Formation of ring-like structures in flared α-discs with X-ray/FUV photoevaporation

ABSTRACT Protoplanetary discs are complex dynamical systems where several processes may lead to the formation of ring-like structures and planets. These discs are flared following a profile where the vertical scale height increases with radius. In this work, we investigate the role of this disc flaring geometry on the formation of rings and holes. We combine a flattening law change with X-ray and FUV photoevaporative winds. We have used a semi-analytical 1D viscous α approach, presenting the evolution of the disc mass and mass rate in a grid of representative systems. Our results show that changing the profile of the flared disc may favour the formation of ring-like features resembling those observed in real systems at the proper evolutionary times, with proper disc masses and accretion rate values. However, these features seem to be short-lived and further enhancements are still needed for better matching all the features seen in real systems.

MRI-accreting inner regions of protoplanetary discs

<p>Short-period super-Earths and mini-Neptunes have been shown to be common, yet it is still not understood how and where inside protoplanetary discs they could have formed. To form these planets at the short periods at which they are detected, the inner regions of protoplanetary discs must be enriched in dust. Dust could accumulate in the inner disc if the innermost regions accrete via the magneto-rotational instability (MRI). We developed a model of the inner disc which includes MRI-driven accretion, disc heating by both accretion and stellar irradiation, vertical energy transport, dust opacities, dust effects on disc ionization, thermal and non-thermal sources of ionization. The inner disc is assumed to be in steady state, and the dust is assumed to be well-mixed with the gas. Using this model, we explore how various disc and stellar parameters affect the structure of the inner disc and the possibility of dust accumulation. We show that properties of dust strongly affect the size of the MRI-accreting region and whether this region exists at all. Increasing the dust-to-gas ratio increases the size of this region, suggesting that dust may accumulate in the inner disc without suppressing the MRI. Overall, conditions in the inner disc may be more favourable to planet formation earlier in the disc lifetime, while the disc accretion rate is higher.</p>

Which planets trigger longer-lived vortices: low-mass or high-mass?

Recent ALMA observations have found many protoplanetary discs with rings that can be explained by gap-opening planets less massive than Jupiter. Meanwhile, recent studies have suggested that protoplanetary discs should have low levels of turbulence. Past computational work on low-viscosity discs has hinted that these two developments might not be self-consistent because even low-mass planets can be accompanied by vortices instead of conventional double rings. We investigate this potential discrepancy by conducting hydrodynamic simulations of growing planetary cores in discs with various aspect ratios (H/r = 0.04, 0.06, 0.08) and viscosities (1.5 × 10−5 ≲ α ≲ 3 × 10−4), having these cores accrete their gas mass directly from the disc. With α < 10−4, we find that sub-Saturn-mass planets in discs with H/r ≤ 0.06 are more likely to be accompanied by dust asymmetries compared to Jupiter-mass planets because they can trigger several generations of vortices in succession. We also find that vortices with H/r = 0.08 survive >6000 planet orbits regardless of the planet mass or disc mass because they are less affected by the planet’s spiral waves. We connect our results to observations and find that the outward migration of vortices with H/r ≥ 0.08 may be able to explain the cavity in Oph IRS 48 or the two clumps in MWC 758. Lastly, we show that the lack of observed asymmetries in the disc population in Taurus is unexpected given the long asymmetry lifetimes in our low viscosity simulations (α ∼ 2 × 10−5), a discrepancy we suggest is due to these discs having higher viscosities.

MRI-active inner regions of protoplanetary discs. I. A detailed model of disc structure

ABSTRACT Short-period super-Earth-sized planets are common. Explaining how they form near their present orbits requires understanding the structure of the inner regions of protoplanetary discs. Previous studies have argued that the hot inner protoplanetary disc is unstable to the magnetorotational instability (MRI) due to thermal ionization of potassium, and that a local gas pressure maximum forms at the outer edge of this MRI-active zone. Here we present a steady-state model for inner discs accreting viscously, primarily due to the MRI. The structure and MRI-viscosity of the inner disc are fully coupled in our model; moreover, we account for many processes omitted in previous such models, including disc heating by both accretion and stellar irradiation, vertical energy transport, realistic dust opacities, dust effects on disc ionization, and non-thermal sources of ionization. For a disc around a solar-mass star with a standard gas accretion rate ($\dot{M}\, \sim \, 10^{-8}$ M⊙ yr−1) and small dust grains, we find that the inner disc is optically thick, and the accretion heat is primarily released near the mid-plane. As a result, both the disc mid-plane temperature and the location of the pressure maximum are only marginally affected by stellar irradiation, and the inner disc is also convectively unstable. As previously suggested, the inner disc is primarily ionized through thermionic and potassium ion emission from dust grains, which, at high temperatures, counteract adsorption of free charges on to grains. Our results show that the location of the pressure maximum is determined by the threshold temperature above which thermionic and ion emission become efficient.

Warm millimetre dust in protoplanetary discs near massive stars

Dust plays a key role in the formation of planets and its emission also provides one of our most accessible views of protoplanetary discs. If set by radiative equilibrium with the central star, the temperature of dust in the disc plateaus at around 10 − 20 K in the outer regions. However sufficiently nearby massive stars can heat the outer disc to substantially higher temperatures. In this paper we study the radiative equilibrium temperature of discs in the presence of massive external sources and gauge the effect that it has on millimetre dust mass estimates. Since millimetre grains are not entrained in any wind we focus on geometrically simple 2D-axisymmetric disc models using radiative transfer calculations with both the host star and an external source. Recent surveys have searched for evidence of massive stars influencing disc evolution using disc properties as a function of projected separation. In assuming a disc temperature of 20 K for a disc a distance D from a strong radiation source, disc masses are overestimated by a factor that scales with D−1/2 interior to the separation that external heating becomes important. This could significantly alter dust mass estimates of discs in close proximity to θ1C in the Orion Nebular Cluster. We also make an initial assessment of the effect upon snow lines. Within a parsec of an O star like θ1C a CO snow line no longer exists, though the water snow line is virtually unaffected except for very close separations of ≤0.01 pc.