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The Future Development of GroundBased OpticalIR Interferometry

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Title: The Future Development of GroundBased OpticalIR Interferometry

1
The Future Development of Ground-Based Optical/IR
Interferometry

Chris Haniff
MRO Astrophysics Group
Cavendish Laboratory
Cambridge UK

2
Outline

Where we are today
Radio vs Optical.
Todays implementations.
Typical science.
Current limitations
Critical shortcomings.
Future prospects
Science possibilities.
Conclusions (aka crystal ball gazing)

3
Optical/IR interferometers (0.4?m-2.4?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) lt typical
single dish diameter.
E.g. r0 10cm at ?500nm gt Limits useful
aperture diameter.
Atmospheric seeing timescale ltlt Earth-rotation
smearing time.
E.g. t0 5 msec at ?500nm gt Limits useful
coherent integration time.
Cannot, even in principle, take advantage of
amplifiers.
Radiation degeneracy parameter (photons per
mode) ?W ltlt 1 when looking at thermal sources
with temperatures lt 20,000K.
More baselines ? splitting light more ways ?
reducing signal-to-noise ratio.

4
Comparison 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).

5
Todays 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-waycombiner, 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.

6
Todays 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.

7
Direct measurements of stellar pulsation

Miras
Data for ? Cyg from COAST.
Diameter in 905nm contaminated bandpass.
Indicative of changes in outer envelope but
probably not physical motion.

Cepheids
Data for ? Gem from PTI.
Visibilities on 110m baseline at 2.2?m.
Allows a geometric check on the calibration of
the Cepheid distance scale.

8
Imaging interferometry

Giroletti et al, AA, 399, 899, 2003

Tuthill et al, ApJ, 543, 284, 2000
9
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.

10
Angular resolution

300m baseline gets nearest BLRs sub-AU scales
at Taurus
Need a factor of gt30 in range of resolution
any useful array must be re-configurable

11
Sensitivity

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
H14 gets 150 quasars
K13 gets to H-burninglimit at Taurus
Requires apertures gt1.4m

12
Imaging

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.

IRC10216 at 2.2?m (Tuthill et al. Ap J, 2000)
Helix Nebula at 1.4GHz (Rodriguez et al. Ap J,
2002)
13
Future 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.

14
Active galactic nuclei

Unified model of an AGN (z0.01)

1 ?as
30-2000 mas
0.1-0.5 mas
10-100 mas
15
Accretion and mass-loss
20mas

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)
16
Star and planet formation

The inner region of a protostellar accretion disc
in Taurus

17
High Precision astrophysics

Simulation of an astrometric observation of HD
162020, which is known to have a companion with m
sin (i) gt 14.1 Mjup. in an 8.42 day orbit.
(Segransan 2003).
Astrometric measurements resolve thesin (i)
ambiguity.

18
Predicting the future

What will an optical VLBA look?
A moderate number (15) of collectors
Fewer wont 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?

19
Conclusions

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.