Download Much of our original knowledge of interstellar dust comes from a

Interstellar Grains
Much of our original knowledge of interstellar dust comes from a 0.60 m telescope dubbed IRAS, the Infrared Astronomical Satellite (1983). IRAS data:
­covered over 96% of the sky at 12, 25, 60, and 100 m
­provided our first view of the extragalactic sky at far­IR wavelengths
­had uniform sensitivity limits: 0.5 Jy at 12, 25, 60 m; 1.5 Jy at 100 m
­experienced negligible Galactic extinction
­had a typical positional uncertainty of 4"x15" (1)
In ½ hour, IRAS obtained more >5m data than all previous efforts combined. The IRAS mission lasted 10 months. In addition to cataloging ~250,000 point sources, IRAS detected 20,000 extended sources and diffuse emission from the entire sky. The COBE, WMAP, and Planck satellites subsequently studied the latter in more detail the 1990s, 2000s, and 2010s.
Shameless plug
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Interstellar Grains
Other important space­based infrared telescopes (& don't forget ground­based 2MASS):
Infrared Space Observatory
Spitzer Space Telescope
0.60 m
mid­/far­IR imaging
mid­/far­IR spectroscopy
0.85 m
mid­/far­IR imaging
mid­IR spectroscopy
WISE
Akari
0.67 m
near­/mid­/far­IR imaging
0.40 m
mid­IR imaging
It's big!
SOFIA
2.5 m
mid­/far­IR imaging
IR/submm spectroscopy
JWST
Herschel Space Observatory
3.6 m
far­IR/submm imaging
far­IR/submm spectroscopy
6.5 m
mid­IR imaging
launch: 2018?
ASTR 5470 Physics of the Interstellar Medium
WFIRST
2.4 m
near­IR imaging
near­IR spectro
launch: 2025?
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Interstellar Grains
Why (stratosphere­ and) space­based? The atmosphere both and in the infrared.
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Interstellar Grains
Why (stratosphere­ and) space­based? The atmosphere both absorbs and emits in the infrared.
J
H
K
L
M
L'
Ks
N
Q
gemini.edu/sciops/telescopes­and­sites/observing­condition­constraints?q=node/10789
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Interstellar Grains
If we model the dust distribution in the Milky Way as a uniform slab of thickness 2h, then we'd expect the dust emission to be proportional to the cosecant of the Galactic latitude b:
f(IR) ∝ hcsc(b). This is consistent with observations (below figure).
­Interstellar dust is composed of clumps of atoms and molecules, much like common smoke, soot, fog, chalk dust, ...
­Interstellar dust is typically of size ~100nm  good scatterer of optical light
­Emission by dust is often fit by a single Planck function times a ­dependent emissivity
f ∝ B(Tdust)
e.g., ∝
­Dust can polarize light
 elongated and aligned
­30­50% of a galaxy's luminosity is stellar light reprocessed by dust
­Interstellar space is 106 times 'dirtier' than Earth's atmosphere
­1 Earth volume = 1 pair of dice
Binney & Merrifield 1998
ASTR 5470 Physics of the Interstellar Medium
b
2h
MW HI
MW dust
Shows that dust comes
from the ISM!
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Interstellar Grains
From prior page: Emission by dust is often fit by a single Planck function times a ­dependent emissivity
f∝B(Tdust) e.g., ∝
Dale+ 2012
Calculating a single dust temperature is very common in extragalactic work. In which situations do you think it is unwarranted? One application of the dust T is
computing how much dust mass is required for the observed L and T,
Mdust ∝ f/B(Td) .
Frequently used to compute the
dust­to­gas ratio, and compared to
that of the Milky Way.
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Interstellar Grains
The Milky Way is patchy  extinction by dust
I = Ioe­
for a<<, ∝­4 (Rayleigh)
for a>>, ∝0 (geometrical)
what does this mean?
Extinction is higher towards the  stars appear . Thus the term 'reddening'.
What about the 'blueing' of
reflection nebulae?!
a ~0.01­1m (particles are ~size of visual photons)
Extinction is ~ 1 mag/kpc in solar neighborhood
Early references: Struve 1847; Light scattering by small particles van de Hulst 1957
Definitions
Q = opt/geom
extinction efficiency
scat/geom
scattering efficiency
Qs =
Qa = abs/geom
Q = Qs + Qa
absorption efficiency
total extinction efficiency
=
opt/a2 (for spheres)
Can dust particles absorb?
AV = extinction in V band (~550nm)
AV/NHtot ~ dust/gas ratio
It is found that AV/NHtot ~ mag cm2  a given mass of gas contains a characteristic amount of dust, irrespective of whether the hydrogen is in the form of H or H 2.
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Interstellar Grains
What Do We Observe in the Milky Way?
The Neutral Hydrogen View
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Interstellar Grains
What Do We Observe in the Milky Way?
The Far­Infrared View
COsmic Background Explorer 60­240 m image
Hey, what's that
blue 'S' curve?
Mathis 1990
Galactic extinction curves show peaks characteristic
of a few distinct species of dust grains
Classical grains of size are primarily responsible
for the diffuse Galactic emission at long wavelengths
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Interstellar Grains
IV = IVo10­Av/2.5, NH = nHds = nHD for uniform density
D
I = Ioe­  V=AVln10/2.5 = AV 2.30/2.5 = AV0.921 = AV/1.086
and V = Qa2ndD = optndD and NH = nHD
Now we can estimate the gas­to­dust ratio
V/NH= Qa2nd/nH  nH/nd = Qa2NH/V
dust = 4/3a3grainnd = dust mass/volume
gas = mHnH
 gas/dust = = ¾ mH/a3grain Qa2 NH/V = 1.086 ¾ mH/a Q/grain NH/AV
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Interstellar Grains
Take a~0.1m, grain~ 2 g cm­3, mH = 1.67 10­24 g
Q varies with , but for optical  and typical grain size, Q is of order 100.
 gas/dust = 130 ~ 102
Reference for gas­to­dust ratios in spiral galaxies: Mayya & Rengarajan (1997), Remy­Ruyer+2014
Q=Qa+Qs
This ratio is roughly what one expects if we sum up metal­to­hydrogen ratios for the solar
neighborhood abundances (see table from Ch. 2 notes).
(ni/nH) A.W.i­1 ~ 102
2
What does this mean plot mean, and how can a cross­section
be twice as large as the geometrical value?
1
5
10
x=2a/
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Interstellar Grains
Extinction
The dimming of light A = (m­m0) p.133 Binney & Merrifield 1998
Reddening or a general color excess is the difference between observed and intrinsic color
E(X­Y) = [m(x)­m(y)] – [m0(x)­m0(y)] = Ax­Ay Some standard filters for optical astronomy
 (nm)
 (nm)
U 365 66
u' 356 60
B 445 94
g' 483 138
V 551 88
r' 626 138
R 658 138
i'
767 153
I
806 149
z' 910 137
The standard color excess quoted is E(B­V). For much of the ISM, AV~3.1E(B­V) (see Slide 15).
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Interstellar Grains
Extinction curve
A ­4
The extinction curve broadly maximizes near 0.1m because there are many dust grains in the ISM of this size. The sharper features arise from absorption by other particular dust grain types.
­4
Does AV scale with any parameter in this listing?
Why doesn't this table obey the canonical 1 mag/kpc we
learned about on Slide 7?
A_V (mag)
N_H (cm-2)
n_H (cm-3)
R (pc)
T (K)
M (Msun)
Globule
IR Cloud
Dark Cloud
Diffuse Cloud
Cloud Complex
4
8e21
7000
0.3
10
30
30
6e22
40000
0.4
50
400
4
8e21
2000
1
10
300
0.2
4e20
20
5
80
400
4
8e21
200
10
10
30000
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Milky Way Extinction (Draine 2003) Interstellar Grains
RVAV/(AB­AV) characterizes the slope of the extinction curve over optical wavelengths. Very large why?
grains would produce RV whereas Rayleigh scattering (A∝­4) would produce very steep extinction
with RV1.2. A 'standard' Milky Way value is RV3.1.
We discussed how AV/NH was essentially constant
for every line­of­sight. Work by a UWyo PhD shows
that this is true to within a factor of 2 (below).
Does the trend make sense?
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Interstellar Grains
Milky Way Extinction (Draine 2003) There are over 150 'diffuse interstellar bands' that are probably due to PAHs. They were discovered in 1922, but not a single one has been convincingly identified!
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Interstellar Grains
Thermal Emission from Large Interstellar Dust Grains
Dust absorbs photons, typically in the UV or visible since the cross­section is higher there, and
then radiates (usually in the infrared — a dust grain cannot get too hot or it will evaporate!).
¼
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Interstellar Grains
Blackbody reminder
If the stars radiate
as blackbodies, the
main sequence
profiles are defined
by the effective
stellar temperatures
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Interstellar Grains
where emissivity per unit area per steradian =
QemB
=
QabsB
(by Kirchoff's Law)
Suppose we have very big particles, e.g., planets or asteroids. Now in general, the l.h.s. of
the radiative balance is dominated by optical/UV photons and the r.h.s. by the emission of infrared photons. Let us take the average visual albedo to be A, implying Qabs=1­A, and the average emissivity in the infrared to be Qem= . Let us assume rapid rotation  isothermal planet or asteroid.
Using Wien's Law (peakT ~ 0.29 cm K) gives peak ~ 10m for T=279 K (in the mid­infrared)
Note that if the object were not rotating (and not thermally conductive), we have
¼
In other words, the effective radiating area is a2 rather than 4a2 for the subsolar point (i.e., at 'noon'). The moon could be an example. For a large object like an asteroid we would have Qem~1. However, for a dust particle in the infrared Qem=Qabs<<1 in general because the particle is much smaller than the wavelength. In the visible Qabs~1. All this means that small particles will be hotter than larger ones because they do not radiate very efficiently in the infrared.
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What do we see emitted from dust?
Interstellar Grains
As we've already seen, the extinction curve toward stars is fairly smooth and exhibits a peak near 220nm, which is attributed to large PAH clusters. The bulk of the absorbed starlight heats large dust grains to temperatures ~20K, grains that are thinly spread throughout the ISM. These grains are responsible for the 'diffuse cirrus' emission (see Slide 9; of course those dust particles very near hot stars will be heated to much higher T). According to Wien's Law, the characteristic wavelength of emission is ~150m. fn
In fact, up to 50% of the energy radiated in the Universe is emitted at IR wavelengths.
cirrus spectrum
Dole et al. 2006
150mm
physics.uwyo.edu/~ddale/research/wysh/wysh1.gif
swire.ipac.caltech.edu/swire/astronomers/strategies.html
Galaxy formation & evolution processes contribute about 5% of the electro­magnetic content of the Universe, with about half from starlight and half from dust
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Interstellar Grains
All is not rosy with this simple picture of dust emission, however. In some cases dust is found which is much hotter than you would expect to find on the basis of thermal equilibrium calculations, e.g., the case of the reflection nebula NGC 7023.
L*=104 Lsun
d=0.15 pc
 Td = 17 K (QUV/QIR)1/4 where QUV and QIR are average UV and IR absorption and
emissivity. For a plausible range of QUV and QIR, we'd expect Td = 70­150 K. But this is not what is observed! If dust grains near NGC7023 are in thermal equilibrium with the surrounding interstellar radiation field, then the observations indicate Td >1000 K !!
One possible explanation is that the observations are of reflected
starlight — that light should be bluer than the stellar spectrum, but observations do not support this explanation (it isn't bluer).
What is the answer? Stochastically­heated small grains
If an energetic photon strikes a very small particle, one composed
of maybe 50 atoms (e.g., polycyclic aromatic hydrocarbons), the
particle may significantly heat up.
ASTR 5470 Physics of the Interstellar Medium
Sellgren, Werner, & Dinerstein 1983
20
Stochastic Heating of Small Interstellar Aromatic Grains
For low Td we have the Debye law: Cv ∝ Td3
For high Td Cv = 3Nk
why?
Grain temp
Interstellar Grains
(6N = # degrees of freedom)
Minutes
Time
The dividing line between low and high temperatures is the 'Debye Temperature' which is
200­500 K for typical grain material. It is usually indicated by D.
Suppose the dust becomes very warm when a photon hits it, so that Td>D. The main contribution to the heat capacity will be the high T component, so Cv=3Nk. Then h=3Nk Td.
In NGC 7023, brightness temperatures of ~1000 K are seen from the dust. Supposing ~2000 Å (near the peak of a moderately warm star),
N = h / 3kTd ~ 24
Approximately 24 'atoms' are present in the particle! A more careful analysis gives N=70­90 atoms for silicate or graphite grains. Thus the physics of very small grains was born.
Note that if we assume a lattice spacing of 3Å: N=4/3a3(1/3Å)3
 a=3(¾ N / )1/3 ~ 3(¾ 80 / )1/3 ~ 8Å for the particle size
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Interstellar Grains
Grain temp
Stochastic Heating of Small Interstellar Aromatic Grains – back of the envelope
Minutes
Time
22
Interstellar Grains
Stochastic Heating of Small Interstellar Aromatic Grains – a realistic model
A day in the life of 4 carbonaceous grains, heated by the local interstellar radiation field. abs is the mean time between photon absorptions (Draine 2003; Draine & Li 2001).
Recall from our notes that PAHS smaller than 15Å are responsible for half of the heating of
the neutral ISM, and grains between 15 and 100Å are responsible for the other half. Recall also that
the diffuse cirrus is caused by 'classical' large grains of size 2000Å.
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Interstellar Grains
Naphthalene
Coronene
C24H12
PAHs
(simple ones)
Phenanthrene
Chrysene
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Interstellar Grains
What Do We Observe in the Milky Way?
The Mid­Infrared View
ISO spectrum of: NGC 7023 (reflection nebula)
M 17 (star­forming/HII region)
Cesarsky et al. 1996
Polycyclic Aromatic HydroCarbons
~50 Carbon atom planar structures with Hydrogen atom appendages
Astrochemistry Lab Models
Roelfsema et al. 1996
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Interstellar Grains
Mid­Infrared Emission Features Are Ubiquitous
Such features are typically seen in: planetary nebulae, proto­planetary nebulae, reflection nebulae, H II regions, circumstellar envelopes, Galactic diffuse cirrus, external galaxies, Buicks, ...
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Interstellar Grains
Mid­Infrared PAH Features (Draine 2003)
The mid­infrared features are attributed to vibrational modes (bending, stretching) of PAHs (e.g. Leger & Puget 1984). When H atoms are attached to the edge of an aromatic ring skeleton, the system will vibrate if triggered by an optical or UV photon.
The bending modes can be 'mono' (no adjacent H), 'duo' (2 contiguous H), 'trio' (3 contiguous), or 'quartet' (4 contiguous).
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Interstellar Grains
The Infrared Spectral Energy Distribution for normal galaxies of different star formation activity levels. Note the presence of mid­infrared features (in addition to the far­infrared thermal emission).
Dale+14
lambda
3.0m
9.7
18.0
11
20
3.3,6.2,
7.7,8.6,
11.2,12.0,
12.7,16.4,
17.1
identification
ice (absorption)
silicate (emiss/abs)
silicate (emiss/abs)
SiC
silicate
PAH
PAH
PAH
PAH
PAH
References from ISO+Spitzer:
Helou et al. 2000
Lu et al. 2003
Dale et al. 2001; 2002
Peeters et al. 2002
Smith et al. 2004; 2007
Sturm et al. 2000
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Interstellar Grains
The Infrared Spectral Energy Distribution for normal galaxies of different star formation activity levels. Note the presence of mid­infrared features (in addition to the far­infrared thermal emission).
lambda
3.0m
9.7
18.0
11
20
3.3,6.2,
7.7,8.6,
11.2,12.0,
12.7,16.4,
17.1
identification
ice (absorption)
silicate (emiss/abs)
silicate (emiss/abs)
SiC
silicate
PAH
PAH
PAH
PAH
PAH
References from ISO+Spitzer:
Helou et al. 2000
Lu et al. 2003
Dale et al. 2001; 2002
Peeters et al. 2002
Smith et al. 2004; 2007
Sturm et al. 2000
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Interstellar Grains
Q: What is the physical significance of the different far­infrared shapes below?
Q: Can you make a quantitative prediction for the characteristics of the far­infrared hump?
Q: Why don't the mid­infrared features change in
wavelength as well?
Q: Do you think the infrared spectrum helps or hinders photometric redshift efforts?
Kennicutt et al. 2003
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Interstellar Grains
Q: Do you think the infrared spectrum helps or hinders photometric redshift efforts?
Spitzer infrared color tracks from
z=0 to1
A photometric redshift is a redshift estimated according to the observed colors and a suite of galaxy SED templates. Ideally, a single template at a single redshift will be the unique solution. In practice, different templates at different redshifts can provide equally reasonable fits.
Dale+01
f
Galaxy SED
z
filter filter filter
l
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Interstellar Grains
Challenges to photometric redshift efforts:
­optical spectrum is variable according to star formation history
­optical spectrum can be reddened and attenuated due to dust extinction
­having the proper suite of template SEDs (e.g., AGN add additional confusion)
­far­infrared peaks at different wavelengths for different dust temperatures
­mid­infrared feature (PAHs) can be minimal/absent for particularly hard radiation fields
Thus, ideally one will have photometry from a large number of wavelengths, in order to have
the best chance of constraining the redshift.
f
Galaxy SED
z
filter filter filter
l
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Interstellar Grains
http://lsst.org/files/img/ColorZ.gif
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Interstellar Grains
A k­correction is the correction made to an observed flux or color to account for the redshifting of the SED with respect to the fixed filter(s) bandpass. In the submillimeter there is a unique twist, a 'negative' k­correction. The 850m flux stays relatively fixed for all observable redshifts, making this wavelength particularly powerful for carrying out cosmological surveys of dusty galaxies.
Galaxy SED
f
z
filter
850mm
l
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Interstellar Grains
The Infrared­to­Radio correlation is one of the tightest correlations known for galaxies (see
previous IR­Radio SEDs on p. 28). References: Helou, Soifer, Rowan­Robinson 1985
Condon 1992
Yun, Reddy, Condon 2001
The correlation relies on the trapping of supernovae­ejected electrons within galaxy magnetic field lines, and is defined as
q = log IR/Radio
It is a global correlation, but we are exploring it on small scales with Spitzer and Herschel.
­
e
s spiraling around interstellar
magnetic field linessynchrotron radiation
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Yun, Reddy, Condon 2001 35
Interstellar Grains
The Infrared­to­Radio correlation
We understand the basic outline of the physics of this correlation. Massive stars are responsible
for both heating dust grains which re­emit in the infrared, and for kicking out energetic electrons in supernovae phenomena which spiral around galaxy magnetic field lines producing synchrotron radiation.
However, though all stars can contribute to dust heating not all stars go supernovae. Moreover,
the mean free path for UV photons is only ~100 pc whereas the diffusion length scale for cosmic ray electrons is 1­2 kpc (after this distance the electrons typically become entangled in galaxy magnetic fields). Thus, there must be some sort of global averaging to get the global infrared­to­radio ratio to be so remarkably constant among galaxies.
Also surprising is that this constancy holds for a wide range of galaxies. It holds for galaxy
samples exhibiting a variety of metallicities, magnetic field strengths, masses, grain populations, and star formation rates. An interesting question is what occurs on smaller scales within galaxies, scales for which an
even larger diversity in metallicities, magnetic fields, dust grains, and star formation rates might be explored?
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The Infrared­to­Radio correlation
Locally, the infrared­to­radio relation is higher in spiral arms and other sites of star formation;
HII regions have an excess of
infrared emission relative to
the radio continuum emission.
Left to right:
1. 24m map
2. q70=70m/22cm (FIR vs synchrotron)
3. q24=24m/22cm (MIR vs synchrotron)
Murphy+06
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The Infrared­to­Radio correlation
Aperture masks overlaid
on 24m images
nucleus:
cyan
inner­disk: red
outer­disk: yellow
arm:
blue
inter­arm: green
(Murphy et al. 2005)
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Interstellar Grains
The Infrared­to­Radio correlation
q70 grouped by
different
environments
within each
galaxy. Solid
lines indicate the
expected trend if
the galaxy disk
was characterized
by a constant
radio surface
brightness.
Murphy06
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Interstellar Grains
The Infrared­to­Radio correlation
List four properties of a dwarf irregular galaxy that has recently undergone a burst of star
formation. Be prepared to discuss how each property could affect the infrared­to­radio ratio.
1.
2.
3.
4.
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Interstellar Grains
Much of the PAH emission we observe in the Milky Way is stimulated by the absorption of optical photons with energy of order 1 eV. But the strong mid­infrared PAH features are at ~10m and thus involve photons being released with energy ~0.1 eV. Explain how optical photons can be responsible for PAH features—where does all the 'leftover' energy go?
It is impossible for an isolated electron to absorb a photon and conserve both momentum
and total relativistic energy. Is it possible for an isolated atom to absorb a photon?
The work function of a typical grain is 5 eV. Photons that escape from H II regions have energies at least as high as 11 eV. If a typical “classical” grain is 100 nm across, how ionized
can the grain typically become in an 11 eV radiation field?
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Homework Recap:
Interstellar Grains
8. Emission and absorption: cooling and heating
a) Why isn't the absorption of a photon by an atom or molecule in an interstellar cloud necessarily a heating event?
It deposits energy into the cloud, but what else must happen for it to heat the cloud?
b) Why isn't the emission of a photon by an atom or molecule in an interstellar cloud necessarily a cooling event?
Answers
a) Consider a 3 eV photon (Joe) that leaves his star (Sally) and heads out for his morning work of exciting hydrogen.
Consider a 13.7 eV photon (Jim) that heads out into a 104 K HII region (Sue).
Consider Jim exploring a 50 K HI region (Marta).
b)
Consider Joe, the 3 eV photon again.
Consider an 8 eV photon (Jerry) that leaves a Hydrogen atom within Marta, and runs into a neutral iron atom (Susette).
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Weisz+2011
Interstellar Grains
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