Stefan Gillessen

Detections of protoplanets are rare and protoplanetary disk features mischaracterized as planets are common. PDS 70 is one of only two stars known to host multiple confirmed protoplanets, PDS 70 b and c, and repeat detections of a third point-like source in the system suggest the presence of third inner planet. However, previous observations of this third source are insufficient to distinguish whether it is a planet or a concentrated dust clump in Keplerian motion. Our observations with VLTI/GRAVITY+ did not re-detect this point-like source, suggesting that it is, in fact, a dust clump and not a planet. These observations demonstrate how the angular resolving power of VLTI/GRAVITY+ can be used to distinguish between protoplanets and protoplanetary disk features.

Context. High-precision astrometric observations at the microarcsecond level require a consistent relativistic framework for modelling both the dynamics of test particles and the propagation of light in gravitational fields. Aims. We aim to formulate the post-Newtonian (PN) expansion in the test body limit in a manner that yields simple and broadly applicable expressions for the motion of massive and massless particles in generic, spherically symmetric spacetimes. In particular, we intend to formulate it such that the formalism suits the modelling of observations of stars orbiting the Galactic centre (GC). Methods. We employed the PN expansion scheme to derive equations of motion for test particles and applied a perturbative solution method to solve the boundary value problem of identifying the geodesic connecting a given emitter–observer pair. Results are validated against numerical integration of the photon geodesic equation. Results. We derive the PN equations of motion for test particles in a generic, spherically symmetric spacetime. We derive analytical expressions for the apparent position of stars as measured by an observer on Earth and show the importance of this formalism for the GC science case. We compare our results with other approaches taken in the literature and discuss its implications for the modelling of orbital motion around Sgr A*. Conclusions. The proposed framework is well suited for the analysis of astrometric observations of stars orbiting the GC with microarcsecond accuracy. In this context, measurements performed with the GRAVITY+ instrument at the Very Large Telescope Interferometer can be used to place constraints on metric theories of gravity through the combined modelling of stellar dynamics and relativistic light propagation near Sgr A*.

Residing in the centre of the Milky Way, Sagittarius A* (Sgr A*) is the closest massive black hole ¹ (MBH). Its vicinity has allowed measuring individual stellar orbits around it ^2–4 . The stars act as test particles and probe the gravitational potential around the 4.3 × 10 ⁶ M _⊙ MBH. These observations have determined the central mass to sub-per-cent precision ⁵ , and the mildly relativistic motions of stars have given access to the dominant relativistic corrections, the gravitational redshift ^6,7 , the transverse Doppler effect and the prograde precession imposed by the Schwarzschild metric nature of the potential ⁸ . These effects are of order β ² = ( v / c ) ² (for velocity v and speed of light c ). The Kerr metric for a rotating black hole leads to corrections of order β ³ . Here, we report the discovery of a faint main-sequence star ( m _K = 19.3), S301, on an 8.7-year orbit and with small enough a pericentre distance, such that the peak velocity of the star reaches 25,000 km s ⁻¹ . Within the measurement abilities of current near-infrared interferometry and future spectroscopy on an extremely large telescope, the motion of S301 is directly sensitive to the spin of Sgr A*. The high eccentricity of S301 suggests that it is the captured component of a binary that was torn apart by the Hills mechanism.

We report the direct imaging discovery of a third exoplanet in the β Pictoris ( β Pic) system. We detect β Pictoris d ( β Pic d) in noncoronagraphic observations obtained with the Very Large Telescope (VLT) Enhanced Resolution Imager and Spectrograph (ERIS), as well as multi-epoch archival datasets from the JWST Near Infrared Camera (NIRCam) and VLT/SPHERE. Astrometric measurements over an 11 yr baseline demonstrate that it is consistent with a gravitationally bound source with orbital motion. Joint multi-planet orbit fits of all three planets in the system yield a semimajor axis of 26 . 0 − 6.1 + 2.2 au and inclination 89 . 0 − 0.6 + 0.7 deg for planet d. β Pic d has a larger orbital semimajor axis than the other known planets in the system, but is coplanar with the inner two planets, and its orbit is consistent with sculpting the inner edge of the debris disk. β Pic d has a contrast of Δ L ′ = 12.11 ± 0.15 mag, with colors and luminosity that closely match those of 51 Eri b, another exoplanet in the β Pic moving group. Its VLT/ERIS and JWST/NIRCam colors are distinct from those of free-floating planetary-mass objects of a similar age and temperature. Its red F410M − F444W color indicates strong CO ₂ absorption in its atmosphere and suggests significant enhancement in metals compared to free-floating objects. From the ATMO hot-start evolutionary models, we estimate an effective temperature of 60 0 − 60 + 45 K and mass of 2.4 ± 0.6 M _Jup , which also closely matches similar estimates for 51 Eri b. β Pic d is among the lowest-mass exoplanets imaged from the ground. This discovery highlights the deep sensitivity achievable with ground-based imaging in the mid-infrared and the discovery potential of future high-contrast observations with the Extremely Large Telescope.

Context . Constraining the link between the atmospheric composition of giant exoplanets and their formation history is a key goal of exoplanet studies. In particular, the atmospheric carbon-to-oxygen (C/O) ratio and metallicity – which are readily measurable with direct spectroscopic observations – are believed to be chemical tracers of the birth location of substellar and planetary companions. Aims . We aim to collect observational constraints for planet formation theories by performing a large and systematic survey of the atmospheric C/O ratio and metallicity. Methods . We collected new K -band moderate-resolution ( R ~ 11 000) spectroscopic observations of 13 directly imaged planetary-mass companions spanning the molecular snowlines of CO ₂ , CO, and CH ₄ and with masses 4–30 M _J with VLT/ERIS/SPIFFIER. Additionally, we gathered a large portion of the available archival observations for these targets, amounting to 40 spectra and 140 photometric fluxes. We performed spectral fits using self-consistent grid models and freely parametrisable models. We obtained robust estimates of the key atmospheric parameters by aggregating the results from the different models, thereby reducing modelling systematics. Results . Using molecular mapping, we detected H ₂ O and CO in 12 of our targets as well as ¹³ CO in HR 2562 B. With our multimodel spectral fitting strategy, we obtained stellar to superstellar C/O ratios – ranging between 0.3 and 0.8 – and predominantly superstellar metallicities – between −1 and 1 dex – across all targets. We measured a substantial enrichment of ¹³ CO for HR 2562 B with ¹² CO/ ¹³ CO= 12.0 _−3.3 ^+4.5 . If corroborated by independent observations, it could indicate that the companion might have formed beyond the CO snowline and later migrated inwards to its current location. We find an anti-correlation ( R = −0.64) between the C/O ratio and the companion mass, consolidating a previous result. Conclusions . Our work demonstrates the scientific potential of the ERIS/SPIFFIER instrument for the orbital and atmospheric characterisation of close-in substellar and exoplanet companions.

Context . Protoplanetary disks around young Sun-like stars are the cradles of the vast majority of detected exoplanets. Probing these disks at multiple spatial scales is key to uncovering how planets form. The inner astronomical unit, the star-disk interaction region, is of utmost importance because most detected exoplanets occupy this zone. Aims . We aim to spatially and spectrally resolve the inner disk and star-disk interaction region of the M0.3 T Tauri star DO Tau by combining two complementary techniques. Methods . We used high-resolution near-infrared spectra from CFHT/SPIRou to constrain the magnetospheric star-disk interaction process and optical long-baseline interferometry with ESO VLTI/GRAVITY to determine the sizes of the K-band continuum and Br γ line emitting regions. From the SPIRou spectra, we measured the veiling in the YJHK bands along with the equivalent widths of the HeI λ 1083, Paβ, and Br γ emission lines, from which we estimated the mass accretion rate. We were able to monitor the time variability of these quantities thanks to our long-sequence of observations over about 40 days. We fit the GRAVITY visibilities in the continuum and the differential quantities in the line with geometrical models to obtain the orientation and the size of the inner disk as well as the size and the on-sky displacement of the Br γ emitting region. Results . We derived a mass accretion rate of ∼10 ⁻⁸ −10 ⁻⁷ M _⊙ yr ⁻¹ , which confirms that this ∼0.5 M _⊙ star is a strong accretor. The HI and HeI lines exhibit strong variability on a daily timescale, consistent with the burster classification of DO Tau derived from its K2 light curve. We report a periodic modulation of the intensity of the redshifted high-velocity wings of the Br γ line profile. The modulation occurs at the rotational period of the star (5.128 d), which suggests the existence of corotating magnetospheric funnel flows. We derived an upper limit of 0.35 on the ratio between the magnetospheric truncation radius and the disk corotation radius, indicative of an ordered unstable accretion regime. The size of the Br γ line emitting region obtained from GRAVITY is quite small ( R _Brγ = 0.011 au ∼ 1.3 R _* ), and it is much smaller than the K-band continuum emitting region ( R _K = 0.09 au ∼ 11 R _* ). Such a compact Br γ emission region suggests that most of the line flux originates from the magnetospheric accretion region and/or from an inner wind close to the magnetosphere-disk interface. The on-sky displacements of the blue and red Br γ line velocity channels suggest a rotation pattern of the emitting gas, as they appear to be nearly aligned along the position angle of the disk. The inclination we derived for the inner disk (∼45-55°) differs from that of the outer disk inferred from the ALMA continuum (∼30°). This points toward a misalignment or warp of the outer disk that may originate from the suspected past encounter with the neighboring HV Tau system. Conclusions . Based on combining high-resolution spectroscopy and long baseline interferometry, we find that the T Tauri star DO Tau appears to be a strong accretor undergoing magnetospheric accretion in an ordered unstable regime, with a Br γ line emitting region as compact (∼0.01 au) as the size of its magnetosphere.

GRAVITY+ improves by orders of magnitude the sensitivity, sky-coverage, and contrast of the Very Large Telescope Interferometer (VLTI). A central part of this project is the development of Gravity Plus Adaptive Optics (GPAO), a dedicated high-order and laser-guide star adaptive optics (AO) system for VLTI. GPAO consists of four state-of-the-art AO systems that equip all 8 m class Unit Telescopes (UTs) for the wavefront correction of the VLTI instruments. It offers both visible and infrared natural guide star (NGS) and laser guide star (LGS) operations. The paper presents the design, operations, and performances of GPAO. We illustrate the improvement brought by GPAO with interferometric observations obtained during the commissioning of the NGS mode at the end of 2024. These science results include the first optical interferometry observations of a redshift z ~ 4 quasar, the spectroscopy of a cool brown-dwarf with magnitude K ~ 21.0, the first observations of a Class I young star with GRAVITY, and the first sub-micro arcsecond differential astrometry in the optical. Together with the entire GRAVITY+ project, the implementation of GPAO is a true paradigm shift for observing the optical Universe at very high angular resolution.

The black hole in the Galactic center, Sgr A*, is prototypical of ultra-low-fed galactic nuclei. The discovery of a handful of gas clumps with masses on the order of a few Earth masses in its immediate vicinity provides a gas reservoir sufficient to power Sgr A*. In particular, the gas cloud G2 is of interest due to its extreme orbit, on which it passed at a pericenter distance of around 100 AU and notably lost kinetic energy during the fly-by due to interaction with the black hole accretion flow. Thirteen years prior to G2, a similar gas cloud called G1, passed Sgr A* on a similar orbit. The origin of G2 remained a topic of discussion, with models including a central (stellar) source still proposed as alternatives to pure gas clouds. Here, we report the orbit of a third gas clump moving along (nearly) the same orbital trace. Since the probability of finding three stars on such similar orbits is very low, this strongly argues against stellar-based source models. Instead, we show that the gas streamer G1–2–3 plausibly originates from the stellar wind of the massive binary star IRS 16SW. This claim is substantiated by the fact that the small differences between the three orbits – the orientations of the orbital ellipses in their common plane as a function of time – are consistent with the orbital motion of IRS 16SW.

Residing in the center of the Milky Way, Sgr A* is the closest massive black hole. Its vicinity has allowed measuring individual stellar orbits around it with periods as short as 12 years. The stars act as test particles and probe the gravitational potential around the 4.3 × 10^6 M_sun black hole. These observations have determined the central mass to sub-percent precision, and the mildly relativistic motions of stars have given access to seeing the dominant relativistic corrections, the gravitational redshift, the transverse Doppler effect and the prograde precession imposed by the Schwarzschild metric nature of the potential. These effects are of order (v/c)^2 (for velocity v and speed of light c). The Kerr metric occur- ring for a rotating black hole leads to corrections of order (v/c)^3. Here, we report the discovery of a faint main sequence star (mK = 19.3), S301, the orbit of which has a period of 8.7 years and small enough a pericenter distance, such that the star’s peak velocity reaches 25000 km/s or 8% of c. Within the measurement capabilities of current near-infrared interferometry and future spectroscopy on an extremely large telescope, S301’s motion is directly sensitive to the spin of Sgr A*. The high eccentricity of S301 suggests that it is the captured component of a binary that was torn apart via the Hills mechanism, sending the other component away as a hyper-velocity star.

We present the first near-infrared interferometric data of a QSO at z = 4. The K -band observations were performed with GRAVITY+ on the VLTI using all four UTs, detecting a differential phase signal that traces the spatially resolved kinematics for both the H β and H γ lines in the broad-line region (BLR). We fit the two lines simultaneously with an updated model that includes distinct rotating and conical outflowing components. For the best-fit model, more than 80% of the H I line emission from the BLR originates in an outflow with a velocity up to 10 ⁴ km s ⁻¹ . This is oriented so that our line of sight is along an edge of the conical structure, which produces the prominent blue wing on the line profile. A combination of anisotropic line emission and mid-plane opacity leads to the single-sided phase signal. The model is able to qualitatively match both the outflowing C IV line profile and the systemic O I fluorescent emission. The black hole mass of 8 × 10 ⁸ M _⊙ that we derive is the highest redshift black hole mass measurement to date obtained directly from BLR dynamics. It is an order of magnitude lower than that inferred from various single epoch scaling relations, and it implies that the accretion is highly super-Eddington. With reference to recent simulations, the data suggest that this QSO is emitting close to its radiative limit in a regime where strong outflows are expected around a polar conical region.