CCU Spring School March 23 -26, 2009 CCU Spring School Radio Astronomy for Chemists Lucy M. Ziurys...

CCU Spring School March 23 -26, 2009

CCU Spring SchoolRadio Astronomy for Chemists

Lucy M. ZiurysDepartment of ChemistryDepartment of Astronomy

Arizona Radio ObservatoryUniversity of Arizona

Arizona

ObservatoryRadio

Arizona

ObservatoryRadio


Chemistry and Interstellar Molecules

Our Galaxy at Optical Wavelengths

Columbia-CfA ProjectCO 1-0 All Sky Survey

Our Galaxy in Molecules

• Molecular Astrophysics: 35 Years of Investigation Universe is truly MOLECULAR in nature• Molecular Gas is Widespread in the Galaxy and in External Galaxies

• 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


• 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

From Interstellar Molecules..

CRL 2688

Post-AGB Star

CO in M51

Protostars inOrion: HCN


Known Interstellar Molecules2 3 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 (CH2OH)2 12

SiN c-SiC2 CO2 HCCN HCCNC 6 rings CH3CH2CHO

CH CCO H2CN HNCCC 116 Carbon Molecules 13

CH+ CCS c-SiC3 H2COH+ 20 Refractories HC11N

NH CCP PH3 WHAT ELSE ??? Total = 151


Physical Characteristics of Molecular Gas

CRL 2688

Circumstellar Envelopes of Evolved Stars

• 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

OrionMolecular Clouds

• Primarily Found in Two Types of Objects


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)

• Identification by “Finger Print” Pattern• Unique to a Given Chemical CompoundRotational ~ 10 cm-1

Vibrational ~ 100-1000 cm-1

Electronic ~ 10,000 cm-1

Molecular Energy Levels I = μ r2

I2B

r

• Rotational energy levels Depend on Moments of Inertia

Erot = B J(J+1)


0.6

0.4

0.2

0.0

T R* (

K)

226800226600226400Frequency (MHz)

J = 1.5 - 1.5hf components



G 1 9 . 6 G i a n t M o l e c u l a r C l o u d C N R a d i c a l (N = 2 - 1)

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


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


• Antenna response in terms of Antenna Temperature TA

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


Radio signals comeFrom sky

Signals reflected from primary

Radio Telescope OpticsDirected to

Sub-reflector

To central selection mirrorInto a radioReceiver

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


Dewar windowLens

Feedhorn

Coupler

Mixer

Bias

Isolator

HEMTamplifier

• HETERODYNE RECEIVERS withMULTIPLEXING 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

Millimeter Telescope Receivers

LO

sky

IF

To spectrometer backend

COMPLEX SYSTEMS


Mixer Block

• 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

Incorporation into “Insert”“Insert” put into Dewar


A Complete Receiver… Optics

Card Cage

Cryo linescabling


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)

13CO in LSB (signal sideband)

12CO image from USB

NGC 7027

Typical rejection: > 15 - 20 db EXAMPLE: NGC702712CO: J=2 →1 line TA*~ 8 K - reduced to 0.1 K in image 20.6 dbrejection- LO shift


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

Frequency

steering

AOS

A,B,C

Filterbanks

Rx switch/

Total power/

Attenuators

IF System Block Diagram: SMT

Channel

steering

345Rx

490Rx

NewRx

1.5G

Rx

switch RightFlange

Rx

BE switch

1.5->5G

Converter5G

Rx

switch

Right Rx roomLeft Rx room

Computer room

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


Spectrometer “Backends”

TYPES of BACKENDS• Filter banks: Complex set of capacitors, filters, etc.• Acousto-optic spectrometers (AOS)• Autocorrelators: Digital devices (MAC)

• Backend separates out signal as a function of frequency A spectrum is created…

= 178.323 MHz

Filter Banks at the SMT


Filter Card Block Diagram(one channel)

MuxBPF

Zero DAC

Square law detector

Integrator

Filter Card for 16 channels:1 MHz resolution filters


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


• 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

Observing Techniques


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

Establish pointing constants in az and elv


FOCUS scan on Jupiter

Determine optimal position of sub-reflector


• 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)

Astronomical Sources

“Catalog Tool”at ARO


• 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

Nutate sub-reflector to get ON/OFF positions

Also begin with Cal Scan • Frequency switching

Change frequency of LO ± 1-2 MHz

Spectral Line Techniques

Molecular cloud

Blank sky

• (ON-OFF)/OFF and calibration all done instantly in software


• 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

Data Calibration and Intensity Scales


Sensitivity Limits:

Trms 2Tsys

spec t int

• 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

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 a scan number• Written to computer disk• Average many scans for high S/N

Radiometer Equation


Spectrum after 15 hoursTrms = 0.0014 KMOSTLY NOISE

Spectrum after 30 hoursrms = 0.0010 KMAYBE A LINE ???

Spectrum after 60 hoursrms = 0.0007 K

LINES APPEAR

• Searching for KCN: new molecule

• J(Ka,Kc) = 16(0,16) 15(0,15)

at 150.0433 GHz

Signal Averaging: An Illustration

IRC+10216

KCN UU


Dual Polarization Capabilities

J=2-1 line of HCO+ near 178 GHzOrthogonal

linear

polarizations

for 12 m

receivers: Two

independent

measurements

of the spectra

Then average

two spectra

together for

increased S/N


• 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

From a Spectrum to an Abundance

hfTE

ij

rotRtot

ReS

VTkN

rotgd /20

32/1

5

8

103


Trot = 27 ± 8 KNtot = 1.1 ± 0.4 x1011 cm-2

KCN/H2 ~ 3 x10-11

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• Create “Rotational Diagram”• Also model with more sophisticated excitation code: LVG, Monte Carlo formalism, etc.


Line Profiles Contain Kinematic Information


1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

TR

* (K)

232000231800231600231400231200231000Frequency (MHz)

13C

S

OC

S

C2H

3CN

C2H

3CN

C2H

3CN

CH

3CH

O +

C2H

3CN

CH

3CH

O +

C2H

5CN

C2H

3CN

C2H

3CN

HC

OO

CH

3 +

C2H

3CN

C2H

3CN

C2H

5CN

C2H

5CN

C2H

5CN

C2H

5CN

HC

OO

CH

3

HC

OO

CH

3

HC

OO

CH

3

HC

OO

CH

3

HC

OO

CH

3

CH

3CH

O

C2H

3CN

+ C

2H5C

N

C2H

5OH

C2H

5OH

+ (C

H2O

H) 2

CH

3CH

O

C2H

5OH

CH

3NH

2

HN

CO

CH

3CH

O

CH

3CH

O

HC

OO

H

C2H

5OH

(CH

3)2O

(CH

3)2O

NH

2CH

O

37 Indentified Features35 Unidentified Features~6 lines per 100 km/sTRMS = 0.003 K (theoretical)

U

U U

U U

U

U

U

U


(125, 185)

(390, -30)

(130, -180)

(-15, 270)

(-240, -100)

(-120, 240)

(-372, 0)

(-300, -200)

HCO+ J = 1 → 0:Helix Nebula

Spatial Mapping of Molecular Lines

Beam Size


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 surveyof C-Rich StellarEnvelopewith new ALMABand 3 Receiver


Watch out for the Skunk !

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CCU Spring School March 23 -26, 2009 CCU Spring School Radio Astronomy for Chemists Lucy M. Ziurys...

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