arXiv subjects

S. de Regt

Publications and source records attributed to S. de Regt.

13 recordsLinked to original sources

arXiv2026

Native-resolution retrievals of VHS 1256-1257 b spanning the JWST/NIRSpec wavelength range: Chemical composition of a partially cloudy atmosphere

The wide wavelength coverage and sensitivity offered by JWST enable detailed analyses of extrasolar atmospheres. At its highest resolution (R~2700), NIRSpec measures the absorption from atomic, molecular, and isotopic gases whose abundances reflect the chemical composition of accreted material, making them key tracers of the formation environment. Orbiting an inner M-dwarf binary at a wide separation, the planetary-mass companion VHS 1256 b is one of the most variable sub-stellar objects known, with flux variations of 10-30%. We analyse the 0.97-5.27 micron NIRSpec ERS spectra of VHS 1256 b, update the data reduction and employ petitRADTRANS for atmospheric retrievals at the native spectral resolution. We model a partially cloudy atmosphere in chemical disequilibrium and fit directly for the elemental and isotopic abundances. Our best-fitting model closely matches the observations, bringing the residuals down to ~1%. The results are cautiously interpreted as degeneracies can bias some parameter constraints, most notably the mass and radius. Still, the retrieval finds a partial cloud deck covering ~79% of the visible surface, with a clearer column dominating at short wavelengths. Small changes of 1-3% can account for the high observed variability. From the many detected gases, we infer a metallicity, C/O ratio, and 12C/13C ratio in line with a solar composition, while the 18O isotope appears depleted relative to the Sun and local ISM. The isotope abundances are significantly lower than previous studies suggested, underlining the importance of our updated spectra. The 18O-depletion defies our understanding of the likely top-down formation. Our interpretation of the retrieved composition is complicated further by the lack of host star abundances. Nevertheless, this study demonstrates the value of panchromatic, native-resolution retrievals for characterising complex extrasolar atmospheres.

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arXiv2026

The ESO SupJup Survey XI. Atmospheric properties of six isolated M- and L-type dwarfs with CRIRES+

The distinct formation pathways of brown dwarfs and giant exoplanets may be encoded in their atmospheric composition. We present atmospheric retrievals for six isolated brown dwarfs of spectral types M7-L2.5 from the ESO SupJup Survey, aiming to constrain their thermal structures, chemical compositions, and isotope ratios. We analyse CRIRES+ K-band spectra, coupling the radiative transfer code petitRADTRANS with the nested sampling algorithm PyMultiNest under both free and equilibrium chemistry frameworks. The L0 dwarf 2MASS J09532126-1014205 emerges as one of the fastest-rotating ultracool dwarfs known, with $v\sin i = 85.9\pm0.5$ km s$^{-1}$. H$_2$O is strongly detected in all six targets and $^{12}$CO in five, with a marginal $^{12}$CO detection in the ultra-fast L0 rotator consistent with severe rotational broadening. $^{13}$CO is significantly detected in DENIS J060852.8-275358 and tentatively in three further targets. Retrieved compositions are consistent with isolated brown dwarfs: near-solar C/O ratios ($0.51$-$0.63$), predominantly near-solar metallicities, and $^{12}$C/$^{13}$C ratios of ~91-155, at or above the local ISM value, with constraints for the two fastest rotators resting on the spectral fit but not corroborated by a $^{13}$CO cross-correlation peak. The M7 dwarf 2MASS J04341527+2250309 shows discrepant gravity and metallicity values between chemistry frameworks. Apparent H$_2^{18}$O constraints for two targets are found to be spurious, and their H$_2^{(16)}$O/H$_2^{18}$O ratios are presented as lower limits, demonstrating the importance of cross-correlation validation for minor species detections. The near-solar C/O ratios and metallicities, with $^{12}$C/$^{13}$C ratios at or above the ISM value, support a molecular cloud fragmentation origin for the sample. The agreement of $^{12}$C/$^{13}$C between chemistry frameworks supports the robustness of these ratios.

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arXiv2026

The carbon isotope ratio of β Pic b with high-resolution spectroscopy

Isotopic ratios trace the formation and evolution of planets and link their atmospheres to the chemistry of their natal protoplanetary discs. We measure $^{12}\mathrm{C}/^{13}\mathrm{C} = 58^{+18}_{-15}$ in the atmosphere of the young super-Jupiter $β$ Pic b from 11 nights of CRIRES+ K-band spectroscopy ($\mathcal{R} \approx 100{,}000$) at the Very Large Telescope (VLT). We detect both $^{12}\mathrm{CO}$ and $^{13}\mathrm{CO}$ and constrain $^{12}\mathrm{C}/^{13}\mathrm{C}$ with a Bayesian retrieval jointly fitted with near-infrared photometry. The inferred $^{12}\mathrm{C}/^{13}\mathrm{C}$ is consistent with the present-day interstellar medium (ISM), is below the solar value, and is comparable to measurements in other young super-Jupiters. We also retrieve $T_{\rm eff} = 1629^{+30}_{-28}\,\mathrm{K}$, near-solar to mildly super-solar metallicity ([M/H]$ = 0.20^{+0.16}_{-0.12}$), a solar-like carbon-to-oxygen ratio (C/O$ = 0.52 \pm 0.03$), and tentative evidence for thick clouds. We analyse each night independently and combine the results of the six epochs with the highest signal-to-noise ratio (S/N), propagating night-to-night scatter into the final uncertainties. This provides an isotopic benchmark for a directly imaged planet interior to the CO snow line.

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arXiv2026

The ESO SupJup Survey. X. A carbon isotope contrast in the young ROXs 12 system

Emerging research suggests that elemental and isotopic ratios of exoplanet and brown dwarf atmospheres may serve as potential tracers of their formation pathways. The ESO SupJup Survey aims to shed light on this hypothesis, with a focus on the $^{12}$CO/$^{13}$CO ratio, by investigating the atmospheric composition of substellar companions and isolated brown dwarfs. In this work, we aim to characterize the atmospheres and determine the ratios of $^{12}$CO/$^{13}$CO of the Rho Ophiuchus X-ray source (ROXs) 12 system ($\sim$6Myrs), consisting of an M0 host with an L0 companion, as part of the ESO SupJup survey. Using high-resolution CRIRES+ K band spectra of these objects, we perform atmospheric retrieval analyses to derive their atmospheric properties, including the $^{12}$CO/$^{13}$CO ratio. Our retrieval framework is built on the radiative transfer code petitRADTRANS, with which we generate model spectra based on equilibrium chemistry tables computed with FastChem, coupled with the nested sampling algorithm PyMultiNest. We report the presence of H$_2$O, $^{12}$CO, $^{13}$CO, and HF in both the star and companion, with a tentative detection of H$_2^{18}$O in ROXs 12B. The $^{12}$CO/$^{13}$CO ratios of the two objects show a measurable, though not strongly significant, difference, namely $77\substack{+10 \\ -7}$ and $55\substack{+10 \\ -7}$ for ROXs 12A and B. We measure a C/O ratio of 0.54$\pm$0.01, while the C/O ratio of the star is not reliably constrained due to the absence of atomic oxygen lines in the K band. Furthermore, we retrieve moderate veiling in the host star of $r_k$=$0.17\substack{+0.02 \\ -0.03}$. Systems such as ROXs 12, in which both star and planet can be chemically and isotopically characterized, are crucial for constraining potential formation mechanisms of massive, wide-orbit super-Jupiters.

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arXiv2026

The ESO SupJup Survey IX: Isotopic evidence of a recent formation for Luhman 16AB

The distinct formation pathways of directly-imaged exoplanets and isolated brown dwarfs might leave imprints in the inherited elemental and isotopic abundances, but such measurements require careful characterisation of the atmospheres. In particular, L-T transition objects exhibit signs of dynamics that drive their atmospheres out of chemical equilibrium. In this work, we studied the nearest L-T brown dwarfs, Luhman 16A and B, to assess the chemical dis-equilibrium in their atmospheres and to investigate their elemental and isotopic composition. As part of the ESO SupJup Survey, we obtained high-resolution CRIRES$^+$ K-band spectra of the binary, which were analysed using an atmospheric retrieval framework. We detect and retrieve the abundances of $^{12}$CO, H$_2$O, CH$_4$, NH$_3$, H$_2$S, HF, and the $^{13}$CO isotopologue. Both atmospheres are in chemical dis-equilibrium with somewhat stronger vertical mixing in Luhman 16A compared to B ($K_\mathrm{zz,A}\sim10^{8.7}$, $K_\mathrm{zz,B}\sim10^{8.2}\ \mathrm{cm^2\ s^{-1}}$). The tested chemical models, free- and dis-equilibrium chemistry, yield consistent mixing ratios and agree with earlier work at shorter wavelengths. The gaseous C/O ratios show evidence of oxygen trapping in silicate-oxide clouds. While the C/O ratios are consistent with solar, the metallicities are modestly enhanced with $\mathrm{[C/H]}\sim0.15$. The carbon isotope ratios are measured at $\mathrm{^{12}C/^{13}C_A}=74^{+2}_{-2}$ and $\mathrm{^{12}C/^{13}C_B}=74^{+3}_{-3}$. The coincident constraints of metallicities and isotopes across the binary reinforce their likely shared formation. The $\mathrm{^{12}C/^{13}C}$ ratios are aligned with the present-day interstellar medium, but lower than the solar-system value. This suggests a recent inheritance and corroborates the relatively young age ($\sim500$ Myr) of Luhman 16AB as members of the Oceanus moving group.

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arXiv2025

Disentangling disc and atmospheric signatures of young brown dwarfs with JWST/NIRSpec

Young brown dwarfs serve as analogues of giant planets and provide benchmarks for atmospheric and formation models. JWST has enabled access to near-infrared spectra of brown dwarfs with unprecedented sensitivity. We aim to constrain their chemical compositions, temperature structures, isotopic ratios, and disc emission. We perform retrievals and disc modelling on JWST/NIRSpec medium-resolution ($R \approx 2700$) spectra spanning 0.97--5.27 $μ$m, combining radiative transfer, line-by-line opacities, parameterised temperature profiles, and flexible equilibrium chemistry. We include a disc ring with blackbody continuum and optically thin CO emission. We detect over twenty species, including $^{12}$CO, H$_2$O, CO$_2$, SiO, and hydrides. The CO band at 4.6 $μ$m reveals $^{13}$CO and C$^{18}$O. Carbon isotope ratios are $^{12}$C/$^{13}$C = $79^{+14}_{-11}$ (TWA 27A) and $75^{+2}_{-2}$ (TWA 28); oxygen ratios are $^{16}$O/$^{18}$O = $645^{+80}_{-70}$ and $681^{+53}_{-50}$. Both objects show excess infrared emission, consistent with warm ($\approx 650$ K) blackbody rings, and optically thin CO from hot gas ($\geq 1600$ K) needed to match the red spectra. The atmospheric C/O ratios are $0.54 \pm 0.02$ (TWA 27A) and $0.59 \pm 0.02$ (TWA 28), consistent with solar values. We characterise the atmospheres and discs of two young brown dwarfs through joint constraints on temperature, composition, isotopes, and discs, demonstrating JWST/NIRSpec's ability to probe young objects and circumplanetary discs.

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arXiv2025

The ESO SupJup Survey VIII. Chemical fingerprints of young L dwarf twins

The potentially distinct formation pathways of exoplanets and brown dwarfs may be imprinted in their elemental and isotopic ratios. This work is part of the ESO SupJup Survey, which aims to disentangle the formation pathways of super-Jupiters and brown dwarfs through the analysis of their chemical and isotopic ratios. In this study, we characterize the atmospheres of two young L4 dwarfs, 2MASS J03552337+1133437 (2M0355) and 2MASS J14252798-3650229 (2M1425), in the AB Doradus Moving Group. This involved constraining their chemical composition, $^{12}$CO/$^{13}$CO ratio, pressure-temperature profile, surface gravity, and rotational velocity. We have obtained high-resolution CRIRES+ K-band spectra of these brown dwarfs, which we analyzed with an atmospheric retrieval pipeline. Atmospheric models were generated with the radiative transfer code petitRADTRANS, for which we employed a free and equilibrium chemistry approach, which we coupled with the PyMultiNest sampling algorithm to determine the best fit. We report robust detections of $^{13}$CO (13.4 & 8.0 $σ$) and HF (11.6 & 15.8 $σ$) in 2M0355 and 2M1425, respectively. The objects have similar overall atmospheric properties, including $^{12}$CO/$^{13}$CO isotope ratios of $95.5\substack{+6.8 \\ -6.4}$ for 2M0355 and $109.6\substack{+10.6 \\ -9.6}$ for 2M1425. In both brown dwarfs, we find only tentative hints of H$_2^{18}$O (1.1 & 3.0 $σ$), with lower limits of H$_2^{16}$O/H$_2^{18}$O $\sim$ 1000. Both objects appear to be close to chemical equilibrium, considering the main spectral contributors. Future studies will put them into the context of other objects observed as part of the ESO SupJup Survey.

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arXiv2025

The ESO SupJup Survey VII: Clouds and line asymmetries in CRIRES$^+$ J-band spectra of the Luhman 16 binary

Brown dwarfs at the L-T transition likely experience an inhomogeneous clearing of the clouds in their atmospheres. The resulting surface of thin and thick cloudy patches has been put forward to explain the observed variability, J-band brightening, and re-emergence of FeH absorption. We study the closest brown dwarf binary, Luhman 16A and B, in an effort to constrain their chemical and cloud compositions. As this binary consists of an L7.5 and T0.5 component, we gain insight into the atmospheric properties at the L-T transition. As part of the ESO SupJup Survey, we observed Luhman 16AB at high spectral resolution in the J-band ($1.1-1.4\ \mathrm{μm}$) using CRIRES$^+$. To analyse the spectra, we employ an atmospheric retrieval framework, coupling the radiative transfer code petitRADTRANS with the MultiNest sampling algorithm. For both objects, we report detections of H$_2$O, K, Na, FeH, and, for the first time in the J-band, hydrogen-fluoride (HF). The K doublet at $1250\ \mathrm{nm}$ shows asymmetric absorption in the blue line wings, which are reproduced via pressure- and temperature-dependent shifts of the line cores. We find evidence for clouds in both spectra and we place constraints on an FeH-depletion in the Luhman 16A photosphere. The inferred over-abundance of FeH for Luhman 16B opposes its predicted rainout into iron clouds. A two-column model, which emulates the patchy surface expected at the L-T transition, is weakly preferred ($\sim 1.8σ$) for component B but disfavoured for A ($\sim 5.5σ$). The results suggest a uniform surface on Luhman 16A, which is in good agreement with the reduced variability observed for this L-type component. While the presented evidence is not sufficient to draw conclusions about any inhomogeneity on Luhman 16B, future observations covering a broader wavelength range could help to test the cloud-clearing hypothesis.

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arXiv2025

The ESO SupJup Survey IV. Unveiling the carbon isotope ratio of GQ Lup B and its host star

The carbon isotope ratio ($^{12}\mathrm{C}/^{13}\mathrm{C}$) is a potential tracer of giant planet and brown dwarf formation. The GQ Lup system, hosting the K7 T Tauri star GQ Lup A and its substellar companion GQ Lup B, offers a unique opportunity to investigate this ratio in a young system. We aim to characterise the atmosphere of GQ Lup B, determining its temperature, chemical composition, spin, surface gravity, and $^{12}\mathrm{C}/^{13}\mathrm{C}$, while also measuring the same ratio for its host star, GQ Lup A. High-resolution K-band spectra of GQ Lup were obtained using CRIRES+ at the VLT. We modelled the starlight contribution from GQ Lup A and fitted GQ Lup Bś spectrum with atmospheric models from petitRADTRANS. The $^{12}\mathrm{C}/^{13}\mathrm{C}$ ratio for GQ Lup A was derived using isotope-sensitive PHOENIX models. Atmospheric analysis of GQ Lup B revealed abundances of H$_2$O, $^{12}$CO, $^{13}$CO, HF, Na, Ca, and Ti, along with a C/O ratio of $0.50 \pm 0.01$, consistent with the solar value. The carbon isotope ratio was measured as $^{12}\mathrm{C}/^{13}\mathrm{C} = 53^{+7}_{-6}$ for GQ Lup B and $^{12}\mathrm{C}/^{13}\mathrm{C} = 51^{+10}_{-8}$ for GQ Lup A. Strong veiling of the photospheric lines of GQ Lup A was identified and accounted for. The similar $^{12}\mathrm{C}/^{13}\mathrm{C}$ values in GQ Lup A and B suggest a common origin from a shared material reservoir, supporting formation via disc fragmentation or gravitational collapse.

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arXiv2024

The ESO SupJup Survey II: The 12C/13C ratios of three young brown dwarfs with CRIRES$^+$

Young brown dwarfs exhibit atmospheric characteristics similar to those of super-Jupiters, providing a unique opportunity to study planetary atmospheres. The ESO SupJup Survey, utilizing CRIRES$^+$ on the Very Large Telescope, aims to assess the role of $^{12}$C/$^{13}$C as a formation tracer. We present observations of three young brown dwarfs: 2MASS J12003792-7845082, TWA 28, and 2MASS J08561384-1342242, with the goal of constraining their chemical compositions, thermal profiles, surface gravities, spin rotations, and $^{12}$C/$^{13}$C. Atmospheric retrievals of CRIRES$^+$ K-band spectra were conducted using the radiative transfer code petitRADTRANS coupled with the Bayesian inference algorithm MultiNest, resulting in a detailed characterization of the atmospheres of these objects. We report the volume mixing ratios of main molecular and atomic species, including the novel detection of hydrogen fluoride (HF) in a brown dwarf's atmosphere, and determine $^{12}$C/$^{13}$C values of $81^{+28}_{-19}$ and $79^{+20}_{-14}$ in the atmospheres of TWA 28 and J0856, respectively, with strong significance ($>3σ$). Tentative evidence ($\sim 2σ$) of $^{13}$C in J1200 was found, with $^{12}$C/$^{13}$C = $114^{+69}_{-33}$, along with $^{18}$O detected at moderate significance in J0856 (3.3$σ$) and TWA 28 (2.1$σ$). The retrieved thermal profiles indicate hot atmospheres (2300-2600 K) with low surface gravities and slow spins, consistent with young objects. The consistent carbon isotope ratios among the three objects, showing no significant deviation from the local ISM, suggest a fragmentation-based formation mechanism similar to star formation. The tentative detection of $^{18}$O in two objects highlights the potential of high-resolution spectroscopy to probe additional isotope ratios, such as $^{16}$O/$^{18}$O, in the atmospheres of brown dwarfs and super-Jupiters.

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arXiv2024

The ESO SupJup Survey I: Chemical and isotopic characterisation of the late L-dwarf DENIS J0255-4700 with CRIRES$^+$

It has been proposed that the distinct formation and evolution of exoplanets and brown dwarfs may affect the chemical and isotopic content of their atmospheres. Recent work has indeed shown differences in the $^{12}$C/$^{13}$C isotope ratio, provisionally attributed to the top-down formation of brown dwarfs and the core accretion pathway of super-Jupiters. The ESO SupJup Survey aims to disentangle the formation pathways of isolated brown dwarfs and planetary-mass companions using chemical and isotopic tracers. The survey uses high-resolution spectroscopy with the recently upgraded VLT/CRIRES$^+$ spectrograph, covering a total of 49 targets. Here, we present the first results: an atmospheric characterisation of DENIS J0255-4700, an isolated brown dwarf near the L-T transition. We analyse its K-band spectrum using a retrieval framework where the radiative transfer code petitRADTRANS is coupled to PyMultiNest. Gaussian Processes are employed to model inter-pixel correlations and we adopt an updated parameterisation of the PT-profile. Abundances of CO, H$_2$O, CH$_4$, and NH$_3$ are retrieved for this fast-rotating L-dwarf. The ExoMol H$_2$O line list provides a significantly better fit than that of HITEMP. A free-chemistry retrieval is strongly favoured over equilibrium chemistry, caused by an under-abundance of CH$_4$. The free-chemistry retrieval constrains a super-solar C/O-ratio of $\sim0.68$ and a solar metallicity. We find tentative evidence ($\sim3σ$) for the presence of $^{13}$CO, with a constraint on the isotope ratio of $\mathrm{^{12}C/^{13}C}=184^{+61}_{-40}$ and a lower limit of $\gtrsim97$, suggesting a depletion of $^{13}$C compared to the interstellar medium ($\sim68$). High-resolution, high signal-to-noise K-band spectra provide an excellent means to constrain the chemistry and isotopic content of sub-stellar objects, as is the main objective of the ESO SupJup Survey.

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arXiv2024

Polarimetric differential imaging with VLT/NACO. A comprehensive PDI pipeline for NACO data (PIPPIN)

The observed diversity of exoplanets can possibly be traced back to the planet formation processes. Planet-disk interactions induce sub-structures in the circumstellar disk that can be revealed via scattered light observations. However, a high-contrast imaging technique such as polarimetric differential imaging (PDI) must first be applied to suppress the stellar diffraction halo. In this work we present the PDI PiPelIne for NACO data (PIPPIN), which reduces the archival polarimetric observations made with the NACO instrument at the Very Large Telescope. Prior to this work, such a comprehensive pipeline to reduce polarimetric NACO data did not exist. We identify a total of 243 datasets of 57 potentially young stellar objects observed before NACO's decommissioning. The PIPPIN pipeline applies various levels of instrumental polarisation correction and is capable of reducing multiple observing setups, including half-wave plate or de-rotator usage and wire-grid observations. A novel template-matching method is applied to assess the detection significance of polarised signals in the reduced data. In 22 of the 57 observed targets, we detect polarised light resulting from a scattering of circumstellar dust. The detections exhibit a collection of known sub-structures, including rings, gaps, spirals, shadows, and in- or outflows of material. Since NACO was equipped with a near-infrared wavefront sensor, it made unique polarimetric observations of a number of embedded protostars. This is the first time detections of the Class I objects Elia 2-21 and YLW 16A have been published. Alongside the outlined PIPPIN pipeline, we publish an archive of the reduced data products, thereby improving the accessibility of these data for future studies.

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arXiv2022

A quantitative assessment of the VO line list: Inaccuracies hamper high-resolution VO detections in exoplanet atmospheres

Metal hydrides and oxides are important species in hot-Jupiters since they can affect their energy budgets and the thermal structure of their atmospheres. One such species is VO, which is prominent in stellar M-dwarf spectra. Evidence for VO has been found in the low-resolution transmission spectrum of WASP-121b, but this has not been confirmed at high resolution. It has been suggested that this is due to inaccuracies in its line list. In this paper, we quantitatively evaluate the VO line list and assess whether inaccuracies are responsible for the non-detections at high resolution in WASP-121b. Furthermore, we investigate whether the detectability can be improved by selecting only those lines associated with the most accurate quantum transitions. A cross-correlation analysis was applied to archival HARPS and CARMENES spectra of several M dwarfs. VO cross-correlation signals from the spectra were compared with those in which synthetic VO models were injected, providing an estimate of the ratio between the potential strength (in case of a perfect model) and the observed strength of the signal. This was repeated for the reduced model covering the most accurate quantum transitions. The findings were fed into injection and recovery tests of VO in a UVES transmission spectrum of WASP-121b. We find that inaccuracies cause VO cross-correlation signals in M-dwarf spectra to be suppressed by about a factor 2.1 and 1.1 for the complete and reduced line lists, respectively, corresponding to a reduced observing efficiency of a factor 4.3 and 1.2. The reduced line list outperforms the complete line list in recovering the actual VO signal in the M-dwarf spectra by about a factor of 1.8. Neither line list results in a VO detection in WASP-121b. Injection tests show that with the reduced efficiency of the line lists, the potential signal as seen at low resolution is not detectable in these data.

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