CCU Spring School Radio Astronomy for Chemists - PowerPoint PPT Presentation

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CCU Spring School Radio Astronomy for Chemists

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  1. CCU Spring SchoolRadio Astronomy for Chemists Lucy M. Ziurys Department of Chemistry Department of Astronomy Arizona Radio Observatory University of Arizona

  2. Our Galaxy in Molecules Columbia-CfA Project CO 1-0 All Sky Survey Chemistry and Interstellar Molecules • Molecular Astrophysics: 35 Years of Investigation •  Universe is truly MOLECULAR in nature • Molecular Gas is Widespread in the Galaxy and in External Galaxies Our Galaxy at Optical Wavelengths • 50% of matter in inner 10 kpc of Galaxy is MOLECULAR (~1010 M) • Molecular clouds largest well-defined objects in Galaxy (1 -106 M) • Unique tracers of chemical/physical conditions in cold, dense gas •  New window on astronomical systems - no longer realm of atoms

  3. CRL 2688 Post-AGB Star From Interstellar Molecules.. Protostars in Orion: HCN • Galactic Structure (Milky Way, others) • - Galaxy Morphology • - Galactic Chemical Evolution • Early Star Formation • - Life Cycles of Molecular Clouds • - Creation of Solar Systems • Late Stages of Stellar Evolution • - Properties of Giant Stars, Planetary Nebulae • - Mass Loss and Processing of Material in ISM • - Nucleosynthesis and Isotope Ratios • Molecular Compositionof ISM • - Remarkably Active and Robust Chemistry • - Molecules present in extreme environments • Implications for Astrobiology/Origins of Life • - Limits of Chemical Complexity Unknown CO in M51

  4. Known Interstellar Molecules

  5. Orion Molecular Clouds CRL 2688 Circumstellar Envelopes of Evolved Stars Physical Characteristics of Molecular Gas • Primarily Found in Two Types of Objects • Characteristics of Molecular Regions • Cold: T ~ 10 -100 K • Dense: n ~ 103-107 particles/cm3 (OR 10-13-10-9 mtorr) • Clouds Collapse to Form Stars/Solar Systems • Chemistry occurs primarily via 2-body ION-MOLECULE reactions • Kinetics governs the chemistry, NOT thermodynamics • Timescales for chemistry: 103 - 106 years

  6. r Molecular Energy Levels Electronic ~ 10,000 cm-1 Vibrational ~ 100-1000 cm-1 Rotational ~ 10 cm-1 Rotational Spectroscopy: How Molecules are Detected • Cold Interstellar Gas: Rotational Levels Populated via • Collisions • Spontaneous Decay Produces Narrow Emission Lines • Resolve Individual Rotational Transitions (Gas-Phase) • Rotational energy levels •  Depend on Moments of Inertia I = μ r2 Erot = B J(J+1) • Identification by “Finger Print” Pattern • Unique to a Given Chemical Compound

  7. Spectra obtained with Radio Telescopes C N • High Resolution Spectral Data • Many transitions measured • High signal-to-noise • Resolve fine, hyperfine structure N =2→1 rotational transition: 15 hyperfine components

  8. Radio Telescopes: Some Technical Aspects • Radio Telescope: • - Consists of two main components • - Telescope (antenna) itself with control system • - Receiver plus associated detection electronics • Antenna: • - Panels on a super structure • (aluminum with carbon fiber) • - Power pattern or gain function g(θ,φ) • - Pencil beam on sky with circular aperture • Gain pattern is Airy pattern • - First null at 1.22 λ/D: “diffraction-limited” • - Describes HPBW (θb) of antenna • - At 12 m, θb ~ 75″ – 40″ SMT HPBW

  9. Antenna response in terms of Antenna TemperatureTA • TA = 1/4π ∫ g(θ,φ) TB (θ,φ) d • - convolution of source and antenna properties • - imbed antenna in Blackbody at TBB • TA = T/4π ∫ g(θ,φ) d = TBB • Various Efficiencies for Antenna response • Aperture Efficiency ηA • - Response to a point source • - ηA ~ 0.5 • - a measure of surface accuracy of dish (as good as 15 microns rms) • Main Beam Efficiency ηB • - Percent of power in main beam vs. side lobes • - Response to extended source • TA = 1/4π∫ gTB d ~ <TB> • - ηB ~ 0.7 – 0.9

  10. Directed to Sub-reflector Signals reflected from primary Into a radio Receiver To central selection mirror Radio signals come From sky Radio Telescope Optics • - Cassegrain systems • f/D ratio of primary is ~ 0.4 -0.6

  11. Dewar window Lens Feedhorn Coupler Mixer Bias Isolator HEMT amplifier Millimeter Telescope Receivers sky • HETERODYNE RECEIVERS with • MULTIPLEXING SPECTROMETERS • Sky signal (sky) arrives at mixer • SIS junction in a dewar, cooled to 4.2 K • At Mixer, local oscillator (LO) signal (LO) is mixed with sky signal • Generates a signal at frequency difference • (intermediate frequency), IF •  IF = sky - LO or LO- sky • IF frequency detected by HEMT amplifier • IF Signal sent to the spectrometer (Backend) • Not single signal but range IF 0.5 GHz = sky 0.5 GHz LO To spectrometer backend IF COMPLEX SYSTEMS

  12. Mixer, amplifier, LO coupler etc built into “Insert” • One insert per mixer • Two mixers per frequency band (one for each orthogonal polarization) • Frequency coverage determined by Waveguide Band (WR 10, WR 8, etc) • Inserts into Dewar; cooled to 4.2 K Mixer Block Incorporation into “Insert” “Insert” put into Dewar

  13. A Complete Receiver… Optics Card Cage Cryo lines cabling

  14. Heterodyne Receivers and Image Rejection • With Mixers: observe two frequencies simultaneously • Upper sideband (USB): IF = sky- LO • Lower sideband (LSB): IF = LO- sky • Reject unwanted sideband to avoid confusion (SSB mixer or optics) • “Single” vs. “Double” sideband receiver (SSB vs. DSB) Typical rejection: > 15 - 20 db EXAMPLE: NGC7027 12CO: J=2 →1 line TA*~ 8 K - reduced to 0.1 K in image  20.6 db rejection - LO shift NGC 7027 13CO in LSB (signal sideband) 12CO image from USB

  15. IF System Block Diagram: SMT Left Rx room Right Rx room 345 Rx 1.5G Rx switch 1.5->5G Converter 5G Rx switch Rx switch/ Total power/ Attenuators 490 Rx Right Flange Rx New Rx Channel steering Computer room BE switch AOS A,B,C Frequency steering Filter banks IF Systems at Radio Telescopes • Radio Telescopes: MULTIPLEX ADVANTAGE • Simultaneously collect data over complete BW of IF Amplifier • Must have electronics to cleanlyprocess IF signals • Mix IF signal down to base band • Send into spectrometer

  16. Spectrometer “Backends” • Backend separates out signal as a function of frequency •  A spectrum is created…  = 178.323 MHz Filter Banks at the SMT • TYPES of BACKENDS • Filter banks: Complex set of capacitors, filters, etc. • Acousto-optic spectrometers (AOS) • Autocorrelators: Digital devices (MAC)

  17. Square law detector Integrator Mux BPF Zero DAC Filter Card for 16 channels: 1 MHz resolution filters Filter Card Block Diagram (one channel)

  18. Telescope Control System • Sophisticated Control System • Coordinates telescope motion with • data collection and electronics • Fast data acquisition/processing • Distributed nature of system • Each task controlled by separatecomputers • Computer for telescope tracking, focus position, each backend, etc. • Efficient, synchronous operation • Remote Observing •  Trained operators at site ARO Control System

  19. Observing Techniques • Continuum methods: Observe over broad band: 1.2 GHz (Digital Backend) • 1) Pointing • - Small corrections for gravitational deformation of dish • - one in azimuth, one in elevation • 2) Focus • - Move sub-reflector axially to best position • Spectral Line methods • - Observe spectral lines • - Background noise subtracted out with a switching technique • Telescope Calibration • - Measure a voltage from mixer • - Convert to Temperature Scale (TR*) using “Calibration Scan” • - Voltage on sky (Tsky) and ambient load (Tamb) • - Intrinsic “noise” of system (Tsys), including electronics, antenna, sky

  20. Pointing scan or continuum 5-point: done on planet Jupiter Establish pointing constants in az and elv

  21. FOCUS scan on Jupiter Determine optimal position of sub-reflector

  22. Astronomical Sources • Various sources “visible” at different times of day • Matter of position in sky”, i.e. Celestial Coordinates • Right Ascension (RA or α) and Declination (dec or δ) • Source overhead when RA = LST (Local Sidereal Time) “Catalog Tool” at ARO

  23. Spectral Line Techniques • Position switching • Switch telescope position between the source and blank sky (“off position”: 10-30 arcmin away in azimuth) • Subtract “(ON – OFF)/OFF” to remove background • Calibrate the intensity scale (voltage) by doing a • “Cal scan” :Tscale=TA*( in K) • Beam-switching • Nutatesub-reflector to get ON/OFF positions •  Also begin with Cal Scan • Frequency switching • Change frequency of LO ± 1-2 MHz Blank sky Molecular cloud • (ON-OFF)/OFF and calibration all done instantly in software

  24. Data Calibration and Intensity Scales • Data obtained immediately calibrated with background subtracted • Background given by SYSTEM TEMPERATURE (Tsys) • Tsys changes with time • Tsys ~ 150 – 250 K with new ALMA 3 mm rxr at 12 m • Spectral Line Intensity (TR*) ~ 0.001 – 10 K • Want background subtracted • No further reduction needed • Only cosmetic: • baseline subtraction, “bad channels”, etc) • Look at data and ON-LINE decisions • Change frequency, source, receiver, etc. • Optimize data return • Flexibility for new discoveries

  25. rms = 2mk at 12+ hrs rms = 1 mK at 25 hrs rms = 0.5 mK at 100 hrs Extensive Signal-Averaging • Collect data over 5-6 min as a single “scan” with ascan number • Written to computer disk • Average many scans for high S/N SensitivityLimits: Radiometer Equation • Tsys = system temperature • For a noise level of 0.5 mK, signal average for ~100 hours (Tsys ~ 300 K) • Requires telescope systems to be very stable over long periods of time •  can be accomplished with ARO

  26. Signal Averaging: An Illustration • Searching for KCN: new molecule • J(Ka,Kc) = 16(0,16)  15(0,15) • at 150.0433 GHz IRC+10216 Spectrum after 15 hours Trms = 0.0014 K MOSTLY NOISE Spectrum after 30 hours rms = 0.0010 K MAYBE A LINE ??? KCN U U Spectrum after 60 hours rms = 0.0007 K LINES APPEAR

  27. Dual Polarization Capabilities Orthogonal linear polarizations for 12 m receivers: Two independent measurements of the spectra Then average two spectra together for increased S/N J=2-1 line of HCO+ near 178 GHz

  28. From a Spectrum to an Abundance • Spectrum gives Intensity (TR*) • Convert TR* to TR (in K) via telescope efficiencies • TR related to the opacity τ • TB (or TL) = f Tex (1 – e-τ) • Thin limit: TB (or TL) = f Texτ • Thick limit: TB (or TL) = f Tex • f = beam filling factor (assume f = 1) • Column Density (in cm-2) • - Unsure of distance along line of sight • - Estimate an abundance along a column N (in cm-2) • - Column diameter given by telescope beam size θb • - NJ ~ TB in thin limit • - Ntot = gJ NJe-ΔEg’d/ζrot

  29. Rotational Diagrams • Measure many transitions • More accurate picture of abundance and excitation • Population in the levels governs the intensity of the transitions • By considering multiple transitions, column density (abundance) and temperature governing level population can be derived Trot = 27 ± 8 K Ntot = 1.1 ± 0.4 x1011 cm-2 KCN/H2 ~ 3 x10-11 • Create “Rotational Diagram” • Also model with more sophisticated excitation code: • LVG, Monte Carlo formalism, etc.

  30. Line Profiles Contain Kinematic Information

  31. Spatial Mapping of Molecular Lines (125, 185) (-15, 270) (-120, 240) HCO+ J = 1 → 0: Helix Nebula (390, -30) (-372, 0) Beam Size (130, -180) (-240, -100) (-300, -200)

  32. Observing Plan for School • Divide into three groups • Eight hours of observing per day in shifts • Conducting 2 part sequence of observations and data analysis • Part I: Introduction with various sources and molecules AND calculations • Part II: Real observations could lead to publishable results Part II: Begin a spectral line survey of C-Rich Stellar Envelope with new ALMA Band 3 Receiver

  33. Watch out for the Skunk !