Post on 14-Dec-2015
transcript
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
21July2009 VLBA Astrometry -- AFB2
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
21July2009 VLBA Astrometry -- AFB8
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
21July2009 VLBA Astrometry -- AFB38
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