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ASTR2050 Spring 2005
Lecture 10am 29 March 2005
Please turn in your homework now!
In this class we will discuss the Interstellar Medium:
• Introduction: “Dust” and “Gas”
• Extinction and Reddening
• Physics of Dust Grains
• Atomic and Molecular Gas
1
“Sure it’s beautiful
but I can’t help
thinking about all
that interstellar
dust out there.”
2
Introduction: “Dust” and “Gas”
A Prelude to Star Formation (next week)
The space between stars is not empty:
• Dust: Relatively large grains of relatively
complex materials. Although only about 1%
of the mass of a “cloud”, they are the main
reason the cloud appears dark.
• Gas: Atoms or molecules, making up 99% of
the mass of the cloud. Observed by narrow
emission or absorption lines.
We see gas in the direction of the dark clouds, so we are
pretty sure that they are part of the same thing.
3
Absorption and Scattering
Light scatters more
effectively at short
wavelengths, so the
reflected light is blue.
Therefore, the light
that passes through
appears reddish.
Different effect: H-II
regions are hot gas,
and glow red from
Balmer H-α emission.
4
Example:The Horsehead Nebula
See Textbook Figure 14.3
H-II Region
(Red!)
Reflection Nebulae (Blue!)
5
Dark Cloud
Example:The
Trifid Nebula
More clear examples
of reflection nebulae,
hot H-II regions, and
“dust lanes”.
6
Example:The Black Cloud Barnard 68
This is an extreme example of “reddening”.
Blue,Visible, Infrared
≈1/2 Light Year
Blue, Infrared, K-band
7
Using filters at different wavelengths:
8
Extinction and Reddening
We can quantify the amount of light absorbed by a cloud.
−!
I
=
I
e
Light intensity is exponentially attenuated:
0
This results in a difference in apparent magnitude:
A = m! − m = 2.5 log10(I/I0) = 2.5! log10(e) = 1.086!
i.e. one optical depth gives ≈one magnitude of extinction
So, we can talk about the “true” apparent magnitude:
m = mno extinction + A = M + 5 log(r/10 pc) + A
The distance r to the star may or may not be known.
9
Ratio of Total-to-Selective Extinction
Red light is absorbed less than blue light, so the extinction
for “visible” light AV should be less than AB for blue light.
Note that we can measure AB-AV as follows:
mV =MV + 5 log(r/10 pc) + AV
mB=MB + 5 log(r/10 pc) + AB
(mB − mV )=(MB − MV ) + (AB − AV )
The left side is what we observe directly, and the first term
on the right (the “color” of the star) can be deduced by
using spectral classification.
10
Both AV and AB are proportional to the amount of dust in
between the star and the observer. Therefore, the quantity
AV
R≡
>0
AB − AV
is independent of the amount of dust. R is called the ratio
of total-to-selective extinction.
Observations find R≈3.1 for almost all regions.
This is the standard value we will use.
Therefore if we measure AB-AV then we can infer AV !
(This is a way to determine stellar distances. See Homework.)
11
Extinction Curves
Carry through this
same exercise but
as a function of the
wavelength λ.
The “hump” near
λ= 200nm shows
up all the time!
More on this later.
12
See also Kutner Fig.14.6
Physics of Dust Grains
Physical properties of dust grains include...
Size and shape
Alignment mechanism and polarization
Composition
Temperature
Electric charge
Formation and evolution
•
•
•
•
•
•
We are not going to study these in any detail, but
your textbook has many details.
13
This spectral region is dominated by the strong 3.294 m
ture due to the aromatic C-H stretch. Following the sugge
of Jourdain de Muizon et al. (1990), which was strength
by arguments of Joblin et al. (1996), we attribute the 3.40
band to the asymmetric stretch in methyl groups attache
PAHs. The spectrum of methyl coronene also shows very w
bands near 3.35 and 3.48 m (Joblin 1992). There is no obv
sign of the three strong ethyl coronene bands in this wavele
region. Based upon the detection of the 2-0 overtone of
C-H stretch in IRAS21282+5050 (Geballe et al. 1994), the
ture at 3.435 m is assigned to the 2-1 hot band of the
stretch (Barker et al. 1987). In the same vein, the weak fea
in IRAS21282+5050 at 3.59 m may be due to the 3-2
stretch. HR 4049 shows relatively strong features near 3.43
3.55 m, which may well be the C-H hot bands. In that c
the 2-0 should be readily detectable in this source. Possibly
smallest PAHs, which dominate the emission in these hot ba
survive better in the weaker UV radiation field of this sou
The peak position of the aromatic C-H stretch is somewhat
sitive to molecular structure and internal excitation (cf., Jo
et al. 1995). The weak bands at 3.248 and 3.342 m could
be due to somewhat larger, less excited and somewhat sma
more highly excited PAHs, respectively. There are a numb
weaker features in these three spectra. These could be du
C-C overtone and combination bands (ATB). Other identi
tions are also possible. For example, the weak bands at 3.53
3.64 in NGC 7027 could be due to the C-H stretch in aldeh
groups attached to PAHs.
The 5–8 m region: The strong “6.2" and “7.7" m b
dominate this part of the spectrum and are assigned to arom
Infrared spectra of a late
stage red giant (HR 4049)
and planetary nebulae.
Emission from 20-100 C
atom “polycyclic aromatic
hydrocarbons” (PAH’s)
are shown in red.
This tells us about the
composition, size, and
temperature of grains.
See also Kutner Fig.14.7
14
15
“Dust grain concepts”
Key point: The grain size,
relative to the wavelength,
determines extinction.
See also Kutner Fig.14.8
16
Blocking light with dust grains
Dust is only 1% of the mass of the ISM, but
remember our old discussion about ∞×0!
!=0.5 × c × 1yr
18
=0.5 × 10 cm
6 3
−12
3
n=1/10 m = 10 /cm
2
−7
2
!∼(3µm) = 10 cm
⇒ n!! ≈ 0.05
!1/2 Light Year
17
In other words, for these values,
5% of the light with wavelength
below 3 microns is “blocked”.
Atomic and Molecular Gas
Free atoms and (small) molecules
Some important components:
• Hot (ionized) hydrogen gas (“H-II”)
• Cold (atomic) hydrogen gas (“H-1”)
Cold
molecular
hydrogen
(H
)
2
•
• Other cold molecules (e.g. HD, CN, CO,...)
18
HII Regions: Hot Ionized Hydrogen
Example: Rosette Nebula
Red glow is from Balmer
Hα (3→2) emission
Familiar sight in regions of
massive, hot, young stars.
In fact, this one shows a
“Stromgen sphere”
But this is not a large component of the interstellar medium.
19
Cold Atomic Hydrogen Gas
Key to detection:Very low energy “spin flip” transition
Wavelength (Frequency) of radiation is 21cm (1.4GHz)
See also Kutner Figure 14.11
20
Application: Spiral Arms of our Galaxy
Locate “clumps” of HI gas by
observing Doppler shift from
differential rotation of arms.
Necessary velocity resolution
is easy with radio waves (HW)
-50
0
+50
+100
Radial Velocity (km/sec)
21
Cold molecular gas
Rotational Energy Levels:
Have J=0,1,2,3,... unless the
molecule is diatomic (eg H2)
2
h̄
E = [J(J + 1)]
2I
Important interplay with level
population with temperature:
Energy (eV)
!
"
nj gj
E j − Ei
= exp −
ni gi
kT
(Kutner Equation 3.9)
Example: CO
So, the molecular species can
have a lot to with the local
environment of the ISM.
22
(Optical image)
Example:
CO Map of
a distant
galaxy
viewed
edge-on
23