Astrometry from Long-Baseline OIR Interferometers

A. Boden, R. Akeson, A. Sargent, J. Carpenter – CaltechG. Torres & D. Latham – CFA/Harvard

A. Quirrenbach – HeidelbergM. Colavita & M. Shao – JPL

D. Hutter & J. Benson – USNO FlagstaffD. Boboltz & K. Johnston – USNO

M. Massi & K. Menten – MPIfR BonnL. Loinard & R. Torres – UNAM

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Outline

Intro – Long-Baseline Optical/Near-IR Interferometry

OIR Interferometric Astrometry (Crash Course)

Differential Astrometry Results Survey Absolute Astrometry Results Survey Summary

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Sampling of the incident radiation field

Transport to a common location

(Internal) compensation for geometric (external) delay

Combination ofthe beams

Detection of theresulting output

A cartoon astronomicalinterferometer

CBsd ˆ

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Kinds of Science with OIR Interferometry Visibility Modeling/Parametric Imaging – modeling of

sparse u-v coverage and/or visibility amplitude data: stellar diameters, rotational oblateness, circumstellar material

Differential astrometry over narrow field (mas – 10s arcsec). Narrow-field binaries. Fractional accuracies ~ 10-5 – 10-6

Wide-angle astrometry over wide-ish fields (10s of degrees). Fractional accuracies ~ 10-6 – 10-7

For what follows I want to (mostly) focus on astrometric results…

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Interferometric Astrometry

How a (dilute aperture) interferometer does astrometry… Single telescope beam & coherence length – imaging

(or “parametric” imaging) Multiple beam and/or coherence length – differential

delay Single-beam – measure differential delay in delay “sweeping”,

or measuring delays serially Dual-beam – measure multiple fringe packets simultaneously

& relate the two delays through metrology

Showing examples of all these techniques

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For a field (10s of mas) covered by single beam & coherence length…

Imaging (“parametric” imaging) astrometry proceeds by interpreting fringe observable (e.g. fringe “visibility”) for multiple sources

Usually “parametric” because spatial frequency content is low and “systems” are simple (e.g. binary stars), so images are never synthesized

Most published results (e.g. binary orbits) are done through this parametric imaging

Interferometric “imaging” astrometry

Armstrong et al 2006

Hyades BinaryQ2 Tau

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Binary“Parametric Imaging”

When the scene is simple…

State of the art is integrated visibility & RV modeling to estimate binary orbits (Herbison-Evans et al 1971, Armstrong et al 1993, Hummel et al 1998, Boden et al 1999)

This is what (essentially) everyone in the business does

Boden et al 1999

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Differential delay astrometry

For multiple beams and/or coherence lengths, the delay (OPD) offset between fringes on multiple sources becomes the observable proxy for sky separation

12 Per/CHARABagnolo et al 2006

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Binary StarContributing Facilities

Intensity Interferometer Herbison-Evans et al

Mark III (Mt Wilson) Armstrong, Pan, Hummel

HST FGS e.g. Benedict, Nelan, Henry

PTI (Palomar Observatory) Boden, Konacki, Koresko,

Muterspaugh, Pan

NPOI (Anderson Mesa) Armstrong, Hummel, North,

Zavalla SUSI (Narrabri)

Davis, Tango, North KI (Mauna Kea)

Boden et al, Schafer et al IOTA (Mt. Hopkins)

Krauss et al, Zhao et al CHARA (Mt Wilson)

Bagnuolo, Raghavan, Zhao

HST: Image Credit NASA

PTI: Image Credit National Geographic

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12 Boo Boden et al 2000 & 2005 : 12 Boo

components are (nearly) equal-mass (dynamical masses at 0.3% precision), but a factor of two different in luminosity.

Due to primary evolution off main sequence; primary at the MS Turnoff – entering the subgiant phase, establishing a thick H-burning shell.

“Apparent” (evolutionary model) ages are discrepant at the 10% level (much larger than experimental errors); no single isochrone matches both components.

Miglio et al 2007 proposed convective overshooting differences to explain discrepancy, and astroseismic photometry to test proposal – results pending

Boden, Torres, & Hummel 2005

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Binary dynamicalmasses

Quite a number (34) of systems have been interferometrically analyzed and published over the past 20 years…

Cunha et al 2007

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Measuring both astrometric and physical (3-D) orbits, one can determine system distances free of any model (beyond Keplerian motion) d = aphysical/a”

These distances are typically as good as (or better than) the best available stellar distances (Hipparcos)

Binary-DerivedDistances

V773 Tau ABoden et al 2007

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Atlas/Pleiades/Pan et al 2004 Continuing controversy between

“conventional” and Hipparcos estimates of Pleiades distance

Atlas visual orbit + system mass estimate yields Atlas distance

Result strongly favors “conventional” distance

(Additional eclipsing system reinforces Atlas result – Munari et al 2004; FGS parallaxes Soderblom et al 2005)

Hipparcos sticking with their guns: van Leeuwen 2009 puts Pleiades at 122 +/- 2 pc

Pan, Shao, & Kulkarni 2004, Nature 427, 396Zwahlen et al 2004, A&A 425, L45

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Object DistanceComparisons Van Leeuwen 2007

lamented lack of direct comparisons with Hipparcos parallaxes

Interferometric binaries provide excellent opportunity to assess precision & accuracy of original & revised Hipparcos parallaxes (Tomkin 2005)

No sign of systematic bias, but room for small-scale correlations

Boden & Quirrenbach in prep

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VLBI AstrometryIntegration: V773 Tau A

Including VLBI possible for radio-emitting systems: V773 Tau A

Lestrade et al 1999 estimated distance 148.4 +/- 5.5 pc w/VLBA

Boden et al 2007 (to be) updated by Torres et al 2009 (see Thursday) analysis by joint VLBI, Keck Interferometry, & RV

Derived orbital dist (134.5 +/- 3.2 pc) in excellent agreement with new trigonometric distance (134.7 +/- 3.8 pc); accuracy and precision

D (pc) M Aa (Msun)

M Ab (Msun)

B2007 136.2 (3.7)

1.54 (0.14)

1.33 (0.10)

T2009 134.5 (3.2)

1.48 (0.12)

1.28 (0.07)

Boden et al 2007

Torres et al 2009

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Radiometric Modeling

It’s important to invest similar care in radiometric modeling as in astrometry & kinematics Luminosities, temperatures,

absolute magnitudes, colors, extinction

In the end we want to test/refine astrophysical models

Boden et al 2007

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Differentialdelayastrometry

Muterspaugh et al 2006a

Differential delay results

Technique is to measure (and calibrate!) delay offset between separated fringe packets

Implementations in a single telescope beam & in separate beams

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Single-beamresults

Over a very narrow field (sub-arcsec), technique yields 10-20 uas precision

k Peg (triple)Muterspaugh et al 2006b

Sample of narrow-field results d Equueli (Muterspaugh et al

2005) k Peg (Mutterspaugh et al

2006b) V819 Tau (Muterspaugh et al

2006c) 12 Per (Bagnolo et al 2006)

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Narrow-Field Astrometry:Fractional Precision

(With phase referencing), very high precisions are possible PTI PHASES typically

delivers 15 uas precision over 500 mas field – 3 parts in 105!

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Dual Beam Astrometry

Primary star Used to phase individual

apertures Used to co-phase the

interferometer

Secondary star Used as positional reference for

primary star

Delay line difference Observable proxy for angular

separation between stars Angular separation reflects

periodic reflex motion of stars due to planetary companions

For exo-planet reflex detection 10s of uas (O(10-11 rad))

Delay LineDifferential

“Primary”Star

“Secondary”Star

BeamCombiners

DelayLines

DelayLines

Objective: ground-based astrometric detection of exo-planets ~ 50 -- 200 uas @ PTI (10 uas @ VLTI)

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PTI Astrometry on 61 Cygni

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PTI Astrometry on 61 Cyg (2)

2000xPTI demonstration fractional precision:100 uas/30 arcsec = 3 parts in 106!

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Absolute Astrometry with Interferometers

Long-baseline O/IR interferometers are making absolute (global) astrometry measurements as well…

Where positions are referenced to some external standard (e.g. a priori positions from Hipparcos)

Allow for refining global parameters such as proper motion and parallax

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Mark III results

Mozurkewich et al 1998 Shao et al 1990 Hummel et al 1994

Shao et al 1990

Hummel et al 1994

Precisions ~ 6-10 mas (Shao et al 1990)

Accuracies ~ 15-20 mas (Hummel et al 1994)

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Wide Angle (Absolute) Astrometry with an Interferometer

Measure d, calculate s

Voila…stellar position

But it’s not that simple…

B

s

•Measured delays corrupted by

atmospheric turbulence

• Internal optical paths vary

rapidly from thermal effects

• Baselines vary rapidly due to

mechanical and thermal effects

on siderostats/mounts

CBsd ˆ

Impact of these effects

increases with field!

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Atmospheric Correction

• Delay residuals from predicted delay are dominated by

atmospheric fluctuations (Kolmogorov turbulence)

• Error of mean reduces only as 6th root of Nobs

• Air delay calculated by fitting dispersed fringes - atmosphere dispersive in visible

- vacuum delay lines allow wide bandpass

• Corrected delays (Fig. 1 minus Fig. 2)

= 3.09 mm

White noise mean = 3.09 mm/√(N)

For 100s observation (500, 200ms frames) mean = 0.13 mm

astrometric precision = 1.3 mas (20 m baseline)

Fig. 1

Fig. 2

Fig. 3

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Internal Path Length (C-term) Metrology

Internal feed beam metrology injection

Feed beam metrology cube corner reflector

Benson et al 2004Johnston et al 2006

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External metrology

Apply baseline metrology data:

Laser metrology beams monitor hemispherical “cat’s-eye” reflector on each siderostat mirror

Laser source and distribution optics on temperature-stabilized reference table

Reference table referenced to bedrock by “optical anchors”

Benson et al 2004Johnston et al 2006

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Preliminary Astrometric Solutions

• 14 Stars, delta Ra, delta Dec: 28 parameters

• 4 Baseline parameters, 3 low-order polynomials

• 28 or 35 parameters (non-trivial problem)

• Applied robust Bayesian modeling techniques

Delay residuals after stellar positions fit

• Dispersion & C-term corrected delays

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Preliminary Astrometric Solutions

Star Name # Scans mv DeclinationFK5 357 8 4.56 69.83 12.87 23.87FK5 427 12 4.044 6.029 11.13 121.5FK5 1275 12 4.689 31.97 8.76 24.99FK5 569 9 3.027 71.83 8.64 19.62FK5 423 12 3.32 15.29 8.61 52.29FK5 416 12 2.36 56.38 7.83 17.08FK5 472 10 3.88 69.78 6.42 19.79FK5 509 16 1.853 49.31 6.12 11.33

• Single night, single baseline (East-West)

• Precisions of ~ 10 mas in RA

• Fractional precision 10 mas/30 deg ~ 1 part in 107!!!

ra dec

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Summary

Ground-based LB OIR Interferometers making important astrometric contributions: Resolving and analyzing binary systems inaccessible any

other way (stellar astrophysics in many HR-diagram sectors – e.g. PMS systems)

Demonstrating potential relevance to astrometric exo-planet studies (e.g. PTI PHASES, VLTI PRIMA – just coming on line!)

Potential future contributions in global astrometry (in advance of GAIA and SIM) (NPOI)

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Backup

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Differential Delay Astrometry

Multiple sources => multiple fringe patterns

Metrology measuring the relative delay

This relative delay is the astrometric observable

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Intro

Talking about long-baseline (LB) optical/near-IR (OIR) interferometry in general, and interferometric astrometry in particular

I will not be talking about filled-aperture (speckle) interferometry

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Interference fringes are variations in detected power vs relative delay (OPD)

Polychromatic interference fringe packet centered at “zero OPD” (packet size Lcoh l02/dl)

This zero OPD (and the internal delay at which is occurs) is the obs. proxy for astrometric measurements

Fringes & polychromatic response

DkD

D

coh

coh0cos

/

/sin 1

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Differential Astrometry Survey

Survey of Differential Astrometry results from LB OIR interferometers Interferometric “imaging” results Differential delay results

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Classical imaging/relative astrometric techniques

Speckle Long-baseline interferometry

Capella with Mt Wilson Interferometer Spica (a Vir) with intensity

interferometer Mark III HST FGS NPOI PTI SUSI KI IOTA CHARA

Binary Studies ByInterferometers

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HD 98800: PMS quad system, B an SB2 with 315d period & mid-IR excess

Physical orbit from KI V2, HST FGS, & RV data; dynamical masses of two low-mass PMS components

Suggestion that HD 98800 (& TW Hya stars) have sub-solar metallicity?

Verified (?) in Laskar et al 2009

KI/PMS Binary HD 98800 B

Boden et al 2005

14 Apr 2006

18 May 2006

2 May 2007

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Interferometric Astrometry Technology

Technologies relevant to LB OIR interferometric astrometry “Dual-star” feed mechanisms Beam combination/fringe measurement Metrology (internal, external) Phase referencing of multiple beam combiners

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Dual-star feed schematic (PTI, KI, VLTI)

SSSM

Collimator& FSM

Collimator& FSM

Field separator

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PTI Central Optics

PrimarySecondary

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Dual-Beam Phase Referencing

HD 1777244 Aug 1999

Lane & Colavita 2003

Objectives: long synthetic coherence time for faint-object detection &x-combination delay comparison

Phase referenced interferometry: the analog of single-aperture AO

Fringe tracking piston correction signal on one object is used to correct the piston on a second, nearby (isoplanatic separation) object.

Required for KI (and VLTI) faint-object interferometry

Phase error with and without loop closed between the two PTI fringe trackers.

Two data segments taken within 200 s of each other.

Lane & Colavita 2003