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CCU Spring School Radio Astronomy for Chemists

CCU Spring School Radio Astronomy for Chemists Lucy M. Ziurys Department of Chemistry Department of Astronomy Arizona Radio Observatory University of Arizona Our Galaxy in Molecules Columbia-CfA Project CO 1-0 All Sky Survey Chemistry and Interstellar Molecules

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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 !

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