arXiv subjects

Arthur Vigan

Publications and source records attributed to Arthur Vigan.

At least 19 recordsLinked to original sources

arXiv2026

The Roman Coronagraph Community Participation Program: corgisim - a simulation suite for the Nancy Grace Roman Space Telescope Coronagraph Instrument

NASA's Roman Space Telescope will feature a pathfinder Coronagraph Instrument to demonstrate advanced high-contrast imaging from space, paving the way for future missions like the Habitable Worlds Observatory. The Coronagraph Instrument could obtain imaging, polarimetry and spectroscopy of Jupiter analogs in reflected visible light for the first time. We present the development of an open-source simulation package ``corgisim'' as part of the Roman Coronagraph Community Participate Program. Built on established optical propagation libraries including PROPER and CGISim, corgisim provides a user-friendly, publicly available Python framework for end-to-end simulations of the Coronagraph Instrument observations. The package produces high-fidelity, format-compliant data for pre-launch calibration, pipeline testing, and community applications such as target selection and observation planning. We will give an overview of corgisim's infrastructure, functionalities, and current implementation across planned imaging, polarimetry, and spectroscopy modes, including the ability to simulate host stars, injected companions, and extended disks. We will also highlight suitable applications of corgisim and provide guidance on how users can access and employ the software.

astro-ph.IM↗

arXiv2026

The Roman Coronagraph Community Participation Program: Using the Zernike wavefront sensor for a full characterisation of the Roman Space Telescope and the Coronagraph Instrument

The Roman Space Telescope Coronagraph Instrument (CGI) will demonstrate a series of technologies and techniques to enable the direct detection of reflected-light planets with space-based observatories. Among the several available observing modes and coronagraphic devices embarked in CGI, there is the transmissive dual-path Zernike wavefront sensor (ZWFS) that could be used to directly measure optical aberrations in the system. The dual-path ZWFS is currently unsupported, but in this work we advocate for the commissioning of this unique observing mode. We investigate the sensitivity of the ZWFS using CGI simulator and other tools developed and supported by the Roman community participation program (CPP). This type of analysis is crucial for understanding the stability of the Roman observatory and to prepare the path towards HWO.

astro-ph.IM↗

arXiv2026

ForMoSA: Forward Modeling tool for Spectral Analysis

ForMoSA (FORward MOdeling tool for Spectral Analysis) is an open-source Python package to fit spectroscopic and photometric observations using a Bayesian framework. It can utilize different self-consistent atmospheric models to perform robust parameter space exploration. It has been mainly designed for fitting directly imaged young planetary-mass brown dwarfs and exoplanets. The developments within ForMoSA are supported by an international collaboration of several laboratories in France (IPAG, LIRA, LAM, and Lagrange), Germany (MPIA), USA (NASA Goddard), and Chile (FCLA, Universidad Diego Portales, and Universidad de Chile). The evolution of the code and the growing interest from the scientific community has led to the need for this dedicated publication alongside the release of ForMoSA v2.0, which has been refactored into a class architecture with user-friendly features and extended functionalities.

astro-ph.IM↗

arXiv2026

Latitudinal chemical and cloud variations in the atmosphere of a brown dwarf

Brown dwarfs are massive analogues of extrasolar giant planets. Compared to exoplanets whose observations are generally limited by the presence of their bright host star, brown dwarfs are ideal targets for studying substellar atmospheric physics, chemistry and dynamics. Previous observations and simulations of their atmospheres suggest preferential cloud formation around the equator, associated with an equator-pole thermal gradient. Here we show that this atmospheric structure should induce latitudinal chemical variations detectable by the Doppler effect. We introduce a new method - Differential Molecular Rotational Broadening - which consists in comparing the apparent rotational broadening of individual molecules from high-resolution spectra. Application of this approach to VLT-CRIRES observations for different molecules (CO, H2O, CH4 and NH3) in the atmosphere of the brown dwarf DENIS J0255-4700 confirms the existence of latitudinal chemical variations. Our data suggest a depletion of CH4 and NH3 at low latitudes, consistent with an equatorial cloud belt. Our method could be applied to multiple brown dwarfs and exoplanets to map their atmospheres and to study various atmospheric processes.

astro-ph.EP↗

arXiv2026

SAXO+, the second-stage adaptive optics for SPHERE: NCPA compensation and dark-hole loop with a pyramid wavefront sensor

The SAXO+ upgrade of the VLT/SPHERE adaptive optics system introduces a second-stage near-infrared pyramid wavefront sensor to improve high-contrast imaging, making accurate calibration of non-common path aberrations (NCPAs) essential to fully exploit its performance. This work refines the expected level of NCPAs in SAXO+ and presents the calibration procedures developed for static NCPA compensation and focal-plane dark-hole control. Monte Carlo simulations based on an updated Zemax optical model were used to estimate the NCPA error budget. These simulations are in good agreement with previous measurements on SPHERE and with the assumptions adopted in earlier performance studies. We also propose a calibration strategy that offloads most static aberration correction to the first-stage deformable mirror while preserving the second-stage mirror stroke for high-speed adaptive optics correction. These results validate the expected SAXO+ optical quality and establish the calibration framework required for efficient NCPA compensation and focal-plane wavefront control during future on-sky operations.

astro-ph.IM↗

arXiv2026

Panchromatic View of the Frigid Jovian Exoplanet COCONUTS-2 b

We use a high signal-to-noise MIRI-LRS spectrum (5.45 - 11 $\mu$m, R$_\lambda$ $\sim100$) of COCONUTS-2~b revealing prominent molecular features of H$_2$O, CH$_4$ and NH$_3$. This dataset is combined with spectra from Gemini/FLAMINGOS-2 and JWST/NIRSpec (G395H), as well as photometry from WISE and Spitzer, resulting in almost continuous wavelength coverage from 1 to 15 $\mu$m. We analyze the data using five grids of self-consistent atmospheric models, spanning a wide range of T$_\text{eff}$, log(g), and [M/H]. We also investigate the use of Gaussian Processes to account for correlated noise either caused by the spectrograph or by systematic departures of models in the inversion framework. All models manage to fit the overall combined observations but predict fainter flux in Y- and N-bands. Classical model comparison suggests that the ATMO2020++ synthetic specra (with and without PH$_3$) are statistically preferred. Fitting for the correlated noise of the three spectroscopic instruments, ATMO2020++ models yields constraints consistent with previous studies and evolutionary models predictions: T$_\text{eff}$ $=496^{+5}_{-3}$ K, log(g) $=4.30^{+0.04}_{-0.02}$ dex, [M/H] $=-0.02^{+0.03}_{-0.02}$ dex, and R $=1.03^{+0.01}_{-0.02}$ R$_\text{jup}$. The extended wavelength coverage provided by MIRI (accounting for 41% of the bolometric flux) completes the SED, yielding a precise luminosity estimation of log(L/L$_{\odot}$) $=-6.166\pm0.002$ dex. Combined with a previous estimate of the system age ($414\pm23$ Myr), cooling models predict a mass of M $=7.3\pm0.3$ M$_\text{jup}$.

astro-ph.EP↗

arXiv2025

YSES 2b is a background star: Differential astrometric M-dwarf measurements in time

We wish to confirm the nature of YSES 2b, a purportedly faint companion of the young star YSES 2. We used on-sky observations from SPHERE and GRAVITY to measure the astrometric position of 2b with respect to the star YSES 2, and examined the competing hypotheses of (i) a bound substellar companion versus (ii) a distant unrelated background source with a non-zero proper motion. YSES 2b appears to be a late-type M-dwarf star over 2 kiloparsecs behind the star YSES 2. It has a transverse velocity of about 300 km/s and is located within one of the spiral arms of the Galaxy. The main discriminant was multiple epochs of GRAVITY astrometry that identified the sub-milliarcsecond parallactic motion of the star.

astro-ph.EP↗

arXiv2025

Spectral analysis of two directly imaged benchmark L dwarf companions at the stellar-substellar boundary

We used multiple epochs of high-contrast imaging spectrophotometric observations to determine the atmospheric characteristics and thermal evolution of two previously detected benchmark L dwarf companions, HD 112863 B and HD 206505 B. We analyzed IRDIS and IFS data from VLT/SPHERE of each companion, both of which have dynamical masses near the stellar-substellar boundary. We compared each companion with empirical spectral standards, as well as constrained their physical properties through atmospheric model fits. From these analyses, we estimate that HD 112863 B is spectral type $\rm{L}3\pm1$ and that HD 206505 B is spectral type $\rm{L}2\pm1$. Using the BT-Settl atmospheric model grids, we find a bimodal solution for the atmospheric model fit of HD 112863 B, such that $T_{\rm{eff}}=1757^{+37}_{-36}$ K or $2002^{+23}_{-24}$ K and $\log{g}=4.973^{+0.057}_{-0.063}$ or $5.253^{+0.037}_{-0.033}$, while for HD 206505 B, $T_{\rm{eff}}=1754^{+13}_{-13}$ K and $\log{g}=4.919^{+0.031}_{-0.029}$. Comparing the bolometric luminosities of both companions with evolutionary models imply that both companions are likely above the hydrogen burning limit.

astro-ph.EP↗

arXiv2025

Differentiating Formation Models with New Dynamical Masses for the PDS 70 Protoplanets

Hot- and cold-start planet formation models predict differing luminosities for the young, bright planets that direct imaging surveys are most sensitive to. However, precise mass estimates are required to distinguish between these models observationally. The presence of two directly imaged planets, PDS 70 b and c, in the PDS 70 protoplanetary disk provides us a unique opportunity for dynamical mass measurements, since the masses for these planets are currently poorly constrained. Fitting orbital parameters to new astrometry of these planets, taken with VLTI/GRAVITY in the $K$~band, we find $2\sigma$ dynamical upper mass limits of 4.9 $M_{\rm Jup}$ for b and 13.6 $M_{\rm Jup}$ for c. Adding astrometry from the newly proposed planet candidate PDS 70 d into our model, we determine $2\sigma$ dynamical upper mass limits of 5.3 $M_{\rm Jup}$, 7.5 $M_{\rm Jup}$ and 2.2 $M_{\rm Jup}$ for b, c, and the candidate d respectively. However, $N$-body analysis of the orbits fit in this case suggest that the inclusion of $d$ makes the system unstable. Using the upper mass limits for b and c we rule out the coldest-start formation models for both planets, calculating minimum post-formation entropies ($S_i$) of 9.5 $k_{\rm B}/{\rm baryon}$ and 8.4 $k_{\rm B}/{\rm baryon}$ respectively. This places PDS 70 b and c on the growing list of directly-imaged planets inconsistent with cold-start formation.

astro-ph.EP↗

arXiv2024

VLTI/GRAVITY Observations of AF Lep b: Preference for Circular Orbits, Cloudy Atmospheres, and a Moderately Enhanced Metallicity

Direct imaging observations are biased towards wide-separation, massive companions that have degenerate formation histories. Although the majority of exoplanets are expected to form via core accretion, most directly imaged exoplanets have not been convincingly demonstrated to follow this formation pathway. We obtained new interferometric observations of the directly imaged giant planet AF Lep b with the VLTI/GRAVITY instrument. We present three epochs of 50$\mu$as relative astrometry and the K-band spectrum of the planet for the first time at a resolution of R=500. Using only these measurements, spanning less than two months, and the Hipparcos-Gaia Catalogue of Accelerations, we are able to significantly constrain the planet's orbit; this bodes well for interferometric observations of planets discovered by Gaia DR4. Including all available measurements of the planet, we infer an effectively circular orbit ($e<0.02, 0.07, 0.13$ at $1, 2, 3 \sigma$) in spin-orbit alignment with the host, and a measure a dynamical mass of $M_\mathrm{p}=3.75\pm0.5\,M_\mathrm{Jup}$. Models of the spectrum of the planet show that it is metal rich ([M/H]$=0.75\pm0.25$), with a C/O ratio encompassing the solar value. This ensemble of results show that the planet is consistent with core accretion formation.

astro-ph.EP↗

arXiv2024

ESCAPE project: testing active observing strategies for high-contrast imaging in space on the HiCAT testbed

The Roman Space Telescope will be a critical mission to demonstrate high-contrast imaging technologies allowing for the characterisation of exoplanets in reflected light. It will demonstrate $10^{-7}$ contrast limits or better at 3--9 $\lambda / D$ separations with active wavefront control for the first time in space. The detection limits for the Coronagraph Instrument are expected to be set by wavefront variations between the science target and the reference star observations. We are investigating methods to use the deformablel mirrors to methodically probe the impact of such variations on the coronagraphic PSF, generating a PSF library during observations of the reference star to optimise the starlight subtraction at post-processing. We are collaborating with STScI to test and validate these methods in lab using the HiCAT tested, a high-contrast imaging lab platform dedicated to system-level developments for future space missions. In this paper, we will present the first applications of these methods on HiCAT.

astro-ph.IM↗

arXiv2024

ESCAPE project. CAPyBARA: a Roman Coronagraph simulator for post-processing methods development

The Roman Coronagraph Instrument will be the first space facility equipped with deformable mirrors (DMs). These will lead to reach a contrast of $10^{-8}$ or better in a dark hole between $3-9 \lambda/D$. Post-processing techniques play an important role in increasing the contrast limits. Our work investigates how DMs can be used to calibrate the instrument response to controlled wavefront error maps and to improve the post-processing performance. To this goal, we are developing a simulation pipeline, CAPyBARA, that includes both a propagation model of the Coronagraph and a post-processing module and produces starlight subtracted images of a science target. This pipeline will allow us to investigate alternative observing strategies and test their performance for the Roman Coronagraph. Here we present the first version of the simulator: it currently reproduces the optical propagation, which consists in the hybrid Lyot coronagraph optical structure and dark-hole digging technique (Electric Field Conjugation coupled with $\beta$-bumping), the environment (quasi-static aberration) and the post-processing. With it, we mimic a Coronagraph Instrument observing sequence, which consists in first acquiring reference star data before slewing to the scientific target, and we investigate how the evolution of quasi-static aberrations deteriorate the contrast limit in the dark hole. We simulate a science target with planets at high contrast with their star and we perform a first post-processing analysis with classical subtraction techniques. Here we present the CAPyBARA simulator, as well as some first results. The next step will be to generate PSF libraries by injecting pre-calibrated probes on the DMs (in open loop) during the reference star acquisition and compute a PCA model. Later, we will compare the performance gain obtained with the modulated-DM reference library over standard approaches (RDI).

astro-ph.IM↗

arXiv2024

A new atmospheric characterization of the sub-stellar companion HR\,2562\,B with JWST/MIRI observations

Context: HR2562B is a planetary-mass companion located 0.56arcsec (19au) from its host star. It is one of a few L/T transitional objects orbiting a young star. This companion provides insight into the evolution of young objects in the L/T transition. However, its key physical properties, such as Teff and mass, remain poorly constrained, with large uncertainties (34% for Teff, 22% for log(g)) based on near-infrared observations alone. Aims: We aim to refine these uncertainties, especially for Teff (1200-1700K) and log(g) (4-5), using new MIR data from the JWST/MIRI filters (10.65, 11.40, and 15.50 microns), and better understand the companion's chemical composition and its role in the L/T transition. Methods: MIRI data were processed using reference star differential imaging, revealing HR2562B at high S/N (16) in all 3 filters. We used 2 atmospheric models, ATMO and ExoREM, to fit the SED, combining MIR and NIR datasets. Additionally, we used CMD with brown dwarfs to explore the chemical composition of HR2562B's atmosphere and compare it to another L/T transition object, VHS1256b. Results: Our analysis improved the temperature precision (Teff=1255+-15K) by 6x compared to previous estimates. We also narrowed its luminosity to -4.69+-0.01 dex. Surface gravity remains uncertain (4.4-4.8), and its mass is estimated between 8 and 18.5Mj, depending on modeling and astrometry. Sensitivity analysis revealed the ability to detect objects between 2-5Mj at 100au. Conclusions: HR2562B likely has a near cloud-free atmosphere, with the ATMO model fitting better than ExoREM. Silicate absorption features are weak, requiring further spectroscopic observations. While HR2562B and VHS1256b share similarities, they are in different evolutionary stages, making HR2562B key to understanding young objects in the L/T transition. It is likely a planetary-mass companion, suggesting a reclassification as HR2562b.

astro-ph.EP↗

arXiv2024

ESCAPE project: investigating active observing strategies and post-processing methods for exoplanet high-contrast imaging with future space missions

The search for biosignatures in potentially habitable exoplanets is one of the major astrophysics' drivers for the coming decades, and the prime science goal of the HWO NASA mission, a large UV-Optical-IR space telescope to be launched in the 2040s. To reach this goal, it will be equipped with state-of-the-art high-contrast spectro-imaging capabilities enabling the detection of exoplanets 10^10 times fainter than their host stars, a formidable challenge given today's best detection limits at ~10^-6 contrast levels. This goal puts stringent constraints on the entire observatory, and demands the optimization at the system level to leverage the performance of individual sub-systems. However, while image processing techniques are a key asset to reach the ultimate performance, the science and technological definition of the mission concepts mostly rely on the coronagraph and wavefront control to reject the starlight, assuming a conservative gain of ~10 in sensitivity from image processing, extrapolated from performance obtained with classical techniques on Hubble observations. In the ESCAPE project, we investigate integrated solutions for optimizing the observing methods and data processing techniques with future space telescopes, making use of their wavefront sensors and deformable mirrors. The Roman Space Telescope, scheduled for launch in 2026, will be a critical milestone to demonstrate key technologies ahead of HWO with the Coronagraph instrument, and is thus a unique opportunity to also test and validate innovative image processing techniques. Here we present the rational, methodology, and timeline of the ESCAPE project.

astro-ph.EP↗

arXiv2024

Upgrading SPHERE with the second stage AO system SAXO+: non-common path aberrations estimation and correction

SAXO+ is a planned enhancement of the existing SAXO, the VLT/ SPHERE adaptive optics system, deployed on ESO's Very Large Telescope. This upgrade is designed to significantly enhance the instrument's capacity to detect and analyze young Jupiter-like planets. The pivotal addition in SAXO+ is a second-stage adaptive optics system featuring a dedicated near-infrared pyramid wavefront sensor and a second deformable mirror. This secondary stage is strategically integrated to address any residual wavefront errors persisting after the initial correction performed by the current primary AO loop, SAXO. However, several recent studies clearly showed that in good conditions, even in the current system SAXO, non-common path aberrations (NCPAs) are the limiting factor of the final normalized intensity in focal plane, which is the final metric for ground-based high-contrast instruments. This is likely to be even more so the case with the new AO system, with which the AO residuals will be minimized. Several techniques have already been extensively tested on SPHERE in internal source and/or on-sky and will be presented in this paper. However, the use of a new type of sensor for the second stage, a pyramid wavefront sensor, will likely complicate the correction of these aberrations. Using an end-to-end AO simulation tool, we conducted simulations to gauge the effect of measured SPHERE NCPAs in the coronagraphic image on the second loop system and their correction using focal plane wavefront sensing systems. We finally analyzed how the chosen position of SAXO+ in the beam will impact the evolution of the NCPAs in the new instrument.

astro-ph.IM↗

arXiv2024

The discovery of two new benchmark brown dwarfs with precise dynamical masses at the stellar-substellar boundary

Aims. Measuring dynamical masses of substellar companions is a powerful tool to test models of mass-luminosity-age relations, as well as determining observational features that constrain the boundary between stellar and substellar companions. In order to dynamically constrain the mass of such companions, we use multiple exoplanet measurement techniques to remove degeneracies in the orbital fits of these objects and place tight constraints on their model-independent masses. Methods. We combine long-period radial-velocity data from the CORALIE survey with relative astrometry from direct imaging with VLT/SPHERE, along with astrometric accelerations from Hipparcos-Gaia eDR3 to perform a combined orbital fit and measure precise dynamical masses of two newly discovered benchmark brown dwarfs. Results. We report the discovery of HD112863B and HD206505B, which are two new benchmark likely brown dwarfs that sit at the substellar-stellar boundary, with precise dynamical masses. We perform an orbital fit which yields dynamical masses for HD112863B and HD206505B to be $77.1^{+2.9}_{-2.8}~M_{\rm{Jup}}$ and $79.8\pm1.8~M_{\rm{Jup}}$ respectively. The orbital period for HD112863B is determined to be $21.59\pm0.05$ years and the orbital period of HD206505B is determined to be ${50.9}_{-1.5}^{+1.7}$ years. From the $H$ and $K$ band photometry from IRDIS data taken with VLT/SPHERE, we estimate the spectral types of both HD112863B and HD206505B to be early-mid L-types.

astro-ph.SR↗

arXiv2024

Behind the Mask: can HARMONI@ELT detect biosignatures in the reflected light of Proxima b?

Proxima b is a rocky exoplanet in the habitable zone of the nearest star system and a key test case in the search for extraterrestrial life. Here, we investigate the characterization of a potential Earth-like atmosphere around Proxima b in reflected light via molecule mapping, combining high resolution spectroscopy (HRS) and high contrast imaging, using the first-generation integral field spectrograph HARMONI on the $39$-m Extremely Large Telescope. We simulate comprehensive observations of Proxima b at an assumed $45^{\circ}$ inclination using HARMONI's High Contrast Adaptive Optics mode, with spatial resolution $\sim 8$mas ($3.88$mas/spaxel) and spectral resolving power $R\simeq17,000$ between $1.538$--$1.678 \mu m$, containing the spectral features of water, carbon dioxide and methane. Tellurics, stellar features, and additional noise sources are included, and removed using established molecule mapping techniques. We find that HARMONI's current focal plane mask (FPM) is too large and obscures the orbit of Proxima b and thus explore smaller and offset FPMs to yield a detection. A $\rm{S/N}=5$ detection of Proxima b's reflected light, suitable for atmospheric characterisation, is possible with such modifications, requiring a minimum of $20$ hours, but ideally at least $30$ hours of integration time. We highlight that such detections do not scale with the photon noise, hence suitably detailed simulations of future instruments for the ELTs are needed to fully understand their ability to perform HRS observations of exoplanet atmospheres. Alterations to the HARMONI FPM design are feasible at this stage, but must be considered in context of other science cases.

astro-ph.EP↗

arXiv2023

$β$ Pictoris b through the eyes of the upgraded CRIRES+

Context: High-resolution spectrographs fed by adaptive optics (AO) provide a unique opportunity to characterize directly imaged exoplanets. Observations with such instruments allow us to probe the atmospheric composition, spin rotation, and radial velocity of the planet, thereby helping to reveal information on its formation and migration history. The recent upgrade of the Cryogenic High-Resolution Infrared Echelle Spectrograph (CRIRES+) at the VLT makes it a highly suitable instrument for characterizing directly imaged exoplanets. Aims: In this work, we report on observations of $β$ Pictoris b with CRIRES+ and use them to constrain the planets atmospheric properties and update the estimation of its spin rotation. Methods: The data were reduced using the open-source \textit{pycrires} package. We subsequently forward-modeled the stellar, planetary, and systematic contribution to the data to detect molecules in the planet's atmosphere. We also used atmospheric retrievals to provide new constraints on its atmosphere. Results: We confidently detected water and carbon monoxide in the atmosphere of $β$ Pictoris b and retrieved a slightly sub-solar carbon-to-oxygen ratio, which is in agreement with previous results. The interpretation is hampered by our limited knowledge of the C/O ratio of the host star. We also obtained a much improved constraint on its spin rotation of $19.9 \pm 1.0$ km/s, which gives a rotation period of $8.7 \pm 0.8$ hours, assuming no obliquity. We find that there is a degeneracy between the metallicity and clouds, but this has minimal impact on the retrieved C/O, $v\sin{i}$, and radial velocity. Our results show that CRIRES+ is performing well and stands as a highly useful instrument for characterizing directly imaged planets.

astro-ph.EP↗