Science cases for GRAVITY
In the following sections the science cases for GRAVITY are briefly outlined, beginning with the broad range of science opportunities that have opened up at the Galactic Centre of the Milky Way.
The Galactic Centre is by far the closest galactic nucleus and the best studied SMBH. There are still a number of fundamental open issues and just to name a few that we want to answer with GRAVITY: What is the nature of the flares in Sgr A*? What is the spin of a BH? How can we resolve the “Paradox of Youth” of the stars in its vicinity? Even tests of fundamental physics may come into reach with GRAVITY: Does the theory of general relativity hold in the strong field around SMBHs? Do BHs really have “no hair”?
Uncovering the true nature of the Sgr A* flares
The Galactic Centre BH is surprisingly faint — its average luminosity is only about 10E–8 of the Eddington luminosity, emitted predominantly at radio to submm wavelengths. On top of this quasisteady component there is variable emission in the X-ray and IR bands. Some of this variable emission comes as flares, typically a few times per day, lasting for about one to two hours, and reaching the brightness of massive main-sequence stars. The three most plausible explanations for the origin of these flares are: a jet with clumps of ejected material; hot spots orbiting a BH; or statistical fluctuations in the accretion flow (Figure 1). The jet model seems natural from the presence of jets in active galactic nuclei. The orbiting hot-spot model would be a natural explanation for the observed quasi-periodicity in the light curves of flares and associated changes of the IR polarisation. However, the long-term light curves are well described by a pure, red power-law noise, indicating that statistical fluctuations in the accretion flow are responsible for the observed variability. Time-resolved astrometric measurements with GRAVITY will settle the debate. Even without pushing GRAVITY to its ultimate performance, the observed distribution of flare positions and its periodic variation will distinguish between these models.
Measuring spin and inclination of the Galactic Center black hole
The mass of the Galactic Center black hole is well known from stellar orbits. If the currently favored orbiting hot-spot model is correct, GRAVITY will take the next step and measure its spin and inclination. These measurements are more difficult because the astrometric signature from the spin is a factor few smaller than the orbital motion and lensing effects. However, the combined signal from the periodic light-curves and astrometry is much stronger. Already the simple correlation between the observed position variation and flux variability gives first insights into the source geometry. The next step is a simultaneous fit to the observed motion and light-curve to quantify the underlying model parameters (Figure 1). Finally, the periodic flux can be used to trace the orbital phase to coherently co-add measurements from multiple flares, such that higher order signatures can be directly identified.
Resolving the Paradox of Youth of the Galactic Center stars
Most stars in the central light month are young, massive early type main sequence stars. It is currently not understood how these stars have formed or moved so close to the supermassive black hole, because the tidal forces should have prevented in-situ formation, and because these stars are too young to have migrated so far within the time scale of classical relaxation. Precise orbit measurements with GRAVITY offer a route to solve this Paradox-of-Youth. In particular the orbital eccentricities can distinguish between the various scenarios. The currently favored Hills scenario, in which massive binaries are scattered down to the central black hole and one component is ejected in a three-body interaction with the supermassive black hole, will lead to predominantly high eccentricities. In contrast, the competing migration scenario, in which the stars migrate from circum-nuclear stellar disks, will result mainly in initially low eccentricities. First results from adaptive optics observations slightly favor the Hills scenario, but the significance is still marginal. GRAVITY will significantly enlarge the number of stars with known eccentricities, and will tell apart the formation scenarios without doubt (Figure 2).
Testing general relativity in the strong field regime
The unprecedented astrometric accuracy of GRAVITY may even open up the possibility to test General Relativity in the so far unexplored strong field around supermassive black holes. The observed orbit of a hot spot on the last stable orbit will be dominated by strong gravitational effects like gravitational lensing and red-shift (Figure 1). GRAVITY observations will thus directly probe space-time in the immediate vicinity of the black hole’s event horizon. And even stellar orbits will be notably affected by higher-order general relativistic effects, for example the relativistic peri-astron shift and the Lense-Thirring precession of the orbital angular momentum around the black hole spin axis (Figure 3). These effects will be strongest for stars within the central light week, which will be observed with GRAVITY in its interferometric imaging mode. In the most optimistic case, GRAVITY may even be able to test the so-called “no-hair” theorem, which says that a black hole is fully characterized by its mass and spin. In particular the black hole spin and its quadrupole moment should be strictly related. Because spin and quadrupole moment couple differently to the inclination of stellar orbits, they can be measured independently. Therefore GRAVITY has been proposed to test the no-hair theorem for black holes.
Active galactic nuclei
The standard unified model for active galactic nuclei postulates that accreting supermassive black holes are surrounded by an obscuring torus, whose orientation determines if the central engine is hidden from the observer’s view or not. The direct proof that this absorber is really a torus rather than another structure is still pending. Indeed most resolved gaseous structures on the putative scale of the torus appear more disk-like, for example the maser disk, the radio continuum emission and the mid-infrared emission of the prototypical active galactic nuclei NGC 1068 (see Figure 4). Observing simultaneously six baselines, GRAVITY will image with unprecedented quality the inner edge of the torus where the dust is close to the sublimation limit. GRAVITY will thus put strong constraints on the absorber models. These models are very much inspired by the observations of NGC 1068, but the few active galactic nuclei with interferometric observations show a puzzling variance. It is up to GRAVITY to significantly extend the sample to finally draw statistically sound conclusions.
Even the broad line region may come into reach for GRAVITY. It is seen in those active galactic nuclei for which we have a direct view onto the supermassive black hole. The size of the broad line region can currently only be measured indirectly, looking at the time delay between the variations of the ultraviolet continuum and the emission lines. Broad line regions of nearby active galactic nuclei are typically smaller than 0.1 milliarcsecond, and thus too small to be resolved in GRAVITY’s images. But the astrometric accuracy of GRAVITY will allow measuring the velocity gradient across it. This will strongly constrain the broad line region geometry and determine dynamically the central Black Hole mass.
Intermediate mass black holes
The tight correlation between the bulge mass of a galaxy and the mass of the central super-massive black hole suggests that the rapid formation of a spheroidal stellar system also collects up to about 1% of the initial mass in a central black hole. Such a core collapse and collisional build-up may have also led to the formation of intermediate mass black holes in massive, dense star clusters. Recent searches in globular clusters show evidence for such intermediate mass black holes (Figure 5). However, the sphere of influence of the postulated black holes is typically less than a few arcseconds, such that only a few stars are available for these statistical studies. GRAVITY will dramatically change this situation in a few suitable cases for which accelerations will be detected, thus directly probing the gravitational potential without suffering from the small number statistics of velocity dispersion measurements.
X-ray binaries are the best place to study neutron stars and stellar black holes. These ultra-compact objects are just too faint when living in isolation. But some X-ray binaries are bright enough for GRAVITY observations. We expect that the orbital displacement from the compact companion will be detectable in the interferometric closure phase. Even the absolute astrometric displacement of the binary-system’s photo-center will be observable with GRAVITY in few nearby systems, for which a suitable astrometric reference star is available. Combined with spectroscopy, these observations will provide the orbital elements and distance of the system, as well as the mass of the two components. On top of that GRAVITY will characterize the wind from the stellar companion at a scale of a few stellar radii. The physical properties of this wind are particularly interesting as it is the main source for feeding the compact object.
Masses of the most massive stars and brown dwarfs
There still exists a discrepancy by up to a factor two in the mass estimates for the most massive main-sequence stars. We don’t even know the maximum mass a star can have. Comparison of spectra with atmospheric models yields upper mass limits of typically 60 solar masses, whereas evolutionary tracks and observed luminosities suggest a mass of up to 120 solar masses. Clearly, dynamical mass estimates are required. Quite a number of spectroscopic binary O-stars are known in the cores of starburst clusters like Arches, 30Dor, and the Galactic Center. GRAVITY will resolve some of the longer-period spectroscopic binaries, and will monitor the astrometric motions of the photo-centers for the short-period, close binaries. In this way, GRAVITY will directly yield dynamical mass estimates for many of these systems, and finally provide the most crucial input to calibrate the stellar evolutionary tracks.
The situation is similar for brown dwarfs, the lowest mass stars. Most current mass estimates are based on evolutionary models and model atmospheres, which have not yet been accurately calibrated to observations. Dynamical masses for brown dwarfs have only been derived for few objects. In general, the observed masses for sub-stellar objects with ages larger than a few 100 million years seem to be in good agreement with theoretical models. But there are significant uncertainties and discrepancies for the very young, very low mass objects like AB Dor C. If indeed these objects are more massive than indicated by stellar evolutionary models, many putative planets would be rather in the brown dwarf and not in the planetary mass regime. GRAVITY will probe many more multiple systems like AB Dor C, deriving the individual component masses, and even probe the sub-stellar companions themselves for binarity, thus clarifying the situation.
Jet formation in young stars
Jets are omnipresent in the universe, from gamma-ray bursts to active galactic nuclei, from young stars to micro-quasars. Understanding the formation of jets is still one of the big open challenges of modern astrophysics. It is now known that jets are powered by magneto-hydrodynamic engines, tapping on the energy of the accretion disk. Young stars are the ideal objects to study these processes at highest resolution. Matter from the disk surface couples to the open, highly inclined star-disk magnetic field lines, and gets accelerated up to the Alfvén surface. The rotating magnetic field lines then become more and more twisted, wind up and collimate the jet. But surprisingly, some stellar jets are found only on one side of the disk. Clearly, some basic ingredient is missing in our understanding of jet formation. The relevant processes take place within about one astronomical unit from the star, which at the typical distance to the nearest star forming regions of about 150 parsec translates into an angular size of about 6 milliarcsecond, slightly larger than GRAVTIY’s four milliarcsecond angular resolution. By repeatedly imaging the time-dependent ejection just outside the engine at high spectral resolution, GRAVITY will provide key observational tests of time-dependent jet simulations (Figure 6). Even more, the astrometric signal across the emission line will directly probe the central engine on sub-milliarcsecond scale, i.e. far within one astronomical unit.
Planet formation in circumstellar discs
Circum-stellar disks are the cradles of planet formation. Planets are thought to form rapidly in a few million years through the fast evolution of the disk structure. Dust processing, settling, coalescence are accompanied by an increase of particle size, leading to the formation of planetesimals that eventually aggregate to form planetary systems. The planet formation process is expected to leave strong imprints in the disk structure such as inner disk clearing, gap opening, and tidally induced spiral structures. GRAVITY will hunt down all these signs. Its unique sensitivity in the near-infrared will allow increasing the sample of observed young stars towards the poorly explored solar mass regime. This will be done at sub-astronomical unit resolutions for the closest star forming regions. It will reveal the disk structure evolution, the so-called transitional step, search for disk disruptions signatures, and will be used to probe the presence of hot, young sub-stellar and planetary companions.
Astrometric planet detection
Several hundred of exoplanets have been detected to date, mostly from radial-velocity measurements and photometric transit observations. However, these methods are biased towards detecting massive planets on close orbits. Moreover, radial velocity measurements alone cannot provide the inclination of the orbit, and can thus only give a lower limit for the mass of the planet. In contrast, the reflex motion of a star observed in astrometry allows retrieving the orbital solution and thus unambiguously measuring the mass of the planet. The astrometric planet detection is also a scientific goal of the PRIMA facility currently commissioned at the VLTI. While the planet search with PRIMA is targeting mostly isolated stars or wide binaries, GRAVITY will focus on detecting brown dwarfs and exoplanets in close binary systems. For Sun-like stars, GRAVITY’s survey volume would extend out to more than 200 parsec. Even the much fainter M-stars, which have just 20% of the mass of the Sun, can be observed out to about 25 parsec (Figure 7). GRAVITY has the potential to detect exoplanets as small as three earth masses around an M5V star at a distance of five parsec or less than two Neptune masses around an M3V star at a distance 25 parsec.
The transit of a planet in front of its host star causes an apparent motion of the photo-center of the star and it introduces a slight asymmetry in the image of the star. The first effect can be measured using GRAVITY’s astrometric observing mode, the second effect is seen in the closure phases of the interferograms. GRAVITY observations of such transits have the potential to measure the radius of the planet and its parent star. For a star like HD 189733, a 0.8 solar mass star at a distance of about 20 parsec, and its Jupiter size planet on a very close, two-day orbit, the apparent motion is about ten microarcecond. This is at the limit of GRAVITY’s capability, but transiting planets around later-type dwarfs would be easier to detect. This type of measurement will also give the position angle of the orbit on the sky, which combined with the direction and amount of polarization of the light reflected by the planet might ultimately even place constraints on the distribution of surface features like clouds and weather zones.