Search for: All records

Abstract Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto stars via funnel flows, which follow stellar magnetic field lines and shock on the stellar surfaces 1–3 , leaving hot spots with density gradients 4–6 . Previous work has provided observational evidence of varying density in hot spots 7 , but these observations were not sensitive to the radial density distribution. Attempts have been made to measure this distribution using X-ray observations 8–10 ; however, X-ray emission traces only a fraction of the hot spot 11,12 and also coronal emission 13,14 . Here we report periodic ultraviolet and optical light curves of the accreting star GM Aurigae, which have a time lag of about one day between their peaks. The periodicity arises because the source of the ultraviolet and optical emission moves into and out of view as it rotates along with the star. The time lag indicates a difference in the spatial distribution of ultraviolet and optical brightness over the stellar surface. Within the framework of a magnetospheric accretion model, this finding indicates the presence of a radial density gradient in a hot spot on the stellar surface, because regions of the hot spot with different densities have different temperatures and therefore emit radiation at different wavelengths. 

ABSTRACT We present results of global 3D magnetohydrodynamic simulations of accretion on to magnetized stars where both the magnetic and rotational axes of the star are tilted about the rotational axis of the disc. We observed that initially the inner parts of the disc are warped, tilted, and precess due to the magnetic interaction between the magnetosphere and the disc. Later, larger tilted discs form with the size increasing with the magnetic moment of the star. The normal vector to the discs are tilted at different angles, from ∼5°–10° up to ∼30°–40°. Small tilts may result from the winding of the magnetic field lines about the rotational axis of the star and the action of the magnetic force which tends to align the disc. Another possible explanation is the magnetic Bardeen–Petterson effect in which the disc settles in the equatorial plane of the star due to precessional and viscous torques in the disc. Tilted discs slowly precess with the time-scale of the order of ∼50 Keplerian periods at the reference radius (∼3 stellar radii). Our results can be applied to different types of stars where signs of tilted discs and/or slow precession have been observed. 

ABSTRACT Planets are thought to form at the early stage of stellar evolution when mass accretion is still ongoing. RY Tau is a T Tauri type star at the age of a few Myr, with an accretion disc seen at high inclination, so that the line of sight crosses both the wind and accretion gas flows. In a long series of spectroscopic monitoring of the star over the period 2013–2020, we detected variations in H$$\, {\alpha }$$ and Na i D absorptions at radial velocities of infall (accretion) and outflow (wind) with a period of about 22 d. The absorptions in the infalling and outflowing gas streams vary in antiphase: an increase of infall is accompanied by a decrease of outflow, and vice versa. These ‘flip-flop’ oscillations retain phase over several years of observations. We suggest that this may result from the magnetohydrodynamics processes at the disc–magnetosphere boundary in the propeller mode. Another possibility is that a massive planet is modulating some processes in the disc and is providing the observed effects. The period, if Keplerian, corresponds to a distance of 0.2 au, which is close to the dust sublimation radius in this star. The presence of the putative planet can be confirmed by radial velocity measurements: the expected amplitude is ≥90 m s−1 if the planet mass is ≥2 MJ.