Polygram Stars: Resonant Tidal Excitation of Fundamental Oscillation Modes in Asynchronous Stellar Coalescence

Morgan Macleod, Michelle Vick, Dong Lai, James McLellan Stone

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7 Scopus citations


The prevalence of binary stars at close separations implies that many of these systems will interact or merge during the binary's lifetime. This paper presents hydrodynamic simulations of the scenario of binary coalescence through unstable mass transfer, which drives the pair to closer separations. When the donor star does not rotate synchronously with respect to the orbit, dynamical tidal waves are excited in its envelope. We show that resonance crossings with high azimuthal order (m ∼ 3 to 6) fundamental modes induce a visible "polygram" distortion to the star. As the binary orbit tightens, the system sweeps through resonance with modes of decreasing azimuthal order, which are selectively excited. We compare our hydrodynamic simulations to predictions from linear theory of resonant-mode excitation. The linear theory provides an estimate of mode amplitudes to within a factor of two, even as the oscillations become quite nonlinear as the stars coalesce. We estimate that a wave with 10% radial amplitude generates approximately 1% photometric variability; this may be detectable if such a binary coalescence is caught in action by future photometric all-sky surveys.

Original languageEnglish (US)
Article number28
JournalAstrophysical Journal
Issue number1
StatePublished - May 20 2019

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • binaries: close
  • methods: numerical
  • stars: oscillations


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