arXiv subjects

Loic Albert

Publications and source records attributed to Loic Albert.

At least 19 recordsLinked to original sources

arXiv2026

The MIRI Excess around Degenerates (MEAD) Survey II: A Probable Planet detected via IR Excess around WD 0644+025

The MIRI Excess Around Degenerates (MEAD) Survey is a cycle 2 JWST program designed to image nearby white dwarfs with MIRI at 10 and 15 microns. This survey targeted 56 white dwarfs within 25 pc to search for mid-infrared excesses, flux deficits from collision-induced absorption, and resolved substellar companions. In this paper we present our analysis of WD 0644+025, an unusually massive white dwarf (0.95 Msun) and the MEAD target exhibiting the most significant mid-infrared excess. The observed JWST MIRI photometry shows a 7.3 sigma excess at 15 microns and a 3.6 sigma excess at 10 microns, which may be associated with either a planetary companion or a circumstellar dust disk. This excess corresponds to a companion mass of 6.8 Mjup (Teff=261 +/- 9 K) with orbital distance <11.8 au, although substantially lower masses are possible if we consider a closely orbiting insolated companion. No spatially resolved sources are detected within 200 au, with contrast curve analysis excluding planets more massive than 2 Mjup beyond ~12 au. Metal pollution is confirmed in both archival Keck HIRES spectra from 1999 and new observations from 2025, with no evidence suggesting the accretion rate has substantially changed over the decades. We explore possible dust disk morphologies to describe the observed IR excess, and find that traditional debris disks struggle to fit our data. WD 0644+025 thus represents a compelling case study in the growing population of white dwarfs with cold infrared excesses, and highlights JWST's ability to probe planetary system remnants inaccessible to prior infrared observatories.

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arXiv2025

Follow-up Observations of Candidate White Dwarf Planets with MIRI

We report on second-epoch imaging of two candidate planet-hosting white dwarfs stars, WD2105-82 and WD1202-232. Both stars showed evidence of resolved, planet-mass candidate companions in observations using the MIRI mid-infrared imager on JWST. WD2105-82 also showed evidence of an infrared excess consistent with an unresolved 1.4 Jupiter mass companion with an orbital separation of <4 au. Our second epoch observations confirm that the source of the excess shares common proper motion with the star. The excess is almost certainly due to a companion planet or debris disk. However, neither of the two resolved sources with projected separations of >1" in the first epoch of JWST observations show measurable proper motion and are thus likely faint, unresolved background galaxies. We also search for common proper motion companions out to hundreds of au, but find no evidence of widely separated companions.

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arXiv2025

Composition of planetary debris around the white dwarf GD 362

White dwarf stars with high abundances of heavy elements in their atmospheres and infrared excesses are believed to be accreting planetary material. GD 362 is one of the most heavily polluted white dwarfs and has an exceptionally strong mid-infrared excess, reprocessing 2.4% of the star's light into the mid-infrared. We present a high signal-to-noise, medium-resolution spectrum of GD 362 obtained with JWST, covering 0.6 to 17 microns, along with photometry out to 25.5 microns. The mid-infrared spectrum is dominated by an exceptionally strong 9 to 11 micron silicate feature, which can be explained by a combination of olivine and pyroxene silicate minerals. Grains such as carbon, hotter than silicates, are required to explain the near-infrared emission. The silicates and carbon reside in a disk from 140 to 1400 stellar radii, and the disk scale height is greater than half the stellar radius. The elemental abundances of the solid material, relative to Si, are within a factor of 2 of meteoritic (CI chondrites) for C, O, Mg, Al, and Fe, with Al elevated and O slightly depleted. A similar pattern is observed for the abundances of accreted material in the stellar photosphere. Hydrogen is an exception, because no significant H-bearing minerals or water were detected in the disk, despite a large H abundance in the photosphere.

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arXiv2025

Empirical Modeling of Zodiacal Backgrounds to Improve JWST NIRISS/SOSS Data Reduction

Zodiacal light -- arising from both the thermal emission and scattered sunlight by interplanetary dust -- is the dominant component of the sky background in NIRISS Single Object Slitless Spectroscopy (SOSS) observations. The GR700XD grism disperses the zodiacal background across multiple diffraction orders, producing a characteristic two-dimensional, spectral order-dependent background pattern on the detector that reflects the combined contribution of overlapping orders. Observations reveal significant variability in background intensity driven by JWST's sky pointing and seasonal changes, underscoring the need for precise background subtraction during SOSS data reduction. Current methods rely on a generic background template derived during commissioning, scaled to match individual exposures. However, mis-scaled templates can leave behind structured residuals that may compromise the precision of exoplanet transit depth measurements. To improve background modeling, we conducted two calibration programs (PID 4479 and 6658) using the FULL frame readout mode and a 5-row by 2-column mosaic pattern to sample a range of sky positions. These observations enable empirical reconstruction of the sky background and provide detailed insights into its spatial and spectral characteristics. We present a library of empirically derived background templates and evaluate their performance, alongside the current template, by benchmarking against the PID 2113 dataset, which includes contemporaneous background exposures. This work supports aims to enhance background subtraction for SOSS time-series to achieve higher-precision exoplanet spectroscopy with JWST.

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arXiv2025

JWST NIRISS Transmission Spectroscopy of the Super-Earth GJ 357b, a Favourable Target for Atmospheric Retention

We present a JWST NIRISS/SOSS transmission spectrum of the super-Earth GJ 357 b: the first atmospheric observation of this exoplanet. Despite missing the first $\sim$40 % of the transit due to using an out-of-date ephemeris, we still recover a transmission spectrum that does not display any clear signs of atmospheric features. We perform a search for Gaussian-shaped absorption features within the data but find that this analysis yields comparable fits to the observations as a flat line. We compare the transmission spectrum to a grid of atmosphere models and reject, to 3-$\sigma$ confidence, atmospheres with metallicities $\lesssim$100$\times$ solar ($\sim$4 g/mol) with clouds at pressures down to 0.01 bar. We analyse how the retention of a secondary atmosphere on GJ 357 b may be possible due to its higher escape velocity compared to an Earth-sized planet and the exceptional inactivity of its host star relative to other M2.5V stars. The star's XUV luminosity decays below the threshold for rapid atmospheric escape early enough that the volcanic revival of an atmosphere of several bars of CO$_2$ is plausible, though subject to considerable uncertainty. Finally, we model the feasibility of detecting an atmosphere on GJ 357 b with MIRI/LRS, MIRI photometry, and NIRSpec/G395H. We find that, with two eclipses, it would be possible to detect features indicative of an atmosphere or surface. Further to this, with 3-4 transits, it would be possible to detect a 1 bar nitrogen-rich atmosphere with 1000 ppm of CO$_2$.

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arXiv2024

A Novel Survey for Young Substellar Objects with the W-band Filter. VII. Water-Bearing Objects in the Core of the Rho Ophiuchi Cloud Complex

We present a study of very low-mass stars and brown dwarfs in the rich star-forming core of the Rho Ophiuchi cloud complex. The selection of the sample relies on detecting the inherent water absorption characteristic in young substellar objects. Of the 22 water-bearing candidates selected, 15 have a spectral type of M6 or later. Brown dwarf candidates too faint for membership determination by Gaia have their proper motions derived by deep-infrared images spanning six years. Astrometric analysis confirms 21/22 sources as members, one identified as a contaminant. Infrared colors and the spectral energy distribution of each water-bearing candidate are used to diagnose the mass, age, and possible existence of circumstellar dust. 15 sources exhibit evidence of disks in their spectral energy distributions, as late as in M8-type objects. Spectroscopy for bright candidates has confirmed one as an M8 member and verified two sources (with disks) exhibiting signatures of magnetospheric accretion.

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arXiv2024

JWST Resolves CIA Features in White Dwarfs

Infrared-faint white dwarfs are cool white dwarfs exhibiting significant infrared flux deficits, most often attributed to collision-induced absorption (CIA) from H$_2$-He in mixed hydrogen-helium atmospheres. We present James Webb Space Telescope (JWST) near- and mid-infrared spectra of three such objects using NIRSpec (0.6-5.3 $\mu$m) and MIRI (5-14 $\mu$m): LHS 3250, WD J1922+0233, and LHS 1126. Surprisingly, for LHS 3250, we detect no H$_2$-He CIA absorption at 2.4 $\mu$m, instead observing an unexpected small flux bump at this wavelength. WD J1922+0233 exhibits the anticipated strong absorption feature centered at 2.4 $\mu$m, but with an unexpected narrow emission-like feature inside this absorption band. LHS 1126 shows no CIA features and follows a $\lambda^{-2}$ power law in the mid-infrared. LHS 1126's lack of CIA features suggests a very low hydrogen abundance, with its infrared flux depletion likely caused by He-He-He CIA. For LHS 3250 and WD J1922+0233, the absence of a 1.2 $\mu$m CIA feature in both stars argues against ultracool temperatures, supporting recent suggestions that infrared-faint white dwarfs are warmer and more massive than previously thought. This conclusion is further solidified by Keck near-infrared spectroscopy of seven additional objects. We explore possible explanations for the unexpected emission-like features in both stars, and temperature inversions above the photosphere emerge as a promising hypothesis. Such inversions may be common among the infrared-faint population, and since they significantly affect the infrared spectral energy distribution, this would impact their photometric fits. Further JWST observations are needed to confirm the prevalence of this phenomenon and guide the development of improved atmospheric models.

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arXiv2024

JWST Directly Images Giant Planet Candidates Around Two Metal-Polluted White Dwarf Stars

We report the discovery of two directly imaged, giant planet candidates orbiting the metal-rich DAZ white dwarfs WD 1202-232 and WD 2105-82. JWST's Mid-Infrared Instrument (MIRI) data on these two stars show a nearby resolved source at a projected separation of 11.47 and 34.62 au, respectively. Assuming the planets formed at the same time as their host stars, with total ages of 5.3 and 1.6Gyr, the MIRI photometry is consistent with giant planets with masses about 1-7 Jupiter Masses. The probability of both candidates being false positives due to red background sources is approximately 1 in 3000. If confirmed, these would be the first directly imaged planets that are similar in both age and separation to the giant planets in our own solar system, and they would demonstrate that widely separated giant planets like Jupiter survive stellar evolution. Giant planet perturbers are widely used to explain the tidal disruption of asteroids around metal-polluted white dwarfs. Confirmation of these two planet candidates with future MIRI imaging would provide evidence that directly links giant planets to metal pollution in white dwarf stars.

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arXiv2023

A MIRI Search for Planets and Dust Around WD 2149+02

The launch of JWST has ushered in a new era of high precision infrared astronomy, allowing us to probe nearby white dwarfs for cold dust, exoplanets, and tidally heated exomoons. While previous searches for these exoplanets have successfully ruled out companions as small as 7-10 Jupiter masses, no instrument prior to JWST has been sensitive to the likely more common sub-Jovian mass planets around white dwarfs. In this paper, we present the first multi-band photometry (F560W, F770W, F1500W, F2100W) taken of WD 2149+021 with the Mid-Infrared Instrument (MIRI) on JWST. After a careful search for both resolved and unresolved planets, we do not identify any compelling candidates around WD 2149+021. Our analysis indicates that we are sensitive to companions as small as ~0.34 MJup outwards of 1."263 (28.3 au) and ~0.64 MJup at the innermost working angle (0."654, 14.7 au) with 5 sigma confidence, placing significant constraints on any undetected companions around this white dwarf. The results of these observations emphasize the exciting future of sub-Jovian planet detection limits by JWST, which can begin to constrain how often these planets survive their host stars evolution.

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arXiv2023

The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope -- III. Single Object Slitless Spectroscopy

The Near Infrared Imager and Slitless Spectrograph instrument (NIRISS) is the Canadian Space Agency (CSA) contribution to the suite of four science instruments of JWST. As one of the three NIRISS observing modes, the Single Object Slitless Spectroscopy (SOSS) mode is tailor-made to undertake time-series observations of exoplanets to perform transit spectroscopy. The SOSS permits observing point sources between 0.6 and 2.8 um at a resolving power of 650 at 1.25 um using a slit-less cross-dispersing grism while its defocussing cylindrical lens enables observing targets as bright as J=6.7 by spreading light across 23 pixels along the cross-dispersion axis. This paper officially presents the design of the SOSS mode, its operation, characterization, and its performance, from ground-based testing and flight-based Commissioning. On-sky measurements demonstrate a peak photon conversion efficiency of 55% at 1.2 um. The first time-series on the A-type star BD+60o1753 achieves a flux stability close to the photon-noise limit, so far tested to a level of 20 parts per million on 40-minute time-scales after simply subtracting a long-term trend. Uncorrected 1/f noise residuals underneath the spectral traces add an extra source of noise equivalent to doubling the readout noise. Preliminary analysis of a HAT-P-14b transit time-series indicates that it is difficult to remove all the noise in pixels with partially saturated ramps. Overall, the SOSS delivers performance at the level required to tackle key exoplanet science programs such as detecting secondary atmospheres on terrestrial planets and measuring abundances of several chemical species in gas giants.

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arXiv2023

The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope -- I. Instrument Overview and in-Flight Performance

The Near-Infrared Imager and Slitless Spectrograph (NIRISS) is the science module of the Canadian-built Fine Guidance Sensor (FGS) onboard the James Webb Space Telescope (JWST). NIRISS has four observing modes: 1) broadband imaging featuring seven of the eight NIRCam broadband filters, 2) wide-field slitless spectroscopy (WFSS) at a resolving power of $\sim$150 between 0.8 and 2.2 $\mu$m, 3) single-object cross-dispersed slitless spectroscopy (SOSS) enabling simultaneous wavelength coverage between 0.6 and 2.8 $\mu$m at R$\sim$700, a mode optimized for exoplanet spectroscopy of relatively bright ($J<6.3$) stars and 4) aperture masking interferometry (AMI) between 2.8 and 4.8 $\mu$m enabling high-contrast ($\sim10^{-3}-10^{-4}$) imaging at angular separations between 70 and 400 milliarcsec for relatively bright ($M<8$) sources. This paper presents an overview of the NIRISS instrument, its design, its scientific capabilities, and a summary of in-flight performance. NIRISS shows significantly better response shortward of $\sim2.5\,\mu$m resulting in 10-40% sensitivity improvement for broadband and low-resolution spectroscopy compared to pre-flight predictions. Two time-series observations performed during instrument commissioning in the SOSS mode yield very stable spectro-photometry performance within $\sim$10% of the expected noise. The first space-based companion detection of the tight binary star AB Dor AC through AMI was demonstrated.

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arXiv2023

Awesome SOSS: Transmission Spectroscopy of WASP-96b with NIRISS/SOSS

The future is now - after its long-awaited launch in December 2021, JWST began science operations in July 2022 and is already revolutionizing exoplanet astronomy. The Early Release Observations (ERO) program was designed to provide the first images and spectra from JWST, covering a multitude of science cases and using multiple modes of each on-board instrument. Here, we present transmission spectroscopy observations of the hot-Saturn WASP-96b with the Single Object Slitless Spectroscopy (SOSS) mode of the Near Infrared Imager and Slitless Spectrograph, observed as part of the ERO program. As the SOSS mode presents some unique data reduction challenges, we provide an in-depth walk-through of the major steps necessary for the reduction of SOSS data: including background subtraction, correction of 1/f noise, and treatment of the trace order overlap. We furthermore offer potential routes to correct for field star contamination, which can occur due to the SOSS mode's slitless nature. By comparing our extracted transmission spectrum with grids of atmosphere models, we find an atmosphere metallicity between 1x and 5x solar, and a solar carbon-to-oxygen ratio. Moreover, our models indicate that no grey cloud deck is required to fit WASP-96b's transmission spectrum, but find evidence for a slope shortward of 0.9$\mu$m, which could either be caused by enhanced Rayleigh scattering or the red wing of a pressure-broadened Na feature. Our work demonstrates the unique capabilities of the SOSS mode for exoplanet transmission spectroscopy and presents a step-by-step reduction guide for this new and exciting instrument.

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arXiv2023

A novel survey for young substellar objects with the W-band filter VI: Spectroscopic census of sub-stellar members and the IMF of $\sigma$ Orionis cluster

Low-mass stars and sub-stellar objects are essential in tracing the initial mass function (IMF). We study the nearby young $\sigma$ Orionis cluster (d$\sim$408 pc; age$\sim$1.8 Myr) using deep NIR photometric data in J, W and H-bands from WIRCam on the Canada-France-Hawaii Telescope. We use the water absorption feature to photometrically select the brown dwarfs and confirm their nature spectroscopically with the IRTF-SpeX. Additionally we select candidate low-mass stars for spectroscopy and analyze their membership and that of literature sources using astrometry from Gaia DR3. We obtain the near-IR spectra for 28 very low-mass stars and brown dwarfs and estimate their spectral type between M3-M8.5 (mass ranging between 0.3-0.01 M$_{\odot}$). Apart from these, we also identify 5 new planetary mass candidates which require further spectroscopic confirmation of youth. We compile the comprehensive catalog of 170 spectroscopically confirmed members in the central region of the cluster, for a wide mass range of $\sim$19-0.004 M$_{\odot}$. We estimate the star/BD ratio to be $\sim$4, within the range reported for other nearby star forming regions. With the updated catalog of members we trace the IMF down to 4 M$_\mathrm{Jup}$ and we find that a two-segment power-law fits the sub-stellar IMF better than the log-normal distribution.

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arXiv2022

A Novel Survey for Young Substellar Objects with the W band Filter.V. IC 348 and Barnard 5 in the Perseus Cloud

We report the discovery of substellar objects in the young star cluster IC 348 and the neighboring Barnard 5 dark cloud, both at the eastern end of the Perseus star-forming complex. The substellar candidates are selected using narrowband imaging, i.e., on and off photometric technique with a filter centered around the water absorption feature at 1.45 microns, a technique proven to be efficient in detecting water-bearing substellar objects. Our spectroscopic observations confirm three brown dwarfs in IC 348. In addition, the source WBIS 03492858+3258064, reported in this work, is the first confirmed brown dwarf discovered toward Barnard 5. Together with the young stellar population selected via near- and mid-infrared colors using the Two Micron All Sky Survey and the Wide-field Infrared Survey Explorer, we diagnose the relation between stellar versus substellar objects with the associated molecular clouds. Analyzed by Gaia EDR3 parallaxes and kinematics of the cloud members across the Perseus region, we propose the star formation scenario of the complex under influence of the nearby OB association.

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arXiv2022

The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope -- II. Wide Field Slitless Spectroscopy

We present the wide field slitless spectroscopy mode of the NIRISS instrument on the James Webb Space Telescope. This mode employs two orthogonal low-resolution (resolving power $\approx 150$) grisms in combination with a set of six blocking filters in the wavelength range 0.8 to $2.3\,\mu$m to provide a spectrum of almost every source across the field-of-view. When combined with the low background, high sensitivity and high spatial resolution afforded by the telescope, this mode will enable unprecedented studies of the structure and evolution of distant galaxies. We describe the performance of the as-built hardware relevant to this mode and expected imaging and spectroscopic sensitivity. We discuss operational and calibration procedures to obtain the highest quality data. As examples of the observing mode usage, we present details of two planned Guaranteed Time Observations programs: The Canadian NIRISS Unbiased Cluster Survey (CANUCS) and The NIRISS Survey for Young Brown Dwarfs and Rogue Planets.

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arXiv2020

A wide planetary-mass companion to a young low-mass brown dwarf in Ophiuchus

We present the discovery of a planetary-mass companion to CFHTWIR-Oph 98, a low-mass brown dwarf member of the young Ophiuchus star-forming region, with a wide 200-au separation (1.46 arcsec). The companion was identified using Hubble Space Telescope images, and confirmed to share common proper motion with the primary using archival and new ground-based observations. Based on the very low probability of the components being unrelated Ophiuchus members, we conclude that Oph 98 AB forms a binary system. From our multi-band photometry, we constrain the primary to be an M9-L1 dwarf, and the faint companion to have an L2-L6 spectral type. For a median age of 3 Myr for Ophiuchus, fits of evolutionary models to measured luminosities yield masses of $15.4\pm0.8$ M$_\mathrm{Jup}$ for Oph 98 A and $7.8\pm0.8$ M$_\mathrm{Jup}$ for Oph 98 B, with respective effective temperatures of $2320\pm40$ K and $1800\pm40$ K. For possible system ages of 1-7 Myr, masses could range from 9.6-18.4 M$_\mathrm{Jup}$ for the primary, and from 4.1-11.6 M$_\mathrm{Jup}$ for the secondary. The low component masses and very large separation make this binary the lowest binding energy system imaged to date, indicating that the outcome of low-mass star formation can result in such extreme, weakly-bound systems. With such a young age, Oph 98 AB extends the growing population of young free-floating planetary-mass objects, offering a new benchmark to refine formation theories at the lowest masses.

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arXiv2018

Simulated JWST/NIRISS Transit Spectroscopy of Anticipated TESS Planets Compared to Select Discoveries from Space-Based and Ground-Based Surveys

The Transiting Exoplanet Survey Satellite (TESS) will embark in 2018 on a 2-year wide-field survey mission, discovering over a thousand terrestrial, super-Earth and sub-Neptune-sized exoplanets potentially suitable for follow-up observations using the James Webb Space Telescope (JWST). This work aims to understand the suitability of anticipated TESS planet discoveries for atmospheric characterization by JWST's Near InfraRed Imager and Slitless Spectrograph (NIRISS) by employing a simulation tool to estimate the signal-to-noise (S/N) achievable in transmission spectroscopy. We applied this tool to Monte Carlo predictions of the TESS expected planet yield and then compared the S/N for anticipated TESS discoveries to our estimates of S/N for 18 known exoplanets. We analyzed the sensitivity of our results to planetary composition, cloud cover, and presence of an observational noise floor. We found that several hundred anticipated TESS discoveries with radii from 1.5 to 2.5 times the Earth's radius will produce S/N higher than currently known exoplanets in this radius regime, such as K2-3b or K2-3c. In the terrestrial planet regime, we found that only a few anticipated TESS discoveries will result in higher S/N than currently known exoplanets, such as the TRAPPIST-1 planets, GJ1132b, and LHS1140b. However, we emphasize that this outcome is based upon Kepler-derived occurrence rates, and that co-planar compact multi-planet systems (e.g., TRAPPIST-1) may be under-represented in the predicted TESS planet yield. Finally, we apply our calculations to estimate the required magnitude of a JWST follow-up program devoted to mapping the transition region between hydrogen-dominated and high molecular weight atmospheres. We find that a modest observing program of between 60 to 100 hours of charged JWST time can define the nature of that transition (e.g., step function versus a power law).

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arXiv2014

Near-infrared Thermal Emission Detections of a number of hot Jupiters and the Systematics of Ground-based Near-infrared Photometry

We present detections of the near-infrared thermal emission of three hot Jupiters and one brown-dwarf using the Wide-field Infrared Camera (WIRCam) on the Canada-France-Hawaii Telescope (CFHT). These include Ks-band secondary eclipse detections of the hot Jupiters WASP-3b and Qatar-1b and the brown dwarf KELT-1b. We also report Y-band, $K_{CONT}$-band, and two new and one reanalyzed Ks-band detections of the thermal emission of the hot Jupiter WASP-12b. We present a new reduction pipeline for CFHT/WIRCam data, which is optimized for high precision photometry. We also describe novel techniques for constraining systematic errors in ground-based near-infrared photometry, so as to return reliable secondary eclipse depths and uncertainties. We discuss the noise properties of our ground-based photometry for wavelengths spanning the near-infrared (the YJHK-bands), for faint and bright-stars, and for the same object on several occasions. For the hot Jupiters WASP-3b and WASP-12b we demonstrate the repeatability of our eclipse depth measurements in the Ks-band; we therefore place stringent limits on the systematics of ground-based, near-infrared photometry, and also rule out violent weather changes in the deep, high pressure atmospheres of these two hot Jupiters at the epochs of our observations.

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