Long orbital period pre-polars containing an early K-type donor stars. Bottleneck accretion mechanism in action

TL;DR

This paper investigates two long-period cataclysmic variables with early K-type donors, revealing a magnetic wind accretion mechanism in detached binaries that mimics traditional magnetic CVs, expanding understanding of binary evolution.

Contribution

It introduces the concept of detached, magnetic, wind-accreting binaries with early K-type donors, providing new insights into pre-polars and magnetic CV formation.

Findings

V1082 Sgr shows low luminosity state dominated by a chromospherically active K star.

Accretion occurs via stellar wind through magnetic coupling, not Roche lobe overflow.

Similar systems like V479 And suggest a class of detached, magnetic binaries before contact.

Abstract

We studied two objects identified as a Cataclysmic Variables (CVs) with periods exceeding the natural boundary for Roche lobe filling ZAMS secondary stars. We present observational results for V1082 Sgr with 20.82 h orbital period, an object that shows low luminosity state, when its flux is totally dominated by a chromospherically active K- star with no signs of ongoing accretion. Frequent accretion shut-offs, together with characteristics of emission lines in a high state, indicate that this binary system is probably detached and the accretion of matter on the magnetic white dwarf takes place through stellar wind from the active donor star via coupled magnetic fields. Its observational characteristics are surprisingly similar to V479 And, a 14.5 h binary system. They both have early K-type stars as a donor star. We argue, that similar to the shorter period pre-polars containing M-dwarfs, these are detached binaries with strong magnetic components. Their magnetic fields are coupled, allowing enhanced stellar wind from the K star to be captured and channeled through the bottleneck connecting the two stars onto the white dwarf's magnetic pole, mimicking a magnetic CV. Hence, they become interactive binaries before they reach contact. This will help to explain an unexpected lack of systems possessing white dwarfs with strong magnetic fields among detached white + red dwarf systems.