arXiv subjects

L. Arnold

Publications and source records attributed to L. Arnold.

At least 19 recordsLinked to original sources

arXiv2026

Revisiting TOI-4438 and TOI-442 planetary systems with new observations from SPIRou and TESS

We present a comprehensive re-analysis of two star-planet systems: TOI-4438, an M3.5V star hosting a mini-Neptune in a 7.4-day orbit, and TOI-442, an M1V star with a 4-day period planet located within the hot Neptune desert. Both systems were originally identified as transiting planet candidates by TESS and subsequently validated through the radial velocity (RV) method. Our work incorporates new TESS transit data and high-resolution spectroscopy from the SPIRou near-infrared spectropolarimeter. We detect a persistent and relatively strong Zeeman signature in TOI-442, while TOI-4438 exhibits weaker and intermittent magnetic activity, and we infer the stellar rotation periods of both stars from the variability of the longitudinal magnetic field. We jointly fit photometry and RV models for each system. For TOI-4438\,b we combine archival CARMENES data with 81 SPIRou observations and five TESS sectors. This yields a refined planetary mass of $M_{\rm p} = 4.11^{+0.40}_{-0.38}\,M_{\oplus}$ and a radius of $R_{\rm p} = 2.40^{+0.09}_{-0.10}\,R_{\oplus}$, consistent with the previous estimate within 1.1$\sigma$ while improving by 53\% the precision on the mass and 22\% on the radius. For TOI-442\,b, we add 29 SPIRou RV measurements to an extensive archival dataset, significantly extending the temporal baseline. With a new TESS sector, we tighten the mass to $M_{\rm p} = 28.38^{+0.77}_{-0.73}\,M_{\oplus}$ and radius to $R_{\rm p} = 4.25^{+0.10}_{-0.08}\,R_{\oplus}$, which agrees to the previous values within 1.5$\sigma$ and improves the precisions by 46\% and 67\% respectively. We find no clear signs of additional planets in the available RV data, although we detect a single-transit event in the TOI-4438 light curve. We compare various RV models and find that those accounting for stellar variability-induced signals yield improved constraints on the planetary parameters.

astro-ph.EP↗

arXiv2026

The SPIRou Legacy Survey: Detection of a nearby world orbiting in the habitable zone of Gl725B achieved by correcting strong telluric contamination in near-infrared radial velocities with WAPITI

M dwarfs are prime targets in the search for exoplanets because of their prevalence and because low-mass planets can be better detected with radial velocity (RV) methods. In particular, the near-infrared (NIR) spectral domain offers an increased RV sensitivity and potentially reduced stellar activity signals. Howevern precise NIR RV measurements can be strongly affected by telluric absorption lines from the Earth's atmosphere. We searched for planets orbiting Gl 725 B, a nearby late-M dwarf at $3.5$ pc, using high-precision SPIRou RV observations. We assessed the impact of telluric contamination and evaluated the performance of the weighted principal component analysis reconstruction method (WAPITI), designed to mitigate these systematics and improve planet detectability. Using synthetic and observational SPIRou data, we simulated telluric effects on RVs under varying barycentric Earth radial velocity (BERV) conditions and applied WAPITI to correct line-by-line RVs. The method was tested through injection-recovery experiments and applied to real SPIRou observations of Gl 725 B. WAPITI efficiently corrects telluric contamination in simulated and real datasets, enhancing the detectability and accuracy of planetary signals. We identify a two-planet system around Gl 725 B composed of a candidate inner planet (Gl 725 Bb) with a period of $4.765 \pm 0.004$ days and semi-amplitude $1.4 \pm 0.3$ m.s$^{-1}$, and a confirmed outer planet (Gl 725 Bc) with a period of $37.90 \pm 0.17$ days and semi-amplitude $1.7 \pm 0.3$ m.s$^{-1}$. Their minimum masses are $1.5 \pm 0.4$ and $3.5 \pm 0.7$ M$_\oplus$, respectively, and the outer planet lies in the habitable zone. Using a multi-dimensional Gaussian process framework to model stellar activity, we also recover a stellar rotation period of $105.1 \pm 3.3$ days.

astro-ph.EP↗

arXiv2025

A magnetic field study of two fast-rotating, radio bright M dwarfs. StKM 1-1262 and V374 Peg

Radio observations at low frequencies are sensitive to the magnetic activity of stars and the plasma environment surrounding them. The accurate interpretation of the processes underlying the radio signatures requires a detailed characterisation of the stellar magnetism. We study two M dwarfs, StKM 1-1262 (M0 type, P$_\mathrm{rot}=1.24$ d) and V374 Peg (M4 type, P$_\mathrm{rot}=0.4455$ d), which were detected with the LOw Frequency ARray (LOFAR). StKM 1-1262 exhibited a type-II radio burst, potentially resulting from a coronal mass ejection event. V374 Peg manifested low-frequency radio emission typical of an electron-cyclotron maser instability emission mechanism. We analysed spectropolarimetric observations of these M dwarfs collected with the SpectroPolarim\`etre InfraRouge (SPIRou). Firstly, we refined the stellar parameters such as effective temperature, surface gravity, and metallicity, and measured the average surface magnetic flux via modelling of Zeeman broadening in unpolarised spectra. We then applied Zeeman-Doppler imaging to least-squares deconvolution line profiles in circular polarisation to reconstruct their large-scale magnetic fields. StKM 1-1262 has a total, unsigned magnetic field of $3.53\pm0.06$ kG on average and the large-scale magnetic field topology is dipolar and moderately axisymmetric, with an average strength of 300 G. V374 Peg has an unsigned magnetic field of $5.46\pm0.09$ kG and the large-scale field is dipolar and axisymmetric, with an average strength of 800 G. For StKM 1-1262, we found a strong anti-correlation between the total magnetic field and the effective temperature which is reminiscent of the tight link between small-scale magnetic fields and surface inhomogeneities. For V374 Peg, we found a moderate anti-correlation, possibly due to a more even distribution of surface features. (Abridged)

astro-ph.SR↗

arXiv2025

Characterizing planetary systems with SPIRou: questions about the magnetic cycle of 55 Cnc A and two new planets around B

One of the first exoplanet hosts discovered thirty years ago, the star 55 Cnc has been constantly observed ever since. It is now known to host at least five planets with orbital periods ranging from 17 hours to 15 years. It is also one of the most extreme metal rich stars in the neighbourhood and it has a low-mass secondary star. In this article, we present data obtained at the Canada-France-Hawai'i Telescope with the SPIRou spectropolarimeter on both components of the 55 Cnc stellar system. We revisit the long-period radial-velocity signals of 55 Cnc A, with a focus on the role of the magnetic cycle, and propose the existence of a sixth planet candidate, whose period falls close to that of the magnetic cycle, or half of it. The other massive outer planet has a revised period of 13.15 years and a minimum mass of 3.8 MJup. Although some uncertainty remains on these outer planets, the characterization of the four inner planets is very robust through the combination of many different data sets, and all signals are consistent in the nIR and optical domains. In addition, the magnetic topology of the solar-type primary component of the system is observed by SPIRou at the minimum of its activity cycle, characterized by an amplitude ten times smaller than observed during its maximum in 2017. For the low-mass component 55 Cnc B, we report the discovery of two exoplanets in the system, with a period of 6.799+-0.0014 and 33.75+-0.04 days and a minimum mass of 3.5+-0.8 and 5.3+-1.4 MEarth, respectively. The secondary magnetic field is very weak and the current data set does not allow its precise characterization, setting an upper limit of 10 G. The system 55 Cnc stands out as the sixth binary system with planetary systems around both components, and the first one with non equal-mass stellar components.

astro-ph.EP↗

arXiv2025

Mass, Gas, and Gauss around a T Tauri Star with SPIRou

Studies of young planets help us understand planet evolution and investigate important evolutionary processes such as atmospheric escape. We monitored IRAS 04125+2902, a 3 Myr-old T Tauri star with a transiting planet and a transitional disk, with the SPIRou infrared spectropolarimeter on the Canada-France-Hawaii Telescope. Using these data, we constrained the mass and density of the Jupiter-size companion to <0.16 M_J and <0.23 g/cm^3, respectively (90\% upper limits). These rule out a Jovian-like object and support the hypothesis that it is an ancestor to the numerous sub-Neptunes found around mature stars. We unambiguously detect magnetic fields at the stellar surface, small-scale fields reaching 1.5 kG and the large-scale field mostly consisting of a 0.80-0.95 kG dipole inclined by 5-15deg to the rotation axis. Accretion onto the star is low and/or episodic at a maximum rate of ~10^{-11} Msun/yr, indicating that IRAS 04125+2902 is most likely in a magnetic 'propeller' regime, possibly maintaining the star's slow rotation (11.3~d). We discover persistent Doppler-shifted absorption in a metastable He I line, clear evidence for a magnetized wind from a gaseous inner disk. Variability in absorption suggests structure in the disk wind that could reflect disk-planet interactions.

astro-ph.SR↗

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.

astro-ph.EP↗

arXiv2024

TOI-3568 b: a super-Neptune in the sub-Jovian desert

The sub-Jovian desert is a region in the mass-period and radius-period parameter space, typically encompassing short-period ranges between super-Earths and hot Jupiters, that exhibits an intrinsic dearth of planets. This scarcity is likely shaped by photoevaporation caused by the stellar irradiation received by giant planets that have migrated inward. We report the detection and characterization of TOI-3568 b, a transiting super-Neptune with a mass of $26.4\pm1.0$ M$_\oplus$, a radius of $5.30\pm0.27$ R$_\oplus$, a bulk density of $0.98\pm0.15$ g cm$^{-3}$, and an orbital period of 4.417965(5) d situated in the vicinity of the sub-Jovian desert. This planet orbiting a K dwarf star with solar metallicity, was identified photometrically by TESS. It was characterized as a planet by our high-precision radial velocity monitoring program using MAROON-X at Gemini North, supplemented by additional observations from the SPICE large program with SPIRou at CFHT. We performed a Bayesian MCMC joint analysis of the TESS and ground-based photometry, MAROON-X and SPIRou radial velocities, to measure the orbit, radius, and mass of the planet, as well as a detailed analysis of the high-resolution flux and polarimetric spectra to determine the physical parameters and elemental abundances of the host star. Our results reveal TOI-3568 b as a hot super-Neptune, rich in hydrogen and helium with a core of heavier elements with a mass between 10 and 25 M$_\oplus$. We analyzed the photoevaporation status of TOI-3568 b and found that it experiences one of the highest EUV luminosities among planets with a mass M$_{\rm p}$ $<2$ M$_{\rm Nep}$, yet it has an evaporation lifetime exceeding 5 Gyr. Positioned in the transition between two significant populations of exoplanets on the mass-period and energy diagrams, this planet presents an opportunity to test theories concerning the origin of the sub-Jovian desert.

astro-ph.EP↗

arXiv2024

Search for Very-Short-Baseline Oscillations of Reactor Antineutrinos with the SoLid Detector

In this letter we report the first scientific result based on antineutrinos emitted from the BR2 reactor at SCK CEN. The SoLid experiment uses a novel type of highly granular detector whose basic detection unit combines two scintillators, PVT and 6LiF:ZnS(Ag), to measure antineutrinos via their inverse-beta-decay products. An advantage of PVT is its highly linear response as a function of deposited particle energy. The full-scale detector comprises 12800 voxels and operates over a very short 6.3--8.9 m baseline from the reactor core. The detector segmentation and its 3D imaging capabilities facilitate the extraction of the positron energy from the rest of the visible energy, allowing the latter to be utilised for signal-background discrimination. We present a result based on 280 reactor-on days (55 MW mean power) and 172 reactor-off days, respectively, of live data-taking. A total of 29479 $\pm$ 603 (stat.) antineutrino candidates have been selected, corresponding to an average rate of 105 events per day and a signal-to-background ratio of 0.27. A search for disappearance of antineutrinos to a sterile state has been conducted using complementary model-dependent frequentist and Bayesian fits, providing constraints on the allowed region of the Reactor Antineutrino Anomaly.

hep-ex↗

arXiv2024

Characterizing planetary systems with SPIRou: a temperate sub-Neptune exoplanet orbiting the nearby fully-convective star GJ 1289 and a candidate around GJ 3378

We report the discovery of two new exoplanet systems around fully convective stars, found from the radial-velocity (RV) variations of their host stars measured with the nIR spectropolarimeter CFHT/SPIRou over multiple years. GJ 3378 b is a planet with minimum mass of $5.26^{+0.94}_{-0.97}$ Mearth in an eccentric 24.73-day orbit around an M4V star of 0.26 Msun. GJ 1289 b has a minimum mass of $6.27\pm1.25$ Mearth in a 111.74-day orbit, in a circular orbit around an M4.5V star of mass 0.21 Msun. Both stars are in the solar neighbourhood, at respectively 7.73 and 8.86 pc. The low-amplitude RV signals are detected after line-by-line post-processing treatment. These potential sub-Neptune class planets around cool stars may have temperate atmospheres and be interesting nearby systems for further studies. We also recovered the large-scale magnetic field of both stars, found to be mostly axisymmetric and dipolar, and with a polar strength of 20-30 G and 200-240 G for GJ 3378 (in 2019-21) and GJ 1289 (in 2022-23), respectively. The rotation periods measured with the magnetic field differ from the orbital periods, and in general, stellar activity is not seen in the studied nIR RV time series of both stars. GJ 3378 b detection is not confirmed by optical RVs and is therefore considered a candidate at this point.

astro-ph.EP↗

arXiv2024

SPIRou spectropolarimetry of the T Tauri star TW Hydrae: magnetic fields, accretion and planets

In this paper we report near-infrared observations of the classical T Tauri star TW Hya with the SPIRou high-resolution spectropolarimeter and velocimeter at the 3.6-m Canada-France-Hawaii Telescope in 2019, 2020, 2021 and 2022. By applying Least-Squares Deconvolution (LSD) to our circularly polarized spectra, we derived longitudinal fields that vary from year to year from -200 to +100 G, and exhibit low-level modulation on the 3.6 d rotation period of TW Hya, despite the star being viewed almost pole-on. We then used Zeeman-Doppler Imaging to invert our sets of unpolarized and circularly-polarized LSD profiles into brightness and magnetic maps of TW Hya in all 4 seasons, and obtain that the large-scale field of this T Tauri star mainly consists of a 1.0-1.2 kG dipole tilted at about 20{\deg} to the rotation axis, whereas the small-scale field reaches strengths of up to 3-4 kG. We find that the large-scale field is strong enough to allow TW Hya to accrete material from the disc on the polar regions at the stellar surface in a more or less geometrically stable accretion pattern, but not to succeed in spinning down the star. We also report the discovery of a radial velocity signal of semi-amplitude $11.1^{+3.3}_{-2.6}$ m/s (detected at 4.3$\sigma$ at a period of 8.3 d in the spectrum of TW Hya, whose origin may be attributed to either a non-axisymmetric density structure in the inner accretion disc, or to a $0.55^{+0.17}_{-0.13}$ Jupiter mass candidate close-in planet (if orbiting in the disc plane), at an orbital distance of $0.075\pm0.001$ au.

astro-ph.SR↗

arXiv2024

The classical T Tauri star CI Tau observed with SPIRou: magnetospheric accretion and planetary formation

We report new observations of the classical T~Tauri star CI~Tau with the SPIRou near-infrared spectropolarimeter and velocimeter at the Canada-France-Hawaii Telescope (CFHT) in late 2019, 2020 and 2022, complemented with observations obtained with the ESPaDOnS optical spectropolarimeter at CFHT in late 2020. From our SPIRou and ESPaDOnS spectra, to which we applied Least-Squares Deconvolution, we infer longitudinal fields clearly modulated with the 9-d rotation period of CI~Tau. Using Zeeman-Doppler imaging, we reconstruct the large-scale magnetic topology, first from SPIRou data only in all three seasons, then from our 2020 SPIRou and ESPaDOnS data simultaneously. We find that CI~Tau hosts a mainly axisymmetric poloidal field, with a 1~kG dipole slightly tilted to the rotation axis and dark spots close to the pole that coincide with the footpoints of accretion funnels linking the star to the inner disc. Our results also suggest that CI~Tau accretes mass from the disc in a stable fashion. We further find that radial velocities (RV) derived from atomic and CO lines in SPIRou spectra are both rotationally modulated, but with a much lower amplitude than that expected from the putative candidate planet CI~Tau~b. We confirm the presence of a RV signal at a period of 23.86~d reported in a separate analysis, but detect it clearly in CO lines only and not in atomic lines, suggesting that it likely traces a non-axisymmetric structure in the inner disc of CI~Tau rather than a massive close-in planet.

astro-ph.SR↗

arXiv2024

TOI-4860 b, a short-period giant planet transiting an M3.5 dwarf

We report the discovery and characterisation of a giant transiting planet orbiting a nearby M3.5V dwarf (d = 80.4 pc, $G$ = 15.1 mag, $K$=11.2 mag, R$_\star$ = 0.358 $\pm$ 0.015 R$_\odot$, M$_\star$ = 0.340 $\pm$ 0.009 M$_\odot$). Using the photometric time series from TESS sectors 10, 36, 46, and 63 and near-infrared spectrophotometry from ExTrA, we measured a planetary radius of 0.77 $\pm$ 0.03 R$_J$ and an orbital period of 1.52 days. With high-resolution spectroscopy taken by the CFHT/SPIRou and ESO/ESPRESSO spectrographs, we refined the host star parameters ([Fe/H] = 0.27 $\pm$ 0.12) and measured the mass of the planet (0.273 $\pm$ 0.006 M$_J$). Based on these measurements, TOI-4860 b joins the small set of massive planets ($>$80 M$_E$) found around mid to late M dwarfs ($<$0.4 R$_\odot$), providing both an interesting challenge to planet formation theory and a favourable target for further atmospheric studies with transmission spectroscopy. We identified an additional signal in the radial velocity data that we attribute to an eccentric planet candidate ($e=0.66\pm0.09$) with an orbital period of $427\pm7$~days and a minimum mass of $1.66\pm 0.26$ M$_J$, but additional data would be needed to confirm this.

astro-ph.EP↗

arXiv2023

The SOPHIE search for northern extrasolar planets-XIX. A system including a cold sub-Neptune potentially transiting a V = 6.5 star HD88986

Transiting planets with orbital periods longer than 40 d are extremely rare among the 5000+ planets discovered so far. The lack of discoveries of this population poses a challenge to research into planetary demographics, formation, and evolution. Here, we present the detection and characterization of HD88986b, a potentially transiting sub-Neptune, possessing the longest orbital period among known transiting small planets (< 4 R$_{\oplus}$) with a precise mass measurement ($\sigma_M/M$ > 25%). Additionally, we identified the presence of a massive companion in a wider orbit around HD88986. Our analysis reveals that HD88986b, based on two potential single transits on sector 21 and sector 48 which are both consistent with the predicted transit time from the RV model, is potentially transiting. The joint analysis of RV and photometric data show that HD88986b has a radius of 2.49$\pm$0.18 R$_{\oplus}$, a mass of 17.2$^{+4.0}_{-3.8}$ M$_{\oplus}$, and it orbits every 146.05$^{+0.43}_{-0.40}$ d around a subgiant HD88986 which is one of the closest and brightest exoplanet host stars (G2V type, R=1.543 $\pm$0.065 R$_{\odot}$, V=$6.47\pm 0.01$ mag, distance=33.37$\pm$0.04 pc). The nature of the outer, massive companion is still to be confirmed; a joint analysis of RVs, Hipparcos, and Gaia astrometric data shows that with a 3$\sigma$ confidence interval, its semi-major axis is between 16.7 and 38.8 au and its mass is between 68 and 284 M$_{Jup}$. HD88986b's wide orbit suggests the planet did not undergo significant mass loss due to extreme-ultraviolet radiation from its host star. Therefore, it probably maintained its primordial composition, allowing us to probe its formation scenario. Furthermore, the cold nature of HD88986b (460$\pm$8 K), thanks to its long orbital period, will open up exciting opportunities for future studies of cold atmosphere composition characterization.

astro-ph.EP↗

arXiv2023

Magnetic fields & rotation periods of M dwarfs from SPIRou spectra

We present near-infrared spectropolarimetric observations of a sample of 43 weakly- to moderately-active M dwarfs, carried with SPIRou at the Canada-France-Hawaii Telescope in the framework of the SPIRou Legacy Survey from early 2019 to mid 2022. We use the 6700 circularly polarised spectra collected for this sample to investigate the longitudinal magnetic field and its temporal variations for all sample stars, from which we diagnose, through quasi-periodic Gaussian process regression, the periodic modulation and longer-term fluctuations of the longitudinal field. We detect the large-scale field for 40 of our 43 sample stars, and infer a reliable or tentative rotation period for 38 of them, using a Bayesian framework to diagnose the confidence level at which each rotation period is detected. We find rotation periods ranging from 14 to over 60d for the early-M dwarfs, and from 70 to 200d for most mid- and late-M dwarfs (potentially up to 430d for one of them). We also find that the strength of the detected large-scale fields does not decrease with increasing period or Rossby number for the slowly rotating dwarfs of our sample as it does for higher-mass, more active stars, suggesting that these magnetic fields may be generated through a different dynamo regime than those of more rapidly rotating stars. We also show that the large-scale fields of most sample stars evolve on long timescales, with some of them globally switching sign as stars progress on their putative magnetic cycles.

astro-ph.SR↗

arXiv2023

Characterizing planetary systems with SPIRou: M-dwarf planet-search survey and the multiplanet systems GJ 876 and GJ 1148

SPIRou is a near-infrared spectropolarimeter and a high-precision velocimeter. The SPIRou Legacy Survey collected data from February 2019 to June 2022, half of the time devoted to a blind search for exoplanets around nearby cool stars. The aim of this paper is to present this program and an overview of its properties, and to revisit the radial velocity (RV) data of two multiplanet systems, including new visits with SPIRou. From SPIRou data, we can extract precise RVs using efficient telluric correction and line-by-line measurement techniques, and we can reconstruct stellar magnetic fields from the collection of polarized spectra using the Zeeman-Doppler imaging method. The stellar sample of our blind search in the solar neighborhood, the observing strategy, the RV noise estimates, chromatic behavior, and current limitations of SPIRou RV measurements on bright M dwarfs are described. In addition, SPIRou data over a 2.5-year time span allow us to revisit the known multiplanet systems GJ~876 and GJ~1148. For GJ~876, the new dynamical analysis including the four planets is consistent with previous models and confirms that this system is deep in the Laplace resonance and likely chaotic. The large-scale magnetic map of GJ~876 over two consecutive observing seasons is obtained and shows a dominant dipolar field with a polar strength of 30~G, which defines the magnetic environment in which the inner planet with a period of 1.94~d is embedded. For GJ~1148, we refine the known two-planet model.

astro-ph.EP↗

arXiv2023

Near-IR and optical radial velocities of the active M dwarf star Gl 388 (AD Leo) with SPIRou at CFHT and SOPHIE at OHP: A 2.23 day rotation period and no evidence for a corotating planet

Context: The search for extrasolar planets around the nearest M dwarfs is a crucial step towards identifying the nearest Earth-like planets. One of the main challenges in this search is that M dwarfs can be magnetically active and stellar activity can produce radial velocity (RV) signals that could mimic those of a planet. Aims: We aim to investigate whether the 2.2 day period observed in optical RVs of the nearby active M dwarf star Gl 388 (AD Leo) is due to stellar activity or to a planet that corotates with the star as suggested in the past. Methods: We obtained quasi-simultaneous optical RVs of Gl 388 from 2019 to 2021 with SOPHIE (R~75k) at the OHP in France, and near-IR RV and Stokes V measurements with SPIRou at the CFHT (R~70k). Results: The SOPHIE RV time series displays a periodic signal with a 2.23+-0.01 day period and 23.6+-0.5 m/s amplitude, which is consistent with previous HARPS observations obtained in 2005-2006. The SPIRou RV time series is flat at 5 m/s rms and displays no periodic signals. RV signals of amplitude higher than 5.3 m/s at a period of 2.23 days can be excluded with a confidence level higher than 99%. Using the modulation of the longitudinal magnetic field (Bl) measured with SPIRou, we derive a stellar rotation period of 2.2305+-0.0016 days. Conclusions: SPIRou RV measurements provide solid evidence that the periodic variability of the optical RVs of Gl 388 is due to stellar activity rather than to a corotating planet. The magnetic activity nature of the optical RV signal is further confirmed by the modulation of Bl with the same period. The SPIRou campaign on Gl 388 demonstrates the power of near-IR RV to confirm or infirm planet candidates discovered in the optical around active stars. SPIRou observations reiterate how effective spectropolarimetry is at determining the stellar rotation period.

astro-ph.SR↗

arXiv2023

The magnetic field and multiple planets of the young dwarf AU~Mic

In this paper we present an analysis of near-infrared spectropolarimetric and velocimetric data of the young M dwarf AU Mic, collected with SPIRou at the Canada-France-Hawaii telescope from 2019 to 2022, mostly within the SPIRou Legacy Survey. With these data, we study the large- and small-scale magnetic field of AU Mic, detected through the unpolarized and circularly-polarized Zeeman signatures of spectral lines. We find that both are modulated with the stellar rotation period (4.86 d), and evolve on a timescale of months under differential rotation and intrinsic variability. The small-scale field, estimated from the broadening of spectral lines, reaches $2.61\pm0.05$ kG. The large-scale field, inferred with Zeeman-Doppler imaging from Least-Squares Deconvolved profiles of circularly-polarized and unpolarized spectral lines, is mostly poloidal and axisymmetric, with an average intensity of $550\pm30$ G. We also find that surface differential rotation, as derived from the large-scale field, is $\simeq$30% weaker than that of the Sun. We detect the radial velocity (RV) signatures of transiting planets b and c, although dwarfed by activity, and put an upper limit on that of candidate planet d, putatively causing the transit-timing variations of b and c. We also report the detection of the RV signature of a new candidate planet (e) orbiting further out with a period of $33.39\pm0.10$ d, i.e., near the 4:1 resonance with b. The RV signature of e is detected at 6.5$σ$ while those of b and c show up at $\simeq$4$σ$, yielding masses of $10.2^{+3.9}_{-2.7}$ and $14.2^{+4.8}_{-3.5}$ Earth masses for b and c, and a minimum mass of $35.2^{+6.7}_{-5.4}$ Earth masses for e.

astro-ph.SR↗

arXiv2023

Optical and near-infrared stellar activity characterization of the early M dwarf Gl~205 with SOPHIE and SPIRou

The stellar activity of M dwarfs is the main limitation for discovering and characterizing exoplanets orbiting them since it induces quasi-periodic RV variations. We aim to characterize the magnetic field and stellar activity of the early, moderately active, M dwarf Gl205 in the optical and nIR domains. We obtained high-precision quasi-simultaneous spectra in the optical and nIR with the SOPHIE spectrograph and SPIRou spectropolarimeter between 2019 and 2022. We computed the RVs from both instruments and the SPIRou Stokes V profiles. We used ZDI to map the large-scale magnetic field over the time span of the observations. We studied the temporal behavior of optical and nIR RVs and activity indicators with the Lomb-Scargle periodogram and a quasi-periodic GP regression. In the nIR, we studied the equivalent width of Al I, Ti I, K I, Fe I, and He I. We modeled the activity-induced RV jitter using a multi-dimensional GP regression with activity indicators as ancillary time series. The optical and nIR RVs have similar scatter but nIR shows a more complex temporal evolution. We observe an evolution of the magnetic field topology from a poloidal dipolar field in 2019 to a dominantly toroidal field in 2022. We measured a stellar rotation period of Prot=34.4$\pm$0.5 d in the longitudinal magnetic field. Using ZDI we measure the amount of latitudinal differential rotation (DR) shearing the stellar surface yielding rotation periods of Peq=32.0$\pm$1.8 d at the stellar equator and Ppol=45.5$\pm$0.3 d at the poles. We observed inconsistencies in the activity indicators' periodicities that could be explained by these DR values. The multi-dimensional GP modeling yields an RMS of the RV residuals down to the noise level of 3 m/s for both instruments, using as ancillary time series H$α$ and the BIS in the optical, and the FWHM in the nIR.

astro-ph.SR↗