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What is Millimetre-Wave Astronomyand why is it different?
Michael Burton
University of New South Wales
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)……
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
Transparancies
• Electromagnetic Spectrum
• MM transmission for 4mm H2O
• MM transmission for 11mm H2O
• Some bright MM-lines
Brightness Temperature
Atmospheric Transmission
The 3mm Millimetre Spectrum
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!!
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
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
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
Methanol Maser-selected Hot Molecular Core SurveyCH3CN CH3OH HCO+ H13CO+ N2H+ HCN HNC
7 lines; 86 Sources
Purcell
‘On the Fly’ Mapping with Mopra:The Horsehead Nebula
Optical 12CO 13CO
6 arcmin
Tony Wong
Tony Wong 0.17 km/s channel spacing
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
How many photons have we lost (or gained)?
00sec(z)
z
Signal on-source:
( ){ })sec()sec( 001 zsou
zatmrecSig eTeTTCT ττ −− +−+=
Trec
Tsou
Tatm
Sky (Reference, Off)
Source (On)
Difference
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
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
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
Calibrating Data:
• {[TSig-TRef]/[TRef]} / {[GTPPaddle - GTPRef] / GTPRef }
= TSource / TAtmosphere
• Actually TSource = T’Source / Efficiency
– Usually written as TMB = TA* /
(note the different notation)
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……
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??
Water Vapour and Phase Fluctuations
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
ALMAAtacama Large Millimetre Array
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Antarctica??