arXiv subjects

J. Gagné

Publications and source records attributed to J. Gagné.

10 recordsLinked to original sources

arXiv2026

A temperate sub-Neptune transiting the M4 Dwarf TOI-210 identified by NIRPS and TESS -- Uncovering hidden M-dwarf planetary systems in the near-infrared

Super-Earths and sub-Neptunes dominate the exoplanet population, yet their compositions, formation mechanisms, and capability of retaining atmospheres remain poorly understood. M-type stars offer a unique opportunity to investigate these questions, thanks to their favorable planet-to-star radius and mass ratios and the diversity of planetary systems they host. We aim to characterise the planetary system of the M4 dwarf TOI-210, in which TESS during its primary mission identified one transiting exoplanet candidate at $P= 9.01$ days. We combine multi-technique observations, including transits from 40 sectors of TESS and ground-based follow-up with LCOGT and ExTrA, and radial velocities (RV) from NIRPS obtained as part of its Guaranteed Time Observations program. These data were analysed using a Bayesian framework to constrain stellar, planetary, and orbital parameters. We confirm TOI-210 b, a temperate sub-Neptune ($T_{\rm eq} = 368 \pm 9$ K) with a mass of 6.75 $\pm$ 1.25 M$_{\oplus}$ and a radius of 2.234 $\pm$ 0.074 R$_{\oplus}$. Its bulk density implies a volatile-rich composition, either as an extended H/He atmosphere with mass fraction of approximately 1% (gas dwarf), a substantial water reservoir comprising at least 29% of the mass (2$σ$ lower limit; water world), or a mixture of both. The NIRPS RVs show moderate evidence ($Δ\ln Z = 5.9$) for at least one non-transiting exoplanet interior to TOI-210 b. In a targeted search, we identify two Keplerian signals at 2.15 and 3.76 days, with minimum masses of 3.2 $\pm$ 0.8 M$_{\oplus}$ and 4.4 $\pm$ 1.0 M$_{\oplus}$, respectively, that provide a plausible explanation for the observed RV variations. If these signals are planets, the absence of transits in the TESS photometry implies mutual inclinations with TOI-210 b above 2.3-3.5$^{\circ}$.

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arXiv2026

Upside down: GJ3090 b the first retrograde exoplanet around an M dwarf detected with NIRPS

The angle between stellar spin axis and planetary orbital plane can provide key insights into the formation and dynamical evolution of planetary systems. In particular, this measurement in multi-planet systems can be used to further discriminate between different competing migration scenarios. We present six transit observations of the sub-Neptune GJ3090 b obtained with NIRPS and HARPS. GJ3090 b is the inner planet of a confirmed multi-planet system orbiting an M dwarf (M2). Using high spectral resolution and high temporal cadence spectroscopic observations, we analyzed the Rossiter-McLaughlin (RM) effect induced by GJ3090 b to determine its orbital obliquity. Through the RM revolutions technique, we find that the planet is on a retrograde orbit with a derived 3D obliquity of $ψ= 136^{+24}_{-18}\,\mathrm{deg}$. We find no evidence of massive outer planetary or wide stellar binary companions, which disfavors scenarios involving gravitational perturbations from a massive body and instead points toward a primordial misalignment of the protoplanetary disk. Our results establish GJ3090 b as the first planet on a retrograde orbit discovered around an M dwarf and the first highly misaligned confirmed multi-planet system without a known massive companion. We further propose late secondary disk accretion around GJ3090, in which the disk is not expected to be aligned with the stellar spin axis, followed by disk-driven migration as the most likely mechanisms to explain the observed architecture. This work also illustrates the capability of the RM revolutions technique when applied to near-infrared data to probe orbital architectures of the smallest planets around M dwarfs that have remained mostly inaccessible.

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arXiv2026

Clouds with a silicate lining: Using JWST spectra to probe atmospheric diversity in young AB Dor L dwarfs

We present the first full JWST NIRSpec Prism and MIRI LRS 0.6 - 14 $μ$m (R ~ 100) spectra and analysis of five ~ 133 Myr L dwarf members of the AB Doradus moving group and one probable $\sim 500$ Myr T dwarf of the Oceanus moving group with known inclination angles between ~ $23 - 90^{\circ}$: W0047+68, 2M0355+11, 2M0642+41, W1741-46, 2M2206-42, and 2M2244+20. We construct near-complete spectral energy distributions of each of our objects to measure their bolometric luminosities, and estimate their fundamental parameters ($T_{\text{eff}}$, radius, $M$ and $\log g$). We use cross-sections of relevant gases to identify the species that are present in each atmosphere. Of particular interest is the silicate absorption feature at 8 - 11 $μ$m, which provides insight into the complex cloud structure of brown dwarfs. We examine this silicate absorption feature in detail and also test whether there exists a latitudinal dependence in the silicate absorption feature within a coeval sample of brown dwarfs. Various molecular absorption bands are visible in our spectra, including H$_2$O, CH$_4$, CO and CO$_2$. The shape of the silicate absorption feature varies within our sample, and we find that 4/5 of our L type objects agree with previously observed trends stating that objects viewed equator-on have deeper silicate absorption. We highlight W1741-46 as an outlier in our sample with an unusually strong silicate absorption given its near pole-on orientation. We also present a tentative correlation between the wavelength of peak silicate absorption and inclination, which may suggest variations in cloud chemical composition or physical properties. We find an unexpected spectral diversity within our sample, which motivates future studies on these objects through atmospheric retrievals, which will determine the silicate cloud composition and reveal whether there exists a trend with inclination.

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arXiv2025

Characterizing planetary systems with SPIRou: Detection of a sub-Neptune in a 6-day period orbit around the M dwarf Gl 410

The search for exoplanets around nearby M dwarfs represents a crucial milestone in the census of planetary systems in the vicinity of our Solar System. Since 2018 our team is carrying a radial-velocity blind search program for planets around nearby M dwarfs with the near-IR spectro-polarimeter and velocimeter SPIRou at the CFHT and the optical velocimeter SOPHIE at the OHP in France. Here we present our results on Gl 410, a 0.55 Msun 480+-150 Myr old active M dwarf distant 12 pc. We used the line-by-line (LBL) technique to measure the RVs with SPIRou and the template matching method with SOPHIE. Three different methods were employed, two based on principal component analysis (PCA), to clean the SPIRou RVs for systematics. We applied Gaussian processes (GP) modeling to correct the SOPHIE RVs for stellar activity. The l1 and apodize sine periodogram analysis was used to search for planetary signals in the SPIRou data taking into account activity indicators. We analyzed TESS data and searched for planetary transits. We report the detection of a M sin(i)=8.4+-1.3 Mearth sub-Neptune planet at a period of 6.020+-0.004 days in circular orbit with SPIRou. The same signal, although with lower significance, was also retrieved in the SOPHIE RV data after correction for activity using a GP trained on SPIRou's longitudinal magnetic field (Bl) measurements. The TESS data indicate that the planet is not transiting. Within the SPIRou wPCA RVs, we find tentative evidence for two additional planetary signals at 2.99 and 18.7 days. In conclusion, infrared RVs are a powerful method to detect extrasolar planets around active M dwarfs. Care should be taken however to correct/filter systematics generated by residuals of the telluric correction or small structures in the detector plane. The LBL technique combined with PCA offers a promising way to reach this objective. Further monitoring of Gl 410 is necessary.

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arXiv2020

Discovery of a Nearby Young Brown Dwarf Disk

We report the discovery of the youngest brown dwarf with a disk at 102 pc from the Sun, WISEA~J120037.79-784508.3 (W1200-7845), via the Disk Detective citizen science project. We establish that W1200-7845 is located in the 3.7$\substack{+4.6 \\ -1.4}$ Myr-old $\varepsilon$~Cha association. Its spectral energy distribution (SED) exhibits clear evidence of an infrared (IR) excess, indicative of the presence of a warm circumstellar disk. Modeling this warm disk, we find the data are best fit using a power-law description with a slope $α= -0.94$, which suggests it is a young, Class II type disk. Using a single blackbody disk fit, we find $T_{eff, disk} = 521 K$ and $L_{IR}/L_{*} = 0.14$. The near-infrared spectrum of W1200-7845 matches a spectral type of M6.0$γ\pm 0.5$, which corresponds to a low surface gravity object, and lacks distinctive signatures of strong Pa$β$ or Br$γ$ accretion. Both our SED fitting and spectral analysis indicate the source is cool ($T_{eff} = $2784-2850 K), with a mass of 42-58 $M_{Jup}$, well within the brown dwarf regime. The proximity of this young brown dwarf disk makes the system an ideal benchmark for investigating the formation and early evolution of brown dwarfs.

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arXiv2020

The ASAS-SN Catalog of Variable Stars VIII: "Dipper" Stars in the Lupus Star-Forming Region

Some young stellar objects such as T Tauri-like "dipper" stars vary due to transient partial occultation by circumstellar dust, and observations of this phenomenon inform us of conditions in the planet-forming zones close to these stars. Although many dipper stars have been identified with space missions such as $Kepler$/$K2$, ground-based telescopes offer longer term and multi-wavelength perspectives. We identified 11 dipper stars in the Lupus star forming region in data from the All-Sky Automated Survey for SuperNovae (ASAS-SN), and further characterized these using observations by the Las Cumbres Global Observatory Telescope (LCOGT) and the Transiting Exoplanet Survey Satellite $TESS$, as well as archival data from other missions. Dipper stars were identified from a catalog of nearby young stars and selected based on the statistical significance, asymmetry, and quasi-periodicity or aperiodicity of variability in their ASAS-SN light curves. All 11 stars lie above or red-ward of the zero-age main sequence and have infrared excesses indicating the presence of full circumstellar disks. We obtain reddening-extinction relations for the variability of 7 stars using our combined ASAS-SN-$TESS$ and LCOGT photometry. In all cases the slopes are below the ISM value, suggesting larger grains, and we find a tentative relation between the slope (grain size) and the $\text{K}_\text{s}-[22\:μ\text{m}]$ infrared color regarded as a proxy for disk evolutionary state.

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arXiv2017

CFBDSIR 2149-0403: young isolated planetary-mass object or high-metallicity low-mass brown dwarf??

We conducted a multi-wavelength, multi-instrument observational characterisation of the candidate free-floating planet CFBDSIR~J214947.2-040308.9, a late T-dwarf with possible low-gravity features, in order to constrain its physical properties. We analyzed 9 hours of X-Shooter spectroscopy with signal detectable from 0.8--2.3$μ$m, as well as additional photometry in the mid-infrared using the Spitzer Space Telescope. Combined with a VLT/HAWK-I astrometric parallax, this enabled a full characterisation of the absolute flux from the visible to 5$μ$m, encompassing more than 90\% of the expected energy emitted by such a cool late T-type object. Our analysis of the spectrum also provided the radial velocity and therefore the determination of its full 3-D kinematics. While our new spectrum confirms the low gravity and/or high metallicity of CFBDSIR2149, the parallax and kinematics safely rule out membership to any known young moving group, including AB~Doradus. We use the equivalent width of the KI doublet at 1.25$μ$m as a promising tool to discriminate the effects of low-gravity from the effects of high-metallicity on the emission spectra of cool atmospheres. In the case of CFBDSIR2149, the observed KI doublet clearly favours the low-gravity solution. CFBDSIR2149 is therefore a peculiar late-T dwarf that is probably a young, planetary-mass object (2--13Mjup, $<$500Myr) possibly similar to the exoplanet 51Erib, or perhaps a 2--40Mjup brown dwarf with super-solar metallicity.

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arXiv2016

MASSIVE: A Bayesian analysis of giant planet populations around low-mass stars

Direct imaging has led to the discovery of several giant planet and brown dwarf companions. These imaged companions populate a mass, separation and age domain (mass>1MJup, orbits>5AU, age<1Gyr) quite distinct from the one occupied by exoplanets discovered by the radial velocity or transit methods. This distinction could pinpoint that different formation mechanisms are at play. We aim at investigating correlations between the host star's mass and the presence of wide-orbit giant planets, and at providing new observational constraints on planetary formation models. We observed 58 young and nearby M-type dwarfs in L'-band with the VLT/NaCo instrument and used ADI algorithms to optimize the sensitivity to planetary-mass companions and to derive the best detection limits. We estimate the probability of detecting a planet as a function of its mass and physical separation around each target. We conduct a Bayesian analysis to determine the frequency of substellar companions orbiting low-mass stars, using a homogenous sub-sample of 54 stars. We derive a frequency of $4.4^{+3.2}_{-1.3}\%$ for companions with masses in the range of 2-80MJup}, and $2.3^{+2.9}_{-0.7}$\% for planetary mass companions (2-14MJup), at physical separations of 8 to 400AU for both cases. Comparing our results with a previous survey targeting more massive stars, we find evidence that substellar companions more massive than 1MJup with a low mass ratio Q with respect to their host star (Q<1%), are less frequent around low-mass stars. This may represent an observational evidence that the frequency of imaged wide-orbit substellar companions is correlated with stellar mass, corroborating theoretical expectations. On the opposite, we show statistical evidence that intermediate-mass ratio (1% 2MJup might be independent from the mass of the host star.

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arXiv2013

Direct imaging discovery of 12-14 Jupiter mass object orbiting a young binary system of very low-mass stars

Context. Though only a handful of extrasolar planets have been discovered via direct imaging, each of these discoveries had tremendous impact on our understanding of planetary formation, stellar formation and cool atmosphere physics. Aims. Since many of these newly imaged giant planets orbit massive A or even B stars we investigated whether giant planets could be found orbiting low-mass stars at large separations. Methods. We have been conducting an adaptive optic imaging survey to search for planetary-mass companions of young M dwarfs of the solar neigbourhood, to probe different initial conditions of planetary formation. Results. We report here the direct imaging discovery of 2MASS J01033563-5515561(AB)b, a 12-14 MJup companion at a projected separation of 84 AU from a pair of young late M stars, with which it shares proper motion. We also detected a Keplerian-compatible orbital motion. Conclusions. This young L-type object at planet/brown dwarf mass boundary is the first ever imaged around a binary system at a separation compatible with formation in a disc.

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arXiv2012

CFBDSIR2149-0403: a 4-7 Jupiter-mass free-floating planet in the young moving group AB Doradus ?

Using the CFBDSIR wide field survey for brown dwarfs, we identified CFBDSIRJ214947.2-040308.9, a late T dwarf with atypically red J-Ks colour. We obtained an X-Shooter spectra, with signal detectable from 0.8 to 2.3 micron, which confirmed a T7 spectral type with an enhanced Ks-band flux indicative of a potentially low-gravity, young, object. The comparison of our near infrared spectrum with atmosphere models, for solar metallicity, shows that CFBDSIRJ214947.2-040308.9 is probably a 650-750 K, log g=3.75-4.0 substellar object. Using evolution models, this translates into a planetary mass object, with an age in the 20-200 Myr range. An independent Bayesian analysis from proper motion measurements results in a 87% probability that this free-floating planet is a member of the 50-120 Myr old AB Doradus moving group, which strengthens the spectroscopic youth diagnosis. By combining our atmospheric characterisation with the age and metallicity constraints arising from the probable membership to the AB Doradus moving group, we find that CFBDSIRJ214947.2-040308.9 is probably a 4-7 Jupiter masses free-floating planet with an effective temperature of ~700K and a log g of ~4.0, typical of the late T-type exoplanets that are targeted by direct imaging. We stress that this object could be used as a benchmark for understanding the physics of the similar T-type exoplanets that will be discovered by the upcoming high contrast imagers.

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