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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

CCU Spring SchoolRadio Astronomy for Chemists

Lucy M. Ziurys

Department of Chemistry

Department of Astronomy

Arizona Radio Observatory

University of Arizona

slide2

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
slide3

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

slide5

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
slide6

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
slide7

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

slide8

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

slide9

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
slide10

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
slide11

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

slide12

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

slide13

A Complete Receiver…

Optics

Card Cage

Cryo lines

cabling

slide14

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

slide15

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
slide16

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)
slide17

Square law detector

Integrator

Mux

BPF

Zero DAC

Filter Card for 16 channels:

1 MHz resolution filters

Filter Card Block Diagram

(one channel)

slide18

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

slide19

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
slide20

Pointing scan or continuum 5-point: done on planet Jupiter

Establish pointing constants in az and elv

slide21

FOCUS scan on Jupiter

Determine optimal position of sub-reflector

slide22

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

slide23

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
slide24

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
slide25

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
slide26

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

slide27

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

slide28

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
slide29

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.
slide32

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)

slide33

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

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