CCU Spring School Radio Astronomy for Chemists

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CCU Spring SchoolRadio Astronomy for Chemists

• Lucy M. Ziurys
• Department of Chemistry
• Department of Astronomy
• Arizona Radio Observatory
• University of Arizona

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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

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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 Composition of 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
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Known Interstellar Molecules
2 2 3 3 4 4 5 6 7 8 9
H2 CN H2O C3 NH3 CH3 SiH4 CH3OH CH3CHO CH3COOH CH3CH2OH
OH CF H2S MgNC H3O C3N- CH4 NH2CHO CH3NH2 HCOOCH3 (CH3)2O
SO CO SO2 NaCN H2CO HCNO HCOOH CH3CN CH3CCH CH3C3N CH3CH2CN
SO CS N2H CH2 H2CS HSCN HC3N CH3NC CH2CHCN C7H HC7N
SiO C2 HNO MgCN HNCO CH2NH CH3SH HC5N H2C6 CH3C4H
SiS SiC HCP HOC HNCS NH2CN C5H C6H CH2OHCHO C8H
NO CP NH2 HCN CCCN H2CCO HC2CHO C6H- HC6H C8H-
NS CO H3 HNC HCO2 C4H C2H4 c-CH2OCH2 CH2CCHCN CH3CONH2
HCl SH N2O AlNC CCCH C4H- H2C4 CH2CHOH CH2CHCHO CH3CHCH2
NaCl HD HCO SiCN c-C3H c-C3H2 HC3NH NH2CH2CN
KCl HF HCO SiNC CCCO CH2CN HC4H 10 11
AlCl PO OCS H2D CCCS C5 HC4N CH3COCH3 HC9N
AlF AlO CCH HD2 HCCH SiC4 C5N CH3C5N CH3C6H
PN HCS KCN HCNH H2C3 20 ions 20 ions (CH2OH)2 12
SiN c-SiC2 CO2 HCCN HCCNC 6 rings 6 rings CH3CH2CHO
CH CCO H2CN HNCCC 116 Carbon Molecules 116 Carbon Molecules 13
CH CCS c-SiC3 H2COH 20 Refractories 20 Refractories HC11N
NH CCP PH3 WHAT ELSE ??? WHAT ELSE ??? Total 151
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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

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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(J1)

• Identification by Finger Print Pattern
• Unique to a Given Chemical Compound

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Spectra obtained with Radio Telescopes

• High Resolution Spectral Data
• Many transitions measured
• High signal-to-noise
• Resolve fine, hyperfine structure

C
N
N 2?1 rotational transition 15 hyperfine
components
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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(?,f)
• - Pencil beam on sky with circular aperture
• Gain pattern is Airy pattern
• - First null at 1.22 ?/D diffraction-limited
  
• - Describes HPBW (?b) of antenna
• - At 12 m, ?b 75? 40?

SMT
HPBW
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• Antenna response in terms of Antenna Temperature
  TA
• TA 1/4p ? g(?,f) TB (?,f) d?
• - convolution of source and antenna
  properties
• - imbed antenna in Blackbody at TBB
• TA T/4p ? g(?,f) d? TBB
• Various Efficiencies for Antenna response
• Aperture Efficiency ?A
• - Response to a point source
• - ?A 0.5
• - a measure of surface accuracy of dish (as
  good as 15 microns rms)
• Main Beam Efficiency ?B
• - Percent of power in main beam vs. side lobes
• - Response to extended source
• TA 1/4p? gTB d? ltTBgt
• - ?B 0.7 0.9

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Radio signals come From sky
Radio Telescope Optics

• - Cassegrain systems
• f/D ratio of primary is
  
  0.4 -0.6

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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
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• 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
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A Complete Receiver
Optics
Card Cage
Cryo lines cabling
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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 gt 15 - 20 db EXAMPLE
NGC7027 12CO J2 ?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
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IF Systems at Radio Telescopes

• Radio Telescopes MULTIPLEX ADVANTAGE
• Simultaneously collect data over complete BW of
  IF Amplifier
• Must have electronics to cleanly process IF
  signals

• Mix IF signal down to base band
• Send into spectrometer

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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)

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Filter Card for 16 channels 1 MHz resolution
filters
Filter Card Block Diagram (one channel)
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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
• separate computers
• Computer for telescope tracking,
• focus position, each backend, etc.
• Efficient, synchronous
• operation
• Remote Observing
• ? Trained operators at site

ARO Control System
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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

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Pointing scan or continuum 5-point done on
planet Jupiter
Establish pointing constants in az and elv
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FOCUS scan on Jupiter
Determine optimal position of sub-reflector
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Astronomical Sources

• Various sources visible at different times of
  day
• Matter of position in sky, i.e. Celestial
  Coordinates
• Right Ascension (RA or a) and Declination (dec
  or d)
• Source overhead when RA LST (Local Sidereal
  Time)

Catalog Tool at ARO
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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 TscaleTA( in K)
• Beam-switching
• ? Nutate sub-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

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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

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Extensive Signal-Averaging

• Collect data over 5-6 min as a single scan
  with a scan number
• Written to computer disk
• Average many scans for high S/N

Sensitivity Limits
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

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Signal Averaging An Illustration

• Searching for KCN new molecule
• J(Ka,Kc) 16(0,16) ? 15(0,15)
• at 150.0433 GHz

IRC10216
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
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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
J2-1 line of HCO near 178 GHz
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From a Spectrum to an Abundance

• Spectrum gives Intensity (TR)
• Convert TR to TR (in K) via telescope
  efficiencies
• TR related to the opacity t
• TB (or TL) f Tex (1 e-t)
• Thin limit TB (or TL) f
  Text
• 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
  ?b
• - NJ TB in thin limit
• - Ntot gJ NJe-?Egd /?rot

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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.

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Line Profiles Contain Kinematic Information
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Spatial Mapping of Molecular Lines
HCO J 1 ? 0 Helix Nebula
Beam Size
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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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Watch out for the Skunk !