The GRAVITY interferometer and the Milky Way’s Nuclear Star Cluster Oliver Pfuhl M¨unchen 2012 The GRAVITY interferometer and the Milky Way’s Nuclear Star Cluster Oliver Pfuhl Dissertation an der Fakult¨at f¨ur Physik der Ludwig–Maximilians–Universit¨at M¨unchen vorgelegt von Oliver Pfuhl aus M¨unchen M¨unchen, den 23.04.2012 Erstgutachter: Prof. Dr. R. Genzel Zweitgutachter: Prof. Dr. A. Burkert Tag der m¨undlichen Pr¨ufung: 17.7.2012 Contents Zusammenfassung xv Abstract xvii 1 Introduction 1 2 Narrow-angleinterferometricastrometry 3 2.1 Atmosphere ................................... 4 2.2 Baselinedefinition ............................... 8 2.3 Baselinestability ............................... 10 2.4 Opticalstability................................ 11 2.5 Phase errors of science and fringe tracking channel . .......... 14 2.5.1 Lateral pupil offset and no tip-tilt error . ..... 16 2.5.2 Tip-tilterrorandnopupiloffset. .. 16 2.5.3 Tip-tilt error in conjunction with pupil offsets . ....... 17 2.6 Phaseerrorsofthemetrology . .. 17 2.7 Errorbudget .................................. 18 3 GRAVITY overview 21 3.1 GRAVITYsubsystems ............................. 23 3.1.1 Wavefrontsensor ............................ 23 3.1.2 Cryostat................................. 23 3.1.3 Fibercoupler .............................. 25 3.1.4 Fibercontrolunit............................ 25 3.1.5 Integratedoptics ............................ 26 3.1.6 Spectrometers.............................. 27 3.1.7 Acquisitioncamera ........................... 29 3.1.8 Guidingsystem ............................. 29 3.1.9 Metrology ................................ 30 4 Fiber coupler 33 4.1 Overview..................................... 34 4.2 Optics ...................................... 36 vi CONTENTS 4.2.1 Notesonsingle-modecoupling. 38 4.2.2 Parabolicrelay ............................. 38 4.2.3 Dual-fieldversussingle-fieldmode . ... 39 4.2.4 Half-waveplate ............................. 40 4.2.5 H/Kdichroic .............................. 45 4.2.6 Opticalperformance .......................... 46 4.3 Mechanics .................................... 49 4.3.1 Structure ................................ 49 4.3.2 Entranceshutter ............................ 49 4.3.3 K-mirror................................. 52 4.3.4 Retro-reflector.............................. 52 4.3.5 Diamond-turnedmirrors . 53 4.3.6 Roof-prismandmount . 54 4.4 Opto-mechanics................................. 54 4.4.1 Rotationstages ............................. 54 4.4.2 Tip/tilt/pistonactuator . 56 4.4.3 Pupilactuator.............................. 57 4.4.4 Fiberpositioner............................. 58 5 Guiding system 59 5.1 Overview..................................... 59 5.2 Tip/tiltguidingsystem. .. 60 5.2.1 Tunnelatmosphereandinjectionloss . ... 60 5.2.2 Tip/tiltlaserbeacon .......................... 62 5.2.3 Guidingreceiver............................. 66 5.2.4 Sensor .................................. 68 5.3 Control-loopanalysis . .. 72 5.3.1 Tip/tiltcontrol-loop . 73 5.3.2 Modeling the acquisition camera response . .... 74 5.3.3 ModelingthePSDresponse . 76 5.3.4 Systemresponse............................. 78 5.3.5 Tip/tiltguidingsystemperformance . ... 80 5.4 Pupilguidingsystem .............................. 81 5.4.1 Pupiltrackingconcept . 82 5.4.2 MeasurementcampaigninParanal . 82 5.4.3 Pupillaserbeacon ........................... 84 5.4.4 Pupiltracker .............................. 86 5.4.5 Pupilcontrol-loop............................ 87 5.4.6 Pupilguidingperformance . 88 5.4.7 Importance of the guiding system for astrometry . ...... 90 Table of content vii 6 The star formation history of the Milky Way’s Nuclear Star Cluster 91 6.1 Introduction................................... 91 6.2 Observationsanddataprocessing . .... 93 6.2.1 Imagingandphotometry . 94 6.2.2 Spectroscopy .............................. 95 6.2.3 Sourceselection............................. 96 6.2.4 Detectionprobability . 98 6.3 Spectralclassification. ... 98 6.3.1 DeepcensusoftheGCpopulation. 100 6.3.2 SupergiantIRS7 ............................ 103 6.3.3 COindexdefinition. 104 6.3.4 Temperaturecalibration . 105 6.3.5 Red clump spectrum: evidence for old ages and near solar metallicity 106 6.4 ConstructionoftheH-Rdiagram . 112 6.4.1 FeaturesoftheH-Rdiagram. 114 6.5 Calculationofthestarformationhistory . ....... 117 6.5.1 Fittingprocedure ............................ 119 6.6 Results...................................... 120 6.6.1 Starformationrateovercosmictime . 120 6.6.2 The mass composition of the nuclear cluster . .... 126 6.7 Discussion.................................... 130 6.7.1 InitialMassFunctionintheGC . 130 6.7.2 StarformationinthevicinityoftheGC . 130 6.7.3 Alternative explanation for the young giants . ..... 132 7 Conclusions and future outlook 135 7.1 Instrumentationresults. 135 7.1.1 Fiber coupler development and implementation . ..... 135 7.1.2 Guiding system development and control-loop performance . 136 7.1.3 Outlook ................................. 138 7.2 Star formation history of Milky Way’s Nuclear Star Cluster......... 138 List of Acronyms and Abbreviations 149 Acknowledgements 151 viii Table of content List of Figures 2.1 Dual-feedastrometry . 4 2.2 Paranalatmosphere............................... 6 2.3 Astrometricerror ................................ 7 2.4 Baselinedefinition ............................... 9 2.5 Mechanicalbaselineerror. ... 11 2.6 Dual-fieldopticalpath . 13 2.7 Fiberinjectionerrors . .. 15 2.8 Pupildefinition ................................. 19 3.1 GRAVITYintheGC.............................. 22 3.2 Systemoverview ................................ 24 3.3 Fibercontrolunit................................ 26 3.4 Integratedoptics ................................ 27 3.5 Spectrometer .................................. 28 3.6 Acquisitioncamera ............................... 30 3.7 Metrologyconcept ............................... 32 4.1 Opticaldesignofthefibercoupler . .... 35 4.2 Opticaldesignofthefibercoupler(3D) . ..... 37 4.3 Roof-prismconcept ............................... 40 4.4 Roof-prismpicture ............................... 41 4.5 PSF-splittingmode ............................... 42 4.6 Half-waveplateprinciple . .. 43 4.7 Half-waveplateretardation . .. 44 4.8 Half-waveplatepicture. 45 4.9 Dichroiccurve.................................. 46 4.10Wavefronterror ................................. 48 4.11FibercouplerCADmodel ........................... 50 4.12 Assembledfibercoupler . 51 4.13K-mirror..................................... 52 4.14Retro-reflector................................. 53 4.15Off-axisparabola ................................ 55 4.16Roof-prismmount................................ 56 x LIST OF FIGURES 4.17 Piezotransferfunction . ... 57 5.1 Guidingconcept................................. 61 5.2 Powerspectrumoftunnelatmosphere. .... 62 5.3 Coupling efficiency as function of tunnel tip/tilt RMS . ......... 63 5.4 Laserinjectionconcept . .. 64 5.5 Tip/tiltlauncheronSTS. 64 5.6 Scatteringefficiency.............................. 65 5.7 Guidingreceiveropticaldesign. ..... 67 5.8 Guidingreceivermountedonfibercoupler . ..... 68 5.9 Guidingreceiverinterior . ... 69 5.10Diodesketch................................... 70 5.11Diodenoisemodel ............................... 71 5.12Noisemeasurement ............................... 73 5.13 Tip/tiltcontrol-loopscheme . ..... 75 5.14 Acquisitioncameraresponse . ... 76 5.15 PSDcontrol-loopblockdiagram . .... 77 5.16 Positionsensitivedioderesponse. ....... 78 5.17 Completesystemresponse . .. 80 5.18 Correctedtip/tiltpower-spectrum . ....... 81 5.19 Pupilmeasurementconcept . .. 83 5.20PupilmotionofVLTI.............................. 85 5.21Pupillaserbeacon ............................... 86 5.22Pupiltracker .................................. 87 5.23 Pupilcontrol-loop. .. 88 5.24 Pupilguidingtransferfunction. ...... 89 5.25 Pupilguidingperformance . ... 89 6.1 Three-colorimageoftheGalacticCenter . ...... 97 6.2 Completenessmap ............................... 99 6.3 Early-typespectra ............................... 101 6.4 K-bandluminosityfunction . 102 6.5 COindexdefinitions .............................. 104 6.6 Temperaturecalibration . 107 6.7 Temperature-agerelation. 108 6.8 Medianspectrumofredclump. 110 6.9 CO bandheads and their temperature dependence . ...... 111 6.10 Hertzsprung-Russell diagram of the GC population . ........ 113 6.11 Radialdistributionofgiants . ..... 116 6.12 Velocity diagram of the giant population . ....... 118 6.13 Fittingresiduals............................... 121 6.14 Starformationrateasfunctionoftime . ...... 124 6.15 Integratedmassasfunctionoftime . ..... 125 List of figures xi 6.16 Masscompositionofthecluster . .... 128 6.17 DiffuseH-bandbackground . 131 xii List of figures List of Tables 2.1 Atmosphereparameters . 6 6.1 COindexdefinition............................... 98 6.2 Earlytypecandidates. 103 6.3 Starformationrate ............................... 122 xiv List of tables Zusammenfassung Die vorliegende Arbeit ist thematisch zweigeteilt in einen instrumentellen Teil und einen astrophysikalischen Teil. Der instrumentelle Teil beschreibt die Entwicklung und Im- plementierung des Fiber Coupler und des Guiding Subsystemes, des zuk¨unftigen VLTI Instrumentes GRAVITY. Der astrophysikalische Teil handelt von der Sternentstehungs- geschichte des Sternhaufens im Zentrum der Milchstraße, welche anhand von Bild- und Spektroskopiedaten, aufgenommen mit dem Very Large Telescope, abgeleitet wurde. Das zuk¨unftige VLTI Instrument GRAVITY