Atacama Large Millimeter/submillimeter ArrayExpanded Very Large Array

Robert C. Byrd Green Bank TelescopeVery Long Baseline Array

Observing the Pluto System with ALMA (and EVLA)

Bryan ButlerNational Radio Astronomy Observatory

Why Long Wavelengths?

Spectrum of a body at 30

AU, albedo=0.1,

emissivity=0.95

reflectedsolar

thermalemission

1 μm 10 μm 100 μm 1 mm 1 cm

ALMA andEVLA

• Subsurface probe (depth ~10)• Relatively accurate diameters• Accurate astrometry• Potential to resolve bodies• Simple spectroscopy (isolated

transitions)• Observe any time of day

Why Long Wavelengths?

But, the emission is so weak!

• Put in units of Jy (10-29 W/m2/Hz) natural units of radio astronomy

• 200 km diameter body at 35 AU• Need mJy to μJy sensitivity• Beyond (or right at the edge of)

capability of current mm and cm instruments 100 100 μμmm 1 mm1 mm

F ~2kTb

2

πR 2

D 2

Expected flux density for a body in thermal equilibrium is:

Tb =εTε ~εkF(1−A)

σ⎡⎣⎢

⎤⎦⎥1/4

1 cm1 cm

A Success Story - SMA Observes Pluto/Charon

• Gurwell & Butler observed Pluto/Charon in 2005 with the SMA at 1.4 mm

• SNR ~ 12 on Pluto, which is not resolved; SNR ~ 4 on Charon

• Resolves Pluto from Charon (first time at thermal wavelengths)

• (Just approved for more time!)

• Need more sensitivity and resolution

Enter ALMA• Millimeter/submillimeter wavelength

interferometer• International collaboration between North

America (NRAO, Canada, Mexico), Europe (ESO), and Japan (NAOJ), and other partners (Taiwan/ASIAA, e.g.)

• Total cost > $1B (contract for North American built antennas was largest single contract ever entered into by the NSF, ~$250M)

What ALMA Will Be• 50 12-m antennas, with extremely accurate

surfaces and pointing• 4 additional 12-m antennas and 12 additional 7-m

antennas for imaging large spatial scales• wavelengths from 6mm to 350m, with incredibly

sensitive receivers• antenna separations from 15 m to 15 km• powerful and flexible correlator; bandwidths to 16

GHz• on an extremely high and dry site in the Chilean

Andes• Sensitivity of ~ 200 Jy/min at main bands (100-

350 GHz)

Sensitivity And SNR For Thermal Emission

“Figure of Merit” for thermal objects: Ψ =1λ 2( )

ΔFλ

noise Figure of Merit

Integration Times For Pluto SystemExpected flux densities for a body at 31 AU, with 50% albedo and 90% emissivity, and time to make a 5-σ detection (with 25% calibration overhead):

Body D (km) F (Jy) Dt (hr) Dt* (hr)

Nix/Hydra? 50 15 88 780

Nix/Hydra? 100 60 6 50

Nix/Hydra? 150 140 1 10

~Charon 1200 9000 < 1 min < 1 min

~Pluto 2200 30000 < 1 min < 1 min

Diameter UncertaintyWith some not unrealistic assumptions, one can show:

Or, the error in diameter is half the error in the flux density determination. So even with just a 5-σ detection (error of 20%), the uncertainty on radius is only ~10%.

dRR

~dF

2F

Linear ResolutionALMA, at 345 GHz in the 15 km configuration, has a resolution of ~12 masec, or in linear extent:

Or ~300 km at Pluto. If the shorter wavelengths work well, factor of ~2 better.

θl ~ 9 DAU (km)

Contrast (Dynamic Range)Since we are on the Rayleigh-Jeans tail of emission, we have little problem with contrast between Pluto and the satellites, which goes like:

Since the temperatures are (roughly) the same, the contrast is only of order a few 100s, which is trivial for modern interferometers.

TpRp2

TsRs2

Astrometry

Astrometric resolution of ALMA:

So with a 5-σ detection, the linear astrometric precision is about 30 km at the Pluto system.

F = θHPBW2 SNR

This is the key!

SpectroscopyALMA is a spectroscopic instrument, naturally. At least CO should be detectable, and possibly others (HCN?).

Observing Pluto/Charon

• Can both be detected trivially (in minutes) across the entire frequency span

• Can easily be separated• Pluto will be resolved with 10’s of pixels,

Charon with a few pixels (but lightcurve is easy)

• CO should be detectable on Pluto, potentially other species

Observing Nix/Hydra

• Both bodies should be detectable with 10’s of hours of integration

• Uncertainty on diameters should be of order 10’s of %

• If they are on the larger side, orbits will be well-determined

• Crude constraints on surface properties may result from such observations

ALMA Status• 12 antennas now on site• First ALMA production receivers on site (not yet tested)• First quarter of the correlator installed on site• Concrete poured for 10’s of antenna pads• Two antenna transporters on site• Operations Support Facility (OSF) technical buildings in use• Array Operation Site (AOS) building in use• Roads, antenna buildings, etc., in use• Software used for antenna verification and 2-element

interferometry• Early science observing to commence in 2011 - full

operations in 2013

ALMA Timeline

ALMA Pluto System Observing Plan

• For early science, propose to observe the Pluto/Charon system, but with a focus on Pluto and Charon themselves, with an outside chance at Nix and Hydra (if they are on the larger end). This will likely be some 10’s of hours. Call for proposals likely December 2010, proposals due spring 2011, observing in summer 2011.

• With full array, propose to observe the system again, but this time explicitly including Nix and Hydra. Will likely be > 100 hours. Dates uncertain, but perhaps 2012.

The EVLA has upgraded:• Front Ends (feeds + Rx)• LO• Data transmission• Correlator• SoftwareMain result is increased

sensitivity: 25 Jy/min

The VLA is the world’s most powerful radio wavelength interferometer, but was designed and built in the 1960’s/70’s, and completed in 1980 - the dark ages relative to “modern” electronics! But the infrastructure (antennas, rails, buildings, etc…) are sound.

The EVLA - Improving the VLA

The Pluto System With EVLA• EVLA does not have the SNR of ALMA for

thermal bodies and cannot observe Nix and Hydra in a reasonable amount of time, but can do Pluto and Charon quite well (and is observing now so no wait!)

• Optimal wavelength is 1 cm• Depth probed is factor of 10 deeper than for

ALMA• Pluto barely resolved, but lightcurves easy• Butler, Gurwell, and Moullet have approved

time to observe this fall (part of a broader program to also observe large KBOs and Pluto/Charon at longer wavelengths)

Summary• ALMA and the EVLA will be revolutionary ground-based

astronomical facilities for the next decade• Both to start operating soon (EVLA now, ALMA in 2011)• They will both provide new and exciting results for the Pluto

system:– EVLA will provide long wavelength brightness

temperatures and lightcurves for Pluto and Charon (Pluto slightly resolved)

– ALMA, in early science, will provide brightness temperatures and lightcurves for Pluto and Charon, and possibly detections of Nix and Hydra

– ALMA, in full operations, will provide resolved images of Pluto and Charon, and certain detections of Nix and Hydra, giving relatively accurate diameters and potentially orbit and surface composition information; it will also yield detections (or upper limits) on atmospheric species