astronomy 16: the interstellar medium 1 evidence for the ism how do we know there is an interstellar...

Astronomy 16: The Interstellar Medium 1

Evidence for the ISM• How do we know there is an interstellar medium (ISM)?

1) The Oort Limit

• Hydrostatic equilibrium, but for the whole Galaxy!

- gravity of Galactic disk balanced by "pressure" (= individual velocities) of stars

- measure velocities of stars → density of disk

• Total density: ρ0 0.08 M/pc3

• Density of stars: ρstars 0.06 M/pc3

• What's left? ρISM 0.02 M/pc3 1.3 x 10-24 g/cm3

nISM 0.8 H atoms / cm3

(but in very few places is the actual value close to this average!)

http://map.gsfc.nasa.gov/m

_uni/uni_101mw

.html

number density

massdensity


Extinction – Discrete Clouds2) Extinction

• Clearly present in discrete clouds spread throughout Galaxy

Dark cloud Barnard 68 (ESO / VLT ANTU)

Horsehead Nebula (Nigel Sharp / NOAO / NSF; © AURA)

http://www.astro.lu.se/Resources/Vintergatan/


Extinction – Diffuse Gas• Robert Trumpler (1930) :

- catalog of 100 open clusters spread throughout Galaxy

- cluster fitting: distance estimates for each cluster

→ "photometric distance"

- nearby clusters: diameter depends on concentration, number of stars → "diameter distance"

- plot "photometric" vs "diameter" distance:

Distant clusters are fainter than they should be! → ~0.7 mag/kpc (modern value: ~2 mag/kpc) of extinction

No globular clusters or background galaxies close to Galactic plane ("zone of avoidance")

photometric distance equalsdiameter distance

photometric distance more thandiameter distance

from T

rumpler, P

ublications of the A

stronomical Society of the P

acific, 42, 214 (1930)


Reddening & Spectra3) Reddening

- stars in same MK class have different B – V ; B – V increases with overall extinction

→ ISM also makes stars redder

4) Interstellar absorption lines

- in binary systems, some lines do not show Doppler shift due to binary motion

Dar

k cl

oud

Bar

nard

68

(ES

O /

VL

T A

NT

U)


Trumpler’s “Reddening”from

Trum

pler, Pub

licatio

ns of the

Astronom

ical Society of th

e Pacific, 42, 249

, 267


• Extinction is due to small dust particles in the ISM - combination of absorption and scattering

Absorption:

Scattering:

• At a given distance, a star appears fainter than implied by its distance modulus:

“AV = 3” means star is 3 magnitudes fainter in V filter due to dust

Towards Galactic center, AV 30 !

Aλ = kλ d , where kλ mag/pc is extinction coefficient at wavelength λ

Extinction & Dust

AdMm +−=− 5log5 10

extinction inmagnitudes (A > 0)


Recall optical depth, τλ : (stopped at this slide Tuesday)

In "stellar structure", we wrote:

Same situation here, but we convert ρ, to number density, n.

We thus now write:

where σλ is cross section (units m2 or cm2) of each dust grain.If dust grains were hard spheres of radius a & photons were bullets, then σλ = πa2 . But if light diffracts, σλ = Qλπa2 , whereQλ is "extinction efficiency factor" at wavelength λ.

E.g. graphite grainsof various radii

Note: Qext = Qabs + Qscat

Optical Depth & Cross Section

L

λτ−= eILI )0()(

rρκτ λλ =

ndλλ στ =

from Draine & Lee, The Astrophysical Journal, 285, 89 (1984)


Qλ is "extinction efficiency factor" at wavelength λ.

Optical Depth & Cross Section

or, a cartoon view…


So and

But how do we measure Aλ (or equivalently τλ) ?

Direct observation: VSpectrum: MV AV

Parallax: d

But if star is close enough for parallax, AV is probably small!

If we don't know d , we can't get AV !

Can resolve this because dust produces selective extinction - blue light gets scattered more than red light (blue skies, red sunsets) - more extinction → more reddening

Extinction & Optical Depth

λτ−−=

−=−

e

F

Fmm

dustwithout

observed

dustwithoutobserved

10

10

log5.2

log5.2

λλ τ 086.1=A nk λλ σ 086.1=

VV AdMV +−=− 5log5 10

}


Extinction curve:

So longer wavelengths show less extinction. Thus extinction not only changes magnitude, it changes color index also!

From shape of extinction curve, can show that (roughly!):

Reddeninghttp://w

ww

.iras.ucalgary.ca/~volk/figs1.html

note: inverse wavelength units!

V

B

bluered

0)( )( VBVBE VB −−−=−"color excess"

observedcolor index

intrinsiccolor index,

equal to MB – MV

1.3≈=−VB

VV E

AR


Example: O6III star is observed with V = +12.4 & B = +13.8

From HR diagram, we know thatO6III stars have (B – V)0 = -0.30 and MV = -5.5

What is distance to star?

Relation between dust & gas:

• Star's color excess gives amount of extinction

• Star's spectrum shows ISM absorption lines of H, from which equivalent width gives column density, NH = ∫ n dl

• EB-V vs NH gives straight line:

Comparison to dust in this room?

Color Excess & Dust/Gas Ratio

-1-221 mag cm 108.5 VBH EN −×=

from Diplas & Savage,

The Astrophysical Journal, 427, 274 (1994)


• Size of interstellar dust grains: 50 Å – 0.25 μm (cf. sand: 50-2000 μm, silt 2-50 μm, toner ~10 μm)

• Tiny part of ISM – 1 dust particle every 106 m3 ! - by mass, ISM is 99% gas, 1% dust

• Temp: absorbs photons, reradiates as 20-40 K blackbody

• Composition: silicates, graphite, water ice

• Formation: need high pressure, temperature steadily falling - condensation in winds of cool giants & of AGBs - expanding/cooling ejecta of novae & supernovae

• Critical role in astrochemistry : site of molecule formation - e.g. H2 molecule can never form by 2 H atoms colliding: tcollision 10-13 sec, tbond formation 10-9 sec → so atoms will usually just rebound

But H atoms can stick to dust grain & bond, then escape

Dust Properties & Formationhttp://w

ww

.ipac.caltech.edu/Outreach/G

allery/IRA

S/allsky.htm

l


• Reddened light is polarized

- grains preferentially absorb one pol. and leave other

- need something to break symmetry

→ dust grains are elongated, not round!

• But only works if all grains aligned in same direction

- global Galactic magnetic field causes alignment

Grain Shape & Polarization

from Worm & Blum,

The Astrophysical Journal, 529, L57 (2000)

from Han & Wielebinski, Chinese Journal of Astronomy & Astrophysics, 2, 293 (2002)


• Dust is important, but remember that 99% is gas!

• Abundances: 85% H, 10% He, 5% rest (by number)

• Gaseous ISM exists in (at least) five phases- molecular medium (MM)- cold neutral medium (CNM)- warm neutral medium (WNM)- warm ionized medium (WIM)- hot ionized medium (HIM) (aka "coronal gas")

The Gaseous ISMfrom

Wilm

s et al, The A

strophysical Journal, 542:914 (2000)


• Grouped into "clouds" – ill-defined variety of structures• M ~ 1 – 106 M; GMCs have M > 104 M

• size ~ 1-100 pc ; T ~ 10 K ; nH ~ 102 – 106 cm-3 • Opaque! E.g. nH = 104 cm-3 and L ~ 1 pc. What is AV ?

• 1% of ISM volume (f = 0.01), 50% of ISM mass!

• Almost entirely molecular hydrogen (H2), but H2 has few emission lines at low T & so is hard to see

• Best tracer: carbon monoxide - only 0.01% by number, but has rotational transition at λ = 2.6 mm (ν = 115.27 GHz) Not absorbed by dust – can see through whole Galaxy!

• 100s of molecules now detected: C2H5OH, C24H12, glycine…

• Only known site of star formation

Molecular Medium (MM)

from Dame et al, The Astrophysical Journal, 547, 792 (2001)


The Molecular Ring

from Clemens et al, The Astrophysical Journal, 327, 139 (1988)

• CO observations show inner Galaxy dominated by molecular ring at R ~ 4 kpc

• Many supernovae, H II regions, open clusters here also


• Cold Neutral Medium (CNM) - atomic hydrogen, nH ~ 20 cm-3, T ~ 100K, f ~ 0.02

• Warm Neutral Medium (WNM) - atomic hydrogen, nH ~ 0.3 cm-3, T ~ 6000K, f ~ 0.5

• Seen through "spin-flip" or "hyperfine" transition of H I - λ = 21.1 cm, ν = 1420.4 MHz (discovered at Harvard, 1951) - not absorbed by dust; most useful tracer of ISM

Neutral Mediumht

tp:/

/ins

truc

t1.c

it.c

orne

ll.e

du/c

ours

es/a

stro

101/

lec0

8.ht

mhttp://w

ww

.ras.ucalgary.ca/CG

PS

/gallery

CNM

(spontaneous transitionfrom high to low occurs once every 11 million years!)


• Ionization potential of H in ground state = 13.6 eV - photon w. E > 13.6 eV (UV: λ < 911 Å) can ionize H - H will recombine → Hα seen at 656.3 nm (why not Lyα?)

• Discrete component: "H II regions" (f ~ 0.03) - ionized bubbles produced by UV photons around hot stars - seen in Hα, in IR (hot dust), in radio ("free-free" emission)

Warm Ionized Medium

• Orion Nebula (Messier 42) - top left: Hα - top right: infrared - bottom left: radio

N.B.: extinction seen in optical, but not in IR/radio

NR

AO

/VL

A/G

BT

http

://w

ww

.am

tsgy

m-s

dbg.

dk/a

s/or

ion-

2002

/http://w

ww

.ipac.caltech.edu/2mass/


H II Regions


• Theorist's H II region: "Strömgren Sphere" - "photoionization equilibrium" between ionizations & recombinations

- LHS = total no. of ionizations per second

- RHS = total no. of recombinations per second

- S* = no. of ionizing photons emitted per second (can be derived from Planck equation)

e.g. O5 star: S* 5 x 1049 photons/sec B1 star: S* 3 x 1045 photons/sec

- R = radius of H II region (cm)

- nH = density of gas being ionized (cm-3)

- αB = "recombination coefficient" 2.5 x 10-13 cm3/sec

• UV has short mean free path: H II regions have sharp edges - 100% ionized inside, 0% ionized outside

• Oxygen and nitrogen ions in H II regions act as thermostat: T ~ 8000-10000 K regardless of central star

Strömgren Spheres

BHnRS απ 23

34

=∗


Hypothetical & Real HII Regions

Strömgren Says “Spheres,” with Radii…:

O Star will destroy it’s birthplace rather thoroughly.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

The Rosette Nebula

NGC 3603

Nature says…


• Diffuse component of WIM recently identified (f ~ 0.20 ?)

- aka "Diffuse Ionized Gas" (DIG) or "Reynolds Layer"

- faint Hα from recombinations over entire sky (hard to map)

- T ~ 8000 K, ne = nH+~ 0.1 cm-3

- H II regions confined to thin disk of height ~100-200 pc, but DIG is in disk of height ~ 1000 pc

- ionization source unknown: escaped photons from O stars?

Diffuse WIMhttp://w

ww

.skymaps.info/


• "Coronal gas" - n ~ 0.003 cm-3 ; T ~ (5-10) x 106 K ; f ~ 0.40? - first seen in O VI absorption lines towards stars - also seen in X-ray/UV emission (but absorbed by gas) - hot interiors of supernova remnants?

Hot Ionized Medium

• Left: optical image of edge-on spiral galaxy NGC 4631

John

Vic

kery

and

Jim

Mat

thes

/A

dam

Blo

ck/N

OA

O/A

UR

A/N

SF

• Right: X-rays (blue), UV from stars & H II regions (orange)

X-ray: N

AS

A/C

XC

/UM

ass/D.W

ang et al., U

V: N

AS

A/G

SF

C/U

IT)


• 1960s: "two phase ISM" (Field, Goldsmith & Habing 1969)

- cold (neutral) clouds, embedded in warm (10% ionized) intercloud medium; two phases in pressure balance

- occasional hot cavities produced by SNe, but not part of big picture

• 1970s: "3 phase ISM" (Cox & Smith 1974; McKee & Ostriker 1977)

- hot cavities left by old SNRs merge & interconnect

→ HIM is persistent & pervasive phase of ISM

- CNM=clouds; WNM/WIM=cloud envelopes; HIM=cavities

- pressure balance:

- probably not completely correct, but useful complete picture

The Multi-phase ISM

3cmK 1000/ −≈→= kPnkTP

3cmK 30002500~/ −−kP

from McKee & Ostriker, The Astrophysical Journal, 218, 148 (1977)


• Over billions of years, gas moves through all phases!

Recycling in the ISM

dust

starlight

SNRs

cooling

SNRs

star-light

recomb-ination

Adadpted from Dopita & Sutherland, "Astrophysics of the Diffuse Universe" (Springer, 2003)

See the reading, instead…


• Basic three-phase picture assumes SNe are randomly located

- but in reality SN progenitors found in "OB associations"

- clustered SNe: >100 stars, all going SNe within ~ 1 Myr!

→ "supershell" : similar evolution to SNR, but 100x energy

→ can escape from Galaxy's gravity to form "chimney"

Clustered Supernovae

~ 1 kpc !

from McClure-Griffiths et al, The Astrophysical Journal, 594, 833 (2003)

21cm H I

(WNM)

cooling?

HIM

astronomy 16: the interstellar medium 1 evidence for the ism how do we know there is an interstellar...

Documents