Ref HAL: hal-04195331_v1
Ref Arxiv: 2306.15391
Ref INSPIRE: 2693340
DOI: 10.1051/0004-6361/202346675
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Ref Arxiv: 2310.04497
DOI: 10.1051/0004-6361/202347498
Ref. & Cit.: NASA ADS
Résumé:

Molecular bands of metal oxides and hydrides dominate the optical and near-infrared spectra of M dwarfs. High-resolution spectra of these bands have immense potential for determining many properties of these stars, such as effective temperature, surface gravity, elemental abundances, radial velocity, or surface magnetic fields. Techniques are being developed to do this but remain limited by the current availability and accuracy of molecular data and spectral line lists. This paper reports metal monohydride line lists selected from near-infrared and visible laboratory data to show that specific bands in several electronic transitions can be used to identify CrH, NiH, and FeH in M stars and to determine radial velocities from Doppler shifts. The possibility of measuring magnetic fields is also investigated for FeH and CrH. We used systematic cross-correlation analysis between unpolarised spectra from a selection of M stars and state-specific laboratory line lists. These lists were generated from a combination of existing data and new laboratory laser-excitation spectra recorded at Doppler-limited resolution, in zero-field conditions or in magnetic fields up to 0.6 tesla. Results. We show that transitions at visible wavelengths in FeH and NiH, usually neglected in the analysis of the spectra of M-type stars, do in fact contribute to the spectra, and we demonstrate the influence of magnetic sensitivity on selected transitions in CrH and FeH. Although the new line lists focus on transitions recorded at temperatures significantly lower than those of stellar objects, they remain pertinent because they cover some band-head regions of high spectral density. FeH bands can provide a useful supplement to atomic lines for the analysis of high-resolution optical and near-infrared spectra of M dwarfs. We demonstrate the influence of a magnetic field on CrH signatures around 862 nm.

Commentaires: Accepted for publication in A&A, 10 pages, 12 figures, 2 tables, additional material will be made available at CDS/Vizier

DOI: 10.1051/0004-6361/202245131
Résumé:

Context. The stellar activity of M dwarfs is the main limiting factor in the discovery and characterization of the exoplanets orbiting them, because it induces quasi-periodic radial velocity (RV) variations. Aims: We aim to characterize the magnetic field and stellar activity of the early, moderately active M dwarf Gl 205 in the optical and near-infrared (NIR) domains. Methods: 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 Zeeman-Doppler imaging (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 Gaussian process regression (GPR). 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 GPR with activity indicators as ancillary time series. Results: The optical and NIR RVs show 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 ± 0.5 days 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 ± 1.8 days at the stellar equator and Ppol = 45.5 ± 0.3 days at the poles. We observed inconsistencies in the periodicities of the activity indicators 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−1 for both instruments while using Hα and the BIS in the optical and the full width at half maximum (FWHM) in the NIR as ancillary time series. Conclusions: The RV variations observed in Gl 205 are due to stellar activity, with a complex evolution and different expressions in the optical and NIR revealed thanks to an extensive follow-up. Spectropolarimetry remains the best technique to constrain the stellar rotation period over standard activity indicators, particularly for moderately active M dwarfs.

DOI: 10.1093/mnras/stad1193
Résumé:

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 time-scale of months under differential rotation and intrinsic variability. The small-scale field, estimated from the broadening of spectral lines, reaches 2.61 ± 0.05 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 ± 30 G. We also find that surface differential rotation, as derived from the large-scale field, is ≃30 per cent 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 ± 0.10 d, i.e. near the 4:1 resonance with b. The RV signature of e is detected at 6.5σ while those of b and c show up at ≃4σ, yielding masses of $10.2^{+3.9}_{-2.7}$ and $14.2^{+4.8}_{-3.5}$ M⊕ for b and c, and a minimum mass of $35.2^{+6.7}_{-5.4}$ M⊕ for e.

DOI: 10.1051/0004-6361/202245660
Résumé:

Context. The search for extrasolar planets around the nearest M dwarfs is a crucial step toward 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 RVs of Gl 388 from 2019 to 2021 with SOPHIE, the optical échelle spectrograph (R ~75k) at the Observatoire de Haute-Provence in France, and RV and Stokes V measurements with SPIRou, the near-infrared spectropolarimeter at the Canada France Hawaii Telescope (R ~70k). Results: The SOPHIE RV time series (precision of 3-5 m s-1 per visit) displays a periodic signal with a 2.23 ± 0.01 day period and 23.6 ± 0.5 m s-1 amplitude, which is consistent with previous HARPS observations obtained in 2005-2006. The SPIRou RV time series (precision of 2 ms-1 per visit) is flat at 5 ms-1 rms and displays no periodic signals. RV signals of amplitude higher than 5.3 m s-1 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 as a proxy of stellar rotation, we measure a 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-infrared RV to confirm or infirm planet candidates discovered in the optical around active stars. Our SPIRou observations additionally reiterate how effective spectropolarimetry is at determining the stellar rotation period from the variations of Bℓ.

Ref HAL: hal-04169468_v1
Ref Arxiv: 2304.09896
Ref INSPIRE: 2680053
DOI: 10.3847/2041-8213/acce38
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Ref HAL: hal-04187255_v1
Ref Arxiv: 2307.01016
DOI: 10.1051/0004-6361/202346845
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