arXiv subjects

Ray Jayawardhana

Publications and source records attributed to Ray Jayawardhana.

At least 19 recordsLinked to original sources

arXiv2026

High-Resolution Dayside Spectroscopy of the Ultra-Hot Jupiter WASP-178b with GHOST/Gemini South

We present high-resolution dayside spectroscopy of the ultra-hot Jupiter WASP-178b obtained with the Gemini High-resolution Optical SpecTrograph (GHOST) at the Gemini South Observatory. The observations cover pre- and post-eclipse orbital phases, lasting approximately 9 hours in total, and represent the first published high-resolution optical dayside emission spectroscopy of WASP-178b's atmosphere, complementing previous near-infrared dayside spectroscopy. We verify the presence of a thermal inversion in the dayside atmosphere with a high-significance (8.3$σ$) detection of neutral iron emission via the Doppler cross-correlation technique, alongside hints of neutral Si and Ca emission. We also carry out model injection/recovery tests for other atmospheric species, which indicate that we do not expect to detect the majority of species searched for in this work, assuming our models are accurate representations of the planet's atmosphere. Finally, we place our results into context with previous work, showing how our tentative detection of neutral Si complements existing space-based observations and adds a new data point to previous detections of Si in ultra-hot Jupiter atmospheres. Our high-resolution observations provide important context about the dayside of WASP-178b, shedding light on 3D atmospheric processes and the extreme conditions present in ultra-hot Jupiter atmospheres.

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arXiv2026

TESS Photometry and Radial Velocity Analysis of the sub-Neptune Exoplanet π Mensae c and the Wider π Mensae Planetary System

Exoplanet characterization relies on precise measurements of planetary orbital and physical parameters. This is particularly important for planetary dynamics and atmospheric evolution, as orbital parameters help constrain system evolution, resolve ambiguities, and gauge atmospheric retention. The first exoplanet discovered by the Transiting Exoplanet Survey Satellite (TESS), $π$ Men c, is a warm sub-Neptune orbiting a bright Sun-like star in a system containing (at least) one other planet with a wildly different period and size. Lying near the 1.5-2.0 $R_{\oplus}$ radius gap, $π$ Men c is expected to have lost its primordial hydrogen and helium, but kept heavier compounds like H$_2$O and CO$_2$. The $π$ Men system is well observed with decades of radial velocity measurements, and TESS has continued to observe $π$ Men c, yielding six years and 21 sectors of photometry. We present a comprehensive analysis of these TESS data and 22 years of radial velocity measurements to provide updated orbital ephemerides for $π$ Men b, c, and the proposed third planet, $π$ Men d. Our newly derived $π$ Men c period error margins are an order of magnitude improved from previous estimates, and we estimate the mass range of $π$ Men d to be 13.4 $\leq$ M$_d$ $<$ 20 M$_{\oplus}$. We find that $π$ Men c is a uniquely interesting target for future transmission spectroscopy studies with JWST, and that existing radial velocity data are consistent with the existence of a third planet.

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arXiv2026

Mapping atmospheric features of the planetary-mass brown dwarf SIMP 0136 with JWST NIRISS

In this paper, we analyze James Webb Space Telescope Near Infrared Imager and Slitless Spectrograph time-series spectroscopy data to characterize the atmosphere of the planetary-mass brown dwarf SIMP J01365662+093347. Principal component analysis reveals that 81\% of spectral variations can be described by two components, implying that variability within a single rotational phase is induced by at least three distinct spectral regions. By comparing our data to a grid of Sonora Diamondback atmospheric models, we confirm that the time-averaged spectrum cannot be explained by a single model but require a linear combination of at least three regions. Projecting these models onto the principal component plane shows that the overall variability is highly correlated with changes in temperature, cloud coverage, and possibly effective metallicity. We also extract brightness maps from the lightcurve and establish North-South asymmetry in the atmosphere. A combined multidimensional analysis of spectro-photometric variability links the three spectral regions to three atmospheric layers. Forsterite cloud and water abundance at each level form unique harmonics of atmospheric variability observed in different spectral bands. Atmospheric retrievals on the time-averaged spectrum are consistent with an optically thick iron cloud deck beneath a patchy forsterite cloud layer and with the overall adiabatic curve. We also demonstrate two new analysis methods: a regionally-resolved spectra retrieval that relies on multi-wavelength spherical harmonics maps, and a method to constrain brightness maps using Doppler information present in the spectra. Future observations of variable brown dwarfs of higher spectral resolution or spanning multiple rotations should help break mapping degeneracy.

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arXiv2026

NIRPS tightens the mass estimate of GJ 3090 b and detects a planet near the stellar rotation period

We present an updated characterization of the planetary system orbiting the nearby M2 dwarf GJ 3090 (TOI-177; $d = 22$ pc), based on new high-precision radial velocity (RV) observations from NIRPS and HARPS. With an orbital period of 2.85 d, the transiting sub-Neptune GJ 3090 b has a mass we refine to $4.52 \pm 0.47 M_{\oplus}$, which, combined with our derived radius of $2.18 \pm 0.06 R_{\oplus}$, yields a density of $2.40^{+0.33}_{-0.30}$ g cm$^{-3}$. The combined interior structure and atmospheric constraints indicate that GJ 3090 b is a compelling water-world candidate, with a volatile-rich envelope in which water likely represents a significant fraction. We also confirm the presence of a second planet, GJ 3090 c, a sub-Neptune with a 15.9 d orbit and a minimum mass of $10.0 \pm 1.3 M_{\oplus}$, which does not transit. Despite its proximity to the star's 18 d rotation period, our joint analysis using a multidimensional Gaussian process (GP) model that incorporates TESS photometry and differential stellar temperature measurements distinguishes this planetary signal from activity-induced variability. In addition, we place new constraints on a non-transiting planet candidate with a period of 12.7 d, suggested in earlier RV analyses. This candidate remains a compelling target for future monitoring. These results highlight the crucial role of multidimensional GP modelling in disentangling planetary signals from stellar activity, enabling the detection of a planet near the stellar rotation period that could have remained undetected with traditional approaches.

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arXiv2025

Diversity in the haziness and chemistry of temperate sub-Neptunes

Recent transit observations of K2-18b and TOI-270d revealed strong molecular absorption signatures, lending credence to the idea that temperate sub-Neptunes (T$_\mathrm{eq}$=250-400K) have upper atmospheres mostly free of aerosols. These observations also indicated higher-than-expected CO$_2$ abundances on both planets, implying bulk compositions with high water mass fractions. However, it remains unclear whether these findings hold true for all temperate sub-Neptunes. Here, we present the JWST NIRSpec/PRISM 0.7-5.4$\mathbfμ$m transmission spectrum of a third temperate sub-Neptune, the 2.4R$_\oplus$ planet LP 791-18c (T$_\mathrm{eq}$=355K), which is even more favorable for atmospheric characterization thanks to its small M6 host star. Intriguingly, despite LP 791-18c's radius, mass, and equilibrium temperature being in between those of K2-18b and TOI-270d, we find a drastically different transmission spectrum. While we also detect methane on LP 791-18c, its transit spectrum is dominated by strong haze scattering and there is no discernible CO$_2$ absorption. Overall, we infer a deep metal-enriched atmosphere (246-415$\times$solar) for LP 791-18c, with a CO$_2$-to-CH$_4$ ratio smaller than 0.07 (at 2$σ$), indicating less H$_2$O in the deep envelope of LP 791-18c and implying a relatively dry formation inside the water ice-line. These results show that sub-Neptunes that are near-analogues in density and temperature can show drastically different aerosols and envelope chemistry, and are intrinsically diverse beyond a simple temperature dependence.

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arXiv2025

Precise Constraints on the Energy Budget of WASP-121 b from its JWST NIRISS/SOSS Phase Curve

Ultra-hot Jupiters exhibit day-to-night temperature contrasts upwards of 1000 K due to competing effects of strong winds, short radiative timescales, magnetic drag, and H2 dissociation/recombination. Spectroscopic phase curves provide critical insights into these processes by mapping temperature distributions and constraining the planet's energy budget across different pressure levels. Here, we present the first NIRISS/SOSS phase curve of an ultra-hot Jupiter, WASP-121 b. The instrument's bandpass [0.6 - 2.85 micron] captures an estimated 50-83% of the planet's bolometric flux, depending on orbital phase, allowing for unprecedented constraints on the planet's global energy budget; previous measurements with HST/WFC3 and JWST/NIRSpec/G395H captured roughly 20% of the planetary flux. Accounting for the unobserved regions of the spectrum, we estimate effective day and nightside temperatures of T_day = 2717 +/- 17 K and T_night = 1562 +/- 19 K corresponding to a Bond albedo of A_B = 0.277 +/- 0.016 and a heat recirculation efficiency of epsilon = 0.246 +/- 0.014. Matching the phase-dependent effective temperature with energy balance models yields a similar Bond albedo of 0.3 and a mixed layer pressure of 1 bar consistent with photospheric pressures, but unexpectedly slow winds of 0.2 km/s, indicative of inefficient heat redistribution. The shorter optical wavelengths of the NIRISS/SOSS Order 2 yield a geometric albedo of A_g = 0.093 +/- 0.029 (3 sigma upper limit of 0.175), reinforcing the unexplained trend of hot Jupiters exhibiting larger Bond albedos than geometric albedos. We also detect near-zero phase curve offsets for wavelengths above 1.5 micron, consistent with inefficient heat transport, while shorter wavelengths potentially sensitive to reflected light show eastward offsets.

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arXiv2025

NIRPS and TESS reveal a peculiar system around the M dwarf TOI-756: A transiting sub-Neptune and a cold eccentric giant

The Near InfraRed Planet Searcher (NIRPS) joined HARPS on the 3.6-m ESO telescope at La Silla Observatory in April 2023, dedicating part of its Guaranteed Time Observations (GTO) program to the radial velocity follow-up of TESS planet candidates to confirm and characterize transiting planets around M dwarfs. We report the first results of this program with the characterization of the TOI-756 system, which consists of TOI-756 b, a transiting sub-Neptune candidate detected by TESS, as well as TOI-756 c, an additional non-transiting planet discovered by NIRPS and HARPS. TOI-756 b is a 1.24-day period sub-Neptune with a radius of 2.81 $\pm$ 0.10 $R_\oplus$ and a mass of 9.8$^{+1.8}_{-1.6}$ $M_\oplus$. TOI-756 c is a cold eccentric (e$_c$ = 0.45 $\pm$ 0.01) giant planet orbiting with a period of 149.6 days around its star with a minimum mass of 4.05 $\pm$ 0.11 $M_\mathrm{jup}$. Additionally, a linear trend of 146$~\mathrm{m\,s}^{-1}\,\mathrm{yr}^{-1}$ is visible in the radial velocities, hinting at a third component, possibly in the planetary or brown dwarf regime. This system is unique in the exoplanet landscape, standing as the first confirmed example of such a planetary architecture around an M dwarf. With a density of 2.42 $\pm$ 0.49 g cm$^{-3}$, the inner planet, TOI-756 b, is a volatile-rich sub-Neptune. Assuming a pure H/He envelope, we inferred an atmospheric mass fraction of 0.023 and a core mass fraction of 0.27, which is well constrained by stellar refractory abundances derived from NIRPS spectra. It falls within the still poorly explored radius cliff and at the lower boundary of the Neptune desert, making it a prime target for a future atmospheric characterization with JWST to improve our understanding of this population.

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arXiv2025

A complex structure of escaping helium spanning more than half the orbit of the ultra-hot Jupiter WASP-121\,b

Atmospheric escape of planets on short orbital periods, driven by the host star's irradiation, influences their evolution, composition, and atmospheric dynamics. Our main avenue to probe atmospheric escape is through the near-infrared metastable helium triplet, which has enabled mass loss rate measurements for tens of exoplanets. Among them, only a few studies show evidence for out-of-transit absorption, supporting the presence of a hydrodynamic outflow. However, none of these observations precisely identified the physical extent of the outflow, either due to non-continuous or short-duration observations. This limits our measurements of accurate mass loss rates. Here we present the first continuous, full-orbit helium phase curve monitoring of an exoplanet, the ultra-hot Jupiter WASP-121b, obtained with JWST/NIRISS. It reveals helium absorption for nearly 60% of the orbit at >3sigma significance. Our results show that WASP-121b sustains a strong outflow, separating into two tails trailing and leading the planet. The persistent absorption from these tails, together with their measured radial velocity shifts, suggests that they remain in a collisional fluid regime at large distances from the planet and display very different dynamics. The leading trail has a higher density and moves toward the star, whereas the trailing trail is being pushed away from the star, with the latter being blue-shifted due to stellar irradiation pressure. While qualitatively agreeing with theoretical expectations, the observed structure of helium is not self-consistently reproducible by current models, limiting constraints on the mass loss rate. Furthermore, we show that while ground-based observations of the helium triplet are essential to measure the outflow dynamics precisely, they ideally should be combined with continuous JWST phase curves to constrain the absolute level of helium absorption.

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arXiv2025

Quantifying thermal water dissociation in the dayside photosphere of WASP-121 b using NIRPS

The intense stellar irradiation of ultra-hot Jupiters results in some of the most extreme atmospheric environments in the planetary regime. On their daysides, temperatures can be sufficiently high for key atmospheric constituents to thermally dissociate into simpler molecular species and atoms. This dissociation drastically changes the atmospheric opacities and, in turn, critically alters the temperature structure, atmospheric dynamics, and day-night heat transport. To this date, however, simultaneous detections of the dissociating species and their thermally dissociation products in exoplanet atmospheres have remained rare. Here we present the simultaneous detections of H$_2$O and its thermally dissociation product OH on the dayside of the ultra-hot Jupiter WASP-121 b based on high-resolution emission spectroscopy with the recently commissioned Near InfraRed Planet Searcher (NIRPS). We retrieve a photospheric abundance ratio of log$_{10}$(OH/H$_2$O) $= -0.15\pm{0.20}$ indicating that there is about as much OH as H$_2$O at photospheric pressures, which confirms predictions from chemical equilibrium models. We compare the dissociation on WASP-121 b with other ultra-hot Jupiters and show that a trend in agreement with equilibrium models arises. We also discuss an apparent velocity shift of $4.79^{+0.93}_{-0.97} $km s$^{-1}$ in the H$_2$O signal, which is not reproduced by current global circulation models. Finally, in addition to H$_2$O and OH, the NIRPS data reveal evidence of Fe and Mg, from which we infer a Fe/Mg ratio consistent with the solar and host star ratios. Our results demonstrate that NIRPS can be an excellent instrument to obtain simultaneous measurements of refractory and volatile molecular species, paving the way for many future studies on the atmospheric composition, chemistry, and the formation history of close-in exoplanets.

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arXiv2025

The substellar population in Corona Australis

The substellar initial mass function (IMF) and the formation mechanisms of brown dwarfs (BDs) remain key open questions in star formation theory. IMF characterization in a large number of star-forming regions (SFRs) is essential for constraining these processes. We aim to identify and spectroscopically confirm very low-mass members of the Corona Australis (CrA) SFR to refine its substellar census, determine its low-mass IMF, and compare it to other clusters. Using deep I-band photometry from SuprimeCam/Subaru and data from the VISTA Hemisphere Survey, we identified low-mass BD candidates in CrA. We subsequently obtained NIR spectra of 173 of these candidates with KMOS/VLT, as well as optical spectra for 8 kinematic candidate members using FLOYDS/LCO. The kinematic candidates are confirmed as low-mass stellar members with spectral types M1 to M5. In contrast, all 173 BD candidates observed with KMOS are identified as contaminants. Although the follow-up yielded no new substellar members, it places strong constraints on the number of undetected substellar objects in the region. Combined with literature data, this enables us to derive the substellar IMF, which is consistent with a single power-law slope of alpha = 0.95+-0.06 in the range 0.01-1 MSun or alpha = 0.33+-0.19 in the range 0.01-0.1 MSun. The star-to-BD ratio in CrA is about 2. We also provide updated IMFs and star-to-BD ratios for Lupus3 and ChaI from the SONYC survey, reflecting revised distances from Gaia. Finally, we estimate surface densities and median FUV fluxes for 6 SFRs and clusters and compare their substellar populations as a function of environmental properties. The IMF and star-to-BD ratio show no clear dependence on stellar density or ionizing flux from the massive stars. A combined effect - where one factor enhances and the other suppresses BD formation - also appears unlikely. (Abridged)

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arXiv2025

NIRPS joining HARPS at ESO 3.6 m. On-sky performance and science objectives

The Near-InfraRed Planet Searcher (NIRPS) is a high-resolution, high-stability near-infrared (NIR) spectrograph equipped with an AO system. Installed on the ESO 3.6-m telescope, it was developed to enable radial velocity (RV) measurements of low-mass exoplanets around M dwarfs and to characterise exoplanet atmospheres in the NIR. This paper provides a comprehensive design overview and characterisation of the NIRPS instrument, reporting on its on-sky performance, and presenting its GTO programme. The instrument started its operations on 1 Apr 2023 after intensive on-sky testing phases. The spectral range continuously covers the Y, J, and H bands from 972.4 to 1919.6 nm. The thermal control system maintains 1 mK stability over several months. The NIRPS AO-assisted fibre link improves coupling efficiency and offers a unique high-angular resolution capability with a fibre acceptance of only 0.4 arcsec. A high spectral resolving power of 90 000 and 75 000 is provided in HA and HE modes, respectively. The overall throughput from the top of the atmosphere to the detector peaks at 13 percent. The RV precision, measured on the bright star Proxima with a known exoplanetary system, is 77 cm/s. NIRPS and HARPS can be used simultaneously, offering unprecedented spectral coverage for spectroscopic characterisation and stellar activity mitigation. Modal noise can be aptly mitigated by the implementation of fibre stretchers and AO scanning mode. Initial results confirm that NIRPS opens new possibilities for RV measurements, stellar characterisation, and exoplanet atmosphere studies with high precision and high spectral fidelity. NIRPS demonstrated stable RV precision at the level of 1 m/s over several weeks. The instrument high throughput offers a notable improvement over previous spectrographs, enhancing our ability to detect small exoplanets.

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arXiv2025

Diving into the planetary system of Proxima with NIRPS -- Breaking the metre per second barrier in the infrared

We obtained 420 high-resolution spectra of Proxima, over 159 nights, using the Near Infra Red Planet Searcher (NIRPS). We derived 149 nightly binned radial velocity measurements with a standard deviation of 1.69 m/s and a median uncertainty of 55 cm/s, and performed a joint analysis combining radial velocities, spectroscopic activity indicators, and ground-based photometry, to model the planetary and stellar signals present in the data, applying multi-dimensional Gaussian process regression to model the activity signals. We detect the radial velocity signal of Proxima b in the NIRPS data. All planetary characteristics are consistent with those previously derived using visible light spectrographs. In addition, we find evidence of the presence of the sub-Earth Proxima d in the NIRPS data. When combining the data with the HARPS observations taken simultaneous to NIRPS, we obtain a tentative detection of Proxima d and parameters consistent with those measured with ESPRESSO. By combining the NIRPS data with simultaneously obtained HARPS observations and archival data, we confirm the existence of Proxima d, and demonstrate that its parameters are stable over time and against change of instrument. We refine the planetary parameters of Proxima b and d, and find inconclusive evidence of the signal attributed to Proxima c (P = 1900 d) being present in the data. We measure Proxima b and d to have minimum masses of 1.055 $\pm$ 0.055 Me, and 0.260 $\pm$ 0.038 Me, respectively. Our results show that, in the case of Proxima, NIRPS provides more precise radial velocity data than HARPS, and a more significant detection of the planetary signals. The standard deviation of the residuals of NIRPS after the fit is 80 cm/s, showcasing the potential of NIRPS to measure precise radial velocities in the near-infrared.

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arXiv2025

Blind search for activity-sensitive lines in the near-infrared using HARPS and NIRPS observations of Proxima and Gl 581

Stellar activity variability is one of the main obstacles to the detection of Earth-like planets using the RV method. The aim of this work is to measure the effect of activity in the spectra of M dwarfs and detect activity-sensitive lines in the NIR. We took advantage of the simultaneous observations of HARPS and the newly commissioned NIRPS spectrograph to carry out a blind search of the most activity-sensitive spectral lines in the NIR using NIRPS spectra and known activity indicators in the optical from HARPS as a reference. We analysed the spectra of Proxima (M5.5V) and Gl 581 (M3V), two M dwarfs with different activity levels and internal structures. Spectral lines were identified for both stars and their profiles were fitted using different models. We found hundreds of lines sensitive to activity for both stars; the Proxima spectra were more affected. For Proxima, 32% of the identified lines can be used to measure the rotation period of the star, while for Gl 581 the numbers drops to 1%. The fraction of lines sensitive to activity increases with increasing line depth. A list of 17 lines with rotation period detection for both stars is provided. Stellar activity is able to affect a significant number of spectral lines in the NIR, and methods should be developed to mitigate those effects at the spectral level. The line distortions detected here are expected to come mainly from the flux effect due to temperature contrasts between active regions and the quiet photosphere; however, we cannot rule out the possibility that core-emission from chromospheric activity or Zeeman splitting are also affecting some lines. The new line lists presented here can be used to improve the RV extraction and the detection of RV variability due to stellar activity signals, and to help false positive detection and the modelling of activity variability, thereby enhancing exoplanet detection in the NIR.

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arXiv2025

NIRPS detection of delayed atmospheric escape from the warm and misaligned Saturn-mass exoplanet WASP-69b

Near-infrared high-resolution echelle spectrographs unlock access to fundamental properties of exoplanets, from their atmospheric escape and composition to their orbital architecture, which can all be studied simultaneously from transit observations. We present the first results of the newly commissioned ESO near-infrared spectrograph, NIRPS, from three transits of WASP-69b. We used the RM Revolutions technique to better constrain the orbital architecture of the system. We extracted the high-resolution helium absorption profile to study its spectral shape and temporal variations. Then, we made 3D simulations from the EVE code to fit the helium absorption time series. We measure a slightly misaligned orbit for WASP-69b (psi of 28.7+/-5.7 deg). We confirm the detection of helium with an average excess absorption of 3.17+/-0.05%. The helium absorption is spectrally and temporally resolved, extends to high altitudes and has a strong velocity shift up to -29.5+/-2.5 km/s 50 minutes after egress. EVE simulations put constraints on the mass loss of 2.25 10^11 g/s and hint at reactive chemistry within the cometary-like tail and interaction with the stellar winds that allow the metastable helium to survive longer than expected. Our results suggest that WASP-69b is undergoing a transformative phase in its history, losing mass while evolving on a misaligned orbit. This work shows how combining multiple observational tracers such as orbital architecture, atmospheric escape, and composition, is critical to understand exoplanet demographics and their formation and evolution. We demonstrate that NIRPS can reach precisions similar to HARPS for RM studies, and the high data quality of NIRPS leads to unprecedented atmospheric characterization. The high stability of NIRPS combined with the large GTO available for its consortium, enables in-depth studies of exoplanets as well as large population surveys.

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arXiv2025

ExoGemS The Effect of Offsets from True Orbital Parameters on Exoplanet High-Resolution Transmission Spectra

High-resolution spectroscopy (HRS) plays a crucial role in characterizing exoplanet atmospheres, revealing detailed information about their chemical composition, temperatures, and dynamics. However, inaccuracies in orbital parameters can affect the result of HRS analyses. In this paper, we simulated HRS observations of an exoplanet's transit to model the effects of an offset in transit midpoint or eccentricity on the resulting spectra. We derived analytical equations to relate an offset in transit midpoint or eccentricity to shifted velocities, and compared it with velocities measured from simulated HRS observations. Additionally, we compared velocity shifts in the spectrum of the ultra-hot Jupiter WASP-76b using previously reported and newly measured transit times. We found that transit midpoint offsets on the order of minutes, combined with eccentricity offsets of approximately $0.1$, lead to significant shifts in velocities, yielding measurements on the order of several kilometers per second. Thus, such uncertainties could conflate derived wind measurements.

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arXiv2025

Spectroscopy of Free-Floating Planetary-Mass Objects and their disks with JWST

Free-floating planetary-mass objects (FFPMOs) are known to harbor disks at young ages. Here, we present 1-13 $μm$ spectra for eight young FFPMOs with masses of 5-10 M$_\mathrm{Jup}$ (at ages of 1-5 Myr), using the NIRSpec and MIRI instruments on the James Webb Space Telescope. We derive fundamental properties of these targets, and find spectral types of M9.5 to L4, with effective temperatures of 1600-1900 K. The photospheric spectra of our targets show a clear diversity at similar temperatures, especially in the 3-5 $μm$ range, unaccounted for by existing atmospheric models. We find a silicate absorption feature in the photosphere of one of our targets, the first such detection in very young FFPMOs, indicating silicate clouds in their cool atmospheres. Six of our objects show mid-infrared excess emission above the photosphere, as well as silicate emission features, demonstrating the presence of disks. The shape and strength of the latter features constitute strong evidence of grain growth and crystallization, similar to what is seen in more massive brown dwarfs and stars. We also detect emission lines from hydrocarbon molecules in the disks of several targets. These are the lowest mass isolated objects found so far with silicate and hydrocarbon emission features arising in their disks. The presence of disks and their characteristics point to the potential for the formation of rocky companions around free-floating planetary-mass objects.

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arXiv2025

Continuous helium absorption from the leading and trailing tails of WASP-107b

The detection of helium escaping the atmosphere of exoplanets has revolutionized our understanding of atmospheric escape and exoplanetary evolution. Using high-precision spectroscopic observations from the James Webb Space Telescope (JWST) NIRISS-SOSS mode, we report the detection of significant helium absorption during the pre-transit phase of WASP-107b (17$σ$), as well as in the transit and post-transit phases. This unique continuous helium absorption begins approximately 1.5 hours before the planet's ingress and reveals the presence of an extended thermosphere. The observations show a maximum transit depth of 2.395$\% \pm$ 0.01$\%$ near the helium triplet (36$σ$; at NIRISS-SOSS resolution $\sim$ 700). Our ellipsoidal model of the planetary thermosphere matches well the measured light curve suggesting an outflow extending to tens of planetary radii. Furthermore, we confidently detect water absorption (log10 H2O=-2.5 $\pm$ 0.6), superimposed with a short-wavelength slope which we attribute to a prominent signature from unocculted stellar spots (5.2$σ$), rather than a small-particle haze slope. We place an upper limit on the abundance of K (log10 K$<$-4.86, or K/H$<$ 75$\times$ stellar) at 2$σ$, which is consistent with the O/H super-solar metallicity estimate. This study underscores the transformative potential of JWST for tracing atmospheric and mass-loss processes, while offering a benchmark for future studies targeting helium escape and its implications for planetary evolution.

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arXiv2025

Detection of Hydrocarbons in the Disk around an Actively-Accreting Planetary-Mass Object

We present the 0.6--12-micron spectrum of Cha\,1107-7626, a 6-10 Jupiter-mass free-floating object in the $\sim$2\,Myr-old Chamaeleon-I star-forming region, from observations with the NIRSpec and MIRI instruments onboard the James Webb Space Telescope. We confirm that Cha\,1107-7626 is one of the lowest-mass objects known to harbor a dusty disk with infrared excess emission at wavelengths beyond 4 microns. Our NIRSpec data, and prior ground-based observations, provide strong evidence for ongoing accretion through Hydrogen recombination lines. In the mid-infrared spectrum, we detect unambiguously emission lines caused by methane (CH$_\mathrm{4}$) and ethylene (C$_\mathrm{2}$H$_\mathrm{4}$) in its circum-substellar disk. Our findings mean that Cha 1107-7626 is by far the lowest-mass object with hydrocarbons observed in its disk. The spectrum of the disk looks remarkably similar to that of ISO-ChaI 147, a very low mass star with a carbon-rich disk that is 10 to 20 times more massive than Cha\,1107-7626. The hydrocarbon lines can be accounted for with a model assuming gas temperatures of a few hundred Kelvin in the inner disk. The obvious similarities between the spectra of a low-mass star and a planetary-mass object indicate that the conditions in the inner disks can be similar across a wide range of central object masses.

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