Large-scale magnetic topologies of M dwarfs

Abstract

We present here the first results of a spectropolarimetric analysis of a small sample (about 20) of active stars ranging from spectral type M0 to M8, which are either fully-convective or possess a very small radiative core. This study aims at providing new constraints on dynamo processes in fully-convective stars. Results for stars with spectral types M0-M4 -- i.e. with masses above or just below the full convection threshold (about 0.35Msun) -- are presented. Tomographic imaging techniques allow us to reconstruct the surface magnetic topologies from the rotationally modulated time-series of circularly polarised profiles. We find strong differences between partly and fully convective stars concerning magnetic field topology and characteristic scales, and differential rotation. Our results suggest that magnetic field generation in fully convective stars relies on different dynamo processes than those acting in the Sun and other partly convective stars, in agreement with theoretical expectations.Comment: 6 pages, proceedings of Cool Stars 15, St Andrews, July 2008, to be published in the Conference Proceedings Series of the AI

3D Orbital Architecture of a Dwarf Binary System and Its Planetary Companion

“…A similar calculation can be carried out for GJ 896B. In this case, the observed RV is V obs (B) = 3340 m s −1 (Morin et al 2008), which was obtained also close to the maximum RV of this star. The reference epoch of the RV observations is 2006.0, which is close to orbital location where the radial velocity of GJ 896B is maximum and positive (see Figure 8).…”

“…The GAIA catalog does not contain the radial velocity of GJ 896B. However, recent RV observations of this source indicate that its RV is about 3.34 km s −1 (Morin et al 2008). This means that GJ 896B is red shifted with respect to GJ 896A.…”

“…There is an ambiguity in the position angles of the line of nodes (Ω or Ω + 180 • , where Ω is the fitted angle), which can be disentangled by RV measurements. For the orbital motion of the binary system, recent RV measurements of both stellar components show that GJ 896B is receding (V obs (B) = +3.34 km s −1 ; Morin et al (2008)) and GJ 896A is approaching to us (V obs (A) = −0.02±0.31 km s −1 ; Lindegren et al ( 2018)), thus the correct position angle of the line of nodes is Ω AB = 255.09 • (Ω AB + 180 • ) (see Table 3). However, the planetary companion has no measured RV, so the fitted value of the position angle of the line of nodes of the orbital plane of GJ 896Ab could be either Ω Ab = 45.6 • or 225.6 • , the former is the fitted angle.…”

“…However, since the time baseline used for these orbital determinations is 34% the estimated period, the orbital parameters were not well constrained. The more massive star, GJ 896A, is an M3.5 star with an estimated mass of 0.39 M , a radius of 0.35 R , a rotation period of 1.061 days, and a rotational inclination angle of 60 • , while the less massive star, GJ 896B, is an M4.5 star with an estimated mass of 0.25 M , a radius of 0.25 R , a rotation period of 0.404 days, and a rotational inclination angle of about 60 • ±20 • (e.g., Delfosse et al 1999;Morin et al 2008;Davison et al 2015;Pearce et al 2020).…”

Magnetospheric accretion and spin-down of the prototypical classical T Tauri star AA Tau

“…This is even more radically different from those of both lower and higher mass cTTSs (namely V2247 Oph and MT Ori) for which the large‐scale poloidal field is much weaker and more complex (Donati et al 2010, Skelly et al, 2010, in preparation); similar conclusions were reached for the high‐mass cTTSs CV Cha and CR Cha (Hussain et al 2009). This makes AA Tau grossly similar to mid‐M dwarfs (whose large‐scale fields are also found to be strong, mostly poloidal and axisymmetric; Morin et al 2008), while higher and lower mass cTTSs are closer to early‐ and late‐M dwarfs, respectively (featuring weak and complex large‐scale fields in average, Donati et al 2008c; Morin et al 2010). This strengthens the idea that magnetic fields of cTTSs are produced through dynamo processes like those of M dwarfs (Donati & Landstreet 2009).…”