ESA Voyage 2050 white paper -- Faint objects in motion: the new frontier of high precision astrometry
F. Malbet, U. Abbas, J. Alves, C. Boehm, W. Brown, L. Chemin, A. Correia, F. Courbin, J. Darling, A. Diaferio, M. Fortin, M. Fridlund, O. Gnedin, B. Holl, A. Krone-Martins, A. Léger, L. Labadie, J. Laskar, G. Mamon, B. McArthur, D. Michalik, A. Moitinho, M. Oertel, L. Ostorero, J. Schneider, P. Scott, M. Shao, A. Sozzetti, J. Tomsick, M. Valluri, R. Wyse
WWhite paper for the Voyage 2050 long-term planin the ESA Science Programme
Faint objects in motion:the new frontier of high precision astrometry
Contact Scientist : Fabien MalbetInstitut de Planétologie et d’Astrophysique de Grenoble (IPAG)
Université Grenoble Alpes, CS 40700, F-38058 Grenoble cedex 9, France
Email:
[email protected] , Phone: +33 476 63 58 33version 20190805 a r X i v : . [ a s t r o - ph . I M ] O c t aint objects in motion : the new frontier of high precision astrometry Faint objects in motion: the newfrontier of high precision astrometry
Sky survey telescopes and powerful targeted telesco-pes play complementary roles in astronomy. In order toinvestigate the nature and characteristics of the mo-tions of very faint objects , a flexibly-pointed instrumentcapable of high astrometric accuracy is an ideal comple-ment to current astrometric surveys and a unique tool forprecision astrophysics.
Such a space-based missionwill push the frontier of precision astrometry from ev-idence of earth-massed habitable worlds around thenearest starts, and also into distant Milky way objectsup to the Local Group of galaxies . As we enter theera of the James Webb Space Telescope and the newground-based, adaptive-optics-enabled giant telescopes,by obtaining these high precision measurements on keyobjects that
Gaia could not reach, a mission that focuseson high precision astrometry science can consolidateour theoretical understanding of the local universe,enable extrapolation of physical processes to remoteredshifts, and derive a much more consistent pictureof cosmological evolution and the likely fate of ourcosmos .Already several missions have been proposed to ad-dress the science case of faint objects in motion usinghigh precision astrometry ESA missions: NEAT for M3,micro-NEAT for S1 mission, and
Theia for M4 and M5(Boehm et al. 2017). Additional new mission configura-tions adapted with technological innovations could be en-visioned to pursue accurate measurements of these ex-tremely small motions. The goal of this white paper isto address the fundamental science questions that are atstake when we focus on the motions of faint sky objectsand to briefly review quickly instrumentation and missionprofiles.
Nota Bene: most Figures in this White Paper refersto the Theia specifications (see Boehm et al. 2017, for de-tails) which target at astrometric end-of-mission precisionsof 10 µ as for faint object of R = mag and 0.15 µ as forbright object of R = mag (see Table 2.1) . Europe has always been a pioneer of astrometry, from thetime of ancient Greece to Tycho Brahe, Johannes Kepler,the Copernican revolution and Friedrich Bessel. ESA’s
Hipparcos and
Gaia satellites continued this tradition, rev-olutionizing our view of the Solar Neighborhood and MilkyWay, and providing a crucial foundation for many disci-plines of astronomy.
An unprecedented microarcsec-ond relative precision mission will advance Europeanastrometry still further, setting the stage for breakthroughs on the most critical questions of cosmology, astron-omy and particle physics . The current hypothesis of cold dark matter (CDM) ur-gently needs verification.
Dark matter (DM) is essentialto the Λ + CDM cosmological model ( Λ CDM), which suc-cessfully describes the large-scale distribution of galax-ies and the angular fluctuations of the Cosmic MicrowaveBackground, as confirmed by the ESA / Planck mission.Dark matter is the dominant form of matter ( ∼ ) inthe Universe, and ensures the formation and stability ofenmeshed galaxies and clusters of galaxies. The currentparadigm is that dark matter is made of heavy, hence cold,particles; otherwise galaxies will not form. However, thenature of dark matter is still unknown .There are a number of open issues regarding Λ CDMon small-scales. Simulations based on DM-only predict a1) large number of small objects orbiting the Milky Way,2) a steep DM distribution in their centre and 3) a pro-late Milky Way halo. However, hydrodynamical simula-tions, which include dissipative gas and violent astrophys-ical phenomena (such as supernovae explosions and jetsfrom galactic nuclei) can change this picture. Quantitativepredictions are based on very poorly understood sub-gridphysics and there is no consensus yet on the results. An-swers are buried at small-scales, which are extremely dif-ficult to probe. A new high precision astrometric missionappears to be the best way to settle the nature of DM andwill allow us to validate or refute key predictions of Λ CDM,such as– the DM distribution in dwarf spheroidal galaxies– the outer shape of the Milky Way DM halo– the lowest masses of the Milky Way satellites andsubhalos– the power spectrum of density perturbationsThese observations will significantly advance research in-to DM. They may indicate that DM is warmer than Λ CDMpredicts. Or we may find that DM is prone to self-interac-tions that reduces its density in the central part of the satel-lites of the Milky Way. We may discover that DM has smallinteractions that reduce the number of satellite compan-ions. Alternatively, measurement of the Milky Way DMhalo could reveal that DM is a sophisticated manifestationof a modification of Einstein’s gravity.
Because they are DM-dominated (see Fig. 1.1 where thenumber of stars versus the mass-to-light ratio is present-ed), dwarf Spheroidal galaxies (dSphs) are excellent labo-ratories to test the distribution of DM within the central part1 aint objects in motion : the new frontier of high precision astrometry
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Fig. 1.1:
Number of dwarf spheroidal galaxy stars within a highprecision astrometry missionfield with expected plane-of-sky er-rors lower than half the galaxy’s velocity dispersion as a functionof the galaxy’s estimated mass-to-light ratio within the effective(half-projected-light) radius of the galaxy. Luminosities and totalmasses within the half-light radii are mainly from Walker et al.(2009). of small galaxies and disentangle the influence of complexbaryonic processes from that of DM at these scales.Simulations from Oñorbe et al. (2015) or Read et al.(2016) for example show that the DM distribution (referredto as DM profile) in dSphs strongly depends on their starformation history. More specifically, these simulations findthat CDM can be heated by bursty star formation insidethe stellar half light radius R / , if star formation proceedsfor long enough. As a result, some dSphs like Fornax haveformed stars for almost a Hubble time and so should havelarge central DM cores, while others, like Draco and UrsaMajor2, had their star formation truncated after just ∼ − Gyrs and should retain their steep central DM cusp.Large DM cores could also be attributed however tostrong self-interactions. Hence finding evidence for suchcores in the faintest dSphs (which are even more DMdominated (Wolf et al. 2010) than the classical ones),will bring tremendous insights about the history of bary-onic processes in these objects and could even dramat-ically change our understanding of the nature of DM. In-deed, self-interacting DM (Spergel & Steinhardt 2000) isexpected to scatter in the dense inner regions of dSphs,and thus leads to homogeneous cores. Finding such acore DM distribution in dSphs could then reveal a new typeof particle forces in the DM sector and provide us with newdirections to build extensions of Standard Model of particlephysics. On the other hand, finding cuspy DM profiles in alldSphs (including the faintest ones) will confirm Λ CDM andplace strong constraints on galaxy formation. As shownin Figs. 2.16 and 2.17, a telescope with micro-arcsecondastrometric precision allows us to determine whether theDM distribution in dSphs is cuspy or has a core, and hence can lead to a very significant breakthrough regarding thenature of DM.To determine the inner DM distribution in dSphs, oneneeds to remove the degeneracy between the radial DMprofile and orbital anisotropy that quantifies whether stel-lar orbits are more radial or more tangential in the Jeansequation (Binney & Mamon 1982). This can be done byadding the proper motions of stars in dSphs. Fig. 1.2shows that for the Draco dSph (which was obtained usingsingle-component spherical mock datasets from the
Gaia
Challenge Spherical and Triaxial Systems working group, and the number of stars expected to be observed by a highprecision astrometry mission), the inclusion of proper mo-tions lifts the cusp / core degeneracy that line-of-sight-onlykinematics cannot disentangle.We remark in addition that a high precision astromet-ric mission is able to perform follow-ups of Gaia ’s obser-vations of dSphs streams of stars if needed. Not only willsuch a mission provide the missing tangential velocities forstars with existing radial velocities, but it will also providecrucial membership information - and tangential velocities- for stars in the outer regions of the satellite galaxies thatare tidally disrupted by the Milky Way.
For almost two decades cosmological simulations haveshown that Milky Way-like DM halos have triaxial shapes,with the degree of triaxiality varying with radius (Dubin-ski 1994; Kazantzidis et al. 2004, for example): halos aremore round or oblate at the center, become triaxial at inter-mediate radii, and prolate at large radii (Zemp et al. 2012).Precise measurement of the velocity of distant HyperVelocity Stars (hereafter HVS) can test these departuresfrom spherical symmetry, independently of any other tech-nique attempted so far (such as the tidal streams). HVSswere first discovered serendipitously (Brown et al. 2005;Hirsch et al. 2005; Edelmann et al. 2005), and later dis-covered in a targeted survey of blue main-sequence stars(Brown 2015, and references therein).
Gaia measure-ments demonstrate that candidate HVSs include unbounddisk runaways (Irrgang et al. 2019), unbound white dwarfsejected from double-degenerate type Ia supernovae (Shenet al. 2018), and runaways from the LMC (Erkal et al.2018), however the highest-velocity main sequence starsin the Milky Way halo have trajectories that point from theGalactic center (Brown et al. 2018; Koposov et al. 2019).Because these velocities exceed the plausible limit fora runaway star ejected from a binary, in which one compo-nent has undergone a supernova explosion, the primary See http://astrowiki.ph.surrey.ac.uk/dokuwiki/doku.php?id=tests:sphtri aint objects in motion : the new frontier of high precision astrometry − r [kpc] − − − ρ [ M (cid:12) p c − ] R eff cored DMisotropic orbits LOS only fitPM+LOS fittrue − r [kpc] − − ρ [ M (cid:12) p c − ] R eff cuspy DMisotropic orbits LOS only fitPM+LOS fittrue − r [kpc] − − ρ [ M (cid:12) p c − ] R eff cored DMradial outer orbitsLOS only fitPM+LOS fittrue − − r [kpc] − − ρ [ M (cid:12) p c − ] R eff cuspy DMradial outer orbitsLOS only fitPM+LOS fittrue Fig. 1.2:
Reconstruction of the DM halo profile of the Draco dSph without (blue) and with (red) proper motions using the mass-orbitmodeling algorithm of Watkins et al. (2013). Four mocks of Draco were used, with cored (left) and cuspy (right) DM halos, andwith isotropic velocities everywhere (top) or only in the inner regions with increasingly radial motions in the outer regions (bottom).The effective (half-projected light) radii of each mock is shown with the arrows. The stellar proper motions in the mocks wereperturbed with apparent magnitude dependent errors as expected with 1000 hours of observations spread over 4 years. standardDM halomodel Fig. 1.3:
Illustration of the trajectories of hyper velocity starsejected from Galactic Centre for 3 different outer DM haloshapes: oblate (left), spherical (middle), and prolate (right). mechanism for a star to obtain such an extreme velocity isassumed to be a three-body interaction and ejection fromthe deep potential well of the supermassive black hole atthe Galactic center (Hills 1988; Yu & Tremaine 2003).By measuring the three-dimensional velocity of thesestars, we will reconstruct the triaxiality of the Galactic po-tential. In a spherical potential, unbound HVS ejected fromthe Galactic center should travel in nearly a straight line, asdepicted in Fig.1.3. However, for triaxial halos, the present velocity vector should not point exactly from the GalacticCenter because of the small curvature of the orbit causedby non-spherically symmetric part of the potential (Gnedinet al. 2005; Yu & Madau 2007). While both the halo andstellar disc induce transverse motions, the effect is domi-nated by halo triaxiality at the typical distance of HVS. Thedeflection contributed by the disc peaks around 10 kpcbut quickly declines at larger distances, while the deflec-tion due to the triaxial halo continues to accumulate alongthe whole trajectory. Fig. 1.4 actually shows the spread ofproper motion for one star, HVS5, for different halo shapes(different halo axis ratios and different orientations of themajor axis).Proper motions of several HVSs were measured withthe Hubble Space Telescope (
HST ) by Brown et al.(2015), using an astrometric frame based on backgroundgalaxies. However, these measurements were not suffi-ciently accurate to constrain the halo shape or the originof HVS. A high precision astrometric mission with a suf-ficiently large field of view could include about 10 knownquasars from the SDSS catalog around most HVSs. Thiswill provide a much more stable and accurate astrometric3 aint objects in motion : the new frontier of high precision astrometry
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Fig. 1.4:
Expected proper motions of HVS5 under different as-sumptions about the shape and orientation of the DM halo. Thefamilies of models are shown with the halo major axis along theGalactic X- (red squares), Y- (blue triangles), and Z- (green cir-cles) coordinates. The solid line shows how the centroid of theproper motions will shift with a different distance to HVS5. frame, and will allow us to constrain the halo axis ratios toabout 5 % .Fig.1.5 shows indeed that with a precision of µ as/yrone can constrain the orientation of the halo major axisand measure the axis ratios to an accuracy of δ ( q Z / q X ) < . for the typical HVS distance of 50 kpc. For com-parison, Gaia at the end of its mission will achieve only − µ as/yr, which is highly insufficient to provide use-ful constraints on the axis ratios.Statistical studies of high-precision proper motions ofHVSs can also constrain departures of the halo shapefrom spherical (Gallo, Ostorero,& Diaferio, in preparation).Indeed, numerical simulations of the trajectories of syn-thetic HVSs ejected through the Hills mechanism showthat the distributions of the HVS tangential velocities in theGalactocentric reference frame are significantly differentfrom spherical and non-spherical halos: the significanceis P ≤ . e − for oblate halos with q Z / q X ≤ . and P ≤ . e − for prolate halos with q Z / q X ≥ . . Themedian tangential velocity of a sample of ∼ HVSslocated at heliocentric distances ∼ kpc can differ by ∼ − km / s , implying differences in proper motions of ∼ − µ as/yr between spherical and non-spherical ha-los. Finally, an accurate measurement of HVS velocitiesmay lead to improved understanding of the black hole(s)at the Galactic center. Indeed, theoretical models showthat HVSs will have a different spectrum of ejection veloci-ties from a binary black hole versus a single massive blackhole. Gaia has led to the discovery of several hyperveloc-ity stars (ejection velocities of over 550 km/s Irrgang et al.2018; Hattori et al. 2019; Irrgang et al. 2019), that weredefinitely not ejected from the Galactic Center but wereejected from spiral arms in the MW disk. These most likely
Fig. 1.5:
Example of a reconstruction of the Galactic haloshape from a high precision astrometry mission measurementof proper motion of HVS5. The assumed proper motions corre-spond to a prolate model with q X = q Y = . q Z , marked by ared square. Shaded contours represent confidence limits cor-responding to the expected 1, 2, and σ µ proper motion errors.The outer blue contours show the accuracy that will be achievedby Gaia at the end of its mission, even if its expected error wasreduced by a factor of 2. point to intermediate mass black holes of mass 100 Msun- these could be local remnants of binary BH mergers ofthe kind discovered by LIGO and could have important im-plications for our understanding of stellar evolution. The orbits of DM particles in halos cannot be detected di-rectly since DM particles interact only weakly with normalmatter. However, in a triaxial potential such as describedabove, it is expected that a large fraction of the DM or-bits do have any net angular momentum. Hence theseparticles should get arbitrarily close to the center of thecusp, regardless of how far from the center they were orig-inally. This allows DM particles, which annihilate within thecusp to be replenished on a timescale longer than ina spherical halo (analogous to loss cone filling in the caseof binary black holes Merritt & Poon 2004).Recent work on the orbital properties and kinematicdistributions of halo stars and DM particles show that halostars, especially the ones with lowest metallicities, are rel-atively good tracers of DM particles (Valluri et al. 2013b;Herzog-Arbeitman et al. 2018a,b) and observations with Gaia
DR2 may have already led to the kinematic discov-ery of dark substructure (Necib et al. 2019). The orbits For an analysis of orbital content of DM halos see Valluri et al.(2010, 2012); Bryan et al. (2012); Valluri et al. (2013a). aint objects in motion : the new frontier of high precision astrometry Fig. 1.6:
Face-on view of the evolution of the perturbation of a Galactic Disc due to a DM subhalo of mass 3 % of the mass ofthe disc crossing the disc from above. The upper and lower panels are before and after the crossing, respectively, for differenttimes 125, 75 and 25 Myr before the crossing and 25,75,125 Myr after (from left to right). The mean displacement amplitude isindicated in the color bar, while the contours indicate the amplitude of the bending mode in velocity space, using plain lines forpositive values and dashed lines for negative values. The green line shows the projected orbit of the subhalo (dashed line afterthe impact with the disc). The green triangle shows the current location of the subhalo on its orbit. The red lines are our potentiallines of sight for Theia, spaced by 10 ◦ in longitude with one pointing above the plane and one below the plane, that will allow us tomap the disc perturbation behind the Galactic Center. reflect both the accretion/formation history and the currentshape of the potential because DM halos are dynamicallyyoung (i.e. they are still growing and have not attaineda long term equilibrium configuration where all orbits arefully phase mixed). This opens up the very exciting possi-bility that one can infer the kinematical distribution of DMparticles by assuming that they are represented by thekinematics of halo stars. A central prediction of Λ CDM in contrast to many alter-natives of DM, such as warm DM (e.g. Schaeffer & Silk1984) or interacting DM (e.g. Boehm et al. 2014), is theexistence of numerous to M (cid:12) DM subhalos in theMilky Way halo. Their detection is extremely challenging,as they are very faint and lighter than dSphs. However,N-body simulations of the Galactic Disc show that such aDM halo passing through the Milky Way disc will warp thedisc and produce a motion (bending mode), as shown inFig. 1.6. This opens new avenues for detection as suchperturbations of the disc will result in anomalous motionsof the stars in the disc (e.g. Feldmann & Spolyar 2015,for recent analysis), that could give rise to an astrometricsignal.These anomalous bulk motions develop both in the so-lar vicinity (Widrow et al. 2012) and on larger scales (Feld- mann & Spolyar 2015), see Fig.1.7. Therefore, measuringvery small proper motions of individual faint stars in differ-ent directions towards the Galactic disc could prove the ex-istence of these subhalos and confirm the CDM scenario.Alternatively, in case they are not found, high precise as-trometric observations will provide tantalizing evidence foralternative DM scenarios, the most popular today being awarmer form of DM particle, though these results couldalso indicate DM interactions (Boehm et al. 2014).A field of view of ◦ × ◦ in the direction of the Galacticdisc has ∼ stars with an apparent magnitude of R ≤ (given by the confusion limit). Given the astrometricprecisions per field of view of Fig. 2.17, a high precisionastrometric instrument could detect up to 7 impacts on thedisc from sub-halos as small as a few M (cid:12) . Gaia
DR2 astrometry has led to the discovery of gapsin tidal streams (Price-Whelan & Bonaca 2018) like theGD1 stream. The gaps and off-stream stars (spur) areconsistent with gravitational interactions with compact DMsubhalos. Further more,
Gaia
DR2 data has revealed thatglobular cluster streams (GD1 and Jhelum) show evidencefor cocoon like structures that most likely arise from evolu-tion inside a (dark) subhalo prior to their tidal disruption bythe Milky Way itself (Carlberg 2018; Malhan et al. 2019;Bonaca et al. 2019). The high astrometric precision ofa
Theia -like mission will enable us to measure the smallvelocity perturbations around the gaps in streams and al-5 aint objects in motion : the new frontier of high precision astrometry
ESA Voyage 2050 White paper
Fig. 1.7:
Astrometric signatures in the proper motion along Galactic latitude of the perturbation of disc stars by a subhalo. Theleft and right panels show lines of sight as a function of distance along the line of sight and time, for (cid:96) = − ◦ and (cid:96) = + ◦ respectively for b = + ◦ . The color codes the time in Myr, red for times prior to the crossing of the plane by the satellite, blue forlater times. The green line is Gaia’s expected end of mission performance for a population of red clump stars along these linesof sight. The vertical dashed line is Gaia’s detection limit ( G =20) for the same population. The red lines are Theia’s expected 1 σ accuracy for the same stars and for a 400 h exposure of the field over the course of the mission. low for a much more accurate determination of both themasses and density structures of the perturbing dark sub-halos. In the Λ CDM model, galaxies and other large-scale struc-tures formed from tiny fluctuations in the distribution ofmatter in the early Universe. Inflation predicts a spectrumof primordial fluctuations in the curvature of spacetime,which directly leads to the power spectrum of initial densityfluctuations. This spectrum is observed on large scalesin the cosmic microwave background and the large scalestructure of galaxies, but is very poorly constrained onscales smaller than 2 Mpc. This severely restricts our abil-ity to probe the physics of the early Universe. A high pre-cision astrometric mission could provide a new window onthese small scales by searching for astrometric microlens-ing events caused by ultra-compact minihalos (UCMHs) ofDM.UCMHs form shortly after matter domination (at z ∼ ), in regions that are initially overdense (e.g. δρ / ρ > . in Ricotti & Gould 2009). UCMHs only form fromfluctuations about a factor of 100 larger than their regu-lar cosmological counterparts, so their discovery will in-dicate that the primordial power spectrum is not scale in-variant. This will rule out the single-field models of inflationthat have dominated the theoretical landscape for the pastthirty years. Conversely, the absence of UCMHs can beused to establish upper bounds on the amplitude of the pri-mordial power spectrum on small scales (Bringmann et al.2012), which will rule out inflationary models that predictenhanced small-scale structure (Aslanyan et al. 2016). -2 -1 M i ( M fl )10 -3 -2 -1 f e q GaiaTheia
Fig. 1.8:
Projected sensitivity of a high precision astrometry mis-sionto the fraction of dark matter in the form of ultracompactminihalos (UCMHs) of mass M i at the time of matter-radiationequality. Smaller masses probe smaller scales, which corre-spond to earlier formation times (and therefore to later stagesof inflation). A UCMH mass of 0.1 M (cid:12) corresponds to a scaleof just 700 pc. Expected constraints from Gaia are given forcomparison, showing that a Theia-like mission will provide muchstronger sensitivity, as well as probe smaller scales and earlierformation times than ever reached before. Like standard DM halos, UCMHs are too diffuse tobe detected by regular photometric microlensing searchesfor MAssive Compact Halo Objects (MACHOs). Becausethey are far more compact than standard DM halos, theyhowever produce much stronger astrometric microlensingsignatures (Li et al. 2012). By searching for microlens-ing events due to UCMHs in the Milky Way, a high preci-sion astrometric mission will provide a new probe of infla-tion. A search for astrometric signatures of UCMHs in the
Gaia dataset could constrain the amplitude of the primor-6 aint objects in motion : the new frontier of high precision astrometry
Fig. 1.9:
Limits on the power of primordial cosmological perturbations at all scales, from a range of different sources. a Theia-likemission will provide far stronger sensitivity to primordial fluctuations on small scales than Gaia, spectral distortions or primordialblack holes (PBHs). Unlike gamma-ray UCMH limits, a high precision astrometry mission’s sensitivity to cosmological perturbationswill also be independent of the specific particle nature of dark matter. dial power spectrum to be less than about − on scalesaround 2 kpc (Li et al. 2012). Fig. 1.8 shows that higherastrometric precision (corresponding to that of Fig. 2.17)will provide more than an order of magnitude higher sen-sitivity to UCMHs, and around four orders of magnitudegreater mass coverage than Gaia . These projections arebased on 8000 hr of observations of 10 fields in the MilkyWay disc, observed three times a year, assuming that thefirst year of data is reserved for calibrating stellar propermotions against which to look for lensing perturbations.Fig. 1.9 shows that a high precision astrometric missionwill test the primordial spectrum of perturbations down toscales as small as 700 pc, and improve on the expectedlimits from
Gaia by over an order of magnitude at largerscales.The results will be independent of the DM nature, asastrometric microlensing depends on gravity only, unlikeother constraints at similar scales based on DM annihila-tion, from the
Fermi
Gamma Ray Space Telescope (Bring-mann et al. 2012). An astrometric mission with higherprecision (shown in Fig. 2.17) will have sensitivity four or-ders of magnitude stronger than constraints from the ab-sence of primordial black holes (PBHs), and more than anorder of magnitude better than CMB spectral distortions(Chluba et al. 2012), which give the current best model-independent limit on the primordial power spectrum at sim-ilar scales.
Using the nearest star, Proximan Cen, astrometry couldmeasure the behaviour of gravity at low accelerations. Ahigh precision astrometry mission with an extended base-line of 10 years and a precision of 0.5 µ as could mea-sure the wide binary orbit of Proxima Centauri around al-pha Centauri A and B to distinguish between Newtoniangravity and Milgromian dynamics (MOND). The separa-tion between Proxima cen and the Alpha Centauri sys- tem suggests orbital acceleration that is significantly lessthan MOND acceleration constant a ∼ . × − m/s (Banik & Kroupa 2019). It would be the first direct mea-surement of the departure from Newtonian gravity in thevery weak field limit, as expected in MOND, and the resultscould have profound implications on fundamental physics. The ultimate exoplanetary science goal is to answer theenigmatic and ancient question, “
Are we alone?’ ’ via un-ambiguous detection of biogenic gases and molecules inthe atmosphere of an Earth twin around a Sun-like star(Schwieterman et al. 2016). Directly addressing the age-old questions related to the uniqueness of the Earth as ahabitat for complex biology constitutes today the vanguardof the field, and it is clearly recognized as one unprece-dented, cross-technique, interdisciplinary endeavor.Since the discovery of the first Jupiter-mass compan-ion to a solar-type star (Mayor & Queloz 1995), tremen-dous progress has been made in the field of exoplanets.Our knowledge is expanding ever so quickly due to the dis-covery of thousands of planets, and the skillful combina-tion of high-sensitivity space-borne and ground-based pro-grams that have unveiled the variety of planetary systemsarchitectures that exist in the Galaxy (e.g. Howard 2013;Mayor et al. 2011). Preliminary estimates (e.g. Winn &Fabrycky 2015) are now also available for the occurrencerate η (cid:93) of terrestrial-type planets in the Habitable Zone(HZ) of stars more like the Sun ( η (cid:93) ∼ ) and low-massM dwarfs ( η (cid:93) ∼ ).However, transiting or Doppler-detected HZ terrestrialplanet candidates (including the recent discovery of the m p sin i = . M ⊕ HZ-planet orbiting Proxima Centauri)lack determinations of their bulk densities ρ p . Thus, theHZ terrestrial planets known to-date are not amenable to7 aint objects in motion : the new frontier of high precision astrometry ESA Voyage 2050 White papermake clear statements on their habitability. The K2 , TESS ,and
PLATO missions are bound to provide tens of Earthsand Super Earths in the HZ around bright M dwarfs andsolar-type stars for which ρ p estimates might be obtainedin principle, but atmospheric characterization for the lattersample might be beyond the capabilities of JWST and theExtremely Large Telescopes (ELTs). The nearest stars tothe Sun are thus the most natural reservoir for the identi-fication of potentially habitable rocky planets that might becharacterized via a combination of high-dispersion spec-troscopy and high-contrast imaging with the ELTs (Snellenet al. 2015) or via coronagraphic or interferometric obser-vations in space (Leger 2015).Unlike the Doppler and transit methods, astrometryalone can determine reliably and precisely the true massand three-dimensional orbital geometry of an exoplanet,which are fundamental inputs to models of planetary evo-lution, biosignature identification, and habitability. By de-termining the times, angular separation and position angleat periastron and apoastron passage, exquisitely preciseastrometric position measurements will allow the predic-tion of where and when a planet will be at its brightest(and even the likelihood of a transit event), thus (a) cru-cially helping in the optimization of direct imaging obser-vations and (b) relaxing important model degeneracies inpredictions of the planetary phase function in terms of or-bit geometry, companion mass, system age, orbital phase,cloud cover, scattering mechanisms and degree of polar-ization (e.g. Madhusudhan & Burrows 2012). Only a highprecision astrometric mission’s observations will have thepotential to 1) discover most of the potentially habitableplanets around the nearest stars to the Sun, 2) directlymeasure their masses and system architectures, and 3)provide the most complete target list and vastly improvethe efficiency of detection of potential habitats for complexexo-life with the next generation of space telescopes andELTs.
Surgical single-point positional precision measurements inpointed, differential astrometric mode ( < µ as), could ex-ploit a high precision astrometric mission’s unique capabil-ity to search for the nearest Earth-like planets to the Sun.The amplitude α of the astrometric motion of a star due toan orbiting planet is (in micro-arcseconds): α = (cid:18) M p M ⊕ (cid:19) (cid:16) a p (cid:17) (cid:18) M (cid:63) M (cid:12) (cid:19) − (cid:18) D (cid:19) − µ as (1)where M (cid:63) is the stellar mass, M p is the mass of the planet, a p is the semi-major axis of the orbit of the planet, and D isthe distance to the star. For a terrestrial planet in the HZ ofa nearby sun-like star, a typical value is 0.3 µ as (an Earthat 1.0 AU of a Sun, at 10 pc). This very small motion (thesize of a coin thickness on the Moon as measured from the Earth) will be accessible to a high precision astrometricinstrument by measuring the differential motion of the starwith respect to far-away reference sources.A core exoplanet program could be comprised of 63of the nearest A, F, G, K, and M stars (Fig. 1.10). Manyof them are found in binary and multiple systems. Binarystars are compelling for a high precision space missionfor a number of reasons. They are easier targets thansingle stars. For close Sun-like binaries, the magnitudeof both components is lower than V = mag, which isthe equivalent magnitude of a typical reference star fieldcomposed of 6 V = mag stars.Furthermore, as the photon noise from the referencesis the dominant factor of the error budget, the accuracyfor binaries increases faster with telescope staring timethan around single stars. For binaries, the references onlyneed to provide the plate scale and the reference directionof the local frame, the origin point coordinates are con-strained by the secondary/primary component of the bi-nary. Finally, when observing a binary, the astrometry onboth components is obtained simultaneously: the staringtime is only spent once as both components are within thesame FoV. These two effects combined cause the obser-vation of stars in binary systems to be much more efficient(as measured in µ as × h − / ) than that of single stars.We further stress that the complete census of smalland nearby planets around solar-type stars is unique tohigh-precision astrometry. On the one hand, Sun-like starshave typical activity levels producing Doppler noise of ∼ m/s (or larger), which is still 10 times the signal expectedfrom an Earth-analog (Lovis et al. 2011). High precisionspace astrometry will be almost insensitive to the distur-bances (spots, plages) due to stellar activity, having typicalactivity-induced astrometric signals with amplitude below0.1 µ as (Lagrange et al. 2011).For the full sample of the nearest stars considered inFig. 1.10 we achieve sensitivity (at the − σ level) toplanets with M p ≤ M ⊕ (See section 3.6). If we consider η Earth ∼ , for the sample of 63 stars closest to ourSolar System we thus expect to detect ∼ HZ terrestrialplanets. Of these, 5 will be amenable for further spectro-scopic characterization of their atmospheres. A high preci-son astrometry mission could perform the measurementsof the relevant stars and make a thorough census (95%completeness) of these planets by using less than 10%of a four years mission. As indicated above, this programwill also be valuable for understanding planetary diversity,the architecture of planetary systems (2-d information plusKepler’s laws, results in 3-d knowledge) including the mu-tual inclination of the orbits, a piece of information that isoften missing in our exploration of planetary systems.8 aint objects in motion : the new frontier of high precision astrometry
System rank0 10 20 30 40 50 M a ss de t e c t i on li m i t i n H Z ( i n E a r t h m a ss ) , Cen. A , Cen. BProcyon AAltair Eridani = Ceti 61 Cygni A70 Ophiuchi A70 Ophiuchi B Cassio. A Cassio. B( Herculis A) ( Herculis B)( Herculis A) - Hydri Indi
Prox. Cen. . Leporis A . Leporis B / Pavonis - Tri. Aus. A(36 Ophiuchi A) (36 Ophiuchi B)( . Virginis A) : Bootis Bootis A Bootis B - Aquilae A82 G. Eridani ( Ursae Maj. A) < Draconis / Eridani Lalande 21185 33 G. Librae A33 G. Librae B Tucanae Barnard`s star Bootis A , Cephei p Eridani ALacaille 8760 , Fornacis A - virginis 66 G. Cen. A279 G. Sagit. A . Cephei - Com. Ber. Cephei . Pavonis Lacaille 9352 - Canum Ven. (Psi Velorum A)(10 Ursae Maj. A)( Cassio. A)( = Bootis A)Groombridge 1618 / Gemi. A
A stars (2)F stars (16)G stars (17)K stars (22)M stars (6)
Fig. 1.10:
Minimum masses of planets that can be detected at the center of the HZ of their star for the 63 best nearby A, F, G,K, M target systems. The target systems (either single or binary stars), are ranked from left to right with increasing minimumdetectable mass in HZ around the primary system component, assuming equal observing time per system. Thus for binary stars,A and B components are aligned vertically, as they belong to the same system they share the same rank. When the A and Bmass thresholds are close the name is usually not explicitly written down to avoid overcrowding. B components that have massthresholds above 2.2 M ⊕ are named in gray and binaries that are estimated too close for follow-up spectroscopy are named ingray and in parenthesis. These binaries are expected to be only part of the secondary science program (planet formation aroundbinaries). The star sample that is best for astrometry is similar to that of the best stars for spectroscopy in the visible, or in thermalIR (see text for explanations). Earths and super-Earths with M p ≥ . M ⊕ can be detected and characterized (actual mass andfull orbit) around 22 stars. All Super-Earths with M p < . M ⊕ can be detected and characterized around 59 stars. A secondary program can help elucidate other importantquestions in exoplanetray science. a ) Planetary systems in S-Type binary systems . Ahigh precision astrometry mission’s performance forexoplanet detection around nearby binaries will beof crucial importance in revealing planet formationin stellar systems, the environment in which roughlyhalf of main-sequence stars are born. The discov-ery of giant planets in binaries has sparked a stringof theoretical studies, aimed at understanding howplanets can form and evolve in highly perturbed en-vironments (Thebault & Haghighipour 2015). Giantplanets around one component of a binary (S-typeorbits) have often been found in orbits very close totheoretical stability limits (e.g. Haghighipour 2004;Thebault 2011; Satyal & Musielak 2016), and as formost of the binary targets the HZ of each compo-nent is stable, finding other and smaller bodies in their HZs is a real possibility. The contribution ofa high precision astrometric mission could be deci-sive for these ongoing studies, by allowing the ex-ploration of a crucial range of exoplanetary archi-tectures in binaries. b ) Follow-up of known Doppler systems . Anotherunique use of a high precision astrometric missionwill be the study of non-transiting, low-mass multi-ple-planet systems that have already been detectedwith RVs. High precision astrometry will confirm orrefute controversial detections, remove the sin i am-biguity and measure actual planetary masses. Fur-thermore, it will directly determine mutual inclinationangles, which are critical to study (i) the habitabilityof exoplanets in multiple systems, since they mod-ify the orientation of the spin axes and hence theway the climates change across time (e.g. Laskar &Robutel 1993; Brasser et al. 2013; Armstrong et al.2014) and (ii) the dynamical evolution history of mul-9 aint objects in motion : the new frontier of high precision astrometry ESA Voyage 2050 White paper
Fig. 1.11:
An example where astrometry breaks the degeneracy. Two simulated planetary systems are around a solar-type star at10 pc, with two Jupiter-like planets at 0.5 and 2.5 AU (left). One is co-planar (dotted black line), the other has a mutual inclinationof 30 ◦ (full red line). The two corresponding RV curves are shown (middle), as well as the two astrometric ones (right). Curvesare identical in the former case, but clearly separated in the latter revealing the inclined orbits. tiple systems, as e.g. coplanar orbits are indica-tive of smooth evolution, while large mutual incli-nations and eccentricities point toward episodes ofstrong interactions, such as planet-planet scatter-ing. Fig. 1.11 illustrates a case where degeneracyin RV can be removed by astrometry. c ) Planetary systems on and off the main sequen-ce . Gaia has the potential to detect thousands ofgiant planetary companions around stars of all ages(including pre- and post-main-sequence), spectraltype, chemical abundance, and multiplicity (Caser-tano et al. 2008; Sozzetti et al. 2014; Perryman et al.2014; Sahlmann et al. 2015). A high precision as-trometriy mission could cherry-pick on
Gaia discov-eries and identify systems amenable to follow-up tosearch for additional low-mass components in suchsystems, particularly in the regime of stellar param-eters difficult for radial velocity work like early spec-tral types, young ages, very low metallicity, whitedwarfs. Some of the systems selected might alsocontain transiting companions identified by
TESS and
PLATO (and possibly even
Gaia itself), or plan-ets directly imaged by
SPHERE or E-ELT. d ) Terrestrial planets around Brown Dwarfs . So far,among the few planetary mass objects that havebeen associated to brown dwarf (BD) hosts using di-rect imaging and microlensing techniques, only oneis likely to be a low-mass planet (Udalski et al. 2015,and references therein)). However, there are bothobservational (Scholz et al. 2008; Ricci et al. 2012,2014) as well as theoretical (Payne & Lodato 2007;Meru et al. 2013) reasons to believe that such sys-tems could also be frequent around BDs. The re-cent identification of a trio of short-period Earth-sizeplanets transiting a nearby star with a mass only ∼ more massive than the Hydrogen-burninglimit (Gillon et al. 2016) is a tantalizing element inthis direction. In its all-sky survey, Gaia will observethousands of ultra-cool dwarfs in the backyard ofthe Sun with sufficient astrometric precision to re-veal any orbiting companions with masses as low asthat of Jupiter (Sozzetti 2014). A high precision as-trometry mission could push detection limits of com-panions down to terrestrial mass. If the occurrencerate of P ≤ . d, Earth-sized planets around BDs is η = as suggested by He et al. (2017) based onextrapolations from transit detections around late Mdwarfs, the high precision measurements, probingfor the first time a much larger range of separationswith respect to transit surveys with sensitivity to low-mass planets, will unveil a potentially large numberof such companions, and place the very first upperlimits on their occurrence rates in case of null detec-tion. The brightest Galactic X-ray sources are accreting com-pact objects in binary systems. Precise optical astrome-try of these X-ray binaries provides a unique opportunityto obtain quantities which are very difficult to obtain other-wise. In particular, it is possible to determine the distancesto the systems via parallax measurements and the massesof the compact objects by detecting orbital motion to mea-sure the binary inclination and the mass function. Witha high precision astrometric mission, distance measure-ments are feasible for >
50 X-ray binaries (in 2000h), andorbital measurements will be obtained for dozens of sys-tems. This will revolutionize the studies of X-ray binaries10 aint objects in motion : the new frontier of high precision astrometry in several ways, and here, we discuss goals for neutronstars (NSs), including constraining their equation of state(EoS), and for black holes (BHs).Matter in the NS interior is compressed to densitiesexceeding those in the center of atomic nuclei, openingthe possibility to probe the nature of the strong interac-tion under conditions dramatically different from those interrestrial experiments and to determine the NS composi-tion. NSs might be composed of nucleons only, of strangebaryons (hyperons) or mesons in the core with nucleonsoutside (a hybrid star), or of pure strange quark matter(a quark star). A sketch of the different possibilities isgiven in Fig. 1.12. Via the equation of state (EoS), matterproperties determine the star’s radius for a given mass.In particular, since general relativity limits the mass fora given EoS, the observation of a massive NS can ex-clude EoS models. Presently, the main constraint stemsfrom the measurements of two very massive NSs in ra-dio pulsar/white dwarf systems which have been reportedwith high precision (Demorest et al. 2010; Antoniadis et al.2013; Fonseca et al. 2016).The key to constraining the NS EoS is to measure themasses and radii of NSs. While masses have been mea-sured for a number of X-ray binary and radio pulsar binarysystems (e.g., Lattimer & Prakash (2016); Özel & Freire(2016)) , the errors on the mass measurements for mostX-ray binaries are large (see Fig. 1.13, left). The ultimateconstraint on the EoS will be a determination of radius andmass of the same object, and a small number of such ob-jects might be sufficient to pin down the entire EoS (e.g.Özel & Psaltis (2009)), see Fig. 1.13 (right), where sev-eral M - R relations for different EoSs are shown. Currenttechniques to determine radii rely on spectroscopic mea-surements of accreting neutron stars, either in quiescence(Heinke et al. 2014) or during thermonuclear (type I) X-raybursts (Özel & Freire 2016), and also timing observationsof surface inhomogeneities of rotating NSs (Miller & Lamb2016; Haensel et al. 2016).A high precision astrometric mission will contribute byobtaining precise mass constraints with orbital measure-ments (Tomsick & Muterspaugh 2010) and by improvingdistance measurements. Distances must be known accu-rately to determine the NS radii. For that purpose, newhigh precision data can be combined with existing and fu-ture X-ray data, e.g., from Athena , which is scheduled asan ESA L2 mission. The
Athena
Science Working Groupon the endpoints of stellar evolution has observations ofquiescent neutron star X-ray binaries to determine the NSEoS as its first science goal; however, their target list is re-stricted to systems that are in globular clusters. A high pre-cision astrometric mission will enable distance measure-ments for many more NS X-ray binaries, allowing Athenato expand their target list.Other techniques for constraining the NS EoS mightalso be possible in the future: detecting redshifted absorp-
Fig. 1.12:
Sketch of the different existing possibilities for theinternal structure of a neutron star. Figure courtesy of FridolinWeber. tion lines; determining the moment of inertia of the dou-ble pulsar J0737 − aint objects in motion : the new frontier of high precision astrometry ESA Voyage 2050 White paper
NS mass [M ⊙ R (km) M ( M fl ) P < ∞ v s o u n d < c J1614 − + ωρ NL3GM1SLy9 TM1BSk19SQM1SQM2
Fig. 1.13:
Left: Neutron star mass measurements in X-ray binaries, update from Lattimer & Prakash (2005), http://stellarcollapse.org . Right: M - R relation for different EoS models (adapted from Fortin et al. (2016)): NS with a purelynucleonic core (in blue), with a core containing hyperons at high density (in red), and pure strange quark stars (in green). Thehorizontal grey bars indicate the masses of PSR J1614 − tion measurements (e.g., Mirabel et al. (2001)), but thisnumber will rise dramatically with the astrometry measure-ments that a high precision astrometry mission will pro-vide.Currently, the cutting edge of research in BH X-ray bi-naries involves constraining BH spins, including the rateof spin and the orientation of the spin axis. Techniquesfor determining the rate of spin include measuring of therelativistic broadening of the fluorescent iron K α line in theX-ray emission and the study of the thermal continuumX-ray spectra |(Remillard & McClintock 2006; Miller et al.2007). Concerning the direction of their spin axes, thereis evidence that the standard assumption of alignment be-tween the BH spin and orbital angular momentum axes isincorrect in some, if not many, cases (Maccarone 2002;Tomsick et al. 2014; Walton et al. 2016), likely requiring awarped accretion disc. Theoretical studies show that suchmisalignments should be common (King & Nixon 2016).However, binary inclination measurements rely on mod-eling the ellipsoidal modulations seen in the optical lightcurves (Orosz et al. 2011), which is subject to systematicuncertainties, and a high precision astrometry mission willbe able to provide direct measurements of orbital inclina-tion for many of the BH X-ray binaries that show evidencefor misalignments and warped discs. In 1986 Bohdan Paczy ´nski (Paczynski 1986) proposed anew method for finding compact dark objects, via pho-tometric gravitational microlensing. This technique relieson continuous monitoring of millions of stars in order to spot its temporal brightening due to space-time curvaturecaused by a presence and motion of a dark massive ob-ject. Microlensing reveals itself also in astrometry, sincethe centre of light of both unresolved images (separated by ∼ The measure of cosmological distances has revolution-ized modern cosmology and will continue to be a ma-jor pathway to explore the physics of the early Universe.The age of the Universe ( H − ) is a key measurement in12 aint objects in motion : the new frontier of high precision astrometry Fig. 1.14:
Microlensing event, OGLE3-ULENS-PAR-02, the best candidate for a ∼ (cid:12) single black hole. Left: photometricdata from OGLE-III survey from 2001-2008. Parallax model alone can only provide mass measurement accuracy of 50-100 % .Right: simulated astrometric microlensing path for a similar event if observed with Theia. Combining superb a high precisionastrometry mission’s astrometric accuracy with long-term photometric data will yield mass measurements of black holes and otherdark compact object to 1 % even at faint magnitudes. non-standard DM scenarios. Its exact value is currentlystrongly debated, with a number of scientific papers point-ing at discrepancies in between measurements methodsat the 2-3 σ level. But the most serious tension appearsbetween CMB estimates ( H = . ± . km/s/Mpc) orfor that matter BAO results from the SDSS-III DR12 data,combined with SNIa which indicate H = . ± . km/s/Mpc (see Alam et al. 2016) and measurements based onCepheids and SNIa ( H = . ± . km/s/Mpc) with adiscrepancy at the 3-4 σ level.The tension between the methods can be due to un-known sources of systematics, to degeneracies betweencosmological parameters, or to new physics (e.g. Karwal &Kamionkowski 2016). It is therefore of crucial importanceto consider methods capable of measuring H with no orlittle sensitivity to other cosmological parameters. Uncer-tainties can be drastically reduced by measuring time de-lays (TD) in gravitationally lensed quasars (Refsdal 1964),as this technique only relies on well-known physics (GR).With enough statistics, and a good modeling the massdistribution in the lensing galaxy, TD measurements canlead to percent-level accuracy on H , independently of anyother cosmological probe (e.g. Bonvin et al. (2016); Suyuet al. (2013, 2014). In practice, TDs can be measuredby following the photometric variations in the images oflensed quasars. As the optical paths to the quasar imageshave different lengths and they intersect the lens plane atdifferent impact parameters, the wavefronts along each ofthese paths reach the observer at different times. Hencethe notion of TD.Significant improvements in lens modeling, combinedwith long-term lens monitoring, should allow measuring H at the percent level. The H0LiCOW program ( H Lenses in COSMOGRAIL’s Wellspring), which focuses on improvingthe detailed modeling of the lens galaxy and of the massalong the line of the sight to the background quasar, led to H = . ± . km/s/Mpc (that is 3.8% precision) in a flatLCMD Universe by using deep HST imaging, Keck spec-troscopy and AO imaging and wide field Subaru imaging(Suyu et al. 2017; Rusu et al. 2017; Sluse et al. 2017;Wong et al. 2017; Bonvin et al. 2016). This value is inexcellent agreement with the most recent measurementsusing the distance ladder (though in tension with the CMBmeasurements from Planck) but still lacks of precision.By performing photometric measurements with the re-quired sensitivity and no interruption, the combination ofa high precision astrometric mission and excellent mod-eling of the lens galaxy, will enable to measure H at thepercent level and remove any possible degeneracies be-tween H and other cosmological parameters. This willopen up new avenues to test the DM nature. An alterna-tive technique consists in using trigonometric parallaxes.This is the only (non-statistical and model-independent)direct measurement method and the foundation of the dis-tance scale. A high precision astrometric mission has thepotential to extend the "standard candles" - the more dis-tant pulsating variables: Cepheids, RR Lyrae, Miras andalso Stellar Twin stars - well beyond the reach of Gaia .These distance measurements can be transferred tonearby galaxies allowing us to convert observable quan-tities, such as angular size and flux, into physical qual-ities such as energy and luminosity. Importantly, thesedistances scale linearly with H , which gives the tempo-ral and spatial scale of the universe. With this improvedknowledge, we will then be able to to better understandthe structure and evolution of both our own and more dis-13 aint objects in motion : the new frontier of high precision astrometry ESA Voyage 2050 White paper
ForSclUMiDraUMa2 Ret2HVS5 HVS6HVS7 HVS8HVS9UMi dSph galaxiesHVS5 hyper-velocity starsnearest stars for exo-planetsX-ray binaries: distancesX-ray binaries: orbital proper motionssubhalo & mini-halo search
Fig. 1.15:
Sky map of the targets considered for observations with a high precision astrometric mission. tant galaxies, and the age of our universe.
The different targets considered for observations with ahigh precision astrometry mission have been located inFig. 1.15 on a sky map.
Several mission profiles have been considered in the lastfew years focused in differential astrometry, for instanceNEAT, micro-NEAT and
Theia . Additional new differen-tial astrometry mission configurations adapted with tech-nological innovations will certainly be envisioned to pur-sue accurate measurements of the extremely small mo-tions required by the science cases in this white paper.
To address the science described in this white paper, ahigh precision astrometry mission should stare towards : • dwarf galaxies (Sphs), to probe their DM inner struc-ture; • hyper-Velocity stars (HVSs), to probe the triaxialityof the halo, the existence of mini compact halo ob-jects and the time delay of quasars; • the Galactic disc, to probe DM subhalos and minicompact halo objects; • star systems in the vicinity of the Sun, to find thenearest potentially habitable terrestrial planets; • known X-ray binaries hosting neutron stars or BlackHoles. Fig. 2.16:
Expected plane-of-sky velocity errors from a high pre-cision astrometry mission’s proper motions as a function of dis-tance from Earth. These errors respectively correspond to 40and 1000 cumulative hours of exposures for exo-planets (green)and more distant objects (cyan and blue), during a 4 year inter-val for observations, including the systematic limit from calibra-tion on Gaia reference stars. The expected precision for specificobjects are highlighted. The accuracy for the 5-year Gaia mis-sion is shown in magenta.
For a targeted mission, the objects of interest must besampled throughout the lifetime of the mission. After re-pointing the telescope and while waiting for stabilization,photometric surveys, e.g. for measurements of H usinglensed quasar time delays could be performed, thus opti-mizing the mission scientific throughput. Fig. 1.15 showsa sample sky map with potential targets.As illustrated in Fig. 2.16, high precision astromet-ric missions could measure the plane-of-sky velocities of14 aint objects in motion : the new frontier of high precision astrometry LSST 10yr accuracyGaia 5yr accuracy (~0.8h, V-I=0.75, no prior)Gaia reference stars max grid accuracyTheia >2yr precision: 40hTheia >2yr precision: 1000h pa r a ll a x un c e r t a i n t y ( μ a s ) − R-band magnitude8 10 12 14 16 18 20 22 24Astrometric end-of-mission parallax
LSST 10yr accuracyGaia 5yr accuracy (~0.8h, V-I=0.75, no prior)Gaia reference stars max grid accuracyTheia 4yr precision: 40hTheia 4yr precision: 1000h p r ope r m o t i on un c e r t a i n t y ( μ a s / y r) − R-band magnitude8 10 12 14 16 18 20 22 24Astrometric end-of-mission proper motion
Fig. 2.17: Estimated RMS precision on a high precision astrometry missionrelative parallax ( left , for ecliptic latitude ◦ ) andproper motion ( right ) in the R -band. Also shown for comparison are the estimated accuracies for 10 years LSST (LSST ScienceCollaboration et al. 2009) as well as the 5-year nominal Gaia mission (de Bruijne et al. 2014) (vertical spread caused by position onthe sky, star colour, and bright-star observing conditions). Small-scale spatial correlations ( < ◦ ) between Gaia reference sourceswill limit the maximum reachable absolute parallax and proper motion calibration for a high precision astrometry mission, indicatedby the light blue band for a range of assumed spatial correlations as function of reference star magnitude.Program Used Mission Nb of objects Benchmark target EoM precisiontime (h) fraction per field R mag (and range) (at ref. mag.)Dark Matter 17 000 0.69 10 –10
20 (14–22) 10 µ as & compact objectsNearby Earths 3 500 0.14 <
20 5 (1–18) 0.15 µ as & follow-upOpen observatory 4 000 0.17 10-10 µ asOverall requirements 24 500 1.00 10 -10 µ asTab. 2.1: Summary of science cases with most stringent performance requirements set in each case. Figures are based on a 4year mission, thermal stabilisation ( + slew time) is assumed to take 30 % of the mission time. the faintest objects in the local Universe, with errors assmall as a few mm/s in the case of the hosts of Earth-mass exo-planets in the habitable zone of nearby stars, afew m/s for stars in the Milky Way disc, i.e. for kinemat-ical searches for DM sub-halos, micro-lensing searchesfor ultra-compact mini-haloes, and for the companions ofneutron stars and black holes in X-ray binaries, 200m/sfor hyper-velocity stars whose line of sight velocities aretypically > km/s, and finally 1km/s for R = stars fordwarf spheroidal galaxies.A mission concept with an expected Theia -like astro-metric precision, as shown in Fig. 2.17, surpasses whatwill be achieved by other approved space astrometric sur-veys and ground surveys, thus unlocking science casesthat are still unreachable.Table 2.1 summarizes the science cases with moststringent performance requirements. To cover the sciencequestions from this white paper, any mission concept mustbe flexible, allowing for observing modes covering a wideflux dynamical range. This requires the concepts to copewith
Deep Field Modes , aimed towards objects as dwarfgalaxies, and
Bright Star Modes , focused in the study ofplanetary systems around nearby stars.
The Payload Module (PLM) of a high precision astromet-ric mission must be simple. It is essentially composedfrom four subsystems: telescope, camera, focal plane ar-ray metrology and telescope metrology. In the case of the
Theia /M5 concept, they were designed applying heritagefrom space missions and concepts like
Gaia , HST /FGS,
SIM , NEAT /M3,
Theia /M4 and
Euclid .However achieving micro-arcsecond differential astro-metric precision requires the control of all effects that canimpact the determination of the relative positions of thepoint spread function. The typical apparent size of an un-resolved star corresponds to 0.2 arcseconds for a 0.8 mtelescope operating in visible wavelengths. The challengeis therefore to control systematics effects to the level of 1part per 200 000. The precision of relative position deter-mination in the Focal Plane Array (FPA) depends on i) thephoton noise, which can be either dominated by the targetor by the reference stars; ii) the geometrical stability of theinstrument, iii) the stability of the optical aberrations, iv)the variation of the detector quantum efficiency betweenpixels. The control of these effects impairs other missions15 aint objects in motion : the new frontier of high precision astrometry
ESA Voyage 2050 White paper
M2 + MechanismOptical benchCameraM1-M2 HexapodstructureM1 M3 (folding)Supportbipods M4
Fig. 2.18:
Overall layout of the Theia Payload Module concept. Volume is estimated in . × . × . m . that otherwise could perform micro-arcsecond differentialastrometry measurements, like HST , K epler, WFIRST or Euclid , posing fundamental limits to their astrometric ac-curacy. All these effects must be taken into account in anyhigh precision differential astrometry mission concept.To address the challenges and fulfill the requirementsfrom section 2.1, two different possible concepts can beinvestigated. A
NEAT -like mission consisting on a forma-tion flight configuration (Malbet et al. 2012) or an
Euclid -like mission, but with a single focal plane and additionalmetrology subsystems. Both concepts consist in adoptinga long focal length, diffraction limited, telescope and ad-ditional metrological control of the focal plane array. Theproposed Theia /M5 mission concept was the result of atrade-off analysis between both concepts.
The
Theia
PLM concept consists on a single Three Mir-ror Anastigmatic (TMA) telescope with a single focal plane(see Fig. 2.18) covering a . ◦ field-of-view with a mo-saic of detectors. To monitor the mosaic geometry and itsquantum efficiency, the PLM includes a focal plane metrol-ogy subsystem. While to monitor the telescope geometry,a dedicated telescope metrology subsystem is used.To reach sub-microarcsecond differential astrometry adiffraction limited telescope, with all aberrations controlled,is necessary. A trade-off analysis was performed betweendifferent optical designs, which resulted in two optical con-cepts that could fulfill all requirements. Both are based on Euclid red book: http://sci.esa.int/euclid/48983-euclid-definition-study-report-esa-sre-2011-12 . FPAM1M2 M3M40.8m PrimaryEFL: 32mCorrected FoV: 0.6 o Fig. 2.19:
On-axis Korsch TMA option. Raytracing and spotdiagrams for the entire FoV. This design was adopted as thebaseline for the Theia/M5 proposal. a Korsch Three Mirror Anastigmatic telescope; one is anon-axis solution while the second is an off-axis telescope.In both cases only three of the mirrors are powered mir-rors. While the on-axis solution adopts a single foldingmirror, the off-axis solution adopts two folding mirrors. Theon-axis design was the
Theia /M5 baseline. More recently,however, studies from NASA/JPL show that a customizedand corrected Ritchey-Chretien can reach 5 µ as over a . ◦ FoV, which even if not capable to address habitable16 aint objects in motion : the new frontier of high precision astrometry
Fig. 2.20:
Concept for the Theia/M5 Camera. Concept for theFPA detector plate at the left. Overall view of the camera con-cept on the right.
Launch date
No constraints, allowing launch date in 2029
Orbit
Large Lissajous in L2
Lifetime • • Tecnical operations: 6 months orbit transfer plus instrument commisioning and 1 month decomissioning
Concept
Single spacecraft, single telescope in the PLM, single camera in the focal plane, metrological monitoring of PLM
Communication architecture
75 Mbps, 4h/day
Tab. 2.2:
Theia’s mission main characteristics. exoplanet science cases, would provide a valuable instru-ment for Dark Matter studies.To achieve the precision by centroiding as many starsas possible, a mosaic of detectors (in principle CCD orCMOS) must be assembled on the focal plane. The detec-tors must feature small pixels ( ∼ µ m) and well controlledsystematic errors along the lifetime of the mission. De-tailed in orbit calibration of the focal plane and detector ge-ometry and response must be monitored, and in the Theia concept this is addressed via a dedicated laser metrology.In addition to measuring the FPA, the structure of thetelescope needs monitoring to control time-variable aber-rations at sub µ as level. Even at very stable environmentssuch as L2 the telescope geometry varies for different rea-sons: structural lattice reorganization (as the micro-clanksobserved in ESA/ Gaia ), outgassing and most importantly,thermo-elastic effects due to the necessary variation of theSolar Aspect Angle during the mission due to repointingto the different science targets. In the case of
Theia , thetelescope metrology subsystem to monitor perturbationsto the telescope geometry was based on a concept of aseries of simple and independent linear displacement in-terferometers installed between the telescope mirrors andorganized in a virtual hexapod configuration.
The time baseline to properly investigate the science top-ics of this white paper would be minimally 4 years, includ-ing time devoted to orbit maintenance. A total of approx- imately 6 months has been estimated for the orbit trans-fer including the spacecraft and instrument commission-ing. This estimate is made from the total of ∼ hdedicated for the scientific program, and considering thatabout 15 min per slew will be dedicated to reconfigurationand station-keeping, while thermal stabilization time is inaddition to the slew time.Some instrument key features of the Theia conceptare presented in Fig. 2.21. The concept is inspired on the
Euclid service module with a downscaled size to minimizemass and improve mechanical properties. Similarly to
Eu-clid and
Herschel satellites,
Theia ’s Korsch telescope isaccommodated on top of the service module in a verticalposition leading to a spacecraft height of about 5m. Thisconcept allows to optimize the payload size.
Observations carried out with a mission dedicated to highprecision astrometry will add significant value and will ben-efit from a number of other ground-based and space mis-sions operating in the 2030s and beyond, including ESA’s
Athena , PLATO , Euclid and
Gaia , ESO’s
MICADO and
Gravity , CTA , SKA , JWST and
LSST . For example: • JWST : Estimates suggest that
JWST will be able todetect Lyman Break galaxies with absolute magni-tudes as faint as M UV ∼ − at z ∼ , correspond-ing to halo masses of about . M (cid:12) . The com-bination of a high precision astrometry mission andthe JWST ’s observations will enable unambiguoustests of DM. • PLATO : PLATO will look at planetary transits andstar oscillations in two fields (each covering 2250deg ), for 2-3 years each, in host stars brighter than16 mag. PLATO high cadence continuous monitor-ing of its target stars will provide information on theinternal structure of the stars, allowing determina-tion of their stellar ages and masses. A high pre-cision astrometry mission will benefit from
PLATO characterization of many of the astrometry mission’score star samples. For close ‘
PLATO ’ stars wheretransits were observed this astrometry mission canmeasure additional inclined planets. • SKA:
SKA aims to use radio signals to look for build-ing blocks of life (e.g. amino acids) in Earth-sizedplanets. A high precision astrometry will identify tar-get planets from their astrometric "wobble" that canbe followed-up spectroscopically with the SKA. Fur-thermore, SKA aims to use its immensely fast skycoverage to detect transients, such as supernovaeand gamma ray bursts. With its precise astrometry,17 aint objects in motion : the new frontier of high precision astrometry
ESA Voyage 2050 White paper
THEIA Satellite key features
Structure
Hexagonal Service module built around a 1194mm central tube Korsch Telescope with a M1 diameter of 0.8m – SiC or Si3N4 ceramic truss and secondary structures
Thermal concept
Lateral Sun shield – Vertical V-groove screens Active thermal control of telescope structure Classical thermal concept for Service module with cold faces allowing efficient thermal rejection for dissipative units
AOCS µ -propulsion or possibly mini Radio-frequency Ion Thruster (mini-RIT) Data Handling
Centralized Data Management Unit Mass Memory Unit with several TBytes capacity
TT&C
Propulsion
Hydrazine propulsion system with 1N thrusters
Power
Regulated 28V power bus provided by 1 x Power Conditioning and Distribution Unit 1 x Solar Array panel with 3G30 GaAs/Ge triple Junction Azur Space cells 1 x Battery Li-Ion Sony with 18650 cells ! Fig. 2.21:
Proposed Theia satellite concept aint objects in motion : the new frontier of high precision astrometryTheia will help study the specific locations of suchevents in stellar clusters. • CTA:
The Cherenkov Telescope Array (CTA) in theNorthern and Southern Hemispheres will carry outmeasurements of the gamma-ray flux with almostcomplete sky coverage and unprecedented energyand angular resolution, in the ∼ [0.02,100] TeV en-ergy range. The sub-microarcsecond performanceof a high precision astrometry mission allow us in-vestigating the so-called J-factor which correspondsto the brightness of the gamma-ray flux in dSphsand thus determines the prime candidates for CTA’sobservations. CTA also aims at observing star form-ing systems over six orders of magnitude in forma-tion rate, to measure the fraction of interacting cos-mic rays as a function of the star-formation rate.By combining high precision astrometry and CTAmeasurements, we will better understand the rela-tive importance of cosmic rays and DM in placeswhere star-formation is important. Furthermore, asmall number of black-hole and neutron star binarysystems in our Galaxy is known to emit gamma-rays. The mechanism by which the particle accel-eration is achieved is not well-understood. The sub-microarcsecond performance of a high precision as-trometry mission allow us probing the velocity struc-ture of the nearby gamma-ray bright radio galaxiesof NGC 1275, IC 310, M 87 and Cen A, which com-bined with CTA’s observations will enable importantastrophysics breakthroughs. There have been several propositions for a space missiondedicated to high precision astrometry: a 6 meter base-line visible interferometer on a single satellite like SIM orSIM-Lite (Goullioud et al. 2008); a single mirror off-axisparabola 1 meter diameter telescope based on two space-craft, one carrying the telescope mirror and the other thefocal plane like the NEAT (Malbet et al. 2012); or a single-mirror telescope like
Theia (Malbet et al. 2016; Boehmet al. 2017). The variety of the concepts shows that thereare areas of progress on spacecraft technologies, espe-cially concerning formation flying, actively-controlledlarge structure interferometers .One interesting potential solution to be consideredis the nanosat technology and the cost reduction that islinked to it. There is a huge cost difference between cube-sats (< 10 M e ) and ESA M class mission (400 500 M e )or NASA MIDEX/Discovery mission also 300 500 M$. Thecubesat technology has matured and more than hundreds are launched every year. That technology has now creptinto micro-sats that are up to 200 kg and spacecraft busof this category are now < M e , while only a few yearsago they were ∼ M e . Because of their low cost andthe high number of flying satellites, this technology hasnow demonstrated 5 year typical lifetime, comparable toa more expensive traditional spacecraft. In any case, allthe price scales will change between now and the epochwhen Voyage 2050 will be implemented, and that includesflying heavier payloads (SpaceX is pushing the launchercartel prices down, for instance). Presently, two detector technologies are used: CCD orCMOS. CMOS detectors present a high quantum effi-ciency over a large visible spectral band that can alsoreach infrared wavelengths depending on the sensitivelayer. CMOS detectors also have programmable readoutmodes, faster readout, lower power, better radiation hard-ness, and the ability to put specialized processing withineach pixel. On the other hand there are many knowndetector systematics, even for advanced detectors as theTeledyne H4RG10. The main challenging effects are thefollowing ones: fluence-dependent PSF, correlated readnoise, inhomogeneity in electric field lines and persistenceeffects (e.g. Simms 2009). All mission proposals so farwere based on CCD technology, but detector evolution willcertainly take place on the context of any mission conceptto answer the challenges being posed by the Voyage 2050white papers.If a
Theia -like mission is selected for the 2040â ˘A ´Zs,detector technology might be different from anything wehave in place nowadays. The main requirements are smallpixels, low read-out noise on large format focal plane andmastering intrapixels effects in order to reach the highestprecision astrometry. It should be noticed that the devel-opment of European detector technology for low-RON andlarge-format IR and visible detector matrices, like the Alfadetector that ESA is undertaking with Lynred, is of highinterest for our science cases.
Traditionally systematic errors have been the major chal-lenge µ as-level astrometry from space. Astrometric accu-racy has a lot in common with photometric accuracy, andthe technology development that proceeded the Keplermission demonstrated ∼ − relative photometry. Similaradvances have been made in detector calibration for as-trometry (Crouzier et al. 2016). Photons from stars carrythe astrometric information at exquisite precision, system-atic errors are imparted when those photons strike thetelescope optics and also when they are detected by thefocal plane array. The calibration of optical field distortion19 aint objects in motion : the new frontier of high precision astrometry ESA Voyage 2050 White paperusing reference stars is a technique that is perhaps a cen-tury old and used on ground and space-based telescopes.Metrology laser feed optical fibers placed at the backof the nearest mirror to the detectors can be used to mon-itor distortions of the focal plane array, and to allow the as-sociated systematic errors to be corrected (Crouzier et al.2016). Such detector calibration at − pixel levels shouldbe continued. In addition to measuring the FPA physi-cal shape, the rest of the telescope needs monitoring tocontrol time-variable aberrations at sub µ as level. Even atvery stable space environments such as L2, the telescopegeometry is expected to vary for different reasons: struc-tural lattice reorganization (as the micro-clanks observedin ESA/ Gaia ), outgassing and most importantly, thermo-elastic effects due to the necessary variation of the SolarAspect Angle during the mission for pointings to the dif-ferent science targets. A telescope metrology subsystembased on a concept of linear displacement interferometersinstalled between the telescope mirrors, with the role tomonitor perturbations to the telescope geometry might berequired and developed. Existing space based interferom-eters from TNO, as the ESA/
Gaia
Basic Angle Monitor arealready capable of reaching more precise measurementsthan those required by
Theia /M5 – BAM can perform ∼ . pm optical path difference measurements (Gielesen et al.2013). A Thales telemeter developed for CNES can reach ∼ pm, and the Thales interferometer produced for theMTG (Meteosat Third Generation) satellite can reach 1 nmper measurement (Scheidel 2011) – higher precisions canbe reached by averaging over many measurements.For telescopes that do not have high level stability lev-els, there are some alternatives. One is the diffractivepupil concept that puts a precision array of dots on theprimary, which produces a regular pattern of dots in thefocal plane. One way to use the diffractive pupil is to lookat a very bright star (0 mag) and record the diffraction pat-tern interspersed with observations of a much dimer tar-get star ( ∼ mag). The diffractive pupil can also be usedduring science observations. But when the targetstar is ∼ mag photon noise of the diffracted light can be signifi-cantly higher than the photon noise of the reference stars( ∼ − mag). Conclusion
To solve fundamental questions like– “
What is the nature of dark matter? ”– “
Are there habitable exo-Earths nearby? ”– “
What is the equation of state of matter in extremeenvironments ?”– “
Can we put direct constraints on cosmologicalmodels and dark energy parameters? ”many branches of astronomy need to monitor the mo- tion of faint objects with significantly higher precision thanwhat is accessible today.
Through ultra-precise micro-arcsecond relative astrometry, a high precision as-trometry space mission will address the large num-ber of prime open questions that have been detailedin this white paper .The scientific requirements points toward a space mis-sion that is relatively simple: a single telescope, withmetrology subsystems and a camera. Such a mission canfit as a M-class mission, or even at lower level dependingon the final accuracy which is aimed at.Some technological challenges must be tackled andadvanced: the spacecraft, the focal plane detector and themetrology. We believe that these challenges can be mas-tered well before 2050 and that they will open the com-pelling scientific window of the faint objects in motion.20 aint objects in motion : the new frontier of high precision astrometry
References
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016a, Phys. Rev. Lett., 116, 061102Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016b, ApJ, 833, L1Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017, ApJ, 851, L16Alam, S., Ho, S., & Silvestri, A. 2016, MNRAS, 456, 3743Antoniadis, J., Freire, P. C. C., Wex, N., et al. 2013, Science, 340, 448Armstrong, D. J., Osborn, H. P., Brown, D. J. A., et al. 2014, MNRAS, 444, 1873Aslanyan, G., Price, L. C., Adams, J., et al. 2016, Phys. Rev. Lett. in pressBanik, I. & Kroupa, P. 2019, MNRAS, 487, 1653Binney, J. & Mamon, G. A. 1982, MNRAS, 200, 361Bird, S., Peiris, H. V., Viel, M., & Verde, L. 2011, MNRAS, 413, 1717Boehm, C., Krone-Martins, A., Amorim, A., et al. 2017, arXiv e-prints, arXiv:1707.01348Boehm, C., Schewtschenko, J. A., Wilkinson, R. J., Baugh, C. M., & Pascoli, S. 2014, MNRAS, 445, L31Bonaca, A., Conroy, C., Price-Whelan, A. M., & Hogg, D. W. 2019, arXiv e-prints, arXiv:1906.02748Bonvin, V., Courbin, F., Suyu, S. H., et al. 2016, MNRASBrasser, R., Ida, S., & Kokubo, E. 2013, MNRAS, 428, 1673Bringmann, T., Scott, P., & Akrami, Y. 2012, Phys. Rev. D, 85, 125027Brown, W. R. 2015, ARA&A, 53, 15Brown, W. R., Anderson, J., Gnedin, O. Y., et al. 2015, ApJ, 804, 49Brown, W. R., Geller, M. J., Kenyon, S. J., & Kurtz, M. J. 2005, ApJ, 622, L33Brown, W. R., Lattanzi, M. G., Kenyon, S. J., & Geller, M. J. 2018, ApJ, 866, 39Bryan, S. E., Mao, S., Kay, S. T., et al. 2012, MNRAS, 422, 1863Carlberg, R. G. 2018, arXiv e-prints, arXiv:1811.10084Casertano, S., Lattanzi, M. G., Sozzetti, A., et al. 2008, A&A, 482, 699Chluba, J., Erickcek, A. L., & Ben-Dayan, I. 2012, ApJ, 758, 76Crouzier, A., Malbet, F., Hénault, F., et al. 2016, in Proc. SPIE, Vol. 9904, 99045Gde Bruijne, J. H. J., Rygl, K. L. J., & Antoja, T. 2014, in EAS Publications Series, Vol. 67, 23–29Demorest, P. B., Pennucci, T., Ransom, S. M., Roberts, M. S. E., & Hessels, J. W. T. 2010, Nature, 467, 1081Dubinski, J. 1994, ApJ, 431, 617Edelmann, H., Napiwotzki, R., Heber, U., Christlieb, N., & Reimers, D. 2005, ApJ, 634, L181Erkal, D., Li, T. S., Koposov, S. E., et al. 2018, MNRAS, 481, 3148Feldmann, R. & Spolyar, D. 2015, MNRAS, 446, 1000Fonseca, E., Pennucci, T. T., Ellis, J. A., et al. 2016, ApJ, 832, 167Fortin, M., Providência, C., Raduta, A. R., et al. 2016, Phys. Rev. C, 94, 035804Gielesen, W., de Bruijn, D., van den Dool, T., et al. 2013, Proc. SPIE, 8863, 88630GGillon, M., Jehin, E., Lederer, S. M., et al. 2016, Nature, 533, 221Gnedin, O. Y., Gould, A., Miralda-Escudé, J., & Zentner, A. R. 2005, ApJ, 634, 344Goullioud, R., Catanzarite, J. H., Dekens, F. G., Shao, M., & Marr, J. C., I. 2008, in Proc. SPIE, Vol. 7013, 70134THaghighipour, N. 2004, in AIP Conf. Ser., Vol. 713, The Search for Other Worlds, ed. S. S. Holt & D. Deming, 269–272Haensel, P., Bejger, M., Fortin, M., & Zdunik, L. 2016, European Physical Journal A, 52, 59Hattori, K., Valluri, M., Castro, N., et al. 2019, ApJ, 873, 116He, M. Y., Triaud, A. H. M. J., & Gillon, M. 2017, MNRAS, 464, 2687Heinke, C. O., Cohn, H. N., Lugger, P. M., et al. 2014, MNRAS, 444, 443Herzog-Arbeitman, J., Lisanti, M., Madau, P., & Necib, L. 2018a, Phys. Rev. Lett., 120, 041102Herzog-Arbeitman, J., Lisanti, M., & Necib, L. 2018b, J. Cosmol. Astropart. Phys., 2018, 052Hills, J. G. 1988, Nature, 331, 687Hirsch, H. A., Heber, U., O’Toole, S. J., & Bresolin, F. 2005, A&A, 444, L61Hlozek, R., Dunkley, J., Addison, G., et al. 2012, ApJ, 749, 90Howard, A. W. 2013, Science, 340, 572Irrgang, A., Geier, S., Heber, U., Kupfer, T., & Fürst, F. 2019, arXiv e-prints, arXiv:1907.06375Irrgang, A., Kreuzer, S., & Heber, U. 2018, A&A, 620, A48Karwal, T. & Kamionkowski, M. 2016, Phys. Rev. D, 94, 103523Kazantzidis, S., Kravtsov, A. V., Zentner, A. R., et al. 2004, ApJ, 611, L73King, A. & Nixon, C. 2016, MNRAS, 462, 464 21 aint objects in motion : the new frontier of high precision astrometry
ESA Voyage 2050 White paperKoposov, S. E., Boubert, D., Li, T. S., et al. 2019, arXiv e-prints, arXiv:1907.11725Lagrange, A.-M., Meunier, N., Desort, M., & Malbet, F. 2011, A&A, 528, L9Laskar, J. & Robutel, P. 1993, Nature, 361, 608Lattimer, J. M. & Prakash, M. 2005, Phys. Rev. Lett., 94, 111101Lattimer, J. M. & Prakash, M. 2016, Phys. Rep., 621, 127Leger, A. M. 2015, in Pathways Towards Habitable Planets, 99Li, F., Erickcek, A. L., & Law, N. M. 2012, Phys. Rev. D, 86, 043519Lovis, C., Dumusque, X., Santos, N. C., et al. 2011, ArXiv e-prints, arXiv:1107.5325LSST Science Collaboration, Abell, P. A., Allison, J., et al. 2009, ArXiv e-prints, arXiv:0912.0201Maccarone, T. J. 2002, MNRAS, 336, 1371Madhusudhan, N. & Burrows, A. 2012, ApJ, 747, 25Malbet, F., Léger, A., Anglada Escudé, G., et al. 2016, in Proc. SPIE, Vol. 9904, 99042FMalbet, F., Léger, A., Shao, M., et al. 2012, Experimental Astronomy, 34, 385Malhan, K., Ibata, R. A., Carlberg, R. G., Valluri, M., & Freese, K. 2019, arXiv e-prints, arXiv:1903.08141Mayor, M., Marmier, M., Lovis, C., et al. 2011, ArXiv e-prints, arXiv:1109.2497Mayor, M. & Queloz, D. 1995, Nature, 378, 355Merritt, D. & Poon, M. Y. 2004, ApJ, 606, 788Meru, F., Galvagni, M., & Olczak, C. 2013, ApJ, 774, L4Miller, L., Kitching, T. D., Heymans, C., Heavens, A. F., & van Waerbeke, L. 2007, MNRAS, 382, 315Miller, M. C. & Lamb, F. K. 2016, European Physical Journal A, 52, 63Mirabel, I. F., Dhawan, V., Mignani, R. P., Rodrigues, I., & Guglielmetti, F. 2001, Nature, 413, 139Necib, L., Lisanti, M., & Belokurov, V. 2019, ApJ, 874, 3Oñorbe, J., Boylan-Kolchin, M., Bullock, J. S., et al. 2015, MNRAS, 454, 2092Orosz, J. A., McClintock, J. E., Aufdenberg, J. P., et al. 2011, ApJ, 742, 84Özel, F. & Freire, P. 2016, ARA&A, 54, 401Özel, F. & Psaltis, D. 2009, Phys. Rev. D, 80, 103003Paczynski, B. 1986, ApJ, 304, 1Payne, M. J. & Lodato, G. 2007, MNRAS, 381, 1597Perryman, M., Hartman, J., Bakos, G. Á., & Lindegren, L. 2014, ApJ, 797, 14Price-Whelan, A. M. & Bonaca, A. 2018, ApJ, 863, L20Read, J. I., Agertz, O., & Collins, M. L. M. 2016, MNRAS, 459, 2573Refsdal, S. 1964, MNRAS, 128, 307Remillard, R. A. & McClintock, J. E. 2006, ARA&A, 44, 49Ricci, L., Testi, L., Natta, A., Scholz, A., & de Gregorio-Monsalvo, I. 2012, ApJ, 761, L20Ricci, L., Testi, L., Natta, A., et al. 2014, ApJ, 791, 20Ricotti, M. & Gould, A. 2009, ApJ, 707, 979Rusu, C. E., Fassnacht, C. D., Sluse, D., et al. 2017, MNRAS, 467, 4220Sahlmann, J., Triaud, A. H. M. J., & Martin, D. V. 2015, MNRAS, 447, 287Satyal, S. & Musielak, Z. E. 2016, Astronomische Nachrichten, 337, 300Schaeffer, R. & Silk, J. 1984, Astrophys. J., 281, L13Scheidel, D. 2011, Journées de Telemetrie Laser OCA 2011Scholz, A., Jayawardhana, R., Wood, K., et al. 2008, ApJ, 681, L29Schwieterman, E. W., Meadows, V. S., Domagal-Goldman, S. D., et al. 2016, ApJ, 819, L13Shen, K. J., Boubert, D., Gänsicke, B. T., et al. 2018, ApJ, 865, 15Simms, L. 2009, PhD Dissertation, Stanford UniversitySluse, D., Sonnenfeld, A., Rumbaugh, N., et al. 2017, MNRAS, 470, 4838Snellen, I., de Kok, R., Birkby, J. L., et al. 2015, A&A, 576, A59Sozzetti, A. 2014, Mem. Soc. Astron. Italiana, 85, 643Sozzetti, A., Giacobbe, P., Lattanzi, M. G., et al. 2014, MNRAS, 437, 497Spergel, D. N. & Steinhardt, P. J. 2000, Physical Review Letters, 84, 3760Suyu, S. H., Auger, M. W., Hilbert, S., et al. 2013, ApJ, 766, 70Suyu, S. H., Bonvin, V., Courbin, F., et al. 2017, MNRAS, 468, 2590Suyu, S. H., Treu, T., Hilbert, S., et al. 2014, ApJL, 788, L35Thebault, P. 2011, Celestial Mechanics and Dynamical Astronomy, 111, 29The LIGO Scientific Collaboration, the Virgo Collaboration, Abbott, B. P., et al. 2018, arXiv e-prints, arXiv:1811.1290722 aint objects in motion : the new frontier of high precision astrometry
Thebault, P. & Haghighipour, N. 2015, Planet Formation in Binaries (Springer Geophysics), 309–340Tomsick, J. A. & Muterspaugh, M. W. 2010, ApJ, 719, 958Tomsick, J. A., Nowak, M. A., Parker, M., et al. 2014, ApJ, 780, 78Udalski, A., Jung, Y. K., Han, C., et al. 2015, ApJ, 812, 47Valluri, M., Debattista, V. P., Quinn, T., & Moore, B. 2010, MNRAS (V10), 403, 525Valluri, M., Debattista, V. P., Quinn, T. R., Roškar, R., & Wadsley, J. 2012, MNRAS, 419, 1951Valluri, M., Debattista, V. P., Stinson, G. S., et al. 2013a, ApJ, 767, 93Valluri, M., Debattista, V. P., Stinson, G. S., et al. 2013b, ApJ, 767, 93Walker, M. G., Mateo, M., Olszewski, E. W., et al. 2009, ApJ, 704, 1274Walton, D. J., Tomsick, J. A., Madsen, K. K., et al. 2016, ApJ, 826, 87Widrow, L. M., Gardner, S., Yanny, B., Dodelson, S., & Chen, H.-Y. 2012, ApJ, 750, L41Winn, J. N. & Fabrycky, D. C. 2015, ARA&A, 53, 409Wolf, J., Martinez, G. D., Bullock, J. S., et al. 2010, MNRAS, 406, 1220Wong, K. C., Suyu, S. H., Auger, M. W., et al. 2017, MNRAS, 465, 4895Yu, Q. & Madau, P. 2007, MNRAS, 379, 1293Yu, Q. & Tremaine, S. 2003, ApJ, 599, 1129Zemp, M., Gnedin, O. Y., Gnedin, N. Y., & Kravtsov, A. V. 2012, ApJ, 748, 54 23 aint objects in motion : the new frontier of high precision astrometry
ESA Voyage 2050 White paper
Core proposing team
Sorted by alphabetical order:Name Affiliation CountryU. Abbas INAF/Obs. Torino ITJ. Alves U. Vienna ATC. Boehm U. Sydney AUW. Brown CFA Harvard USL. Chemin U. Antofogasta CLA. Correia U. Coimbra PTF. Courbin EPFL & Ob. Geneva CHJ. Darling U. Colorado USA. Diaferio U. Torino/INFN ITM. Fortin Copernicus Astronomical Center PLM. Fridlund Leiden Obs., NL& Chalmers Univ. SEO. Gnedin U. Michigan USB. Holl U. Geneva CHA. Krone-Martins CENTRA/U. Lisboa PTA. Léger IAS/U. Paris Sud FRL. Labadie U. Cologne DEJ. Laskar IMCCE/Obs. Paris FRF. Malbet IPAG/U. Grenoble Alpes FRG. Mamon IAP FRB. McArthur U. Texas USD. Michalik ∗ ESA/ESTEC NLA. Moitinho CENTRA/U. Lisboa PTM. Oertel LUTH/Obs. Paris/CNRS FRL. Ostorero U. Torino/INFN ITJ. Schneider Obs. Paris FRP. Scott Imperial College London, UK& U. Queensland, AUM. Shao JPL/NASA USA. Sozzetti Obs. Torino/INAF ITJ. Tomsick SSL Berkeley USM. Valluri U. Michigan USR. Wyse Johns Hopkins U. US ∗ ESA Research Fellow
Acknowledgments to the contributors to the