Phase-referencing at the Keck Interferometer -- the ASTRA ...star/Kiel2010/Circumstellar_Disk... ·...

Phase-referencing at the Keck Interferometer --

the ASTRA project

26-May-2010“Circumstellar disks and planets in Kiel”

Jorg-Uwe Pott(MPIA, Heidelberg, Germany)

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Contact: Jorg-Uwe Pott, MPIA, [email protected]

Disclaimer

• I used to work as instrument scientist for the Keck-Interferometer ASTRA upgrade until May 2009

• I will show some fact and ideas about ASTRA, but it is by no means an official presentation

• For the most up-to-date details, please contact the current ASTRA key figures:

– Julien Woillez (System architect, WMKO)– Peter Wizinowich (PI, WMKO)– Josh Eisner (Project scientist, University of Arizona) – NExScI (NASA KI support, http://msc.caltech.edu/software/KISupport)

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ASTrometric and phase-Referenced Astronomy upgrade

Outline of the presentation

• Current capabilities of the Keck Interferometer

• Three phases of the ASTRA upgrade: from on-axis spectroscopy to narrow-angle astrometry

• First results, status of the commissioning, outlook

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Current capabilities of the KI

• Single 85m baseline results in 2.7mas resolution (λ/2B) at 2μm

• Excellent seeing (up to < 0.5”), and large apertures give high differential broad-band V2-precision (up to 1-2%) down to K~10mag in a single night.

• Limited Aperture synthesis with earth rotation

• Good for sensitive binary search and detection of faint sub-AU circumstellar material

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λ/2B=2.7mas

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Current direct fringe detection capabilities of the KI• Low resolution modes:

– 5 pixels across K' (≤ 10.3)– 10 pixels across K‘ (≤ 9.5)– 4 pixels across H (≤ 9.0)– 10 pixels across L (≤ 6)

• Medium resolution mode – 42 pixels across K' (≤ 7.6)

• K/L split pupil mode: – 10 pixels across L

• 100 Hz: 1.3 < L < 4.8 – 5, 10 or 42 pixels across K

• 5 pixels: 5.0 < K' < 8.7 • 10 pixels: 4.2 < K' < 7.9 • 42 pixels: 2.3 < K' < 6.0

• 10um Nulling– 2.4 Jy at maximum

Nulling performance (2.5e-3)4

Current numbers are online availableat NASA Exoplanet Science Institute(NExScI):

http://nexsci.caltech.edu/software/KISupport/

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Current direct fringe detection capabilities of the KI• Low resolution modes:

– 5 pixels across K' (≤ 10.3)– 10 pixels across K‘ (≤ 9.5)– 4 pixels across H (≤ 9.0)– 10 pixels across L (≤ 6)

• Medium resolution mode – 42 pixels across K' (≤ 7.6)

• K/L split pupil mode: – 10 pixels across L

• 100 Hz: 1.3 < L < 4.8 – 5, 10 or 42 pixels across K

• 5 pixels: 5.0 < K' < 8.7 • 10 pixels: 4.2 < K' < 7.9 • 42 pixels: 2.3 < K' < 6.0

• 10um Nulling– 2.4 Jy at maximum

Nulling performance (2.5e-3)5

Some currently unique capabilities in terms sensitivity and wavelength

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Access to the Keck Interferometer

• Unlike ESO, Keck time is shared between separately administeredcommunities

– CIT– University of California– NASA– University of Hawaii– NOAO (sometimes, via TSIP)– Yale / Swinburne

• Each community has its TAC, proposal deadline, rules …

• Browse the list of publications for an idea of the possible science cases and collaborations:

http://nexsci.caltech.edu/software/KISupport/KI_biblio.shtml

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Three phases of the ASTRA upgrade

• Self-Phase-referencing (SPR): on-axis to increase spectral resolution

• Dual-Field phase-referencing (DFPR):to increase limiting magnitude

• Narrow-Angle astrometry (AST):to do precision proper motion studies

• Overview: http://www2.keck.hawaii.edu/realpublic/ASTRA/index.php

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ASTRA core science cases

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ASTRA conceptual particularities• Na-Laser guide stars:

– Keck 1 being equipped with a second LGS-AO system

– (some) independence of optical NGS

• Step-by-step upgrade of existing and well known interferometric instrumentation and infrastructure

– to save time and money

• Metrology based OPD monitor for large parts of the beam train

– to increase fringe tracking sensitivity

• Artificial beacon for post-AO tip-tilt variation

– independence of star light for TT-tracking

• Plans for active monitoring of the astrometric baseline

– to achieve 30uas on a single pair of stars9

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Keck-I delay lines

• to correct for the geometric delay (up to 75m) in air, due to the high dynamic range of the control needed (0.2um/75m-> 10^-9), and high control bandwidth of ≥ 200 Hz, this task is split in two:

Quasi-static long delay linesFast, short DL

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Keck-I delay lines

• Fast delay lines for correcting the remainder of the geometric delay (up to +/-15m in double path) in air, and for the atmospheric and instrumental piston turbulence

– precision <= 0.1um (lam/20)– Piezos dither the OPD to scan the

fringe in delay space with up to 250Hz– Air-filled delay lines can introduce

differential path length errors when fringe tracking outside the science band -> humidity control

– A suite of accelerometers feed forward the longitudinal vibration of the telescope to the FDL

– OPD metrology feed forward the OPD variations at 1kHz

• ASTRA: Use K/N beam splitters and second set of FDL’s (Nuller hardware) to implement and test phase-referencing in SPR mode

Extended Brγ emission

Continuum

)

ASTRA-SPR: Interferometry at R~2000

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Angle tracking system: KAT

• Fast tip-tilt mirrors to correct for “tunnel-seeing” after the AO (including AO-residuals, remember that TT is a big factor for the Strehl calculation)

• Internal vibrations add to that internal TT• Custom made TT-mirrors in 1in space, which can move within 1ms• Infrared camera, now reaching 11mag in H-band, enabling stable angle-

tracking for red targets, like dust-embedded YSOs and AGN• Parametric oscillators tuned to known system vibration frequencies for

efficient damping• ASTRA: replace star light with artificial laser beacon, launched from the

telescope focus -> TipTilt metrology

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Jörg-Uwe Pott, ‘Mod

ern infrar

ed interferometry’;

t

Angle tracking system: KAT

• ASTRA-Upgrade: Tip-Tilt metrology

– Instead of the starlight, use an internal, artificial star

– Advantages: • You can go faster to better

correct for vibrations at large bandwidth

• You can go deeper on the star: no sensitivity limitation

• Dual approach similar to the LDL/FDL approach:

– High frequency correction (10-1000Hz) on internal, artificial star

– Low frequency correction (<10Hz) on star light

– Status: hardware in place, on-sky commissioning has started

• Important for red targets (AGN) and DFPR and AST

Tunnel-seeing tip-tilt:

Uncorrected internally corrected

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Jörg-Uwe Pott, ‘Mod

ern infrar

ed interferometry’;

t

Fringe tracker FATCAT

– Works at H & K band and commands the FDL in closed loop feedback

– Noise optimized pupil plane interferometer (details of the algorithm in Colavita et al. 99)

– 50/50 beamsplitter beam combination scheme enables group delay tracking and phase delay tracking at individually optimized (minimal) dispersion

– Single-mode fibres spatially filter the incoming (TT-stabilized!) starlight, which is essential for stable system transmission and high precision calibration

– Fast Hawaii-detectors are used in the focal plane– If correctly aligned the optics guide ~80% of the encircled

energy onto the white-light pixel– Cooling without closed-cycle coolers to minimize

vibrations, use liquid nitrogen instead– Recently a FATCAT-L band installed

– ASTRA: uses two FATCAT in K-band at two different speeds

• fast to fringe-track, and the slow for higher angular resolution (SPR) and faint objects (DFPR)

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Further ASTRA dual-field and astrometry hardware

Off-axis phase-referencing - a key technology is ‘star separation’ – Split the light already in the telescope foci to get maximum FOV– Key optic is an annular mirror, which passes the on-axis star, and reflects the off-axis

object– Hardware is in place, on-sky star separation commissioning has started, please check

with J. Woillez in San Diego (SPIE 2010) for latest results

Narrow-angle astrometry will rely on differential IR metrology outside the science band

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ASTRA science - ExoPlanets

• Focus is on direct detection of the astrometric proper motion wobble of the host star

• Due to the missing outriggers (Keck version of the AT’s) for political reasons (not technical!) there is currently no effort to compete with an ESPRI like survey program

• ASTRA technology allows to observe both ExoP’s and the Galactic center

• Concentrate on a few key ExoP systems and on being most sensitive

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ASTRA science – disks

• First scientific results in on-axis self-phase-referencing (SPR)

• Science verification on a Be-star disk (lead: J.-U. Pott, submitted)

• YSO observations (lead: J. Eisner, submitted)

• Next slides will show some performance numbers, and ideas on what to do with the data

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ASTRA science – SPR performance

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• On bright targets we achieve unprecedented differential performance:

70 sec ‘shutter open time’:– Δfluxratio = 0.003– ΔV2 = 0.007– Δφ = 3mrad (0.2º) ≈ 3μas on sky

-> centroiding precision:10-3

– SNR limited, longer integrationscould further improve on this

[Pott et al. 2010, submitted]

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ASTRA science – Science Verification on a Be disk

• Both the differential V2 and Δφare important model constraints

19[Pott et al. 2010, submitted]

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ASTRA science – SPR gives new constraints

• SPR is of particular interest, when line profiles can be spectrally resolved (SPR resolution is ‘only’ 150 km/s), and if several lines are available (BrGam, HeI, Pfund series, CO band heads etc.)

• Time resolved measurements can probe precession of disk material

• Spatial resolution can help to distinguish Keplerian disk motion (vel α R-0.5) from outflow / inflow scenarii

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ASTRA science – technical remarks

• New observables require refinement of data analysis:

– refinement of wavelength calibration with internal and external calibration

– use of adequate stellar templates, of the required spectral resolution, to properly calibrate the total and correlated fluxes (NIR spectroscopy not that standard yet)

– line profile modeling is required to interpret the visibilities best

-> Apply single-telescope spectroscopy recipes to IF data

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ASTRA science – technical remarks

• Dispersion may limit the precision of the differential phase estimate and phase-referenced imaging

– Group delay tracking minimizes the dispersion of air-filled compensation of the geometric delay

– broad-band phase tracking alone (like FINITO) is less adequate

– if the fringe tracker does not run in the science band, you need to take into account the additional differential dispersion of water vapor to get optimum Δφ calibration

– use fringe tracker as immediate performance logger -> fringe selection

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Residual phase when GD tracking with air-filled delay lines [from Colavita’99]:

λ[μm]- [rad] 2.0 - 0.722.1 - 0.162.2 - 0.002.3 - 0.132.4 - 0.47

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ASTRA science – imaging with one baseline only?

• Transitional disks appear to be partially depleted of inner dust at (sub-) AU scales

• Is a companion responsible for clearing the hole and for the NIR excess?

• 85m baseline give 10x better resolution than aperture masking, but worse iPSF

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V773 Tau [Duchene+03]

3.8um, 1” across

CoKu Tau/4 [Ireland+ 08]

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• At 99.7% confidence level ≥ 99 % (95 %) of the field-of-view are rejected

• Brighter companion solutions appear to be co-aligned with each other and with the KI baseline -> probably artificial solutions

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GM Aur

DM Tau

LkCa 15

UX TauA

Binaries with one baseline

[Pot

t et a

l. 20

10, A

pJ 7

10..2

65]

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Binaries with one baseline

• Statistical analysis of the position angle of the solutions

• At the 3σ (1σ)-rejection level, there is no solution with FR ≥ 0.08 (0.05) outside a PA: 20±20 deg

• The probability of having two (three) co-aligned binaries in our five-target sample is 3% (0.4%)-> more than half of the sample does not harbor a binary

25Blue: 3σ-rejectionGreen: 2σ-rejectionRed: 1σ-rejection [P

ott e

t al.

2010

, ApJ

710

..265

]

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ASTRA science with DFPR

Summarizing the binary analysis [Pott et al. 2010, ApJ 710..265]:

KI observations are • are sensitive to binary companions with flux ratios

comparable to the near-IR excess fraction observed for these sources

• exclude almost all of the probed binary parameter range, down to companion flux ratios of 0.05 and 2.5 mas (3.5 AU) separations; this flux ratio limit translates into a companion mass limit of 0.1 M for our target stars

• reject stellar binarity as the dominant mechanism in creating a transitional disk appearancetrick: combine statistically the observations of several targets

ASTRA-DFPR will significantly increase the number of YSO targets in which we can search for companions, and resolve the sub-AU scale emission26

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ASTRA - conclusions

• SPR (on-axis phase-referencing) is in full operation and provides differential visibility and phase constraints at the precision-> ASTRA success motivates to spread the technology: MIDI+PRIMA (Mueller, Pott et al. at SPIE 2010), AMBER+PRIMA, MATISSE+PRIMA…

• DFPR (off-axis phase-referencing) is currently being commissioned, and will increase the YSO sample size accessible to the KI-> model-dependend imaging information at mas res.-> first on-sky performance test will soon probe the isopistonic patch to estimate increase of sky coverage

• AST (narrow-angle astrometry) is designed, and the hardware implementation is ongoing. The YSO / planet related science will focus on a few ExoP systems

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