Chap 3 – Interstellar Dust

Problem We have no direct probes / samples of true interstellar dust. Everything we know comes from its interaction with light: absorption, reddening, polarization…

…and in Other Galaxies

How Much is There? ~1% of the ISM by mass. Immediate impact: at least in the visible, stars look redder (cooler) and fainter (misleadingly distance) if seen through dust. Moreover, the dust is patchy, not smooth.

History Long resisted / ignored, on the optimistic grounds that it would not be a general problem. Led Shapley to misleading conclusions about the size of our Milky Way galaxy (1918).

Irrefutable Proof Trumpler (in the 1930s) compared ‘geometrical distances’ of a number of open star clusters (assume they are all roughly the same size) to distance based on intercomparing apparent magnitudes of stars within them (assume the brightest stars in all clusters are ~the same). Think about the sense of this!

Later Studies… … showed that there is a correlation: stars in more obscured clusters tend to be redder.

How Much Exctinction? On average in the Milky Way, in the V band, it is ~ 1 mag per kpc of distance. Worked example with some round numbers!): the Galactic Centre is ~ 10 kpc distant, so (m-M) = 5 log d – 5 = 5 x 4 – 5 = 15. A star like the Sun is M V = +5, so would be V = 20 (accessible!) at the Galactic centre. But behind 10 mag of obscuration, it would be 30 m –utterly unreachable!

How to Beat It? One way: Look through ‘windows’ in the patchy obscuration (e.g. “Baade’s window” allows us to study the cental bulge of the galaxy [but not quite the true center]). Hardly ideal.

Better! Work at wavelengths that are less affected by the dust. This requires us to characterize its nature.

Approach Use spectroscopy to find a number of identical stars of some type. (The spectrum speaks truth, regardless of obscuration!) Determine true distances (e.g. from parallaxes)

Now Do Photometric Measurements Measure the brightesses in various bandpasses, and derive the apparent colours. This allows you to work out, relative to a nearby unobscured star of that type: How erroneously faint they appear to be: this is attributable to absorption in the visible (A V ) How much reddening has affected their colours (E B-V is the color excess in B-V, for instance) Repeat for as many spectral types and photometric bandpasses as you can.

Draw Up the “Extinction Curve” -- and try to figure out what dust grain (size, composition, structure, origin, distribution,…) could explain the effects (or what mix of different kinds of grains, of course!)

Colour Excess (Reddening) E B-V = A B – A V (think about the sense of this) Likewise E U-B = A U – A B E U-V = A U – A V = E U-B + E B-V etc

What Do We Notice?

Total Absorption (Note: not the same in all directions!)

Diagnostics Study the dependence of A λ on λ, and also the ratio of total to selective absorption Thus - a star that appears redder by 1 mag (in B-V) is 3 mags fainter in V.

Features? Linear dependence: absorption falls off as λ -1 Feature at ~220 nm: what causes it? Silicon bump? Graphite?

Why is the Sky Blue? Rayleigh scattering: λ -4 dependence Caused by molecules; small compared to the wavelength of light By contrast, close to a λ -1 dependence in the ISM Particles comparable in size to wavelength

No Unique Species A spectrum of sizes, compositions, etc. But more small grains than large: n(a) α a -3.5

What Composition? Clearly expected to be the common elements, on general arguments. See discussion in the text. Silicates; carbon molecules; frozen volatiles like water, ammonia, methane; etc. Could be core-envelope structures.

DIBs (Diffuse Interstellar Bands) Buckminster Fullerenes (C 60 ) (“Buckyballs”) now shown (in 2015) to be the cause of at least some of the mysterious DIBs seen in stellar spectra.

Whence the Dust? Cool stars? (E.g. R Cor Bor, a carbon-rich star) Red giants that pulsate, perhaps eject ‘sooty’ C debris that condenses in the expanding atmosphere (dimming the star), then gets blown out by radiation pressure

Also Supernovae SN ejecta are metal-rich, can lead to the formation of grains that then disperse into the more general ISM

Dust in Emission Cool dust emits in the IR (can be important for the cooling of clouds, allowing star formation to proceed).