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Description
The protostar TW Hydra features the best studied and one of the most unusual protoplanetary discs. Its dust disc has a cliff-like rollover at 52 AU which coincides with a suspected sub-Neptune mass planet recently detected as an azimuthally elongated AU-scale excess in ALMA 1.3 mm continuum (Tsukagoshi +19). Here we build detailed models of dust growth, dynamics and synthetic disc emission to investigate the origin of TW Hydra's peculiarities.
We show that the standard scenario in which the dust in TW Hydra disc is primordial accounts neither for the dust morphology nor the excess emission. We propose an alternative model in which the primordial dust is long consumed by the star or locked in planets; the dust currently observed in the system is ejected by the suspected ALMA planet. We show that in this model the mm-sized dust particles are blown inside the planetary orbit, naturally explaining the dust disc morphology and its relation to the 1.3 mm excess. Further, dust lost by the planet performs a characteristic U-turn relative to the planet producing an azimuthally elongated emission feature similar to the one observed by ALMA. Finally, the disruption scenario provides an attractive explanation for why one of the oldest protoplanetary discs happens to be tens times more massive in terms of dust than most discs a fraction of TW Hydra's age.
We consider two scenarios for the nature of the dust-loosing planet. In the first, a dusty pre-collapse gas envelope of a massive core growing in the Core Accretion framework is disrupted, e.g., as a result of a catastrophic encounter. In the second, a massive dusty gas giant planet formed in the Gravitational Instability scenario is disrupted by the energy release in its massive core. In the latter case all of TW Hydra protoplanetary disc, including its gaseous component, may originate in such a disruption; the planet mass has to be no larger than 2 Jupiter masses and it must be 5-10 times more abundant in metals than the Sun.
If these ideas are correct then future observations of TW Hydra, and potentially other discs, will allow us to study planet formation in an entirely new way -- by analysing the flows of dust and gas recently belonging to giant planets. Reverse engineering of mass loss from the planets may inform us about their density structure and elemental composition before the disruption.
