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Keywords:

  • accretion, accretion discs;
  • hydrodynamics;
  • binaries: close

ABSTRACT

Recent observations have shown that the direct-impact Algol systems U CrB and RS Vul possess gas located outside of the orbital plane, including a tilted accretion disc in U CrB. Observations of circumstellar gas surrounding the mass donor in RS Vul suggest magnetic effects could be responsible for deflecting the accretion stream out of the orbital plane, resulting in a tilted disc. To determine whether a tilted disc is possible due to a deflected stream, we use three-dimensional hydrodynamic simulations of the mass transfer process in RS Vul. By deflecting the stream 45° out of the orbital plane and boosting the magnitude of the stream's velocity to Mach 30, we mimic the effects of magnetic activity near the first Lagrange point. We find that the modified stream parameters change the direct-impact nature of the system. The stream misses the surface of the star, and a slightly warped accretion disc forms with no more than 3° of disc tilt. The stream–disc interaction for the deflected stream forces a large degree of material above the orbital plane, increasing the out-of-plane flow drastically. Plotting the Hα emissivity in velocity space allows us to compare our results with tomographic observations. Deflecting and boosting the stream increases the emissivity in each vz slice of the out-of-plane flow by at least three and up to eight orders of magnitude compared to the undeflected case. We conclude that a deflected stream is a viable mechanism for producing the strong out-of-plane flows seen in the tomographic images of U CrB and RS Vul.