The Future Development of Ground-Based The Future Development of Ground-Based Optical/IR InterferometryOptical/IR Interferometry

Chris HaniffMRO & Astrophysics Group

Cavendish LaboratoryCambridge UK

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OutlineOutline• Where we are today:

– Radio vs Optical.– Today’s implementations.– Typical science.

• Current limitations:– Critical shortcomings.

• Future prospects:– Science possibilities.

• Conclusions (aka “crystal ball gazing”)

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Optical/IR interferometers (0.4Optical/IR interferometers (0.4m-2.4m-2.4m)m)• These are essentially the same as phase-unstable radio interferometers

operating at a frequency of ~ 300THz.

• But some important differences exist:– Atmospheric seeing scale size (6/5) < typical “single dish” diameter.

• E.g. r0~ 10cm at =500nm => Limits useful aperture diameter.

– Atmospheric seeing timescale << Earth-rotation smearing time.• E.g. t0~ 5 msec at =500nm => Limits useful coherent integration time.

– Cannot, even in principle, take advantage of amplifiers.• Radiation degeneracy parameter (“photons per mode”) W << 1 when looking at thermal

sources with temperatures < 20,000K.• More baselines splitting light more ways reducing signal-to-noise ratio.

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Comparison with the VLBAComparison with the VLBA• This combination of atmospheric and quantum limits marks the real

difference between phase unstable optical and radio arrays:– Example: observing a 12th magnitude quasar.

• Assume r0 = 10cm, t0= 5msec, = 500nm, / = 10%, system efficiency 10%.

• Get 4 photons through a 2.5r0 aperture in a 1.5t0 integration.

– Hence the signal-to-noise-ratio in one integration is almost always small for astrophysically-interesting objects.

– Large amounts of incoherent integration are required to do useful science.• Fortunately, one can accumulate 1000s of exposures in a few minutes• The primary observables are the power spectrum and the bispectrum (closure phase).

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Today’s arraysToday’s arrays• Keck Interferometer:

– 2 x 10m + 4 x 1.8m fixed, 2 x 2-waycombiners, 120m baseline.

– Main goal is differential astrometryfor planet finding.

• VLTI:– 4 x 8m + 4 x 1.8m movable, 3-way

combiner, 200m baselines.– Facility array, multi-mission.

• CHARA:– 6 x 1m, fixed, 6-way combiner,

330m baselines.– Main goal is binary stars.

• NPOI:– 6 x 0.5m, movable, 6-way combiner, 450m baselines.– Split imaging/astrometry goals.

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Today’s scienceToday’s science• Fundamental parameters:

– Radii, effective temperatures, and masses (through binary star orbits).

• Detailed atmospheric studies:– Stratification of cool stellar atmospheres, limb-darkening, stellar surface imaging.

• Dynamical studies of pulsating stars:– Miras, Cepheids.

• Studies of gas and dust shells:– Hot stars: Be star envelopes.

– Cool stars: dust shell emission in evolved systems.

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Direct measurements of stellar pulsationDirect measurements of stellar pulsation

• Cepheids:– Data for Gem from PTI.– Visibilities on 110m baseline at 2.2m. – Allows a geometric check on the

calibration of the Cepheid distance scale.

• Miras:– Data for Cyg from COAST.– Diameter in 905nm “contaminated”

bandpass. – Indicative of changes in outer envelope

but probably not physical motion.

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Imaging interferometryImaging interferometry

Giroletti et al, AA, 399, 899, 2003

Tuthill et al, ApJ, 543, 284, 2000

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How should we interpret these results?How should we interpret these results?• Long-baseline interferometers can be built and made to work:

– Imaging at the level of the VLBA is a realistic possibility.– Scientific results are beginning to predominate now, not technical ones.– All of this is routine:

• Mark III interferometer made ~ 150 measurements/night, ~200nights/year.

• In the future, 3 critical areas need addressing:– Angular resolution:

• To accommodate a suitable range of science targets.– Sensitivity:

• To keep both galactic and extra-galactic astronomers busy.– Imaging quality:

• To allow rapid, high fidelity, high dynamic range imaging.

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Angular resolutionAngular resolution

• 300m baseline gets nearest BLRs& sub-AU scales at Taurus

• Need a factor of >30 in range of resolution: any useful array must be re-configurable

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SensitivitySensitivity• Defined in similar terms to that of an AO system:

– Requirement is for a compact reference bright enough to give a useful error-signal on a timescale short enough to track the atmosphere.

– Thereafter, the faintest structures visible will be determined by the dynamic range.

• Sensitivity 3.6

– Go to long wavelengths

• H=14 gets ~150 quasars• K=13 gets to H-burning

limit at Taurus

• Requires apertures >1.4m

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ImagingImaging• Most astrophysics on small angular scales is poorly understood.

– Need model-independent imaging for robust science.• Goal: 10x10 pixels

– Requires ~ 100 independent (u,v) data points.– Must be measured in less than time taken for source to evolve.– Speed of imaging essential to find targets of opportunity.

Helix Nebula at 1.4GHz (Rodriguez et al. Ap J, 2002) IRC+10216 at 2.2m (Tuthill et al. Ap J, 2000)

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Future science prospectsFuture science prospects

• Active galactic nuclei: resolved imaging of the nuclear dust component, the BLR, synchrotron jets and nuclear and extra-nuclear starbursts.

• Stellar accretion and mass loss: via winds, jets, outflows, and Roche-lobe overflow. Examples in single and binary systems.

• Star and planet formation: detection and characterization of protostellar disks. Accretion, disk-clearing, fragmentation and duplicity.

• High precision interferometry: planet and low-mass companion detection via astrometry, photocentre shifts, and precision closure phases.

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Active galactic nucleiActive galactic nuclei• Unified model of an AGN (z~0.01)

0.1-0.5 mas

1 as

10-100 mas

30-2000 mas

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Accretion and mass-lossAccretion and mass-loss

• “Supergranules” at the surfaces of late-type stars may be associated with aperiodic mass-ejection events– Important in chemical dredge-up

and recycling in late stages of stellar evolution.

Hydrodynamic simulation of convection in an M-supergiant

(Freytag et al 2002)

~20mas

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Star and planet formationStar and planet formation

The inner region of a protostellar accretion disc in Taurus

Outer disc clearing due to planet formation (4-10mas)

Truncation of the inner disk (~0.4mas)

Final mass transfer taking place via accretion streams along magnetic

field lines (0.2-0.4mas).

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High Precision astrophysicsHigh Precision astrophysicsSimulation of an astrometric observation of HD 162020, which is known to have a companion with m sin (i) > 14.1 Mjup. in an 8.42 day orbit. (Segransan 2003).

Astrometric measurements resolve thesin (i) ambiguity.

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Predicting the futurePredicting the future• What will an optical VLBA look?

– A moderate number (15) of collectors:• Fewer won’t image well enough.• Signal-splitting in the correlator limits the number that can be effectively used.• More will be too costly anyhow.

– Moderate sized apertures (2-3m):• Not obvious that larger ones will be necessary.• Larger ones will be too expensive for the predicted science output:

– This array will deliver on a broad but not comprehensive range of astrophysics.It will not be an ALMA.

– Baselines in the range 10-1000m. Longer baselines introduce other problems:• Where would you put it?• What would you look at?

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ConclusionsConclusions• The next 10 years will see:

– The development of a small number of facility interferometers:• The VLTI will be the first of these.

– A progression from single baseline science to imaging interferometry.– The operation of a number of specialized astrometric interferometers for high-

precision science. – A significant increase in scientific results from optical/IR interferometry.– The start of something like the optical equivalent of the VLBA.