arXiv subjects

Allan Denis

Publications and source records attributed to Allan Denis.

4 recordsLinked to original sources

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

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

X-SHYNE: X-Shooter spectra of young exoplanet analogs II. Presentation and analysis of the full library

The X-SHYNE library is a homogeneous sample of 43 medium-resolution (R=8000) infrared (0.3-2.5um) spectra of young (<500Myr), low-mass (<20Mjup), and cold (Teff=600-2000K) isolated brown dwarfs and wide-separation companions observed with the VLT/X-Shooter instrument. To characterize our targets, we performed a global comparative analysis. We first applied a semi-empirical approach. By refining their age and bolometric luminosity, we derived key atmospheric and physical properties, such as Teff, mass, surface gravity (g), and radius, using the evolutionary model COND03. These results were then compared with the results from a synthetic analysis based on three self-consistent atmospheric models. To compare our spectra with these grids we used the Bayesian inference code ForMoSA. We found similar Lbol estimates between both approaches, but an underestimated Teff from the cloudy models, likely due to a lack of absorbers that could dominate the J and H bands of early L. We also observed a discrepancy in the log(g) estimates, which are dispersed between 3.5 and 5.5 dex for mid-L objects. We interpreted this as a bias caused by a range of rotational velocities leading to cloud migration toward equatorial latitudes, combined with a variety of viewing angles that result in different observed atmospheric properties (cloud column densities, atmospheric pressures, etc.). Finally, while providing robust estimates of [M/H] and C/O for individual objects remains challenging, the X-SHYNE library globally suggests solar values, which are consistent with a formation via stellar formation mechanisms. This study highlights the strength of homogeneous datasets in performing comparative analyses, reducing the impact of systematics, and ensuring robust conclusions while avoiding over-interpretation.

astro-ph.EP↗