Double periodic variable V4142 Sgr: A key to approaching the stellar dynamo

Abstract

In this work we focus on the double periodic variable (DPV) star V4142\,Sgr, aiming to provide a deeper understanding of its evolution, the formation of its accretion disk, and the operation of magnetic dynamos within the donor star. We analyze its characteristics in detail as well as the phenomena associated with DPV stars more generally. The model was implemented using the stellar evolution code MESA r22.11.1. The modeling process starts from the zero age main sequence and incorporates differential rotation to facilitate the creation of a stellar dynamo in the donor star. We adjusted the model by employing a chi-square algorithm, minimizing the deviation between theoretical and observed values based on previously published fundamental parameters for this system. Our analysis includes an evaluation of various parameters, such as initial masses, orbital periods, mixing parameters, the thermohaline parameter, and metallicities. We assessed the algorithm convergence and set the stopping criterion at 20 helium core depletion in the donor star. A comprehensive analysis was conducted at each evolutionary stage, utilizing the Tayler-Spruit formalism to understand the mechanism of magnetic dynamos. The model begins by adjusting fundamental parameters published for this system through a chi-squared optimization algorithm, adopting an initial orbital period of 15.0 days and initial masses for the donor and gainer star of $ M_ i,d odot $ and $ M_ i,g odot $, with a metallicity associated with this type of DPV of $ Z = 0.02 $. It successfully converges with six degrees of freedom and 5 confidence, resulting in a chi-squared value of 0.007. In addition, the best-fit model for V4142\,Sgr shows it is in thermal-timescale mass transfer. Our analysis provides insights into the role of differential rotation in facilitating the formation of a stellar dynamo. Additionally, we have determined that our type-B gainer star is located in a region similar to other type-B DPVs that have undergone rejuvenation due to the transfer of matter. The size of the gainer star shrinks considerably, but it rejuvenates thanks to the material acquired from its donor companion. As for the donor star, the creation and amplification of magnetic fields are influenced by the mixing diffusivity, $ D_ ST $, which is activated by advection outside the overshooting zone.