Magnetic fields of intermediate-mass T Tauri stars

Abstract

Context. The origin of the fossil magnetic fields detected in 5 to 10% of intermediate-mass main sequence stars is still highly debated. Aims. We want to bring observational constraints to a large population of intermediate-mass pre-main sequence (PMS) stars in order to test the theory that convective-dynamo fields generated during the PMS phases of stellar evolution can occasionally relax into fossil fields on the main sequence. Methods. Using distance estimations, photometric measurements, and spectropolarimetric data from HARPSpol and ESPaDOnS of 38 intermediate-mass PMS stars, we determined fundamental stellar parameters (Teff, L and v sin i) and measured surface magnetic field characteristics (including detection limits for non-detections, and longitudinal fields and basic topologies for positive detections). Using PMS evolutionary models, we determined the mass, radius, and internal structure of these stars. We compared different PMS models to check that our determinations were not model-dependant. We then compared the magnetic characteristics of our sample accounting for their stellar parameters and internal structures. Results. We detect magnetic fields in about half of our sample. About 90% of the magnetic stars have outer convective envelopes larger than ∼25% of the stellar radii, and heavier than ∼2% of the stellar mass. Going to higher mass, we find that the magnetic incidence in intermediate-mass stars drops very quickly, within a timescale on the order of few times 0.1 Myr. Finally, we propose that intermediate-mass T Tauri stars with large convective envelopes, close to the fully convective limit, have complex fields and that their dipole component strengths may decrease as the sizes of their convective envelopes decrease, similar to lower-mass T Tauri stars.

The accretion rates and mechanisms of Herbig Ae/Be stars

“…Muzerolle et al (2004) showed that the MA models provided qualitative agreement with the observed emission line profiles of the Herbig Ae star UX Ori. In the process, they also found that the B-field geometry should be more complex than the usual dipole field in T Tauri stars and the expected field strength would be much below the usual T Tauri detections, in line with the Villebrun et al (2019) findings.…”

MOBSTER – III. HD 62658: a magnetic Bp star in an eclipsing binary with a non-magnetic ‘identical twin’

“…One prediction of a bistability scenario is that magnetic fields should be ubiquitous during the earliest phases of the PMS, when the star is at least partially convective, and should rapidly disappear once stars become radiative. This is precisely what was found by Villebrun et al (2019) in their study of intermediate mass T Tauri stars (IMTTS). They showed that essentially all IMTTS with convective envelopes are magnetic, while very few of the fully radiative IMTTS host magnetic fields.…”

“…(i) Magnetization of the protostellar cloud provides the seed for the fossil field, and also inhibits fragmentation of the cloud and therefore prevents the formation of binaries (Commerçon et al 2011) (ii) Fossil fields are left over from dynamos powered by stellar mergers (Schneider et al 2016) (iii) Tidal interactions in eccentric binaries lead to the rapid decay of fossil magnetic fields (Vidal et al 2019) The existence of some, rather than no, close binaries including at least one star with a fossil field suggests that i) is unlikely to be universally true. This depends on whether magnetization prevents fragmentation, or simply makes it unlikely.…”

“…Alternatively, fossil fields might be left over from powerful dynamos generated during stellar mergers, an observation compatible with the apparent anomalous youth of some magnetic stars (Schneider et al 2016) as well as with the expected rate of mergers (de Mink et al 2013(de Mink et al , 2014. It has alternatively been suggested that the tidal influence of a close companion might lead to rapid decay of fossil magnetic fields (Vidal et al 2019).…”

The Detection of Dust Gap-ring Structure in the Outer Region of the CR Cha Protoplanetary Disk

“…We perform the radiative transfer calculation to derive the temperature distributions in the disk r − z plane when the dust disk is more compact than the gas disk by using a published code HO-CHUNK (Whitney et al 2003a(Whitney et al ,b, 2004(Whitney et al , 2013. We adopt the stellar mass of 1.657 M ⊙ , the effective temperature of 4800 K, and the stellar radius of 3.113 R ⊙ (Villebrun et al 2019). We also use the dust surface density derived from the observed I cont at T dust = T B,peak, 13 CO .…”