Introduction to mm-wave astronomy

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Introduction to mm-wave astronomy

• Michel Guélin
• Institut de Radioastronomie Millimétrique

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Introduction to mm-wave astronomy

• Interest of the mm/submm domain
• Emission processes at mm/submm wavelengths.
  Absorption.
• Mm/submm-wave telescopes
• The plus of interferometry
• Interstellar molecules which?
• Interstellar molecules where? (Excerpts of
  recent results)

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1. Interest of the mm/submm domain

• h?kT 1.44 K 30 GHz 1 cm-1
• Black-body emission peaks at ?mhc/3kT0.48/T cm
• Dust emission peaks at ?mhc/(3ß)kT0.3/T cm
• Typical energies involved in molecular
  transitions
• SED of galaxies
• SZ effect, interstellar scintillation (VLBI)
• Atmosphere transparency

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Black body emission Cosmic Background radiation
COBE
CBR peaks at 1.76 mm
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Interstellar Clouds
n10-103 cm-3 T20-100 K Avlt1
Diffuse Cloud
n103-106 cm-3 T8-15 K Avgt1
Dark Cloud
Dust emission peaks at 0.3 mm
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Thermal Emission from cold dust peaks at submm
wavelengths. Rayleigh-Jeans approximation (ST)
is valid only at mm vavelengths
NIR
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SED of the quasar PSS23221944 z 4.12 (Cox et
al.)
SED of M82
Maximum at 2 THz 94µm Tdust 32 K
Minimum of continuum emission around 1 cm
wavelength
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Typical energies involved in molecular
transitions

• Electronic transitions
• Vibrational transitions
• Rotational transitions
• Electronic/nuclear Spin interactions

bending
stretching
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De Mello
low-energy rotational transitions of small
molecules lie at mm wavelengths
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Atmosphere

1. Absorbs electromagnetic waves
2. Introduces a phase delay

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Atmospheric refraction at mm/submm waves main
contributors

• Oxygen
• Homonuclear
• No permanent electric dipole moment ?e0
• Triplet state 3? ? ?
• Large magnetic dipole moment ?e10-20 emu
• 16O18O is heteronuclear ?e 10-24 esu 10-6 D
• Scale height 8 km

• Water vapour
• Planar, C2v
• Electric dipole
• Ortho/Para water
• Scale height 2 km
• Broad lines
• Ozone
• alt 11-40km
• Narrow lines
• Mostly above 200 GHz

?
?b1.9 D
?
?b0.5 D
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Opacity of the Atmosphere Altitude 3000 m J.
Cernicharo J. Pardo
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Atmospheric transmission (calculations by J.
Pardo)
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Atmospheric transmission _at_PdB
future
present
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Phase fluctuations caused by unstable atmosphere
scale as d/?
400 m 330 m
56 m
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2. Emission processes at mm/submm wavelengths

• Atoms electronic (spin, Rydberg states)
• Molecules electronic, vibrational, rotational
• Free electrons
• Synchrotron
• Thermal free-free
• Dust particles (grey body radiation)

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Atomic fine structure lines

• Electrons orbital momentum l
• spin s
• Atom total orbital momentum L ? l
• total spin S ? s
• Total electronic angular momentum
• 
  JLS

S
J
L
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Atomic C fine structure transitions

• Selection rules ?S0, ?L1, ?J0,1

CI S1,L1
CII S1/2, L1
E/k
100
2 THz
2P3/2
3P2
158 µm
370µm
A7.9 10-8s-1
A2.4 10-6s-1
3P1
500 GHz
610 µm
A2.7 10-8s-1
2P1/2
0
0
3P0
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?
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Calculation of the rotational constant B(linear
molecule)
I(CO) (1216)/281.132 8.75 B(CO) 57 GHz
Diatomic XY
Linear XYZ
Bent YXY
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Application to HC5N

• 1.0569 1.2087 1.3623 1.2223 1.348 1.2223 1.3636
  1.1606

B01331.3327 MHz rotation constant D00.30102
kHz distortion constant ?J32?31
2B0(J1)-4D0(J1)385201 MHz
Gordy Cook p. 146
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Covalent Radii (Å) and Electronegativity of
atoms (after L. Pauling) rAB rArB-??xA-xB?
Atom Single bond Double bond Triple bond Electronegativity
H 0.32 2.1
C 0.772 0.667 0.603 2.5
N 0.74 0.62 0.55 3.0
O 0.74 0.62 0.55 3.5
F 0.72 0.60 3.9
Si 1.17 1.07 1.00 1.8
P 1.10 1.00 0.93 2.1
S 1.04 0.94 0.87 2.5
Cl 0.99 0.89 3.0
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Rotational constants of some molecules

• CO 57.8 GHz
• CN 56.6
• HCN 44.3
• HC3N 4.54
• HC5N 1.33

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Signal strength radiative transfer in the
optically thin case
I?
I?dI?
dl
I?
I0
Optically thin, homogeneous medium
??
?
Radiation temperature
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(Optically thin case, TBGltltTrot) Units K, cm-2,
GHz, D, km/s
T(CO1-0) (1-e-t) Trot N(CO)/?v 2.8
10-14/Trot (optically thin) T(CO1-0)Trot
optically thick case t 3 10-14 N(CO)/(?vTrot2)
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Signal strength for molecular clouds

• N(H2) 1021 Av 1022 cm-2 ?v1 km/s
  TK20 K
• t(CO1-0) 3 10-14 N(CO)/Trot2 TrotTK
• X(12CO) 10-4 t(CO) 75 Trad(12CO) 20 K
• X(C18O) 2 10-7 t(CO) 0.15 Trad(13CO)
  3 K
• t(HNC1-0) 1 10-11 N(HCN)/Trot2 Trot TK/2
• X(H12CN) 10-8 t(CO) 10 Trad(12CO) 10 K
• X(H13CN) 10-10 t(CO) 0.1 Trad(12CO) 1 K
• Now, if molecular abundance is smaller an/or if
  source filling factor small (1/100 or less),
  signals become really weak!

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Dust thermal emission at 1.3 mm
Cloud with N(H2) 1022 cm-2, Td 20 K
S1300 20 mJy/beam (4 mK at 30-m telescope) less
if source does not fill the beam Signals are
very week.
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Astronomical Source Handy formulae for dusty
molecular cloud(s)

• HI line emission
• Molecular line emission
• VisualIR light extinction
• Thermal dust emission

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3. Mm-wave Radio telescopes (requirements)

• Large collecting surface for sensitivity
• Large physical dimensions for angular resolution
• High altitude to reduce atmospheric water vapor
  absorption
• Heterodyne receivers for high spectral resolution
  (10-7 ?10-8)

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IRAM 30-m telescope (Sierra Nevada,
Spain) Altitude 2900 m surface accuracy 50 µm
(night)
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The Green Bank telescope (Image courtesy
NRAO/AUI)
No aperture blockage!
Surface accuracy300 µm
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APEX 12-m telescope (Atacama, Chile) Altitude
5100 m surface accuracy 17µm
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A Gallery of mm/submm Interferometers
Plateau de Bure (F) 2500 m
SMA (Hawaii) 4000m
CARMA (Ca) 2300 m
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Mmwave inter 2008
Millimeter interferometers in 2008
m²
380 148 532
CARMA SMA PdB
Scon mJy
Slin mJy
2.3 - 0.9
0.36 0.40 0.14
4.6 5.8 2.9
1 2 2
CARMA SMA PdB
220040702550
200
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VLA (up to 7 mm)
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Global mm VLBI Network
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ALMA
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4. The plusses of interferometry

• High angular resolution (_at_ ?1 mm 0.25 with
  PdB 20 µarcsec with VLBI)
• Large collective area
• No need of reference position (factor 2 in
  sensitivity replaced by N(N-1)/N2)
• Flatter baselines (depends less on
  receiver/atmosphere stability). Makes possible
  composite spectra.
• Field of view (much) with many independent pixels
  ?good noise statistics makes possible secure
  detections down to 4 sigma.
• Balanced observations for special observations
  polarimetry, SZ
• Accurate source positions (by stable atmosphere
  HPBW/SNR)
• Eliminates extended (foreground/background)
  emission

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4.b The minus of interferometry

• - Several receivers to build more complex
  correlator, but heterodyne interferometry is easy
• Short spacings filtered out extended source
  emission lost (partly recovered by mosaicing
  techniques)
• Needs a stable atmosphere (or needs phase
  corrections or self-calibration)
• Difficult to observe very strong sources, such as
  planets (unless modelized)

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Interferometers vs single dish telescopesSummary
Plus and Minus
Interferometer Single Antenna
Total area -
Angular resolution -
Baseline quality -
Field of view --
Short spacings ---() (-)
Receiver cost --
Site requirements -
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5. Interstellar molecules which?
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Ref. PCMI/CNRS
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Ref. PCMI/CNRS
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GBT 100-m
IRAM 30-m
NRO 45-m
Kitt Peak 12-m
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Inter- Circumstellar molecules(not counting
isotopologues)

• 135 species (-5 ?)
• 14 ions
• 29 free radicals (part of which identidied in
  space prior to be studied in the laboratory)
• 19 isomers or highly unstable closed-shell
  molecules
• 18 molecules with refractory atoms, amidst which
  8 silicon compounds
• 5 cycles, among which benzene (1 line!). No other
  benzenic ring detected, except perhaps PAHs.

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6. Interstellar Molecules where?

• Diffuse IS clouds
• Cold dark clouds
• Protostellar cores
• Hot cores (star forming regions)
• Circumstellar disks
• Circumstellar envelopes
• Jets and shocked regions
• External galaxies up to z6.4!

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The Orion-A Hot Core
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Same spectrum as previous one, but with line
identifications (in red). Unidentified lines in
green (noted U)
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Same as previous note the differences in the
spectra
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C
B
A
D
IRC2
B
Maps of Orion-IRC2 in the lines of 6 different
molecules The molecules arise from different hot
cores (or corinos) labelled A,B,C and D.
Guelin, Nobel Symposium 2006
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Holes in Proto-planetary Disks
Beam 0.39 x 0.25 PA230
Beam 0.52 x 0.28 PA 220
Inner cavity of 50 AU
GM Aur (Wilner et al. 2006)
LkCa15 (Pietu et al. 2006)
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HH211 at 1.5 resolution Molecular outflow
driven by a Class 0 low-mass protostar
(dynamical age1000 yr, distance of 300pc)
High velocity CO J2-1
(Gueth Guilloteau 1999)
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1.5? 0.3 resolution
Gueth et al. (in prep)
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AGB star envelope IRC10216
Molecular-line emission
IR emission
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Spectral line survey of The C-star envelope
IRC10216 80-250 GHz 3mm-1.2mm Cernicharo,
Guelin, Kahane (2000)
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IRC10216 (CW Leo) VVsys
Guelin et al. 1998
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• External galaxies
• Andromeda galaxy
• High redshift quasars

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Nieten et al. 2006
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Neininger et al. 1998, Nature
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Multiple CO lines from SMMJ16359 (z2.5)
Sum
Weiss et al. (2005)
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P. Cox
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APM082795255
APM082795255 (z3.9)
Wagg et al. 2006
LHCO / LCO(4-3) 0.26
HCO J5-4
Garcia-Burillo et al. 2006
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Credits and References

• J. Cernicharo (IRAM 2003 Summer School)
• PCMI/CNRS website
• UMIST website