13. The interstellar medium: dust
description
Transcript of 13. The interstellar medium: dust
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
13. The interstellar medium: dust
IRA
S v
iew
of
war
m d
ust
in p
lane
of
the
Gal
axy
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Dark clouds, reflection nebulae and Bok globules
• 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.
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Dark clouds:• Typical size ~10 pc across• Typical mass ~ 2000 M⊙
• Number known in Galaxy ~2600• Galactic latitude nearly always |b| < 10º
Distribution of darkclouds in the galacticplane near the Sun
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Distribution of dark clouds in the Milky WayMost dark clouds are found near the galactic equator
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
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:• 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
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Bok globules in the nebula IC2944
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
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
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Reflection nebulae:above: Pleiadescentre: M20 Trifid nebularight: NGC1999
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
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.
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Wolf diagrams
Max Wolf (Heidelberg, 1923) analysed star counts indirection towards a dark cloud to obtain the clouddistance and estimate the amount of absorption (which depends on cloud mass of dust).
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
5log5 dMm
3dNm
constlog3log dNm
const)(6.0log MmNm
For transparent space
The number of stars brighter than magnitude m and within distance d is:
Hence:
and so
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
If a dark cloud intervenes along the line of sight, thenstars 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
and A, a measure of the amount of dust in the cloud, can be measured from the resulting step in the Wolf diagram.
5)pc(log50
dMm
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Left: a schematic Wolf diagramRight: actual Wolf diagram for the dark cloud NGC 6960The vertical axis is the logarithm of the number of starsper square degree brighter than a given apparent magnitude
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
The general dust layer: IS extinction and reddening
• 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
•Reddening
VVVAdMm 5log5
0obs)()( VBVBE
VB
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
• 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
1A
2.3 VB
V
EA
R
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Extinction and reddening by IS dust grains
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Reddening of starlight by interstellar dust
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Dust observed by IRAS (1984)
• 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
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Above: IRAS all-skyimage of the dust layer in the Galaxy from IRthermal emission fromdust grains.
Below: a detail of theGalaxy’s dust layer as revealed by IRAS
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
IRAS infrared cirrusat the north galactic pole.Image constructed from12, 60 and 100 μmwavelengths.
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Statistics for galactic dust
• 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.
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
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.
Hence EB-V = (B-V)obs – (B-V)0 = 0.50Therefore AV = 3.2 EB-V = 1.60 giving mV0 = mV – AV = 14.61 – 1.60 = 13.01
Distance modulus = mV0 – MV = 5logd – 5so 5logd – 5 = 13.01 – 5.0 = 8.01 or logd = 2.602Thus d = 400 pc
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Extinction in ultraviolet (UV)
• 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
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
UV extinction plotversus wavelengthshowing the 220 μmgraphite peak.
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Extinction in infrared
• 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
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Broad IR absorptionfeatures in the spectrumof an IR source arebands produced bysolid grains, such asices and silicates. Theparticles are probablycircumstellar.
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
Nature of interstellar dust grains
• 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
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
A typical dust grain
ASTR112 The GalaxyLecture 10
Pro
f. J
ohn
Hea
rnsh
aw
End of lecture 10
IRA
S s
atel
lite:
who
le s
ky im
age
of
IS
dus
t in
the
Gal
axy