prof. John Hearnshaw
Properties of dense molecular clouds
ASTR112 The Galaxy
Lecture 9
• Also dust surfaces provide a site for the
formation of the H2 molecule. Other
molecules can form from gas phase
reactions.
Prof. John Hearnshaw
• Note that dust shields molecules from
UV radiation from stars, which would
dissociate most molecules.
ASTR112 The Galaxy
Lecture 9
• Dense molecular clouds are under
• They are consequently sites of star
formation
Prof. John Hearnshaw
gravitational collapse because there is
enough mass for self gravity to pull them
together.
Above: galactic distribution of CO in molecular clouds
Below: CO cloud radial velocity vs galactic longitude
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 9
ASTR112 The Galaxy
Lecture 9
1. η Car
2. M20
Trifid nebula
3. Orion nebula
4. M16 Eagle nebula
Prof. John Hearnshaw
Some HII nebulae which are
also associated with
dense molecular clouds
ASTR112 The Galaxy
Lecture 9
Note on pressures in the IS gas
P = nkT
Phase
HII
HI
dense molecular
hot HI
coronal gas
n (m-3)
108
107
109 to 1012
3 × 105
103
T (K)
9000
90
10 – 30
5000
106
P (Pa)
10-11
10-14
10-13 – 10-10
10-14
10-14
Prof. John Hearnshaw
P pressure (Pa);
n number density (m-3);
T absolute temp. (K); k Boltzmann’s constant (J.K-1)
ASTR112 The Galaxy
Lecture 9
• 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.
Prof. John Hearnshaw
• HI clouds are in pressure equilibrium with the
hot HI and coronal gas intercloud medium
ASTR112 The Galaxy
Lecture 9
• Note IS gas pressures are always very low.
On Earth 1 atmosphere ≈ 105 Pa, much
higher than in ISM
Prof. John Hearnshaw
• 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)
ASTR112 The Galaxy
Lecture 9
Interstellar masers
MASER: Microwave Amplification by Stimulated
Emission of Radiation
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.
Prof. John Hearnshaw
Observed in OH lines (λ ~ 18 cm) and sometimes in
lines of H2O (1.35 mm) and SiO (6.95 mm, 3.47 mm)
ASTR112 The Galaxy
Lecture 9
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).
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.
Prof. John Hearnshaw
Maser sources are compact and probably occur in
dusty regions associated with star formation or in
circumstellar dust shells around M-type stars.
ASTR112 The Galaxy
Lecture 9
Prof. John Hearnshaw
END OF LECTURE 9