Abstract
Abstract Dark rings with bright rims are the indirect signposts of planets embedded in protoplanetary discs. In a recent first, an azimuthally elongated AU-scale blob, possibly a planet, was resolved with ALMA in TW Hya. The blob is at the edge of a cliff-like rollover in the dust disc rather than inside a dark ring. Here we build time-dependent models of TW Hya disc. We find that the classical paradigm cannot account for the morphology of the disc and the blob. We propose that ALMA-discovered blob hides a Neptune mass planet losing gas and dust. We show that radial drift of mm-sized dust particles naturally explains why the blob is located on the edge of the dust disc. Dust particles leaving the planet perform a characteristic U-turn relative to it, producing an azimuthally elongated blob-like emission feature. This scenario also explains why a 10 Myr old disc is so bright in dust continuum. Two scenarios for the dust-losing planet are presented. In the first, a dusty pre-runaway gas envelope of a ∼40 M⊕ Core Accretion planet is disrupted, e.g., as a result of a catastrophic encounter. In the second, a massive dusty pre-collapse gas giant planet formed by Gravitational Instability is disrupted by the energy released in its massive core. Future modelling may discriminate between these scenarios and allow us to study planet formation in an entirely new way – by analysing the flows of dust and gas recently belonging to planets, informing us about the structure of pre-disruption planetary envelopes.
Highlights
Recent advent in the high-resolution imaging of protoplanetary discs via scattering light techniques and mm-continuum with ALMA yielded many exciting examples of sub-structures in the discs, such as large-scale asymmetries (Casassus et al 2013), spirals (Benisty et al 2017), dark and bright rings and/or gaps (ALMA Partnership et al 2015; Andrews et al 2018; Dullemond et al 2018; Long et al 2018), clumps and young planets (Mesa et al 2019)
We show that radial drift of mm-sized dust particles naturally explains why the blob is located on the edge of the dust disc
For a very old system such as TW Hydra, we found that only very metal rich (Z ∼ 0.1) Gravitational Instability (GI) planets with masses no larger than 2 MJ can be disrupted via this mechanism at t ∼ 10 Myr (Section 8.1.2 and Section 8.1.3)
Summary
Recent advent in the high-resolution imaging of protoplanetary discs via scattering light techniques and mm-continuum with ALMA yielded many exciting examples of sub-structures in the discs, such as large-scale asymmetries (Casassus et al 2013), spirals (Benisty et al 2017), dark and bright rings and/or gaps (ALMA Partnership et al 2015; Andrews et al 2018; Dullemond et al 2018; Long et al 2018), clumps and young planets (Mesa et al 2019). It is believed that most protoplanetary discs have substructures; those that do not may be those that have not yet been imaged at high enough resolution (Garufi et al 2018). Veronesi et al (2019) shows that the prevalence of annular rather than spiral features in many of the observed discs indicates that the mm-sized particles have rather large Stokes numbers, limiting disc While the radial drift can be slowed down by invoking very massive gas discs, Mgas (0.1 − 0.2)M⊙ (e.g. Powell, Murray-Clay & Schlichting 2017; Powell et al 2019), other arguments point against that. Veronesi et al (2019) shows that the prevalence of annular rather than spiral features in many of the observed discs indicates that the mm-sized particles have rather large Stokes numbers, limiting disc
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