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Register a new userEvidence that the Directly-Imaged Planet HD 131399 Ab is a Background Star
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We present evidence that the recently discovered, directly-imaged planet HD 131399 Ab is a background star with non-zero proper motion. From new JHK1L photometry and spectroscopy obtained with the Gemini Planet Imager, VLT/SPHERE, and Keck/NIRC2, and a reanalysis of the discovery data obtained with VLT/SPHERE, we derive colors, spectra, and astrometry for HD 131399 Ab. The broader wavelength coverage and higher data quality allow us to re-investigate its status. Its near-infrared spectral energy distribution excludes spectral types later than L0 and is consistent with a K or M dwarf, which are the most likely candidates for a background object in this direction at the apparent magnitude observed. If it were a physically associated object, the projected velocity of HD 131399 Ab would exceed escape velocity given the mass and distance to HD 131399 A. We show that HD 131399 Ab is also not following the expected track for a stationary background star at infinite distance. Solving for the proper motion and parallax required to explain the relative motion of HD 131399 Ab, we find a proper motion of 12.3 mas/yr. When compared to predicted background objects drawn from a galactic model, we find this proper motion to be high, but consistent with the top 4% fastest-moving background stars. From our analysis we conclude that HD 131399 Ab is a background K or M dwarf.
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Recent observations of the protoplanetary disc surrounding AB Aurigae have revealed the possible presence of two giant planets in the process of forming. The young measured age of $1-4$Myr for this system allows us to place strict time constraints on the formation histories of the observed planets. Hence we may be able to make a crucial distinction between formation through core accretion (CA) or the gravitational instability (GI), as CA formation timescales are typically Myrs whilst formation through GI will occur within the first $approx10^4-10^5$yrs of disc evolution. We focus our analysis on the $4-13$M$_{rm Jup}$ planet observed at $Rapprox30$AU. We find CA formation timescales for such a massive planet typically exceed the systems age. The planets high mass and wide orbit may instead be indicative of formation through GI. We use smoothed particle hydrodynamic simulations to determine the systems critical disc mass for fragmentation, finding $M_{rm d,crit}=0.3$M$_{odot}$. Viscous evolution models of the discs mass history indicate that it was likely massive enough to exceed $M_{rm d,crit}$ in the recent past, thus it is possible that a young AB Aurigae disc may have fragmented to form multiple giant gaseous protoplanets. Calculations of the Jeans mass in an AB Aurigae-like disc find that fragments may initially form with masses $1.6-13.3$M$_{rm Jup}$, consistent with the planets which have been observed. We therefore propose that the inferred planets in the disc surrounding AB Aurigae may be evidence of planet formation through GI.
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