Christoph U. Keller

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.

We report observations of faint (10 ⁻⁶ ), concentric, planet-wide rings in the polarized flux of sunlight that is reflected by Venus, obtained during a serendipitous, 36-minute run in 2010, with the highly sensitive Extreme Polarimeter (ExPo) on the William Herschel Telescope. The rings appear to be centered slightly downwind of the subsolar point, are visible in different filters across the visible, and are not obvious in the simultaneous total flux observations. ExPo’s dual-beam exchange and double-differencing design strongly suppresses first-order instrumental polarization, and we could not identify an instrumental cause of the observed pattern. Because ExPo was dismantled before the rings were identified in the data, this is the only set of observations of these rings. We are therefore careful in claiming the detection of a new atmospheric phenomenon on Venus. However, numerical radiative transfer simulations show that planet-wide rings in polarization can arise owing to density variations of 5%–10% in the gas above the clouds, consistent with a gravity wave. Our simulations also show that such density variations would not show up in total flux observations. By presenting our observations and numerical simulations, we hope to motivate new polarimetric observations of Venus that could confirm or refute the presence of such planet-wide waves.

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.

Aims . We describe the design, laboratory manufacture, and on-sky testing of the grating vector apodizing phase plate (gvAPP) coronagraph for the Enhanced Resolution Imager and Spectrograph (ERIS) on the Very Large Telescope. Methods . We used both laboratory measurements and on-sky observations to characterise the gvAPP in several different filters, from the K to the L band. Results . In testing, the gvAPP reaches its design specification in the transmission of the optic with 90% in the K bands and 60% in the L band. While the gvAPP reaches its designed raw contrast performance of 1 × 10 ⁻⁵ , it does not reach the post-processed contrast of 5 × 10 ⁻⁵ in on-sky observations. Electronic detector noise, due to the Airy core of the coronagraphic point spread function inducing cross-talk between the readout amplifiers, produces a repeated pattern within the coronagraphic regions of the gvAPP. Conclusions . Despite these limitations, we recommend the gvAPP as a tool for characterising substellar companions with known separations and position angles, which allow them to be placed in the coronagraphic dark holes for observations. The ERIS gvAPP’s leakage term can also be used as a photometric reference for time series observations; however, we caution that the contrast performance may limit such studies to only the brightest targets. ERIS gvAPP data quality may be improved further with better modelling of detector electronic noise. This work is a pathfinder for Extremely Large Telescope instruments including METIS, which will include gvAPP coronagraphs with improved designs based on these results.

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.

Current, realistic numerical simulations of the solar atmosphere reproduce observations in a statistical sense; they do not replicate observations such as a movie of solar granulation. Inversions on the other hand reproduce observations by design, but the resulting models are often not physically self-consistent. Physics-informed neural networks (PINNs) offer a new approach to solving the time-dependent radiative hydrodynamics equations and matching observations as boundary conditions. PINNs approximate the solution of the integro-differential equations with a deep neural network. The parameters of this network are determined by minimizing the residuals with respect to the physics equations and the observations. The resulting models are continuous in all dimensions, can zoom into local areas of interest in space and time, and provide information on physical parameters that are not necessarily directly observed such as horizontal velocities. Here, we present the first proof of concept of this novel approach, explain the underlying methodology in detail, and provide an outlook to the many applications that PINNs enable.

Context. The Enhanced Resolution Imager and Spectrograph (ERIS) is the new adaptive optics (AO) assisted infrared instrument at the Very Large Telescope (VLT). Its refurbished integral field spectrograph (IFS) SPIFFIER leverages a new AO module, enabling high-contrast imaging applications and giving access to the orbital and atmospheric characterisation of super-Jovian exoplanets. Aims. We tested the detection limits of ERIS, and demonstrate its scientific potential by exploring the atmospheric composition of the young super-Jovian AF Lep b. Additionally, we improved its orbital solution by measuring its radial velocity relative to its host star. Methods. We present new spectroscopic observations of AF Lep b in K-band at R ∼ 11 000 obtained with ERIS/SPIFFIER at the VLT. We reduced the data using the standard pipeline together with a custom wavelength calibration routine, and removed the stellar point spread function using principal component analysis along the spectral axis. We computed molecular maps by cross-correlating the residuals with molecular spectral templates and measured the radial velocity of the planet relative to the star. Furthermore, we computed contrast grids for molecular mapping by injecting fake planets. Results. We detect a strong signal from H₂O and CO but not from CH₄ or CO₂. This result corroborates the hypothesis of chemical disequilibrium in the atmosphere of AF Lep b. Our measurement of the RV of the planet yields Δ v_{R, P ⋆} = 7.8 ± 1.7 km s⁻¹. This enables us to disentangle the degeneracy of the orbital solution; specifically, the correct longitude of the ascending node is Ω = 248_−0.7^+0.4 deg and the argument of periapsis is ω = 109₋₂₁⁺¹³ deg. Our detection limits reach a contrast of ΔK = 11.5 mag at 0′′.12 for the spectral templates of H₂O and CO, significantly extending the parameter space available to moderately high spectral resolution towards small angular separation. Conclusions. Our results demonstrate the competitiveness of the new ERIS/SPIFFIER instrument for the orbital and atmospheric characterisation of exoplanets at high contrast and small angular separation.

Current, realistic numerical simulations of the solar atmosphere reproduce observations in a statistical sense; they do not replicate observations such as a movie of solar granulation. Inversions on the other hand reproduce observations by design, but the resulting models are often not physically self-consistent. Physics-informed neural networks (PINNs) offer a new approach to solving the time-dependent radiative hydrodynamics equations and matching observations as boundary conditions. PINNs approximate the solution of the integro-differential equations with a deep neural network. The parameters of this network are determined by minimizing the residuals with respect to the physics equations and the observations. The resulting models are continuous in all dimensions, can zoom into local areas of interest in space and time, and provide information on physical parameters that are not necessarily directly observed such as horizontal velocities. Here we present the first proof of concept of this novel approach, explain the underlying methodology in detail, and provide an outlook to the many applications that PINNs enable.

Hyperluminous infrared galaxies (HyLIRGs) are the rarest and most extreme starbursts and found only in the distant Universe (z ≳ 1). They have intrinsic infrared (IR) luminositiesL_IR ≥ 10¹³ L_⊙and are commonly found to be major mergers. Recently, the Planck All-Sky Survey to Analyze Gravitationally-lensed Extreme Starbursts project (PASSAGES) searched ~10⁴deg²of the sky and found ~20 HyLIRGs. We describe a detailed study of PJ0116-24, the brightest (μL_IR ≈ 2.6 × 10¹⁴ L_⊙, magnified withμ ≈ 17) Einstein-ring HyLIRG in the southern sky, atz = 2.125, with observations from the near-IR integral-field spectrograph VLT/ERIS and the submillimetre interferometer ALMA. We detected Hα, Hβ, [Nii] and [Sii] lines and obtained an extreme Balmer decrement (Hα/Hβ ≈ 8.73 ± 1.14). We modelled the molecular-gas and ionized-gas kinematics with CO(3–2) and Hα data at ~100–300 pc and (sub)kiloparsec delensed scales, respectively, finding consistent regular rotation. We found PJ0116-24 to be highly rotationally supported (v_rot/σ_{0, mol. gas} ≈ 9.4) with a richer gaseous substructure than other known HyLIRGs. Our results imply that PJ0116-24 is an intrinsically massive (M_baryon ≈ 10^11.3 M_⊙) and rare starbursty disk (star-formation rate, SFR = 1,490 M_⊙ yr⁻¹) probably undergoing secular evolution. This indicates that the maximal SFR (≳1,000 M_⊙ yr⁻¹) predicted by simulations could occur during a galaxy’s secular evolution, away from major mergers.

Volume Bragg gratings are a useful tool for spectral manipulation in a variety of settings. In a previous paper [ Astron. & Astrophys. to be published, (2024)] we simulated an astronomical instrument that detects molecules on exoplanets by optically creating matched filters using multiplexed Bragg gratings, which optically stack spectral lines. Here, we confirm that we can freely modify and multiplex Bragg gratings, with an acousto-optical tunable filter (AOTF) and that we can stack spectral features in a lab setup using a commercially available AOTF. We find that we are able to multiplex an additional grating or modify an existing grating without altering other multiplexed gratings, so long as the bandpasses do not overlap. We can also successfully stack emission and absorption lines, producing comparable results when optically overlaying spectral lines and computationally summing images of separate lines.