ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw 13. The interstellar medium: dust IRAS view of warm dust in plane of the Galaxy

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw 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 H II regions. Named ‘holes in the heavens’ by Wm Herschel (1785) Identified as obscuring clouds by E.E.Barnard in early years of the 20 th century.

ASTR112 The Galaxy Lecture 10 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 galactic plane near the Sun

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw Distribution of dark clouds in the Milky Way Most dark clouds are found near the galactic equator

ASTR112 The Galaxy Lecture 10 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: Size 0.05 to 1 pc Mass 0.2 to 60 M ⊙ Often seen against a bright H II background Globules may be individual proto-stars condensing from a dense molecular cloud

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw Bok globules in the nebula IC2944

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw 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 Galaxy Lecture 10 Prof. John Hearnshaw Reflection nebulae: above: Pleiades centre: M20 Trifid nebula right: NGC1999

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw 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 Galaxy Lecture 10 Prof. John Hearnshaw Wolf diagrams 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 Prof. John Hearnshaw For transparent space The number of stars brighter than magnitude m and within distance d is: Hence: and so

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw If a dark cloud intervenes along the line of sight, then stars behind the cloud go from magnitude m 0 to m = (m 0 + A), where A is the extinction caused by the cloud. Both m 0, 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.

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw 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

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw 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

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw Both extinction A V and reddening E B-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

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw Extinction and reddening by IS dust grains

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw Reddening of starlight by interstellar dust

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw 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 Galaxy Lecture 10 Prof. John Hearnshaw Above: IRAS all-sky image of the dust layer in the Galaxy from IR thermal emission from dust grains. Below: a detail of the Galaxy’s dust layer as revealed by IRAS

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 Prof. John Hearnshaw 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 n dust ~ grains/m 3 Compare this to mean gas density of n gas ~ gas atoms/m 3 Mean visual extinction in galactic plane (b = 0º) is A V ~ 1 to 2 mag. for each kpc of distance but the distribution is very patchy.

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw Calculation example for IS extinction Photometry of a star gives m V = 14.61, (B – V) = 1.1; spectroscopy indicates the spectral type is G0 V. For G0 V stars, (B – V) 0 = 0.60 and M V = 5.0. Hence E B-V = (B-V) obs – (B-V) 0 = 0.50 Therefore A V = 3.2 E B-V = 1.60 giving m V0 = m V – A V = – 1.60 = Distance modulus = m V0 – M V = 5logd – 5 so 5logd – 5 = – 5.0 = 8.01 or logd = Thus d = 400 pc

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw 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 Galaxy Lecture 10 Prof. John Hearnshaw UV extinction plot versus wavelength showing the 220 μm graphite peak.

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw 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 Galaxy Lecture 10 Prof. John Hearnshaw 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.

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw 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 (H 2 O 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 Galaxy Lecture 10 Prof. John Hearnshaw A typical dust grain

ASTR112 The Galaxy Lecture 10 Prof. John Hearnshaw End of lecture 10 IRAS satellite: whole sky image of IS dust in the Galaxy