arXiv subjects

C. P. Folsom

Publications and source records attributed to C. P. Folsom.

At least 19 recordsLinked to original sources

arXiv2026

Magnetically guided accretion and extremely slow rotation in a metal-enriched white dwarf

White dwarfs that show atmospheric metal enrichment offer a direct window into the bulk composition of exoplanetary material and provide some of the most direct constraints on the building blocks of rocky exoplanetary systems. Recent work has suggested that magnetic fields may influence how this material is accreted, potentially channelling debris along field lines and retaining it near magnetic poles rather than allowing it to be redistributed uniformly across the stellar surface. Here we report the discovery of photometric, spectroscopic, and spectropolarimetric variability in the magnetic DZ white dwarf WD1532+129, revealing a rotation period of about 289 days, by far the longest yet directly measured for any degenerate star. The observed period may result from the combined effect of angular-momentum loss during the evolution leading to the white dwarf stage and an unusually slowly rotating progenitor, perhaps a magnetic Ap star. The white dwarf exhibits surface abundance inhomogeneities, with metals concentrated near both magnetic poles, and the observed modulation is consistent with localised metal-rich regions aligned with the magnetic geometry. WD1532+129 is the third of the four known magnetic DZ white dwarfs within 20 pc to show evidence for polar metal enhancement, indicating that magnetically guided inhomogeneities are a common property of magnetic metal-enriched white dwarfs.

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arXiv2026

Magnetic field, rotation, and binarity of the first magnetic B[e] star, IRAS 17449+2320

IRAS 17449+2320 is the first identified magnetic B[e] star. It belongs to a subgroup of peculiar hot B-type stars, called FS~CMa stars. presenting the B[e] phenomenon, i.e., permitted and forbidden emission lines, and infrared excess as a result of the large amount of gas and dust surrounding the central star. The origin of this circumstellar material is unclear, with binarity being the most accepted scenario. However, the presence of a magnetic field in IRAS 17449+2320 points to the merger origin. In this paper, we confirm and characterize the magnetic field of this object, and constrain its rotation and binarity. We used a sample of 12 spectropolarimetric observations taken with ESPaDOnS at the CFHT. We obtained the least squares deconvolution (LSD) profile of the Stokes $V$ parameter of each spectropolarimetric observation. From the LSD profiles, we measured the longitudinal magnetic field (B$_z$), and obtained the magnetic field modulus ($|B|$) from the Zeeman splitting of some metallic lines of the spectra. We calculated a Lomb-Scargle periodogram over B$_z$ to obtain the rotational period of this star. B$_z$ presents a sinusoidal variation with a stable period of 36.11 days, with values ranging from 3000 G to -1000 G. $|B|$ measurements range from $\sim$4400~G to $\sim$6000~G, and it shows a second order term periodic variation. The stable sinusoidal variation of B$_z$ implies an approximately dipolar surface field topology. IRAS~17449+2320 is the first magnetic B[e] star detected. It presents Zeeman splitting in individual lines and consistent Stokes $V$ profiles. We see no evidence of binarity in this object, with stable radial velocities and no detectable lines of a secondary star in the spectrum. We consider the possibility that this star is the result of a merger, which could have generated the strong magnetic field and the large amount of circumstellar material.

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arXiv2026

Ariel stellar characterisation IV. Fundamental parameters of 18 hot stars in the Ariel mission candidate sample

The characterisation of exoplanetary systems depends on the accurate determination of host star parameters. The Ariel mission will probe the atmospheres of a statistically significant sample of exoplanets, and so requires a precise characterisation of the stellar properties well before its launch in 2029. The homogeneous determination of stellar parameters for Ariel will enable both the optimisation of the final target list and set roots for a reliable interpretation of the formation and evolution of planetary systems. Such a homogeneous characterisation has thus far only been carried out for the cool (\teff\ $\lesssim 7000\,$K) host stars among the Ariel target candidates. We present a uniform determination of fundamental stellar parameters for 18 hot stars in the Tier 1 candidate list of the Ariel mission candidate sample. We adopted an iterative spectro-trigonometric approach optimised for high-temperature stars. High-resolution spectra were analysed using the \textsc{zeeman} code with $χ^2$ minimisation, combining model fits to metal and Balmer lines. Surface gravity was refined using photometry-based radii and masses from stellar evolutionary tracks. We derived effective temperatures, surface gravities, projected rotational velocities, microturbulent velocities, overall metallicities, iron abundances, stellar masses, and radii for our sample of $18$ hot stars. Our results were validated against a set of benchmark stars previously presented in the literature. The derived parameters provide an internally consistent basis for studying the link between stellar properties and planetary characteristics in intermediate-mass stars. Building on our previous work on FGK host stars, we show that correlations between stellar mass, metallicity, and planetary radii also extend to early-type stars, and stellar properties influence the architecture of multi-planet systems.

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arXiv2026

Magnetic geometry of M dwarfs in the southern PLATO field

M dwarfs are the most abundant stars in the Galaxy and exhibit diverse magnetic behaviours. Understanding their large-scale magnetic fields is essential to study stellar dynamos and assess the impact of magnetic activity on planetary environments, yet their magnetic properties and long-term variability remain poorly characterised. We aim to characterise the large-scale magnetic fields of 6 M dwarfs in the southern PLATO field, with rotation periods from ~1 to 17 days and masses between 0.26 and 0.64 Msun. Five stars are partially convective, one fully convective, extending the mass-rotation diagram to previously unsampled regions. We analysed TESS light curves to determine accurate rotation periods and optimise phase coverage for spectropolarimetric observations. SPIRou data were reduced to obtain LSD profiles and longitudinal field measurements, while synthetic spectra fitting yielded small-scale field strengths. ZDI was applied to reconstruct large-scale magnetic topologies. We report a wide diversity of magnetic topologies among the 6 M dwarfs, with 3 main results: (1) Rapidly-rotating (Prot < 2 d) early M dwarfs can generate dipole-dominated fields of moderate intensity, similar to less massive mid-M dwarfs; (2) rapidly-rotating mid-M dwarfs can generate non-axisymmetric large-scale fields with a significant toroidal component; (3) a moderately-rotating (Prot ~ 17 d) early M dwarf shows a surprisingly weak large-scale field. Our findings highlight the diversity of magnetic configurations, including in previously unexplored regions. Long-term monitoring is crucial to distinguish persistent features from variability-driven excursions and to characterise the evolution of surface magnetic fields. Complementary PLATO photometry, including flare and spot-induced variability analyses, will be essential to link surface activity with magnetic properties.

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arXiv2026

Magnetic field measurements in a sample of Class I and flat-spectrum protostars observed with SPIRou

Magnetic fields play a crucial role throughout stellar evolution, regulating angular momentum, channelling accretion, and launching jets and outflows. While the magnetic properties of Classical T Tauri Stars (CTTS) are well characterised, those of their progenitors, Class I and Flat-Spectrum (FS) protostars, remain poorly constrained due to observational challenges linked to their embedded nature. We aim to detect and characterise large-scale magnetic fields in a sample of Class I and FS protostars, which are expected to host strong dynamo-generated fields. Using SPIRou, a high-resolution near-infrared spectropolarimeter, we analysed polarised spectra and applied the Least Squares Deconvolution (LSD) technique to extract magnetic signatures and measure longitudinal fields from Stokes V profiles. We report new detections of large-scale magnetic fields in 5 FS protostars. Including the previously known magnetic FS protostar V347 Aur, 40% of our sample (15 objects) is confirmed to be magnetic. These stars exhibit clear Zeeman signatures, with longitudinal field strengths ranging from ~80 to ~200 G. The remaining targets show no detectable Stokes V signature, with upper limits on dipolar fields between 500 G and >5 kG. These results indicate that Class I and FS protostars can host large-scale magnetic fields, possibly weaker than in CTTS, supporting the idea that magnetic processes are already active during the main accretion phase and may influence star-disk interactions from the earliest stages.

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arXiv2026

Activity correlation and temporal variation of small-scale magnetic fields on young Sun-like stars

We aim to evaluate how well the variation of small-scale magnetic fields on the stellar surface can be monitored with time-series observations. Further, we aim to establish to what extent the measured total unsigned magnetic field traces other activity indicators. We measured the total unsigned magnetic field on four young, stars using Zeeman splitting of magnetically sensitive spectral lines from high-resolution spectra obtained with the spectropolarimeters ESPaDOnS at CFHT and NARVAL at TBL. We then characterised the magnetic field variations using both sinusoidal variation and Lomb-Scargle periodograms. We evaluated how the rotational variation of the total unsigned magnetic field strength correlates with the activity indicators S-index, H$α$-index, Ca IRT-index, and the large-scale magnetic field obtained from ZDI maps obtained in earlier studies. We find clear signals of rotational modulation of the total magnetic field on HIP 76768 and tentative detection on Mel 25-5. This is supported both by the sinusoidal fitting and the periodogram. For the other stars, we find no modulation signals of the total magnetic field. We find positive correlations between the total magnetic field and activity indices on all four stars, indicating that indirect magnetic activity indicators trace the underlying magnetic field variability. However, comparing the activity-magnetic field relationship between the stars in our sample shows a significant deviation between activity level and measured magnetic field strength. Small-scale magnetic field variability can be traced using the Zeeman effect on magnetically sensitive lines, provided that the star is sufficiently active. It is also possible to self-consistently recover rotational periods from such measurements. The primary limit for the detection of magnetic field variations is the precision of Zeeman broadening and intensification measurements.

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arXiv2025

PIMMS: Pulsation-Informed Magnetic Mapping of Stars with Zeeman-Doppler Imaging I. Formalism and numerical tests

Context. Magneto-asteroseismology is a novel technique allowing for more precise determinations of internal properties of magnetic pulsating stars, but requires an accurate characterisation of the surface magnetic field, not previously possible with Zeeman-Doppler Imaging (ZDI) due to the time-dependent surface velocity of pulsating stars. Aims. We aim to develop a new version of ZDI, which creates an additional surface velocity map, that includes the time-dependent velocities of surface elements due to pulsations. Methods. We present a new code, PIMMS: Pulsation-Informed Magnetic Mapping of Stars, which uses a surface-integrated line profile model that accounts for the additional Doppler shifts of local lines caused by pulsations. It is then possible to fit this model to spectropolarimetric observations, reconstructing maps of the surface brightness, magnetic field, and the velocity field due to the combination of pulsation and rotation. In this paper, we present and test PIMMS extensively, to understand its limitations and data requirements. Results. We find that PIMMS can accurately reproduce the magnetic fields and brightness distributions of realistic models of pulsating hot stars. The required number of observations is higher than that required for ZDI of a non-pulsating star due to the additional velocity map that must be disentangled from surface brightness variations. PIMMS is now ready to be applied to real stars.

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arXiv2025

Stable magnetic fields and changing starspots on Vega -- An ultra-deep decadal survey at Pic du Midi and OHP

Monitoring magnetic and activity variations in A- and B-type stars with ultra-weak magnetic fields is essential to understand the origin and evolution of these fields in this region of the HR diagram, with Vega standing as the prototype of this category. We collected high-resolution spectroscopic and spectropolarimetric data with SOPHIE (OHP, 2018) and with NARVAL/NEO-NARVAL (TBL, 2018, 2023, and 2024), yielding a total of 13108 individual spectra. Magnetic field maps were reconstructed using the Zeeman-Doppler Imaging method, while brightness maps were derived with a dedicated code developed for this purpose. The average magnetic field confirms a negative radial-field spot at the pole with stable strength, while the maps reveal the long-term stability of an oblique dipole together with smaller magnetic structures consistently detected across observing epochs. In contrast, brightness maps show strong variations in the location of surface spots on timescales of years, possibly shorter, although the spot contrast remains nearly unchanged between 2012, 2018, 2023, and 2024, with a normalized spectral amplitude of 0.0003. No direct correlation between magnetic and brightness features could be established in the simultaneous SOPHIE and NARVAL dataset of 2018. These results suggest that Vega hosts both a persistent fossil magnetic field and a dynamo-generated component, most likely concentrated in equatorial regions.

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arXiv2025

Evolving magnetic lives of Sun-like stars. I. Characterisation of the large-scale magnetic field with Zeeman-Doppler imaging

Planets orbiting young, solar-type stars are embedded in a more energetic environment than that of the solar neighbourhood. They experience harsher conditions due to enhanced stellar magnetic activity and wind shaping the secular evolution of a planetary atmosphere. This study is dedicated to the characterisation of the magnetic activity of eleven Sun-like stars, with ages between 0.2 and 6.1 Gyr and rotation periods between 4.6 and 28.7 d. Based on a sub-sample of six stars, we aim to study the large-scale magnetic field, which we then use to simulate the associated stellar wind and environment. Finally, we want to determine the conditions during the early evolution of planetary habitability. We analysed high-resolution spectropolarimetric data collected in 2018 and 2019 with Narval. We computed activity diagnostics from chromospheric lines such as CaII H&K, H$α$, and the CaII infrared triplet, as well as the longitudinal magnetic field from circularly polarised least-squares deconvolution profiles. For six stars exhibiting detectable circular polarisation signals, we reconstructed the large-scale magnetic field at the photospheric level by means of Zeeman-Doppler imaging (ZDI). In agreement with previous studies, we found a global decrease in the activity indices and longitudinal field with increasing age and rotation period. The large-scale magnetic field of the six sub-sample stars displays a strength between 1 and 25 G and reveals substantial contributions from different components such as poloidal (40-90 %), toroidal (10-60 %), dipolar (30-80 %), and quadrupolar (10-40 %), with distinct levels of axisymmetry (6-84 %) and short-term variability of the order of months. Ultimately, this implies that exoplanets tend to experience a broad variety of stellar magnetic environments after their formation.

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arXiv2025

Ariel stellar characterisation III. Fast rotators and new FGK stars in the Ariel Mission Candidate Sample

The next mission dedicated to the study of planetary atmospheres is the Ariel space mission, planned for launch in 2029, which will observe a variety of planetary systems belonging to different classes around stars with spectral types from M to A. To optimise the scientific outcome of the mission, such stars need to be homogeneously characterised beforehand. In this work, we focus on a methodology based on spectral synthesis for the characterisation of FGK-type stars from the Ariel Tier 1 Mission Candidate Sample (MCS) which exhibit fast rotation. In addition, we analyse slow-rotating FGK-type stars, with either new observations or archival spectra available, consistently as in our previous work using the equivalent width (EW) analysis. To ensure consistency between our methods, we re-analysed a sample of FGK-type stars with the spectral synthesis method and compared it to our previous work. The results of our analysis show excellent agreement with the previous set of derived parameters. We also computed their orbital parameters establishing whether they belong to the Galactic thin or thick discs. With the current set of stellar parameters, we almost double the analysed hosts in the Ariel MCS to 353 stars in total. Using our homogeneous set of stellar parameters, we studied the correlations between stellar and planetary properties for the Ariel MCS analysed so far. We confirmed a close relationship between stellar mass (up to 1.8 solar masses) and giant planet radius, with more inflated planets at lower metallicity. We confirm that giant planets are more frequent around more metal-rich stars that belong to the thin disc, while lower-mass planets are also found in more metal-poor environments, and are more frequent than giant planets in the thick disc as also seen in other works in the literature.

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arXiv2024

A BCool survey of stellar magnetic cycles

The magnetic cycle on the Sun consists of two consecutive 11-yr sunspot cycles and exhibits a polarity reversal around sunspot maximum. Although solar dynamo theories have progressively become more sophisticated, the details as to how the dynamo sustains magnetic fields are still subject of research. Observing the magnetic fields of Sun-like stars are useful to contextualise the solar dynamo. The BCool survey studies the evolution of surface magnetic fields to understand how dynamo-generated processes are influenced by key ingredients, like mass and rotation. Here, we focus on six Sun-like stars with mass between 1.02 and 1.06 MSun and with 3.5-21 d rotation period. We analysed high-resolution spectropolarimetric data collected with ESPaDOnS, Narval and Neo-Narval. We measured the longitudinal magnetic field from least-squares deconvolution line profiles and inspected its long-term behaviour with a Lomb-Scargle periodogram and a Gaussian process. We applied Zeeman-Doppler imaging to reconstruct the large-scale magnetic field geometry at the stellar surface for different epochs. Two stars, namely HD 9986 and HD 56124 (Prot ~ 20 d) exhibit repeating polarity reversals of the radial or toroidal field component on time scales of 5 to 6 yr. HD 73350 (Prot = 12 d) has one polarity reversal of the toroidal component and HD 76151 (Prot=17 d) may have short-term evolution (2.5 yr) modulated by the long-term (16 yr) chromospheric cycle. HD 166435 and HD 175726 (Prot =3-5 d), manifest complex magnetic fields without cyclic evolution. Our findings indicate the potential dependence of the magnetic cycles nature with stellar rotation period. For the two stars with likely cycles, the polarity reversal time scale seems to decrease with decreasing rotation period or Rossby number. These results represent important observational constraints for dynamo models of solar-like stars.

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arXiv2024

First Observation of the Complete Rotation Period of the Ultra-Slowly Rotating Magnetic O Star HD 54879

HD 54879 is the most recently discovered magnetic O-type star. Previous studies ruled out a rotation period shorter than 7 years, implying that HD 54879 is the second most slowly-rotating known magnetic O-type star. We report new high-resolution spectropolarimetric measurements of HD 54879, which confirm that a full stellar rotation cycle has been observed. We derive a stellar rotation period from the longitudinal magnetic field measurements of P = 2562+63-58 d (about 7.02 yr). The radial velocity of HD 54879 has been stable over the last decade of observations. We explore equivalent widths and longitudinal magnetic fields calculated from lines of different elements, and conclude the atmosphere of HD 54879 is likely chemically homogeneous, with no strong evidence for chemical stratification or lateral abundance nonuniformities. We present the first detailed magnetic map of the star, with an average surface magnetic field strength of 2954 G, and a strength for the dipole component of 3939 G. There is a significant amount of magnetic energy in the quadrupole components of the field (23%). Thus, we find HD 54879 has a strong magnetic field with a significantly complex topology.

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arXiv2024

Metal accretion scars may be common on magnetic, polluted white dwarfs

More than 30% of white dwarfs exhibit atmospheric metals, which are understood to be from recent or ongoing accretion of circumstellar debris. In cool white dwarfs, surface motions should rapidly homogenise photospheric abundances, and the accreted heavy elements should diffuse inward on a timescale much longer than that for surface mixing. The recent discovery of a metal scar on WD0816-310 implies its magnetic field has impeded surface mixing of metals near the visible magnetic pole. Here, we report the discovery of a second magnetic, metal-polluted white dwarf, WD2138-332, which exhibits periodic variability in longitudinal field, metal line strength, and broadband photometry. All three variable quantities have the same period, and show remarkable correlations: the published light curves have a brightness minimum exactly when the longitudinal field and line strength have a maximum, and a maximum when the longitudinal field and line strength have a minimum. The simplest interpretation of the line strength variability is that there is an enhanced metal concentration around one pole of the magnetic field; however, the variable line-blanketing cannot account for the observed multi-band light curves. More theoretical work is required to understand the efficiency of horizontal mixing of the accreted metal atoms, and the origin of photometric variability. Because both magnetic, metal-polluted white dwarfs that have been monitored to date show that metal line strengths vary in phase with the longitudinal field, we suggest that metal scars around magnetic poles may be a common feature of metal-polluted white dwarfs.

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arXiv2024

Long-term monitoring of large-scale magnetic fields across optical and near-infrared domains with ESPaDOnS, Narval and SPIRou. The cases of EV Lac, DS Leo, and CN Leo

Dynamo models of stellar magnetic fields for partly and fully convective stars are guided by observational constraints. Zeeman-Doppler imaging has revealed a variety of magnetic field geometries and, for fully convective stars in particular, a dichotomy: either strong, mostly axisymmetric, and dipole-dominated or weak, non-axisymmetric, and multipole-dominated. This dichotomy is explained by dynamo bistability or by long-term magnetic cycles, but there is no definite conclusion on the matter. We analysed optical spectropolarimetric data sets collected with ESPaDOnS and Narval between 2005 and 2016, and near-infrared SPIRou data obtained between 2019 and 2022 for three active M dwarfs with masses between 0.1 and 0.6 MSun: EV Lac, DS Leo, and CN Leo. We looked for changes in time series of longitudinal magnetic field, width of unpolarised mean-line profiles, and large-scale field topology as retrieved with principal component analysis and Zeeman-Doppler imaging. We retrieved pulsating (EV Lac), stable (DS Leo), and sine-like (CN Leo) long-term trends in longitudinal field. The width of near-infrared mean-line profiles exhibits rotational modulation only for DS Leo, whereas in the optical it is evident for both EV Lac and DS Leo. The line width variations are not necessarily correlated to those of the longitudinal field, suggesting complex relations between small- and large-scale field. We also recorded topological changes: a reduced axisymmetry for EV Lac and a transition from toroidal- to poloidal-dominated regime for DS Leo. For CN Leo, the topology remained dipolar and axisymmetric, with only an oscillation in field strength. Our results show a peculiar evolution of the magnetic field for each M dwarf, confirming that M dwarfs with distinct masses and rotation periods can undergo magnetic long-term variations, and suggesting a variety of cyclic behaviours of their magnetic fields.

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arXiv2024

The variable magnetic field of V889 Her and the challenge of detecting exoplanets around young Suns using Gaussian process regression

Discovering exoplanets orbiting young Suns can provide insight into the formation and early evolution of our own solar system, but the extreme magnetic activity of young stars obfuscates exoplanet detection. Here we monitor the long-term magnetic field and chromospheric activity variability of the young solar analogue V889 Her, model the activity-induced radial velocity variations and evaluate the impacts of extreme magnetism on exoplanet detection thresholds. We map the magnetic field and surface brightness for 14 epochs between 2004 and 2019. Our results show potential 3-4 yr variations of the magnetic field which evolves from weak and simple during chromospheric activity minima to strong and complex during activity maxima but without any polarity reversals. A persistent, temporally-varying polar spot coexists with weaker, short-lived lower-latitude spots. Due to their different decay time-scales, significant differential rotation and the limited temporal coverage of our legacy data, we were unable to reliably model the activity-induced radial velocity using Gaussian Process regression. Doppler Imaging can be a useful method for modelling the magnetic activity jitter of extremely active stars using data with large phase gaps. Given our data and using Doppler Imaging to filter activity jitter, we estimate that we could detect Jupiter-mass planets with orbital periods of $\sim$3 d. A longer baseline of continuous observations is the best observing strategy for the detection of exoplanets orbiting highly active stars.

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arXiv2023

The large-scale magnetic field of the M dwarf double-line spectroscopic binary FK Aqr

This work is part of the BinaMIcS project, the aim of which is to understand the interaction between binarity and magnetism in close binary systems. All the studied spectroscopic binaries targeted by the BinaMIcS project encompass hot massive and intermediate-mass stars on the main sequence, as well as cool stars over a wide range of evolutionary stages. The present paper focuses on the binary system FK Aqr, which is composed of two early M dwarfs. Both stars are already known to be magnetically active based on their light curves and detected flare activity. In addition, the two components have large convective envelopes with masses just above the fully convective limit, making the system an ideal target for studying effect of binarity on stellar dynamos. We use spectropolarimetric observations obtained with ESPaDOnS at CFHT in September 2014. Mean Stokes I and V line profiles are extracted using the least-squares deconvolution (LSD) method. The radial velocities of the two components are measured from the LSD Stokes I profiles and are combined with interferometric measurements in order to constrain the orbital parameters of the system. The longitudinal magnetic fields Bl and chromospheric activity indicators are measured from the LSD mean line profiles. The rotational modulation of the Stokes V profiles is used to reconstruct the surface magnetic field structures of both stars via the Zeeman Doppler imaging (ZDI) inversion technique. Maps of the surface magnetic field structures of both components of FK Aqr are presented for the first time. Our study shows that both components host similar large-scale magnetic fields of moderate intensity (Bmean ~ 0.25 kG); both are predominantly poloidal and feature a strong axisymmetric dipolar component. (abridged)

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arXiv2023

Measuring small-scale magnetic fields of 44 M dwarfs from SPIRou spectra with ZeeTurbo

We present the results of an analysis aimed at probing the small-scale magnetic fields of M dwarfs observed with SPIRou, the nIR high-resolution spectro-polarimeter installed at the Canada-France-Hawaii Telescope, in the context of the SPIRou Legacy Survey. Our analysis relies on high-resolution median spectra built from several tens of spectra recorded between 2019 and 2022, and on synthetic spectra computed with the ZeeTurbo code for various combination of atmospheric parameters and magnetic field strengths. We pursue the efforts undertaken in a previous study and focus on 44 weakly to moderately active M dwarfs. We derive average magnetic field strengths (<$B$>) ranging from 0.05 to 1.15 kG, in good agreement with activity estimates and rotation periods. We found that including magnetic fields in our models has virtually no impact on our derived atmospheric parameters, and that a priori assumptions on the stellar surface gravity can affect our estimated <$B$>. Our results suggest that small-scale magnetic fields account for more than 70% of the overall average magnetic field for most targets whose large-scale fields were previously measured. We derived low magnetic fluxes for several targets in our sample, and found no clear evidence that <$B$> decreases with increasing Rossby number in the unsaturated dynamo regime. We even identified counterexamples (GJ 1289 and GJ 1286) where the small-scale field is unusually strong despite the long rotation period. Along with similar results on the large-scale fields, our findings further suggest that dynamo processes may operate in a non-conventional mode in these strongly magnetic, slowly-rotating stars.

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arXiv2023

Star-disk interactions in the strongly accreting T Tauri Star S CrA N

Aims : We aimed at constraining the accretion-ejection phenomena around the strongly-accreting Northern component of the S CrA young binary system (S CrA N) by deriving its magnetic field topology and its magnetospheric properties, and by detecting ejection signatures, if any. Methods : We led a two-week observing campaign on S CrA N with the ESPaDOnS optical spectropolarimeter at the Canada-France-Hawaii Telescope. We recorded 12 Stokes I and V spectra over 14 nights. We computed the corresponding Least-Square Deconvolution (LSD) profiles of the photospheric lines and performed Zeeman-Doppler Imaging (ZDI). We analysed the kinematics of noticeable emission lines, namely He I $λ5876$ and the four first lines of the Balmer series, known to trace the accretion process. Conclusions : The findings from spectropolarimetry are complementary to those provided by optical long-baseline interferometry, allowing us to construct a coherent view of the innermost regions of a young, strongly accreting star. Yet, the strong and complex magnetic field reconstructed for S CrA N is inconsistent with the observed magnetic signatures of the emission lines associated to the post-shock region. We recommend a multi-technique, synchronized campaign of several days to put more constrains on a system that varies on a $\sim$ 1 day timescale.

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