A Formation Crisis of Repeating Partial Tidal Disruption Events

Abstract A number of candidate repeating partial tidal disruption events (rpTDEs) has been reported in recent years. If these events are confirmed, the high fraction of observed rpTDEs among all tidal disruption events is in tension with prediction of the single-star loss-cone channel as noticed in previous studies. We further point out an inequality <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>•</mml:mo> </mml:msub> <mml:mo>≲</mml:mo> <mml:mn>4</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mn>6</mml:mn> </mml:msup> <mml:mspace width="0.25em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>obt</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:mn>10</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>yr</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> <mml:mo>/</mml:mo> <mml:mn>9</mml:mn> </mml:mrow> </mml:msup> </mml:math> that must be satisfied for rpTDEs to be produced by solar-type stars in the loss-cone channel, where M • is the central supermassive black hole (SMBH) mass and T obt is the orbital period of the star. However the majority of reported rpTDE candidates potentially violate this inequality, indicating an alternative formation channel. For the commonly invoked Hills mechanism to explain the rpTDE observations, the majority of tidally disrupted stellar binaries by SMBHs must be near-contact binaries. If the same process operates in the Galactic center, there should exist a population of hypervelocity stars (HVSs) ejected with velocities as high as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>3.6</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mn>3</mml:mn> </mml:msup> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>•</mml:mo> </mml:msub> <mml:mo>/</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mn>6</mml:mn> </mml:msup> <mml:mspace width="0.25em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>/</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:mi>km</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mi mathvariant="normal">s</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> , which however have not been detected. A complete search for HVSs in the Milky Way will be critical for testing this prediction.