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What is Millimetre-Wave Astronomyand why is it different?
Michael BurtonUniversity of New South Wales
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Some Millimetre Basics• MM: 1–~12mm, Sub-MM: 0.3–1mm• CMBR (T = 2.7K = 1mm)• Molecular rotational lines
– Polar molecules (have dipole moment)eg CO (E1 = 5K), HCN, CS, HCO+
• Cold thermal continuum (dust)– Thermal processes: F ~ B ~ 2kT2/c2 . x
• Problem: Atmosphere (O2, H2O)……
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The Millimetre Advantage
• Thermal Processes B 0.5-2 2
• Decay Rates (linear molecules) 3 • Doppler Widths 0.5 [?]• Level Population (T>>TJ; gJJ) • Number of Photons -1
• Energy • Spatial Resolution -1
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Transparancies
• Electromagnetic Spectrum• MM transmission for 4mm H2O
• MM transmission for 11mm H2O• Some bright MM-lines
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Brightness Temperature
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Atmospheric Transmission
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The 3mm Millimetre Spectrum
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Physical Parameters you can derive!
• Temperature: Tex, TBrightness
• Density: nH2 (~ncrit range of densities present!)
• Column Density: N (when optically thin)• Optical Depth: (use isotope ratios)• Mass (with scale length)• Abundances: different species• Velocities: line widths, centres, shapes
Infall, outflow, mass transfer rates
Constrain the properties of your source!!
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16272-4837SEST molecular line survey
–Gradient: Trot = 27 ± 4 K
–Intercept: N(H2) = 1 x 1024 cm-2
( comes in as well)
– Size + Column: n(H2) = 6 x 105 cm-3
– With Volume: Mass = 6 x 103 M
Garay et al, 2002
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16272-4837: SEST kinematical studies
– Evidence for infall(profile of optically thick lines)
- Modelling: Vinfall ~ 0.5 km s-1
- Speed + Density + Size:
dMinfall/dt ~10-2 M yr-1
– Evidence for outflow from wings- Extent: Voutflow = 15 km s-1
Brooks et al, 2002
OpticallyThick
OpticallyThin
WideWings
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Mopra: Current Capabilities
• 22-m Telescope for > ~3mm• 85–115 GHz SIS receiver (2.6 – 3.5 mm)• 35” beam @ 100 GHz• Tsys ~ 150K(@85GHz) – 300K (@115GHz)• Beam Efficiency:
mb (86 GHz) = 0.49, mb (115 GHz) = 0.42 xb (86 GHz) = 0.65, xb (115 GHz) = 0.55
• Bandwidth 64, 128 or 256 MHz (200 - 800 km/s)• 1024 Channels (0.2 - 0.8 km/s per channel)• 2 Polarizations
– 1 frequency or 1 polarization + SiO 86 GHz• Must Nod – No chopping• OTF Mapping
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Methanol Maser-selected Hot Molecular Core SurveyCH3CN CH3OH HCO+ H13CO+ N2H+ HCN HNC
7 lines; 86 Sources
Purcell
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‘On the Fly’ Mapping with Mopra:The Horsehead Nebula
Optical 12CO 13CO
6 arcminTony Wong
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Tony Wong 0.17 km/s channel spacing
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OTF Mapping Specifications
• For a 300” x 300” map:– ~1400 spectra (31 x 46)– ~35” resolution– 0.17 km/s resolution – 120 km/s bandwidth– Dual polarization ~ 0.3K per channel, per beam– ~70 minutes / grid– Upto 7 grids / transit– Processed with LIVEDATA +
GRIDZILLA packages
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The DQS in 13CO: Mopra OTF Mapping
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How many photons have we lost (or gained)?
00sec(z)
z
Signal on-source:
( ){ })sec()sec( 001 zsou
zatmrecSig eTeTTCT ττ −− +−+=
Trec
Tsou
Tatm
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Sky (Reference, Off)
Source (On)
Difference
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Some Radiative Transfer
•Radiative Transfer dI/ds = - I +
•Kirchoff (LTE) / = B(T)
Radiative Transfer dI/d = I + B(T)
•Solution I(s)= I(0)e- (s) + B(T)(1 - e- (s))
Source Atmosphere
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Obtaining Data:Signal from Source and Reference
• TSig = C{TR+TA(1-e-0secz)+TS e-0secz}• TRef = C{TR+TA(1-e-0secz)}• [TSig-TRef]/[TRef] = TS e-0secz
/ {TR+TA(1-e-0secz)}
Show Plots of Opacity + Brightness Temperature
• TBB = C{TR+TA}• [TSig-TRef]/[TBB - TRef] = TS/TA
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Calibrating Data:Gated Total Power
• GTPRef = C’ TRef
• GTPPaddle = C’{TA + TR}
• [GTPPaddle - GTPRef] / GTPRef
= TAe- 0secz / {TR+TA(1-e-0secz)}
• GTPHot - GTPCold = C’{THot - TCold} Atmosphere Liquid Nitrogen
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Calibrating Data:
• {[TSig-TRef]/[TRef]} / {[GTPPaddle - GTPRef] / GTPRef }
= TSource / TAtmosphere
• Actually TSource = T’Source / Efficiency– Usually written as TMB = TA
* / (note the different notation)
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Mopra Upgrades
• 8 GHz Digital Filter Bank– Zoom modes– 4(?) lines simultaneously
• MMIC receiver– Easier tuning– Higher Tsys
– May loose 115 GHz end?• 7 mm receiver
– New ATNF project?• Focal Plane Array???• Ultra-wide band correlator???
– Needs source of funds……
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Australia’s MM–Wave Radio Telescopes
3 mm
12 mm
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Australia Telescope Compact Array• National Facility
– Built for 1–10 GHz operation• MM-upgrades
– 3 mm (85-~105 (115) GHz)• 5 x 22m antennas• EW-array + NS-spur
– Currently 84.9-87.3+88.5-91.3 GHz– 12 mm (22-25 GHz)
• 6 x 22m antennas• 2 GHz bandwidth upgrade • 7 mm (45 GHz) upgrade planned
– 6 antennas• FPAs???
– With ultra-wide-band correlators??
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Water Vapour and Phase Fluctuations
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MillimetreInterferometry
Poses special challenges:• Significant atmospheric opacity, mostly due to H2O
• Fluctuations in H2O produce phase shifts• These increase with both baseline and frequency• Instrumental requirements (e.g. surface, pointing, baseline
accuracy) are more severe• Need more bandwidth to cover same velocity range (1
MHz (mm) km/s)
R SaultDesai 1998
Brightness Temperature H2O Turbulence Seeing
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ALMAAtacama Large Millimetre Array
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Antarctica??