Physical parameters and long-term photometric variability of V1481 Ori, an SB2 member of Orion nebula Cluster with an accreting component

Abstract

We present the results of our analysis on V1481 Ori (JW 239), a young SB2 in the Orion nebula Cluster with a circumbinary disc accreting on the lower mass component. The analysis is based on high-resolution spectroscopic data and high-quality photometric time series about 20-yr long. Thanks to the spectroscopy, we confirm the binary nature of this system consisting of M3 + M4 components and derive the mass ratio M<SUB>B</SUB>/M<SUB>A</SUB> = 0.54, a variable luminosity ratio L<SUB>B</SUB>/L<SUB>A</SUB> = 0.68-0.94, and an orbital period P<SUB>orb</SUB> = 4.433 d. The photometric data allowed us to measure the rotation periods of the two components P<SUB>phot</SUB> = 4.4351 d and they are found to be synchronized with the orbital period. The simultaneous modelling of V-, I-band, and radial velocity curves in the 2005 season suggests that the variability is dominated by one hotspot on the secondary component covering at least ∼3.5 per cent of the stellar surface and about 420 K hotter than the unperturbed photosphere. Such a spot may originate from the material of the circumbinary disc accreting on to the secondary component. We also detect an apparent 6-yr periodic variation in the position of this hotspot, which is inferred from the phase migration of the light-curve maximum, which we interpret as due to either the presence of surface differential rotation as large as 0.065 per cent, a value compatible with the fully convective components, or to a periodic exchange of angular momentum between the disc and the star, which implies a minimum magnetic field strength of 650 G at the stellar surface.