Transcript m V

Prof. John Hearnshaw
IRAS view of warm dust
in plane of the Galaxy
ASTR112 The Galaxy
Lecture 10
13. The interstellar medium: dust
ASTR112 The Galaxy
Lecture 10
• Dust was first found in form of large dark clouds
(e.g. Coalsack, Horsehead etc) which are silhouetted
against bright backgrounds of stars or HII regions.
• Named ‘holes in the heavens’ by Wm Herschel (1785)
• Identified as obscuring clouds by E.E.Barnard in early
years of the 20th century.
Prof. John Hearnshaw
Dark clouds, reflection nebulae and Bok globules
ASTR112 The Galaxy
Lecture 10
Distribution of dark
clouds in the galactic
plane near the Sun
Prof. John Hearnshaw
Dark clouds:
• Typical size ~10 pc across
• Typical mass ~ 2000 M⊙
• Number known in Galaxy ~2600
• Galactic latitude
nearly always |b| < 10º
Distribution of dark clouds in the Milky Way
Most dark clouds are found near the galactic equator
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 10
ASTR112 The Galaxy
Lecture 10
Bok globules:
• Size 0.05 to 1 pc
• Mass 0.2 to 60 M⊙
• Often seen against a bright HII background
• Globules may be individual proto-stars condensing
from a dense molecular cloud
Prof. John Hearnshaw
Also seen are small very dense dark globules of dust,
known as Bok globules (after Bart Bok, who first drew
attention to them).
Bok globules in the nebula IC2944
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 10
Reflection nebulae:
• Light from a nearby star is scattered by dust grains into
the line of sight
• Colour is blue, as blue light is the most readily scattered
• Scattering of light from blue stars, usually type B;
spectrum is also of this type, i.e. absorption lines
• Light is often highly polarized (20 – 30 per cent)
• Amongst best known examples are the reflection
nebulae from circumstellar dust surrounding brightest
members of the Pleiades star cluster; also the reflection
nebula which is part of M20, the Trifid nebula
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 10
Reflection nebulae:
above: Pleiades
centre: M20 Trifid nebula
right: NGC1999
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 10
Other places where interstellar dust is found:
• General diffuse layer between dark clouds in plane
of Galaxy.
• This layer causes
(i) interstellar reddening of stars near the gal. equator,
(ii) interstellar polarization of starlight, and
(iii) diffuse galactic light (DGL).
• Also the infrared cirrus: low density whispy filaments
of dust seen by emission in IR, occurring very near
Sun and hence seen at fairly high galactic latitudes.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 10
ASTR112 The Galaxy
Lecture 10
Wolf diagrams
Prof. John Hearnshaw
Max Wolf (Heidelberg, 1923) analysed star counts in
direction towards a dark cloud to obtain the cloud
distance and estimate the amount of absorption (which
depends on cloud mass of dust).
ASTR112 The Galaxy
Lecture 10
For transparent space
m  M  5 log d  5
The number of stars brighter than magnitude m and
within distance d is:
N d
3
Hence:
log N  3 log d  const
m
and so
log N  0.6(m  M )  const
m
Prof. John Hearnshaw
m
ASTR112 The Galaxy
Lecture 10
If a dark cloud intervenes along the line of sight, then
stars behind the cloud go from magnitude m0 to
m = (m0 + A), where A is the extinction caused by
the cloud.
Both m0, a measure of cloud distance through
0
and A, a measure of the amount of dust in the cloud, can
be measured from the resulting step in the Wolf diagram.
Prof. John Hearnshaw
m  M  5 log d (pc)  5
Left: a schematic Wolf diagram
Right: actual Wolf diagram for the dark cloud NGC 6960
The vertical axis is the logarithm of the number of stars
per square degree brighter than a given apparent magnitude
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 10
ASTR112 The Galaxy
Lecture 10
The general dust layer: IS extinction and reddening
m  M  5 log d  5  A
V
•Reddening
E
B V
V
V
 (B  V )  (B  V )
obs
0
Prof. John Hearnshaw
• General dust layer demonstrated by Robert Trumpler (1930)
• Dust layer causes more distant low latitude stars to be
(a) fainter (IS extinction), and also
(b) redder (IS reddening).
• Extinction
ASTR112 The Galaxy
Lecture 10
• Both extinction AV and reddening EB-V are proportional
to the amount of dust along the line of sight
• In general extinction A(λ) is a function of wavelength, λ
• Whitford extinction law is:

valid from near ultraviolet to the infrared
• Ratio of extinction to reddening is roughly constant for
all stars affected by dust, irrespective of their distance
A
R
 3 .2
E
V
B V
Prof. John Hearnshaw
A 
1
Extinction and reddening by IS dust grains
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 10
Reddening of starlight by interstellar dust
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 10
ASTR112 The Galaxy
Lecture 10
• λ = 12, 25, 60, 100 μm
• Dust often occurs in dense molecular clouds, T ~10 K
which therefore emits most strongly at 100 μm
• But IRAS found many warmer discrete sources in
molecular clouds, corresponding to solar mass
proto-stars inside dusty shells
• IRAS also discovered the infrared cirrus
Prof. John Hearnshaw
Dust observed by IRAS (1984)
ASTR112 The Galaxy
Lecture 10
Below: a detail of the
Galaxy’s dust layer as
revealed by IRAS
Prof. John Hearnshaw
Above: IRAS all-sky
image of the dust layer
in the Galaxy from IR
thermal emission from
dust grains.
ASTR112 The Galaxy
Lecture 10
Prof. John Hearnshaw
IRAS infrared cirrus
at the north galactic pole.
Image constructed from
12, 60 and 100 μm
wavelengths.
ASTR112 The Galaxy
Lecture 10
• Total dust mass is ~1 per cent of mass of ISM
(remainder is gas)
• Mean dust density in the galactic disk is
ndust ~ 10-6 grains/m3
Compare this to mean gas density of
ngas ~ 10+6 gas atoms/m3
• Mean visual extinction in galactic plane (b = 0º) is
AV ~ 1 to 2 mag. for each kpc of distance
but the distribution is very patchy.
Prof. John Hearnshaw
Statistics for galactic dust
ASTR112 The Galaxy
Lecture 10
Calculation example for IS extinction
Photometry of a star gives mV = 14.61, (B – V) = 1.1;
spectroscopy indicates the spectral type is G0 V.
For G0 V stars, (B – V)0 = 0.60 and MV = 5.0.
Distance modulus = mV0 – MV = 5logd – 5
so 5logd – 5 = 13.01 – 5.0 = 8.01 or logd = 2.602
Thus
d = 400 pc
Prof. John Hearnshaw
Hence EB-V = (B-V)obs – (B-V)0 = 0.50
Therefore AV = 3.2 EB-V = 1.60
giving mV0 = mV – AV = 14.61 – 1.60 = 13.01
ASTR112 The Galaxy
Lecture 10
• Satellite observations used for UV stellar photometry
(λ < 300 nm) allow the extinction law A(λ) to be
measured in UV.
• Results show that Whitford law (A(λ)  1/λ) is not
valid in UV.
• Maximum extinction at about 220 nm
• Broad minimum in extinction from λ < 200 nm
down to λ = 125 nm
• The extinction rises steeply in far UV for λ < 125 nm
Prof. John Hearnshaw
Extinction in ultraviolet (UV)
ASTR112 The Galaxy
Lecture 10
Prof. John Hearnshaw
UV extinction plot
versus wavelength
showing the 220 μm
graphite peak.
ASTR112 The Galaxy
Lecture 10
• Extinction is small in infrared
• However some M giant stars have dust shells
around them giving large circumstellar extinction
• These circumstellar grains probably form in the
atmosphere of the M star itself
• Such stars generally show a broad dip in spectrum
at λ ~ 9.7 μm, presumed to be caused by silicate
dust grains
• Silicate dust grains are also thought to be the major
component of interstellar dust grains
Prof. John Hearnshaw
Extinction in infrared
Broad IR absorption
features in the spectrum
of an IR source are
bands produced by
solid grains, such as
ices and silicates. The
particles are probably
circumstellar.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 10
ASTR112 The Galaxy
Lecture 10
• No single grain composition or size fits all the data
• Various possible models include: ice grains, graphite,
silicates, silicates plus ice mantle, polycyclic aromatic
hydrocarbons (PAHs), dirty ice grains (H2O plus
H,C,N,O compounds), metallic grains
• Visual extinction is best explained by silicate cores,
ice mantles, particle size ~ 100 nm
• Graphite grains explain the 220 nm extinction peak;
size ~ 50 nm
• Far UV extinction from silicates, size 5 – 20 nm; also
silicates explain 9.7 μm circumstellar extinction in IR
Prof. John Hearnshaw
Nature of interstellar dust grains
A typical dust grain
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 10
Prof. John Hearnshaw
IRAS satellite: whole sky image
of IS dust in the Galaxy
ASTR112 The Galaxy
Lecture 10
End of lecture 10