This chapter will deal with photometry, that is, the art of
converting a measurement such as elecron/s or photons/s into a
physical SI unit such as W m-2 Hz-1
Motivations underlying photometry are multiple, from the simple
need to understand how stars span a broad range of apparent
magnitude, shine reddish or blueish, etc., to the quest for
precision and accuracy in measuring the universe.
Historically, photometry started with stars, for quite obvious
reasons. Actually, understanding the birth, evolution, and fate,
of stars is central to astrophysics for many reasons.
We may perhaps summarize that stars are the chemical factories of
the universe since they synthesize the stable heavy elements
beyond those produced by the Big Bang, such as carbon, oxygen,
nitrogen, and their isotopologues.
And also, galaxy evolution is largely driven by stars through
their feedback (fountains, cosmic rays, etc).
Indeed, deriving stellar properties (mass, size, composition,
internal motions and state, magnetic fields) is a major task of
astronomy and astrophysics for decades, trying to combine theory
and observations into a comprehensive understanding of stellar
structure and evolution.
In this chapter, we shall not discuss stellar structure but rather
use the properties of the stars to measure distances on a wide
range of scales. As such, stars can tell us about the structure of
our galaxy and similar ones. Moreover, the brightest stars
(e.g. SNe Ia) can be observed up to redshifts ∼ 2 and therefore
provide ways to measure the universe.
In this lecture, you may learn about:
Empirical knowledge of stars:
spectral and luminosity classification; classifying the
stellar ecosystem
Fundamentals of radiation and photometry:
Apparent and absolute magnitudes
Photometric systems (filters, zero-points)
Bolometric correction
Extinction correction and interstellar reddening
How we can measure distances beyond the parallax limit using
standard candles
2 The Classification of Stars
Observed properties of stars: spectra
A-type star: strong and broad H absorption lines
G-type star (like the Sun): many lines (ionized and neutral atoms)
M-star: strong molecular bands (e.g. TiO)
Stellar spectra
Note: our ability to detect absorption lines depend on the
background intensity: strong background allow weak absorption
lines to be detected (see radiative transfer lectures)
2.1 The spectral classification of stars
The spectral classification of stars
Harvard spectral taxonomy in the 1890s: O B A F G K M (if you need
this: Oh Be A Fine Guy/Girl Kiss Me)
The strange choice and ordering of the letters is due to
historical reasons: starting from an alphabetic order (A, B, C,
etc), the order and retained letters had to adapt to the
continuously evolving picture; eventually, no logic is apparent
in the final list.
Numerical subdivision: O0-9 B0-9 etc
Vocabulary: early K type is K0, late G star is B9
Harvard classification: empirical (based on some key lines,
e.g. HeI, HeII, CaII, FeI, etc)
Note: spectroscopic notation HeI stems for neutral He, HeII for
He+, CaII for Ca+, etc
Balmer lines (n=2) are maximum for A0 (Teff = 9520 K)
HeI are strongest for B2 (Teff = 22000 K)
the CaII doublet (H line at λ=396.8 nm and the K line at
λ=393.3 nm) are strongest for K0 (Teff= 5250 K)
spectral type based on absorption line ratios
Catalogues: Henry Draper Catalogue (HDxxxxx) (225,300 spectra
classified by Annie J. Cannon, between 1911 and 1914)
The Harvard College Observatory group
At the Harvard College Observatory, in 1892, a group of women, led by
Williamina P. Fleming (standing middle rear), Henrietta S. Leavitt
(fourth from left, sitting), and Annie Jump Cannon (far right), in the
group of Edward Pickering (rear left).
Example of stellar spectra
Note the clear difference between A-type stars (left) showing few,
isolated, deep lines, and late types (G, K, M) stars (right),
showing more complex spectra with weaker lines.
The spectral classification more or less reflects the effective
temperature of the star, with O stars being the hottest/bluest,
and the M dwarfs being the coldest/redest.
The spectral classification of stars
Several groups, several systems, several variables
The 1912 Draper Catalogue of Stellar Spectra (essentially by
Annie J. Cannon) was validated by an international Committee.
Henry N. Russell supported the non-alphabetical order of the
letters: "This helps to keep the novice from thinking that it is
based on some theory of evolution." Actually this is for
historical reasons, with the letter ordering and choice evolving
with the diversity of stellar spectra.
Karl Schwarshild recommended to keep as little variables as
possible: "ultimately the spectrum of a star might depend on nothing
other than its mass, its age and its temperature"
Annie J. Cannon pursued the work, culminating in the Henry Draper
(HD) catalogue finally published in 1924 (1USD per star).
2.2 From the Harvard to the Morgan-Keenan (MK) classification
Luminosity class
At the same period at which the spectral classification was set
up, Antonia Maury (another woman of the Harvard group) introduced
an idea at the basis of what was later called the luminosity
class. The basic observational fact is that stars of a given
spectral class could have lines with different width and strength
Example of luminosity effects at A0
Limitations of the Harvard classification
The spectral classification does not reflect the origin of
stellar spectra
Example: Hydrogen lines in Vega (A0 by definition) are much
stronger that in the Sun (G2); but Ca lines are much stronger in
the Sun than in Vega
Origin: Teff ? (9500 K for Vega, 5777 K for the Sun); chemical
composition ?
As mentioned earlier, the Harvard classification is empirical. In
particular, it is biased towards the lines in the visible; but
what about other spectral ranges (IR, UV) ?
Later on, the discovery of low-Teff stars in SDSS (Sloan Digital
Sky Survey) and the 2MASS (2-Micron All-Sky Survey) could not fit
into this classification, and new spectral types were added:
L-type (Teff=1300 to 2500 K) and T-type (Teff<1300 K)
Brown dwarfs (deuterium-burning during contraction, but
threshold for H-burning never reached)
Which quantity to classify stars ?
Harvard classification: empirical, essentially based on Teff
But two stars with the same Teff can have different luminosity
Underlying physical questions
What causes the spectrum of a star: Teff, chemical composition?
Answers: both, and even more
chemical composition
physical state (convection zone)
radiative transfer with quantum mechanical description of
ligth/matter interactions (emission, absorption by atoms)
Luminosity classes in the MK classification
Not adopted in Harvard classification
Extended in the MK (Morgan and Keenen) classification
Luminosity classes
I, Ia, Ib, have very narrow, very weak lines: supergiants
II and III have narrow, weak lines: giants
IV has intermediate lines: subgiants
V has very strong, broad lines: dwarfs
Modern notaion for Spectral types:
α Lyr: A0 Va
Type: A
Subtype: 0 to 9, 0 being the hottest
Luminosity class: I to V, I being the largest
The Sun is G2 V in MK classification
3 Magnitudes and photometry
What we will learn
apparent magnitude m and flux density Fν (or Fλ)
absolute magnitude M and luminosity Lν
bolometric magnitude and flux F
bolometric correction: from F to L
3.1 Apparent magnitudes
Visual magnitude \(m\) describes the "brightness" of a star
Classification by Hipparchus (190-120 BC) of ∼800 stars,
further complemented by Ptolemy (150) three centuries later (∼
1000 stars);
\(m=1\): brightest stars
\(m=6\): faintest stars to the naked eye (and w/ dark sky; will
there still be any dark place on Earth in a century from now?)
Note: Brightest star is the Sun: mV=-26.7; next one is Sirius
(Canis Major), mV=-1.5;
Magnitude is a relative measurement (Note: log means log10 ≠
ln, unless stated otherwise):
where \(N(\nu)\) is the photon rate detected over a spectral
interval around a given wavelength
A difference of 5 magnitudes corresponds to a factor 100 in
brightness.
Magnitude, flux density, and photon rate
There is no unique definition of the apparent magnitude.
The definition depends on the device used to measure the magnitude:
energy integrating devices (such as photomultiplier tubes) are
sensitive to the energy, hence to the flux density Fν
for such devices, the magnitude would be defined as
m2-m1 = -2.5log (F2/F1)
most modern detectors are photon-counting detectors: they are
sensitive to N=Fν/hν.
In both cases, the apparent magnitude \(m\) is related to the flux
density Fν
The monochromatic flux density Fν (to be defined in the next
Section) is a quantity that describes the amount of energy
received from a source by an observer, per unit area at the
observer location; its units are W/m2/Hz or W/m2/m;
The total, or integrated, flux density is ∫ Fν dν.
Important: the flux density decreases as d2, where d is the
distance between the source and the observer.
3.2 Absolute magnitudes
Definition: the absolute magnitude M of a source is the apparent
magnitude m of that source if it was moved to a reference distance
d0=10 pc (chosen by convention)
Geometrical dilution of the flux: Fν(d)∝ d-2:
F10pc = Fmeasured x (d/10pc)2; taking the -2.5log(F) gives:
Secondary standards, defined against the primary standards (e.g. Landolt 1992)
Absolute calibrations uses terrestrial standards (1000-2600K
blackbody, tungsten striplamp, photodiodes), the Sun, primary and
secondary stars
Usual primary standard star is Vega (α Lyrae,
Teff=9550K, A0V, d=25 ly)
qλ may be defined by many ways; for instance, such that
color is uniformly zero (UBVRI), or such that mλ ≈
0 for an ideal AOV in all filters (Vega system); in UBVRI
system, V=0.03 for Vega such that m=0.03 in all bands (see
Table)
⇒ Fν = FVega,λ 10-0.4(m-m0,λ)
Relation between apparent magnitude and photon rates:
\[
m_\lambda = m_{{\rm Vega},\lambda} -2.5 \log
\frac{\int R \eta \lambda F_\lambda d\lambda}{\int R \eta \lambda F_0 d\lambda}
\]
with F0 the flux of Vega at λ.
Photometric precision and accuracy
Photometric precision: typically few % (one speaks about
"photometric nights")
Current best precision photometric systems is 1 millimag;
systematic errors in standard photometry is ≈ 0.01 mag,
primarily because of poorly matched passbands, and limited sample
of standard stars across the HR diagram;
Uncertainties budget
Absolute spectrum of Vega: 5% in the UV, 1.5% in V/NIR, 5-6% in
IR
Accuracy of the magnitude measured in a given photometry system
Accuracy in the modeling of the photometric system used,
≈ 2%
Primary standards used in astronomy
The Vega magnitude system
αLyrae, Vega, one of the brightest stars in the night sky
in the northern hemisphere; A0V
In the Vega system, an ideal A0V has B=V=R=I=J=H=K=0, thus
defining the zero-points of each bandpass
In this system, Vega's magnitudes differ from 0 by ≈
±0.1 mag
Vega is the top-used primary standard (HST, Spitzer, SDSS)
Not the only one: Sirius, the Sun (variable)
The Vega magnitude system
Vega has been used as a primary since the 60s, thus enabling its
flux to be measured from the UV (<0.3μm) to the near IR (>5
μm).
Vega's monochromatic flux at 555nm: best value is
3.46(-11)±0.7% W m-2 nm-1 (Megessier 1995)
Validity as a good primary has been questioned; dusty debry disk
hence, extrapolation beyond 5μm risky; yet, if not variable,
that's ok
The UBVRI magnitude system
Questions:
Can you recover the magnitudes of Vega in each band?
What is the flux of a V=2 star ?
Check that you can convert the flux from erg/s/cm2/ang into W/m2/nm
3.5 Bolometric magnitude
Bolometric means 'integrated over all frequencies'; 'bolometers'
are detecting devices which collect the electromagnetic radiation
over a wide range of frequencies.
Bolometric magnitude is thus related to the total, or integrated,
flux: F=∫ Fν dν
mbol = -2.5 log F + C
Equivalently, the bolometric absolute magnitude is related to the
total luminosity;
Constant C is defined with respect to the Sun:
mbol - mbol,\(\odot\) = -2.5 log(F/Fsun)
Mbol - Mbol,\(\odot\) = -2.5 log(L/Lsun)
Stellar luminosity covers a wide range of values:
Brightest stars: ~106 \(L_\odot\), white dwarfs: ~10-3
\(L_\odot\), Brown dwarfs: ~10-7 \(L_\odot\);
In practice
bolometric magnitude is usually I+V+U (main input windows
accessible from the ground)
if one observes a star in a limited spectral range, how to
correct for the missing flux? → bolometric correction
Bolometric correction
Bolometric correction (BC) = difference between the visual and
bolometric magnitude:
BC = mbol - V
mbol = V + BC and also (show it) Mbol = MV + BC
BC is always negative
One expects that BC in the visual is minimum for stars with a
spectral energy density peaking in band V; in contrast, BCV would
be large for very hot, or very cold, stars. A natural choice for
the zero-point of bolometric magnitude should thus be somewhere
around F- or G-type stars.
For the Sun, which is a G2 V star in the MK classification:
BC\(_\odot\) = -0.08
V=-26.75
mbol,\(\odot\)=-26.75-0.08=-26.83
From absolute magnitude to luminosity
Apparent magnitude: m related to Fν
Absolute blometric magnitude: M related to Lν
M = -2.5 log Fν(d0) + C, d0=10pc where Fν(d0) = Lν /
4π d02, is the flux density at a reference distance d0=10pc.
The value of mbol,\(\odot\) (or Mbol,\(\odot\)) is arbitrary;
this is a zero-point value for the bolometric magnitudes, very
much like the qλ for the magnitude scale. The value of
mbol,\(\odot\) is thus a definition and is not a measurement.
See this paper for a useful description and discussion.
The value of BC\(_\odot\) can vary from -0.20 to -0.07; this depends
upon the stellar models used to compute BC.
IAU 2015 Resolution B2: Mbol=0 corresponds to Lref=3.0128E28
W≈ 100Lsun, with Lsun=3.828E26 W, hence Mbol,Sun=4.74:
Mbol = 4.74 -2.5log(L/Lo)
Translation to the apparent bolometric magnitude scale:
mbol,ref=0 corresponds to Fref=Lref/(4π d02)=2.53E-8
W/m2; hence mbol,Sun=-2.5log(Fsun/Fref)=-26.83 (see
above). In the UBVRI system, V=-26.75, hence BCsun=-0.08.
There are tabulated bolometric corrections depending on the
spectral type and luminosity class (see 3.6)
independent of distance (flux ratio) hence apparent or absolute
magnitudes;
shape of the stellar spectrum between two bands
hot stars are blue: B-V \(<0\), \(\approx -0.3\)
cool stars are red: B-V \(>0\), \(\approx 1.5\)
Values
Table from Allen, Astrophyiscal quantitities (4th ed. 2002)
Color index in the UBVRI system: note that the color of a
Vega-like star are ≈ zero, by construction
Note that BC≈-0.20 for the Sun, in contrast with the -0.08
value quoted above, reflecting the underlying stellar models used
to compute BC depending on the MK spectral types.
4 Exercises
Magnitudes
The absolute magnitude of a given star is M; what is the
apparent magnitude of this star at a distance d expressed in pc?
The apparent magnitude of the Sun is V = -26.75; compute MV.
From the CDS, Sirius magnitudes (in Johnson-Cousins system) are:
U: -1.51; B: -1.46; V: -1.46; R: -1.46; I: -1.43; J: -1.36;
H=-1.33; K: -1.35
Compute U-B and B-V ? can you anticipate on the spectral type
of the star ?
Teff ≈ 10000 K: λmax ? consistent with color
indices ?
BC = -0.09: mbol = ?
Absolute magnitude of Vega (d=25ly) in V band?
Give an expression of the distance modulus as a function of the
parallax p in mas.
Show that the bolometric correction is the same for apparent and
absolute magnitudes
Photometric systems
This Table (taken from Bessel et al 1998) gives, for a fictitious
A0 star (based on synthetic photospheric spectrum) observed
through the UBVRIJHKL Cousins-Glass-Johnson system, the effective
λ of the filters, the absolute flux (corresponding to zero
magnitude), and the zeropoint magnitudes.
what is the flux, in the V band, of this A0 star? is this
consistent with the values for Vega shown before?
convert fλ into fν
explain the offset values, 21.1 and 48.598, in the expressions
of magν and magλ
in this photometric system, what is the V-flux of V=2 star?
compute the zp(fν) and zp(fλ). Comment?
Brightness, flux, and luminosity
Based on the Table shown previously for the Vega system, show
that the flux of Vega in the U band is 1964 Jy.
Consider a star of radius R and specific intensity Iν:
Compute the flux Fν measured at a distance d;
From this, what is the flux at the surface of the star
At large distances, show that Fν = ΩS Iν with
ΩS the solid angle of the source
Consider a star as a black-body of temperature Teff:
compute the integrated flux at the surface of the star
from this, obtain the total luminosity L as a function of
Teff;
show that L = 4π2 R2 I0; is this consistent with your
previous result ?
Photon rates from a solar-type starConsider a star with Teff=5500K of radius R=7x108 m, located
at a distance d=10 ly:
Compute the brightness I100, I500, and I1000 of the star at
100, 500, and 1000 nm; give the results in S.I. units (which
you have to explicit) and in MJy/sr;
Compute the wavelength of the peak of Iν and Iλ;
comment;
Compute the fluxes incident on the detecting device at each of
the above wavelengths;
The fluxes are measured through filters centered at each
λ, of equivalent width δλ=0.01λ; the
detector has an efficiency η=80%; compute the power
Pλ measured per unit area at the detector;
Compute the corresponding photon rates Nλ
Proxima Centauri (α Cen C)
From the CDS portal, find the parallax of Proxima Centauri
What is the problem ?
Check that the angular distance between the J2000 and ICRS
coordinates is ≈ 60"
Check that the distance along the RA and DEC axis are δ
x=-58.6" and δ y= 11.9"
Hint: the angular distance ψ between the two positions is:
cos ψ = sin(δ1)sin(δ2) +
cos(δ1)cos(δ2)cos(α1-α2)
Galactic Infrared Emission
At the bottom of the plot, the contribution to the total IR
emission is indicated (in %)
Make sure you understand the units of the plotted quantity;
Can you recover the orders of magnitudes of these relative
contributions ?
Can you recover the total emission of 5x10-24 erg s-1 H-1
5 Radiation fundamentals
In defining the magnitude, we mentionned physical quantities such as
the flux and luminosity without going in the details of how these
quantities are defined. We now go into more details. In doing so, we
will introduce a less familiar quantity, the specific intensity
(also called brightness although this is a more ambiguous word and
we prefer to call it specific intensity). In the process, we will
review our knowledge of the black-body radiation which is so
essential in astrophysics.
Definitions
Luminosity: L or Lν
Flux density: F or Fν
Specific intensity (or brightness): Iν
5.1 Luminosity
This is an intrinsic property of a source
Spectral luminosity Lν: total power per unit bandwidth emitted
by the source, in W/Hz
Energy conservation: Lν,1 = Lν,2
Warning: Luminosity is rarely called luminosity, but more frequently
flux
5.2 Specific intensity
Consider an area dA (e.g. your detector) embedded in a radiation
field
Energy flowing accross dA, carried by the bundle of rays within
solid angle element dΩ, per unit time and frequency range ?
(1) direction is perpendical to dA:
dEν = Iν(θ,φ) dA dΩ dν dt
(2) direction makes an angle θ from the normal to dA:
dEν = Iν(θ,φ) dA⊥ dΩ dν dt
with dA⊥ = dA cos(θ)
Total, or integrated intensity: I=∫ Iν dν
5.3 Flux density
Expose an area dA to some radiation during a time dt. The energy
in bandwidth dν crossing dA from all directions is:
Fν dA dt dν = ∫Ω dEν
In terms of the specific intensity: Fν = ∫Ω Iν cos(θ) dΩ
Units: Fν in W/m2/Hz
Detector of area 1m2 will intercept Fν W/m2/Hz from the
source; obviously, the amount of energy depends upon the
distance d:
Fν = Lν / [4 π d2], W m-2 Hz-1
The flux density is not an intrinsic property of the source
Spatially resolved source: spectral flux density and specific intensity
Spectral flux density detected from \(A_k(\theta_k,\phi_k)\) within
\(d \Omega(\theta_k,\phi_k)\): \[ d F_{\nu,k} =
I_\nu(\theta_k,\phi_k) \cos\theta_k d \Omega(\theta_k,\phi_k) \]
Total spectral flux density detected:
\[ F_\nu =
\int_{\Omega_S} I_\nu(\theta,\phi)
\cos\theta d\Omega(\theta,\phi)\]
Units
Iν is measured in W/m2/Hz/sr
Fν is measured in W/m2/Hz
More convenient unit is the Jansky (see black-body radiation)
1 Jy = 10-26 W/m2/Hz
In cgs units: 1 J = 107 erg
W/m2/Hz = 103 erg/cm2/Hz
5.4 Solid angles
\(\Omega = S(d)/d^2 = \int\int d^2 S \vec{u}\cdot\vec{N}/d^2\)
dΩ = dS cosα/d2
Spherical coordinates: dΩ = sinθ dθ dφ
Uniform sphere: Ω=4π
Cylindrical cone of half-opening angle α:
Ω=2π(1-cosα)≈πα2 for α≪1
Flux emitted by a star within solid angle ω is: Lν =
ω d2 Fν, where Fν is the flux density at a distance
d;
The total luminosity Lν is thus (assuming it is isotropic)
4π d2 Fν
5.5 Fundamental properties of the specific intensity
How is Iν related to the radiation field emitted by the source?
\(d\Omega_A = dA_\perp /d^2\): \(dA_\perp = d A \cos\theta_A\)
\(d\Omega_B = dB_\perp/d^2\): \(dB_\perp = d B \cos\theta_B\)
pνA: power emitted by dA and crossing dB = Iν(A) dA⊥ dΩB
pνB: power flowing through dB looking at dA = Iν(B) dB⊥ dΩA
Assuming propagation in empty space: pνA=pνB
Iν(A) = Iν(B): the detector measures the specific intensity
emitted by the source; intrinsic property of the source
Important: \(I_\nu\) can only be measured for spatially resolved sources
Conservation of the specific intensity (2)
\(I_{\nu,1}\): specific intensity measured on the detector
at distance \(d_1\)
\(I_{\nu,2}\): specific intensity measured on the detector
at distance \(d_2\)
Source area increases as \(d^2\) while dΩdet decreases as
\(d^{-2}\)
Conservation of energy in free space: \(I_{\nu,1} = I_{\nu,2} = I_\nu\)
The detector measures the source brightness: Intrinsic property of the source
Resolved vs unresolved source
Flux dilution when source becomes unresolved: specific intensity
can only be measured for a resolved source unless its angular
size (solid angle) is known;
In contrast, if we do not know the source size, what we measure
is the flux Fν;
More generally, for an unresolved source, one measure the flux;
if the distance is known, the luminosity is recovered; if in
addition the source size is known, one obtains the specific
intensity.
5.6 Spectral Energy Density (SED) and Reduced Brightness
SED: flux, or brightness, over a broad spectral range; here:
sketch of the SED of a normal spiral galaxy
This plot gives the sensation that the galaxy emits much more
energy in the X-ray domain than in the radio domain.
This is because the wavelength are shown on a logarithmic
scale. However, displaying it on a linear scale would completely
wash out the spectral details at short wavelength.
Very broad spectrum: distortion of the spectrum: 1μm in the
radio is tiny, but huge at short visible wavelenghts; Fλ
will under-represent the flux emitted at radio wavelength, and
overrepresent the flux at short wavelength.
Reduced brightness
A very practical way to deal w/ this is to keep plotting the
wavelength on a logarithmic scale, while replacing the flux
Fλ by λ Fλ;
Using the reduced brightness also preserves the link between the
area under the curve and the corresponding integrated energy:
\[
\int I_\nu d\nu = \int I_\lambda d\lambda
\]
and
\[
\int \nu I_\nu d(\log\nu) = \int I_\nu d\nu
\]
so that also
\[
\int \nu I_\nu d(\log\nu) = \int \lambda I_\lambda d(\log \lambda)
\]
Sky background radiation (zodiacal light subtracted); notice the
CMB black-body
By plotting ν Iν againt log(λ), it is possible to
'measure' directly, w/ a ruler, the area under the curve to obtain
the energy radiated within a given spectral range.
6 Black-body radiation
The Cosmic Microwave Background (CMB)
Notice the 1σ error bars which were multiplied by 400
The CMB in other units
1 Jy = 10-26 W/m2/Hz
6.1 Definition
A black-body is an ideal body which absorbs 100% of the
radiation regardless of the wavelength, incidence angle; the
black body has zero reflectance and zero transmittance;
Black-body radiation does not depend upon its geometry
It is unpolarized
Its specific intensity depends on a single parameter: the
temperature
6.2 Mathematical expression
The spectra interval may be frequency or wavelength
Interstellar clouds near bright stars; blue nebulosity is light
scattered by dust; dark lanes are due to absorption;
7.2 Atmospheric absorption
Different processes:
scattering: light pollution
refraction: λ-dependent light deviation
turbulence: degrades the spatial resolution
thermal emission: significant contribution at IR and mm
Atmospheric windows: outside the windows, atm. absorption is very
strong; associated with absorption lines (O2, O3, CO2) or
continuum (UV)
electronic (atomic, molecular) transitions: O, N, CH4, CO,
H2O, O2, O3
ro-vibrational and pure rotation lines: CO2, NO, CO, H2O,
O3
Chemical composition of the atm. is ≈ constant up to
≈ 90 km
Hydrostatic scale height H≈8 km
CO2: mid infrared absorption
Airmass: 1/cos(z) = 1/sin(El) (approximation valid down to
El≈20o)
7.2.1 Atmospheric windows
7.2.2 Atmosphere absorption
7.2.3 Why are observatories in altitude ?
mixing ratio:
r(z) = mass of water per vol. / mass of air per vol;
rapidly varying function of altitude z
quantity of precipitable water: the column density of water
molecules above the observatory:
\[
w(H2O) = \int_{z_0}^\infty n(H2O) dz
\]
and n(H2O) [cm-3] = 4.3x1025 P/P0 T/T0 r(z)
typical values of w(H2{})) are 0 to a few mm;
Atmosphere windows in the mm and sub-mm
Left: NOEMA site (2 km); Right: ALMA site (5 km)
7.3 Interstellar extinction
Space absorption correction
Extinction of light on the way from the source to the atmosphere
Positive correction to the apparent magnitude: A
M = m + 5 -5log10 dpc - A = m - DM - A
Extinction or reddening (in magnitudes):
Fλ,0: the un-reddened flux
Aλ = 2.5 log10 (Fλ,0/Fλ)
wavelength dependent: extinction curves
Aλ is proportional to the opacity τλ:
Aλ = 2.5log10 [exp(τλ)] = 1.086 τλ
In band V, extinction is essentially due to interstellar dust
7.3.1 Reddening and colour excess
Reddening: E(B-V) = AB - AV
Aλ: reddening at one wavelength
Other usual quantity: Aλ-AV = E(λ-V)
By definition: M=m-DM-A, or m=M+DM+A
Color index: B-V = MB - MV + AB - AV
B-V = (B-V)0 + E(B-V)
(B-V)0: intrinsic colour
E(B-V) = AB-AV: colour excess
Extinction curves: relative colour excess vs wavelength
Usual quantities in ordinate: Aλ/E(B-V) or
Aλ/AV, etc…
Important relation that you must know (Bohlin+78):