We describe the ongoing hardware and software developments that shall enable the ESO VLTI to perform narrow-angle differential delay astrometry in K-band with an accuracy of up to 10 μarcsec. The ultimate goal of these efforts is to perform an astrometric search for extrasolar planets around nearby star

Delplancke, F.

Elias, N. M.

Henning, T.

Launhardt, R.

Pepe, F.

Queloz, D.

Quirrenbach, A.

Reffert, S.

Setiawan, J.

Ségransan, D.

Tubbs, R.

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A Giant Step: from Milli- to Micro-arcsecond AstrometryProceedings IAU Symposium No. 248, 2007W. J. Jin, I. Platais & M. A. C. Perryman, eds.c© 2008 International Astronomical Uniondoi:10.1017/S174392130801973XThe ESPRI project:narrow-angle astrometry with VLTI-PRIMAR. Launhardt1,∗, T. Henning1, D. Queloz2, A. Quirrenbach3,F. Delplancke4, N.M. Elias II1,3, F. Pepe2, S. Reﬀert3, D. Se´gransan2,J. Setiawan1, R. Tubbs1 and the ESPRI consortium1,2,3,41Max-Planck-Institut fu¨r Astronomie, Ko¨nigstuhl 17, 69117 Heidelberg, Germany2Observatoire Astronomique de l’Universite` de Gene`ve, 1290 Sauverny, Switzerland3Landessternwarte Ko¨nigstuhl, 69117 Heidelberg, Germany4European Southern Observatory (ESO), Karl-Schwazschild-Str. 2, 85748 Garching∗email: rlau@mpia.deAbstract. We describe the ongoing hardware and software developments that shall enable theESO VLTI to perform narrow-angle diﬀerential delay astrometry in K-band with an accuracyof up to 10 µarcsec. The ultimate goal of these eﬀorts is to perform an astrometric search forextrasolar planets around nearby stars.Keywords. instrumentation: interferometers, techniques: high angular resolution, techniques:interferometric, astrometry, planetary systems, infrared: stars1. Overview: VLTI, PRIMA, and the ESPRI projectThe ESO Very Large Telescope Interferometer (VLTI) consists of four stationary 8.2-mVLT “Unit Telescopes” (UTs), four movable 1.8-m “Auxiliary Telescopes” (ATs), andsix long-stroke dual-beam delay lines (DLs). It provides baselines of up to 200m lengthand covers a wavelength range that extends from the near infrared (1µm) up to 13µm.PRIMA, the instrument for Phase Referenced Imaging and Micro-arcsecond Astrom-etry at the VLTI, is currently being developed at ESO. It will implement the dual-feedcapability for two UTs or ATs to enable simultaneous interferometric observations of twoobjects that are separated by up to 1′. PRIMA is designed to perform narrow-angle as-trometry in K-band with two ATs as well as phase-referenced aperture synthesis imagingwith instruments like Amber (Petrov et al. 2000) and Midi (Leinert et al. 2003). PRIMAis composed of four major sub-systems: Star Separators (STS), a laser metrology system(PRIMET), Fringe Sensor Units (FSUs; see Sahlmann et al. 2008), and Diﬀerential DelayLines (DDLs) (Quirrenbach et al. 1998; Delplancke et al. 2000; Derie et al. 2003). Theﬁrst three subsystems are currently being tested at ESO, but the fate of the DDLs, whichare crucial for reaching high astrometric precision, was insecure until 2004.In order to speed up the full implementation of the 10µarcsec astrometric capabilityand to carry out a large astrometric planet search program, a consortium lead by theObservatoire de Gene´ve (Switzerland), Max Planck Institute for Astronomy, and Lan-dessternwarte Heidelberg (both Germany), is currently building the DDLs for PRIMAand develops the astrometric observation preparation and data reduction software. Thefacility is planned to become fully operational in early 2009. In return for its eﬀort, theconsortium has been awarded guaranteed observing time with PRIMA and two ATs tocarry out a systematic astrometric Exoplanet Search with PRIMA (ESPRI).417at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S174392130801973XDownloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 15:01:29, subject to the Cambridge Core terms of use, available418 R. Launhardt et al.2. The method: narrow-angle astrometry with PRIMAA two-telescope interferometer measures the delay between the wavefront sections froma star as they arrive at the telescopes. However, atmospheric piston perturbations usu-ally prohibit accurate measurements of this delay in absolute terms. To circumvent thisproblem, a dual-star interferometer like PRIMA measures the diﬀerential delay betweentwo stars. When their angular separation is smaller than the isoplanatic angle of theatmosphere (≈10′′ in K-band), the piston perturbations of the two wavefronts are cor-related and the diﬀerential perturbations (∆OPDturb) average to zero rapidly (Shao &Colavita 1992). If one of the stars is bright enough to measure its fringe phase withinthe atmospheric coherence time, it can be used to stabilize the fringes on the other star(fringe-tracking), thus allowing for much longer integrations and hence increasing thenumber of observable objects. In order to obtain fringes on the detector, the externaldelay diﬀerence, which is directly related to the angular separation (∆α) via the interfer-ometer baseline (B), must be compensated with optical Delay Lines in the interferometer.At zero fringe position external and internal delays are equal. For astrometry, the twostars are supposed to have intrinsic phase (Φ) zero. The laser-monitored internal delay(∆OPDint) together with the residual diﬀerential fringe phase (∆OPDFSU) is then theprimary observable of the interferometer (see Fig. 1).In PRIMA, the dual feed is realized with Star Separators (STS) at the Coude´ foci ofthe telescopes, which pick up two sub-ﬁelds with the target and astrometric referencestar and feed them as separate beams to the DLs. In order to minimize the eﬀects ofair turbulence in the tunnels, the two star beams are sent parallel through one mainDL. The large optical path diﬀerence (OPD) between the two telescopes, that is to ﬁrstorder common to both stars, is thus compensated without introducing further diﬀerentialperturbations. Due to the non-zero angular separation between the two stars and thediurnal motion, there is however also a variable diﬀerential OPD between the two stars.After the main DLs, the beams of two stars are therefore sent to separate DDLs thatoperate in vacuum and provide a much smaller stroke (60mm). On a 100m baseline,10µarcsec correspond to 5 nm OPD, which deﬁnes the total error budget for DDLs, fringedetection, and metrology. The beams from the two telescopes are then interferometricallycombined in the PRIMA Fringe Sensor Units (FSU). The measured fringe phase of thebrighter star is used to control the DLs and DDLs and to stabilize the fringes of thefainter star. A second FSU, that can now integrate much longer, measures the fringephase of the fainter star. An approximation to the internal OPD in the interferometer ismeasured by the on-axis end-to-end laser metrology system PRIMET.Based on the error budget and preliminary exposure time calculator, the minimumK-band brightness of reference stars required to reach 10µarcsec is K 14mag (Tubbset al. 2008). The maximum separation between target and reference star is ≈ 15′′. Figure2 shows the dependence of astrometric accuracy on stellar brightness for 10′′ separation.3. Diﬀerential delay lines and astrometric data reduction softwareHardware developments: The design of the DDLs has been developed by the consortiumin close collaboration with ESO. The DDLs consist of Cassegerain-type, all-aluminumretro-reﬂector telescopes (cat’s eyes) with ≈20 cm diameter that are mounted on stiﬀlinear translation stages. A stepper actuator at the translation stage provides the longstroke of up to 60mm, while a piezo actuator at M3 in the cat’s eye provides the 1 nmresolution ﬁne stroke over≈10µm. Together with an internal metrology system, the DDLsare mounted on a custom-made optical bench in non-cryogenic vacuum vessels. The cat’seye optics has been successfully tested at MPIA and is currently being integrated withthe other DDL components and prepared for acceptance tests in Geneva (see Fig. 3).at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S174392130801973XDownloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 15:01:29, subject to the Cambridge Core terms of use, availableESPRI: narrow-angle astrometry with PRIMA 419Figure 1. Measurement principle: astrometry with diﬀerential delay interferometry.Figure 2. Dependence of astrometric accuracy of PRIMA with two ATs on stellar brightnessfor 10′′ separation between primary (PS) and secondary (SeS) star and 30min integration.Figure 3. Final design of the Diﬀerential Delay Lines and photograph of the ﬁrst DDL cat’seye on the translation stage in the Lab at Geneva observatory (setup for acceptance tests).at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S174392130801973XDownloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 15:01:29, subject to the Cambridge Core terms of use, available420 R. Launhardt et al.Software developments by the ESPRI consortium include Observation PreparationSoftware for PRIMA astrometry and the complete astrometric data reduction system(see Fig. 4). Data reduction from raw instrumental data to calibrated delays will proceedfully automatically with two pipelines and a set of calibration parameters that is re-derived every few months from all available PRIMA astrometry data (Elias et al. 2008).The software packages will be delivered to ESO prior to commissioning of the instrumentand will be available to all users. The conversion of calibrated delays into astrophysicalquantities like, e.g., planet orbits, is the responsibility of the user.Figure 4. Overview of astrometric software developments for PRIMA.4. Science goals: the Exoplanet Search programStarting in spring 2009, we want to carry out a systematic astrometric ExoplanetSearch with PRIMA (ESPRI), which will address the following key questions: (i) Precisedetermination of the planetary mass distribution, (ii) Detection of new Saturn- downto Uranus-mass planets around nearby stars, (iii) Formation and evolution of multipleplanetary systems, and (iv) Exploring planet formation as a function of stellar age andmass. With these objectives in mind, we have deﬁned three lists of potential targets,containing in total nearly 900 stars: 1. Stars with known radial velocity planets, 2. Nearbymain-sequence stars within 15 pc around the Sun, and 3. Young stars with ages 5-300Myrwithin 100 pc around the Sun. We are currently carrying out an extensive preparatoryobserving program to identify suitable astrometric reference stars and to characterize theplanet search target stars (Geisler et al. 2008). With a ﬁnal detection rate for referencestars of 10-15%, we will then monitor ≈100 – 150 stars for astrometric signatures ofextrasolar planets. The scientiﬁc program and detection spaces are described in moredetail in Launhardt et al. (2008 - IAU249).ReferencesDelplancke, F., Le´veˆque, S. A., Kervella, P., et al. 2000, Proc. SPIE, 4006, 365Derie, F., Delplancke, F., Glindemann, A., et al. 2003, ESA SP-522Elias, N. M., Tubbs. R. N., Ko¨hler, R., et al. 2008, IAUS, 249, in pressGeisler, R., Setiawan, J., Launhardt, R., et al. 2008, IAUS, 249, in pressLaunhardt, R., Henning, Th., Queloz, D., et al. 2008, Proc. IAU Symp., 249, in pressLeinert, Ch.; Graser, U.; Przygodda, F., et al. 2003, Ap&SS, 286, 73Petrov, R. G., Malbet, F., Richichi, A., et al. 2000, Proc. SPIE, 4006, 68Quirrenbach, A., Coude Du Foresto, V., Daigne, G., et al. 1998, Proc. SPIE, 3350, 807Sahlmann, J., et al. 2008, IAUS, 248, in this volume p. 124Shao, M. & Colavita, M. M. 1992, A&A, 262, 353Tubbs, R., Elias II, N. M., Launhardt, R., et al. 2008, IAUS, 248, in this volume p. 132at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S174392130801973XDownloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 15:01:29, subject to the Cambridge Core terms of use, available

The ESPRI project: narrow-angle astrometry with VLTI-PRIMA

The ESPRI project: narrow-angle astrometry with VLTI-PRIMA

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