Simulating Chemical Abundances in the Circumstellar Nebula of the Late Stage Binary RY Scuti

Abstract RY Scuti, thought to be a Wolf–Rayet (WR) progenitor, is a massive post-main-sequence binary star system undergoing Roche-lobe overflow (RLOF). SOFIA (+FORCAST) spectroscopy of the inner, ionized region of RY Scuti’s double ringed toroidal nebula affirms the previous detection of the well-studied 12.81 μ m Ne ii forbidden transition and reveals four distinct emission lines, including three previously undetected transitions, S iii , Fe iii , and S iii . Cloudy photoionization modeling of the four neon, sulfur, and iron lines was performed to derive fractional abundances (log( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfrac> <mml:mrow> <mml:mi>X</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>H</mml:mi> </mml:mrow> </mml:mfrac> </mml:math> )) of neon at −3.19 ± 0.0251, sulfur at −3.76 ± 0.0487, and iron at −2.23 ± 0.0286. All three species are overabundant with respect to fiducial solar chemical abundances, especially iron. Our analysis suggests that the outer envelope of the primary star in the RY Scuti system is being stripped away via RLOF, leaving helium-rich and hydrogen-poor material visible to observation. This material also exhibits elevated neon, sulfur, and iron fractional abundances, consistent with RY Scuti evolving toward a WR object.