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Astro 2233 Fall 2008 Lect #409 Sept 2008• Homework #1 is due on Thursday

- Includes your 3 minute oral report on current facility- Keep in mind relevance of your facility to Arecibo science

• Kassie is eager to help!• When you work out problems: show your work please!

Coming events:• Paper #1 is due next Tuesday, Sep 16. • It’s not very long. Keep in mind your audience!

• In October: join me observing with Arecibo from SSB!• Final “paper” = project, using real Arecibo ALFALFA data

Thurs: presentations

GMRTLucaChandra RussellVLA/EVLAKatie

Green Bank TelescopeSarah

FacilitySpeaker

• I will provide a slide of your facility

• You tell us about it (briefly!).

• Part of written assignment is a summary (can be bulleted items)

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Arecibo telescope schedule

The Arecibo Radiotelescope

World’s largest radio/radar telescope

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While the primary reflector (the “dish”) is suspended just above ground level over a network of steel cables, the focal structure

with all the receiving equipment – which weighs 900 tons - is suspended 150 meters above the ground, and can be steered to

point at cosmic sources within 20 degrees of zenith.

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The Arecibo Radiotelescope

Gregorian Optics• The dome is referred to as the “Gregorian”.• A “Gregorian” focus refers to one in which the secondary

reflector is placed behind the focal point of the primary reflector.

• In this case, there are 3 reflectors: primary, secondary, tertiary.

• The dome can move up and down in zenith angle and around in azimuth to track sources in R.A., Decl.

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Computer controlled mechanical jacks at three tie-down locations maintain height and attitude of the suspended platform.

Maximum load on each jack is 60 tons -concrete block weighs 60 tons and is free to lift.

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Higher telescope load required additional cables – two more in every location

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The “illuminated” (red) area by the Gregorian system on the surface of the Arecibo reflector. The illuminated area is ~ 213 m x 235 m. It moves radially as the zenith angle (900 – the elevation angle) changes from o0 to 200 (the maximum value). The location shown is for ZA ~ 150.

Receiver systems at Arecibo

1.15-1.73L-wide

1.225-1.525ALFA

0.6075-0.6115610-MHz

0.425-0.435430-MHz

0.312-0.342327-MHz

Freq Range(GHz)

ReceiverName

1.8-3.1S-low

7.8-10.2X5.9-8.1C-high3.85-6C

3-4S-high2.33-2.43S-narrow

Freq Range(GHz)

ReceiverName

L-band: 1.1-1.8 GHz, 18-25 cm21 cm HI line at 1420.4058 MHz (if at rest)

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“Feed”• Smaller antenna (waveguide horn or line feed) that transfers

incoming signal to receiver• Term originated with antennas used for radar transmissions

MH with “ALFA”

430 MHz “line feed”

Radio Telescope Components

• Reflector(s)• Feed horn(s)• Low-noise amplifier• Filter• Downconverter• IF Amplifier• Spectrometer

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Feedhorns

Typical cm-wave feedhorn

ALFA: 7 feed array

Horn feeds

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ALFA: Arecibo L-band Feed Array

A 7-pixel camera

Beam & sidelobes

• Essentially diffraction pattern of telescope functioning as transmitter

• Uniformly illuminated circular aperture: central beam & sidelobe rings

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• Obstructions, non-uniform illumination by feedhorn asymmetry and alter strengths of sidelobes vs. central beam

• Emission received from pattern outside first sidelobe ring often called stray radiation

• Beamwidth FWHM or HPBW refers to the radial distance at which the sensitivity of the beam falls to half its central value. – ALFA beams are elliptical.– The central beam is more sensitive

(higher gain) than the outer beams.– The side/coma lobes are pretty ugly.

(the price we pay for all those photons)

This diagram shows the illumination

pattern of the ALFA instrument with its

7 pixels (feed horns)

Power pattern, sidelobes, “beam”

Resolution of the Arecibo Telescope

Feed illuminates an annulus of 700 ft ~ 213 m = 2.1x104cmL-band λ=21 cm

A homework problem:

Compare the diffraction limit of the whole telescope (305m) with that of the illuminated aperture (213m).

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Research at Arecibo• Atmospheric Science

– Use radar to probe atmosphere: transmit a coded pulse and receive echo back

• Incoherent scatter• Ionospheric modification (inject energy)

– Measures composition, temperature, and density of upper atmosphere

– Measures the growth and decay of disturbances in the ionosphere

• Radar Astronomy– Radio energy is transmitted, reflected and then collected.– Studies surface features, composition, size, shape,

rotation and path of target– Studies objects within our solar system, including Moon,

planets (to Saturn), rings, moons, asteroids (including NEOs), comets…

Don will talk about the Solar system program…

Radio Astronomy at Arecibo

• Continuum Observations: detection, monitoring– Thermal radiation (hot gas)– Non-thermal radiation (synchrotron: electons in magnetic field)

• Pulsars: Periodic signals; Discovery, timing, monitoring• VLBI (Very Long Baseline Interferometry)• Spectral Line Observations

– HI line 1420.4058 MHz/ 21 cm line– OH lines (1612, 1665, 1667, 1720 MHz)– CH, H2CO, methanol, etc.

• Much of the current work uses ALFA: the 7 “pixel” beam feed array to conduct major surveys of areas of the sky

ALFALFA: The Arecibo Legacy Fast ALFA Surveyhttp://egg.astro.cornell.edu/alfalfa

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Blackbody Sources

• Cosmic Microwave Background is about the only relevant blackbody source but nearly constant over sky.

• Peak in cm-wave radio requires very low temperature (2.7 K).• Ignored in most work – essentially constant source of static

(same in all directions) and much weaker than static produced byinstrumentation itself

WMAP results

Continuum Observations• Radio frequency observations

over a wide range of frequencies (wide bandwidth)

• Example: studying synchrotron emission in our own galaxy or other galaxies

High energy electrons, moving at high speed spiral around magnetic field lines => accelerated => radiate energyNon-thermal radiation:

Flux Sν ∝ ν-n

Sources are high energy phenomena: pulsars, active galactic nuclei, supernova remnants, etc.

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Pulsars• Neutron stars were a purely

theoretical concept until observations of the 33-ms pulsar in the Crab Nebula in 1968

• Rapidly rotating neutron stars emit when synchrotron radiation in beam; we see it when beam sweeps past Earth

• Magnetic axis and rotation axis not aligned

• Excellent clocks: used to test relativity (Taylor/Hulse nobelprize)

• First discovery of extra-solar planets (Wolszczan and Frail)

• We’ll discuss more laterCrab Nebula

The Binary Pulsar PSR 1913+16• Discovered in 1974 by Russ

Hulse and Joe Taylor at Arecibo

• Two stars in orbit around center of mass; one is a pulsar, the other is not

• Shrinking of orbit due to loss of energy by gravitational radiation as predicted by Einstein

• 1993 Nobel prize to Hulse and Taylor

• We’ll discuss this more later

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PSR B1257+12 and its Planets

• First detection of an extrasolar planet EVER

• Discovered by Alex Wolszczan & Dale Frail through pulsar timing

• At least 3 bodies of Earth-like masses around PSR B1257+12

• We’ll discuss this more later

Figure from Alex Wolszczan

VLBI - Very Long Baseline Interferometry• Joined the VLBI network in the

late 1990s• NAIC commits 4% of AO’s

telescope time to VLBI • Broad bandwidth video recorders

record signals and are then replayed later in the same location

• Now incorporated into High Sensitivity Array (HSA)

• Track time evolution/motion of sources with extreme precision (micro-arcseconds)

• We’ll discuss this more later

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Spectral Line Sources

• Neutral hydrogen (H I ) spin-flip transition• Recombination lines (between high-lying atomic

states)• Molecular lines (CO, OH, H2O, H2CO, etc)

• HI line: rest frequency is 1420.4058 MHz• Because of Hubble’s Law V = HoD, we observe the HI line from distant galaxies redshifted to lower frequencies

• In frequency units, we write the redshift

• The ALFALFA survey setup covers a frequency range that covers 100 MHz centered at 1385 MHz, corresponding to HI lines with velocities from -2000 to +18000 km/s.

Redshifted HI lines

frestfobs

z = - 1

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The Arecibo Legacy Fast ALFA Survey:

The Search for Low-Mass, Gas-Rich Halos

ALFALFA:

A complete census of HI-bearing objects in the local extragalactic universe

A “blind” survey: does not depend on prior knowledge that a (n optically visible) galaxy exists

Are their gas rich, but optically invisible (no stars!) galaxies?

Spectral Line Observations

• Discrete radio emission over small range of frequencies

• When we search for the 21-cm line, we cannot be sure where to look due to a galaxy’s redshift

• Could be emission or absorption• ALFALFA: HI census of galaxies

in the nearby universe• Do some objects contain a little

HI gas, a lot of dark matter and **no** stars => “dark galaxies”?

• You can help me observe some night if you want to!

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• H I spectral line from galaxy shifted by expansion of universe (“recession velocity”) and broadened by rotation

Frequency

Rest frequency 1420.4058 MHz

∫ SdV HI massV Distance∆V Mass

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ALFALFAhttp://egg.astro.cornell.edu/alfalfa

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How do I get time on the telescope?

• Telescope operates 24 hours a day (although some experiments must run during night (or day!)

• Submit a proposal which is judged by a panel of referees• Deadlines are February 1st, June 1st, and October 1st; special call

for “large projects” like ALFALFA• Oversubscription rate is 6+:1• Open to qualified observers regardless of affiliation; Cornell

scientists do not have preferred access

ALFALFA is approved for up to 900 hours/year;Started Feb 4, 2005. So far 2700 hours = 63%

What is the Sun like?

Radius = 100 REarth

Mass = 1000 MJup = 300,000 MEarth

• We do not see into the Sun.

• We see the “photosphere”.

• How do we know what the interior of the Sun is like?

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The photosphere: the surface we seeGranules:• Bright cells on Sun’s surface• About 100 °K warmer than

surrounding• Maintain identity for 15-20 mins• Tops of convection cells

The photosphere: the surface we seeGranules:• Bright cells on Sun’s surface• About 100 °K warmer than

surrounding• Maintain identity for 15-20 mins• Tops of convection cells

Flares:• Solar storms• Gas heated to 10 million °K• Can burst up to 500,000 km in

~20 mins.

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The Sun in X-rays

What is the Sun like?

Radiative zone: energy transport via photons

Convection zone: energy transport by movement of gas cells

hot cool hot cool hot

• Hot gas rises• Cool gas sinks

Evidence: Mottled appearance of the

Sun’s surface

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The photosphere: the surface we seeGranules:• Bright cells on Sun’s surface• About 100 °K warmer than

surrounding• Maintain identity for 15-20 mins• Tops of convection cells

Flares:• Solar storms• Gas heated to 10 million °K• Can burst up to 500,000 km in

~20 mins.Sunspots:• Cooler areas than surroundings

by up to 1500 °K. • Last hours to months• Number of sunspots: 11 year

cycle

Sunspot magnetic fields

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Sun’s magnetic field

What is the Sun like?Corona:• seen at eclipse• Rarefied; very hot -

1.8 million degrees• Extends millions of km

Photosphere:• “Surface” we see• T ~ 5600 – 6000 °K• Region from which

photons escape

Core:• Inner ¼ radius• T ~ 15 million °K• Region where nuclear

fusion (“hydrogen burning”) takes place

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Simple laws for starsThe luminosity, L, of an object depends on

1. Its surface temperature, T2. Its size (radius), R

L ∝ R2 T4

The most luminous stars have L ~ 100,000 L• These stars must either be larger or hotter

than the Sun (or both!)The faintest stars have L ~ .0001 L .

• These stars must either be cooler or smaller than the Sun (or both!)

H-R diagram

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The Hertzsprung-Russell diagram• A useful tool!

• Allows us to categorize, graphically, the physical properties of a star, especially:

1. Its surface temperature (color or spectral class)

2. Its luminosity

• The Sun is an average star in terms of its physical properties.

• A star’s location on the H-R diagram is determined by:

1. Its mass2. Its age

Spectral ClassificationAnnie J. Cannon

“The Henry Draper Catalog”

Categorized several 100,000 spectra into classes according to the absorption lines in their spectra

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Spectral Classification

As the temperature of the star changes, the atoms/ions/molecules that appear in its spectra change.

Spectral Classification• The details of the

spectral lines (how spread out or narrow they are) tell us about the density of the star’s atmosphere:

• High density: dwarf star

• Low density: giant or supergiant star

Reason => collisionalbroadening depends on density, for same T

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Stellar luminosity classes

Why?

Luminosity classes

Stars of the same surface temperature may have different radii.

• Supergiants• Giants• Main Sequence

(dwarfs)

L ∝ R2 T4

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H-R diagram

The Main Sequence

• Stars on the “Main Sequence” are burning hydrogen into helium in their cores.

• The mass of a star determines its location on the Main Sequence of the H-R diagram

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The Main Sequence

• Stars on the “Main Sequence” are burning hydrogen into helium in their cores.

• The mass of a star determines its location on the Main Sequence of the H-R diagram

Dwarf stars

• Lots of red dwarfs – low mass stars

• “white dwarfs” are hot but very small…; they are not undergoing hydrogen burning(!).

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Giants and supergiants

• Red supergiants and giants have the same surface temperature as low mass main sequence stars but are much bigger.

• Therefore, they are more luminous than Main Sequence stars of the same surface temperature.

L ∝ R2 T4