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The Galactic Centre is known to have undergone a recent star formation episode a few Myr ago, which likely produced many T Tauri stars hosting circumstellar discs. It has been suggested that these discs may be the compact and dusty ionized sources identified as 'G-clouds'. Given the Galactic Centre's hostile environment, we study the possible evolutionary pathways these discs experience. We compute new external photoevaporation models applicable to discs in the Galactic Centre that account for the subsonic launching of the wind and absorption of UV photons by dust. Using evolutionary disc calculations, we find that photoevaporation's rapid truncation of the disc causes them to accrete onto the central star rapidly. Ultimately, an accreting circumstellar disc has a lifetime ≲ 1 Myr, which would fail to live long enough to explain the G-clouds. However, we identify a new evolutionary pathway for circumstellar discs in the Galactic Centre. Removal of disc material by photoevaporation prevents the young star from spinning down due to magnetic braking, ultimately causing the rapidly spinning young star to torque the disc into a 'decretion disc' state which prevents accretion. At the same time, any planetary companion in the disc will trap dust outside its orbit, shutting down photoevaporation. The disc can survive for up to ∼10 Myr in this state. Encounters with other stars are likely to remove the planet on Myr time-scales, causing photoevaporation to restart, giving rise to a G-cloud signature. A giant planet fraction of |$\sim 10{{\ \rm per\ cent}}$| can explain the number of observed G-clouds. [ABSTRACT FROM AUTHOR]

Axisymmetric dust rings containing tens to hundreds of Earth masses of solids have been observed in protoplanetary discs with (sub-)millimetre imaging. Here, we investigate the growth of a planetary embryo in a massive (150 M⊕) axisymmetric dust trap through dust and gas hydrodynamics simulations. When accounting for the accretion luminosity of the planetary embryo from pebble accretion, the thermal feedback on the surrounding gas leads to the formation of an anticyclonic vortex. Since the vortex forms at the location of the planet, this has significant consequences for the planet's growth: as dust drifts towards the pressure maximum at the centre of the vortex, which is initially co-located with the planet, a rapid accretion rate is achieved, in a distinct phase of 'vortex-assisted' pebble accretion. Once the vortex separates from the planet due to interactions with the disc, it accumulates dust, shutting off accretion on to the planet. We find that this rapid accretion, mediated by the vortex, results in a planet containing ≈100 M⊕ of solids. We follow the evolution of the vortex, as well as the efficiency with which dust grains accumulate at its pressure maximum as a function of their size, and investigate the consequences this has for the growth of the planet as well as the morphology of the protoplanetary disc. We speculate that this extreme formation scenario may be the origin of giant planets that are identified to be significantly enhanced in heavy elements. [ABSTRACT FROM AUTHOR]

We used the Atacama Pathfinder Experiment (APEX) telescope to search for C I 1-0 (492.16 GHz) emission towards eight proplyds in NGC 1977, which is an FUV radiation environment two orders of magnitude weaker than that irradiating the Orion Nebular Cluster (ONC) proplyds. C I is expected to enable us to probe the wind launching region of externally photo-evaporating discs. Of the eight targets observed, no 3σ detections of the C I line were made despite reaching sensitivities deeper than the anticipated requirement for detection from prior APEX CI observations of nearby discs and models of external photo-evaporation of quite massive discs. By comparing both the proplyd mass loss rates and C I flux constraints with a large grid of external photo-evaporation simulations, we determine that the non-detections are in fact fully consistent with the models if the proplyd discs are very low mass. Deeper observations in C I and probes of the disc mass with other tracers (e.g. in the continuum and CO) can test this. If such a test finds higher masses, this would imply carbon depletion in the outer disc, as has been proposed for other discs with surprisingly low C I fluxes, though more massive discs would also be incompatible with models that can explain the observed mass loss rates and C I non-detections. The expected remaining lifetimes of the proplyds are estimated to be similar to those of proplyds in the ONC at 0.1 Myr. Rapid destruction of discs is therefore also a feature of common, intermediate UV environments. [ABSTRACT FROM AUTHOR]

The photoevaporation of protoplanetary discs by nearby massive stars present in their birth cluster plays a vital role in their evolution. Previous modelling assumes that the disc behaves like a classical Keplerian accretion disc out to a radius where the photoevaporative outflow is launched. There is then an abrupt change in the angular velocity profile, and the outflow is modelled by forcing the fluid parcels to conserve their specific angular momenta. Instead, we model externally photoevaporating discs using the slim disc formalism. The slim disc approach self-consistently includes the advection of radial and angular momentum as well as angular momentum redistribution by internal viscous torques. Our resulting models produce a smooth transition from a rotationally supported Keplerian disc to a photoevaporative driven outflow, where this transition typically occurs over ∼4–5 scale heights. The penetration of ultraviolet photons predominately sets the radius of the transition and the viscosity's strength plays a minor role. By studying the entrainment of dust particles in the outflow, we find a rapid change in the dust size and surface density distribution in the transition region due to the steep gas density gradients present. This rapid change in the dust properties leaves a potentially observable signature in the continuum spectral index of the disc at mm wavelengths. Using the slim disc formalism in future evolutionary calculations will reveal how both the gas and dust evolve in their outer regions and the observable imprints of the external photoevaporation process. [ABSTRACT FROM AUTHOR]

We use a suite of smoothed particle hydrodynamic simulations to investigate the susceptibility of protoplanetary discs to the effects of self-gravity as a function of star–disc properties. We also include passive irradiation from the host star using different models for the stellar luminosities. The critical disc-to-star mass ratio for axisymmetry (for which we produce criteria) increases significantly for low-mass stars. This could have important consequences for increasing the potential mass reservoir in a proto Trappist-1 system, since even the efficient Ormel et al. formation model will be influenced by processes like external photoevaporation, which can rapidly and dramatically deplete the dust reservoir. The aforementioned scaling of the critical M d/ M * for axisymmetry occurs in part because the Toomre Q parameter has a linear dependence on surface density (which promotes instability) and only an |$M_*^{1/2}$| dependence on shear (which reduces instability), but also occurs because, for a given M d/ M *, the thermal evolution depends on the host star mass. The early phase stellar irradiation of the disc (for which the luminosity is much higher than at the zero age main sequence, particularly at low stellar masses) can also play a key role in significantly reducing the role of self-gravity, meaning that even solar mass stars could support axisymmetric discs a factor two higher in mass than usually considered possible. We apply our criteria to the DSHARP discs with spirals, finding that self-gravity can explain the observed spirals so long as the discs are optically thick to the host star irradiation. [ABSTRACT FROM AUTHOR]

We demonstrate the utility of C  i as a tracer of photoevaporative winds that are being driven from discs by their ambient UV environment. Commonly observed CO lines only trace these winds in relatively weak UV environments and are otherwise dissociated in the wind at the intermediate to high UV fields that most young stars experience. However, C  i traces unsubtle kinematic signatures of a wind in intermediate UV environments (∼1000 G0) and can be used to place constraints on the kinematics and temperature of the wind. In C  i position–velocity (PV) diagrams external photoevaporation results in velocities that are faster than those from Keplerian rotation alone, as well as emission from quadrants of PV space in which there would be no Keplerian emission. This is independent of viewing angle because the wind has components that are perpendicular to the azimuthal rotation of the disc. At intermediate viewing angles (∼30–60°) moment 1 maps also exhibit a twisted morphology over large scales (unlike other processes that result in twists, which are typically towards the inner disc). C  i is readily observable with ALMA, which means that it is now possible to identify and characterize the effect of external photoevaporation on planet-forming discs in intermediate UV environments. [ABSTRACT FROM AUTHOR]

We present a simple model for low-mass planet formation and subsequent evolution within 'transition' discs. We demonstrate quantitatively that the predicted and observed structures of such discs are prime birthsites of planets. Planet formation is likely to proceed through pebble accretion, should a planetary embryo (M ≳ 10-4 M⊕) form. Efficient pebble accretion is likely to be unavoidable in transition disc dust traps, as the dust particles required for pebble accretion are those which are most efficiently trapped in the transition disc dust trap. Rapid pebble accretion within the dust trap gives rise not only to low-mass planets, but also to a large accretion luminosity. This accretion luminosity is sufficient to heat the disc outside the gravitational influence of the planet and makes the disc locally baroclinic, and a source of vorticity. Using numerical simulations, we demonstrate that this source of vorticity can lead to the growth of a single large-scale vortex in ~100 orbits, which is capable of trapping particles. Finally, we suggest an evolutionary cycle: Planet formation proceeds through pebble accretion, followed by vortex formation and particle trapping in the vortex quenching the planetary accretion and thus removing the vorticity source. After the vortex is destroyed, the process can begin anew. This means transition discs should present with large-scale vortices for a significant fraction of their lifetimes, and remnant planets at large 10 au radii should be a common outcome of this cycle. [ABSTRACT FROM AUTHOR]

We present a calculation of the atomic and low-ionization emission line spectra of photoevaporating protoplanetary discs. Line luminosities and profiles are obtained from detailed photoionization calculations of the disc and wind structures surrounding young active solar-type stars. The disc and wind density and velocity fields were obtained from the recently developed radiation-hydrodynamic models of Owen et al. that include stellar X-ray and EUV irradiation of protoplanetary discs at various stages of clearing, from primordial sources to inner hole sources of various hole sizes. Our models compare favourably with currently available observations, lending support to an X-ray driven photoevaporation model for disc dispersal. In particular, we find that X-rays drive a warm, predominantly neutral flow where the O i 6300 Å line can be produced by neutral hydrogen collisional excitation. Our models can, for the first time, provide a very good match to both luminosities and profiles of the low-velocity component of the O i 6300 Å line and other forbidden lines observed by Hartigan et al., which covered a large sample of T-Tauri stars. We find that the O i 6300 Å and the Ne ii 12.8 μm lines are predominantly produced in the X-ray-driven wind and thus appear blueshifted by a few km s−1 for some of the systems when observed at non-edge-on inclinations. We note, however, that blueshifts are only produced under certain conditions: X-ray luminosity, spectral shape and inner hole size all affect the location of the emitting region and the physical conditions in the wind. We caution therefore that while a blueshifted line is a tell-tale sign of an outflow, the lack of a blueshift should not be necessarily interpreted as a lack of outflow. Comparison of spectrally resolved observations of multiple emission lines with detailed model sets, like the one presented here, should provide useful diagnostics of the clearing of gaseous discs. [ABSTRACT FROM AUTHOR]