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Astrometry from Long-Baseline OIR Interferometers

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  1. Astrometry from Long-Baseline OIR Interferometers A. Boden, R. Akeson, A. Sargent, J. Carpenter – Caltech G. Torres & D. Latham – CFA/Harvard A. Quirrenbach – Heidelberg M. Colavita & M. Shao – JPL D. Hutter & J. Benson – USNO Flagstaff D. Boboltz & K. Johnston – USNO M. Massi & K. Menten – MPIfR Bonn L. Loinard & R. Torres – UNAM

  2. Outline • Intro – Long-Baseline Optical/Near-IR Interferometry • OIR Interferometric Astrometry (Crash Course) • Differential Astrometry Results Survey • Absolute Astrometry Results Survey • Summary VLBA Astrometry -- AFB

  3. A cartoon astronomicalinterferometer • Sampling of the incident radiation field • Transport to a common location • (Internal) compensation for geometric (external) delay • Combination ofthe beams • Detection of theresulting output VLBA Astrometry -- AFB

  4. 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… VLBA Astrometry -- AFB

  5. 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 VLBA Astrometry -- AFB

  6. Interferometric “imaging” astrometry Hyades Binary Q2 Tau • 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 Armstrong et al 2006 VLBA Astrometry -- AFB

  7. 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 VLBA Astrometry -- AFB Boden et al 1999

  8. Differential delay astrometry 12 Per/CHARA Bagnolo et al 2006 • For multiple beams and/or coherence lengths, the delay (OPD) offset between fringes on multiple sources becomes the observable proxy for sky separation VLBA Astrometry -- AFB

  9. 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 Binary StarContributing Facilities PTI: Image Credit National Geographic VLBA Astrometry -- AFB HST: Image Credit NASA

  10. 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 VLBA Astrometry -- AFB Boden, Torres, & Hummel 2005

  11. Binary dynamicalmasses • Quite a number (34) of systems have been interferometrically analyzed and published over the past 20 years… Cunha et al 2007 VLBA Astrometry -- AFB

  12. Binary-DerivedDistances • 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) V773 Tau A Boden et al 2007 VLBA Astrometry -- AFB

  13. 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, 396 Zwahlen et al 2004, A&A 425, L45 VLBA Astrometry -- AFB

  14. Boden & Quirrenbach in prep 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 VLBA Astrometry -- AFB

  15. Torres et al 2009 Boden et al 2007 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 VLBA Astrometry -- AFB

  16. Radiometric Modeling • In the end we want to test/refine astrophysical models • It’s important to invest similar care in radiometric modeling as in astrometry & kinematics • Luminosities, temperatures, absolute magnitudes, colors, extinction Boden et al 2007 VLBA Astrometry -- AFB

  17. Differentialdelayastrometry • Differential delay results • Technique is to measure (and calibrate!) delay offset between separated fringe packets • Implementations in a single telescope beam & in separate beams Muterspaugh et al 2006a VLBA Astrometry -- AFB

  18. Single-beamresults • Over a very narrow field (sub-arcsec), technique yields 10-20 uas precision 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) k Peg (triple) Muterspaugh et al 2006b VLBA Astrometry -- AFB

  19. 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! VLBA Astrometry -- AFB

  20. Delay Line Differential Dual Beam Astrometry “Primary” Star “Secondary” Star Objective: ground-based astrometric detection of exo-planets ~ 50 -- 200 uas @ PTI (10 uas @ VLTI) • 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 Lines Delay Lines Beam Combiners VLBA Astrometry -- AFB

  21. PTI Astrometry on 61 Cygni VLBA Astrometry -- AFB

  22. 2000x PTI Astrometry on 61 Cyg (2) PTI demonstration fractional precision: 100 uas/30 arcsec = 3 parts in 106! VLBA Astrometry -- AFB

  23. 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 VLBA Astrometry -- AFB

  24. Mark III results • Mozurkewich et al 1998 • Shao et al 1990 • Hummel et al 1994 Shao et al 1990 • Precisions ~ 6-10 mas (Shao et al 1990) • Accuracies ~ 15-20 mas (Hummel et al 1994) Hummel et al 1994 VLBA Astrometry -- AFB

  25. Wide Angle (Absolute) Astrometry with an Interferometer Measure d, calculate s Voila…stellar position But it’s not that simple… • 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 Impact of these effects increases with field! VLBA Astrometry -- AFB

  26. Atmospheric Correction • Delay residuals from predicted delay are dominated by • atmospheric fluctuations (Kolmogorov turbulence) • • Error of mean reduces only as 6th root of Nobs Fig. 1 • Air delay calculated by fitting dispersed fringes - atmosphere dispersive in visible - vacuum delay lines allow wide bandpass Fig. 2 • 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. 3 VLBA Astrometry -- AFB

  27. Internal Path Length (C-term) Metrology Internal feed beam metrology injection Feed beam metrology cube corner reflector Benson et al 2004 Johnston et al 2006 VLBA Astrometry -- AFB

  28. External metrology Benson et al 2004 Johnston et al 2006 Apply baseline metrology data: Laser source and distribution optics on temperature-stabilized reference table Laser metrology beams monitor hemispherical “cat’s-eye” reflector on each siderostat mirror Reference table referenced to bedrock by “optical anchors” VLBA Astrometry -- AFB

  29. Preliminary Astrometric Solutions • Dispersion & C-term corrected delays • 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 VLBA Astrometry -- AFB

  30. Preliminary Astrometric Solutions ra dec • Single night, single baseline (East-West) • Precisions of ~ 10 mas in RA • Fractional precision 10 mas/30 deg ~ 1 part in 107!!! VLBA Astrometry -- AFB

  31. 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) VLBA Astrometry -- AFB

  32. Backup VLBA Astrometry -- AFB

  33. Differential Delay Astrometry • Multiple sources => multiple fringe patterns • Metrology measuring the relative delay • This relative delay is the astrometric observable VLBA Astrometry -- AFB

  34. 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 VLBA Astrometry -- AFB

  35. Fringes & polychromatic response • 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 VLBA Astrometry -- AFB

  36. Differential Astrometry Survey • Survey of Differential Astrometry results from LB OIR interferometers • Interferometric “imaging” results • Differential delay results VLBA Astrometry -- AFB

  37. Binary Studies ByInterferometers • 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 VLBA Astrometry -- AFB

  38. 14 Apr 2006 18 May 2006 2 May 2007 KI/PMS Binary HD 98800 B • 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 VLBA Astrometry -- AFB Boden et al 2005

  39. 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 VLBA Astrometry -- AFB

  40. Dual-star feed schematic (PTI, KI, VLTI) Field separator SSSM Collimator & FSM Collimator & FSM VLBA Astrometry -- AFB

  41. PTI Central Optics Primary Secondary VLBA Astrometry -- AFB

  42. VLBA Astrometry -- AFB

  43. Dual-Beam Phase Referencing Objectives: long synthetic coherence time for faint-object detection & x-combination delay comparison HD 177724 4 Aug 1999 • 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 Lane & Colavita 2003 VLBA Astrometry -- AFB