Transcript Astronomy 16: Introduction

Evidence for the ISM
• How do we know there is an interstellar medium (ISM)?
1) The Oort Limit
http://map.gsfc.nasa.gov/m_uni/uni_101mw.html
• 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
ρ0  0.08 M/pc3
• Total density:
• Density of stars:
• What's left?
number
density
ρstars  0.06 M/pc3
mass
density
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!)
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Extinction – Discrete Clouds
2) 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/
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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"
photometric distance equals
diameter distance
photometric distance more than
diameter distance
from Trumpler, Publications of the
Astronomical Society of the Pacific, 42, 214 (1930)
- 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")
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Reddening & Spectra
3) Reddening
- stars in same MK class have different B – V ;
B – V increases with overall extinction
Dark cloud Barnard 68 (ESO / VLT ANTU)
→ ISM also makes stars redder
4) Interstellar absorption lines
- in binary systems, some lines do not show Doppler
shift due to binary motion
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Trumpler’s “Reddening”
from Trumpler, Publications of the
Astronomical Society of the Pacific, 42, 249, 267
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Astronomy 16: The Interstellar Medium
Extinction & Dust
• 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:
m  M  5 log10 d  5  A
extinction in
magnitudes (A > 0)
“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
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Optical Depth & Cross Section
Recall optical depth, τλ : (stopped at this slide Tuesday)
I ( L)  I (0) e 
L
In "stellar structure", we wrote:      r
Same situation here, but we convert ρ, to number density, n.
We thus now write:      nd
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 , where
Qλ is "extinction efficiency factor" at wavelength λ.
E.g. graphite grains
of various radii
Note:
Qext = Qabs + Qscat
from Draine & Lee, The Astrophysical Journal, 285, 89 (1984)
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Optical Depth & Cross Section
Qλ is "extinction efficiency factor" at wavelength λ.
or, a cartoon view…
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Extinction & Optical Depth
mobserved  mwithout
dust
Fobserved
 2.5 log10
Fwithout
dust
 2.5 log10 e  
So
A  1.086 
and
k  1.086  n
But how do we measure Aλ (or equivalently τλ) ?
V  MV  5 log10 d  5  AV
Direct observation: V
Spectrum:
MV
Parallax:
d
}
AV
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
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Reddening
Extinction curve:
V
blue
red
note: inverse
wavelength units!
http://www.iras.ucalgary.ca/~volk/figs1.html
B
So longer wavelengths show less extinction. Thus extinction
not only changes magnitude, it changes color index also!
"color
excess"
EBV  ( B  V )  ( B  V )0
observed
color index
intrinsic
color index,
equal to MB – MV
From shape of extinction curve, can show that (roughly!):
AV
RV 
 3.1
EB V
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Color Excess & Dust/Gas Ratio
Example: O6III star is observed with V = +12.4 & B = +13.8
From HR diagram, we know that
O6III 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
from Diplas & Savage,
• EB-V vs NH gives straight line:
The Astrophysical Journal, 427, 274 (1994)
NH  5.8 1021 EBV cm-2 mag-1
Comparison to dust in this room?
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Dust Properties & Formation
• 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
http://www.ipac.caltech.edu/Outreach/Gallery/IRAS/allsky.html
• 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
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Grain Shape & Polarization
• Reddened light is polarized
- grains preferentially
absorb one pol. and
leave other
- need something to break
symmetry
→ dust grains are elongated,
not round!
from Worm & Blum,
The Astrophysical Journal, 529, L57 (2000)
• But only works if all grains aligned in same direction
- global Galactic magnetic field causes alignment
from Han & Wielebinski, Chinese Journal of Astronomy & Astrophysics, 2, 293 (2002)
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The Gaseous ISM
• Dust is important, but remember that 99% is gas!
• Abundances: 85% H, 10% He, 5% rest (by number)
from Wilms et al, The Astrophysical Journal, 542:914 (2000)
• 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")
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Molecular Medium (MM)
• 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!
from Dame et al, The Astrophysical Journal, 547, 792 (2001)
• 100s of molecules now detected: C2H5OH, C24H12, glycine…
• Only known site of star formation
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The Molecular Ring
• CO observations show inner Galaxy dominated by
molecular ring at R ~ 4 kpc
• Many supernovae, H II regions, open clusters here also
from Clemens et al, The Astrophysical Journal, 327, 139 (1988)
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Neutral Medium
• 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
http://instruct1.cit.cornell.edu/courses/astro101/lec08.htm
• 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
(spontaneous transition
from high to low
occurs once every
11 million years!)
http://www.ras.ucalgary.ca/CGPS/gallery
CNM
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Warm Ionized Medium
• 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?)
NRAO/VLA/GBT
http://www.ipac.caltech.edu/2mass/
http://www.amtsgym-sdbg.dk/as/orion-2002/
• 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)
• Orion Nebula (Messier 42)
- top left: Hα
- top right: infrared
- bottom left: radio
N.B.: extinction seen in
optical, but not in IR/radio
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H II Regions
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Strömgren Spheres
• Theorist's H II region: "Strömgren Sphere"
- "photoionization equilibrium" between
ionizations & recombinations
4 3 2
S  R nH  B
3
- 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
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Hypothetical & Real HII Regions
Strömgren Says “Spheres,” with Radii…:
O Star will destroy
it’s birthplace rather thoroughly.
Nature says…
NGC 3603
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
The Rosette Nebula
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Diffuse WIM
• 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?
http://www.skymaps.info/
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Hot Ionized Medium
John Vickery and Jim Matthes/
Adam Block/NOAO/AURA/NSF
• "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?
• Left: optical image of
edge-on spiral galaxy
NGC 4631
X-ray: NASA/CXC/UMass/D.Wang et al.,
UV: NASA/GSFC/UIT)
• Right:
X-rays (blue),
UV from stars
& H II regions
(orange)
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The Multi-phase ISM
• 1960s: "two phase ISM" (Field, Goldsmith & Habing 1969)
- cold (neutral) clouds, embedded in warm (10% ionized)
intercloud medium; two phases in pressure balance
P  nkT  P / k  1000K cm3
- 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: P / k ~ 2500 3000 K cm3
- probably not completely correct, but useful complete picture
from McKee & Ostriker, The Astrophysical Journal, 218, 148 (1977)
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Recycling in the ISM
• Over billions of years, gas moves through all phases!
cooling
SNRs
recombination
starSee the
reading, instead…
light
dust
SNRs
starlight
Adadpted from Dopita & Sutherland,
"Astrophysics of the Diffuse Universe" (Springer, 2003)
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Clustered Supernovae
• 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"
21cm H I
(WNM)
HIM
~ 1 kpc !
cooling?
from McClure-Griffiths et al, The Astrophysical Journal, 594, 833 (2003)
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