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Register a new userDrifting inwards in protoplanetary discs II: The effect of water on sticking properties at increasing temperatures
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Added by
Cynthia Melina Pillich
Publication date
2021
fields
Physics
and research's language is
English
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In previous laboratory experiments, we measured the temperature dependence of sticking forces between micrometer grains of chondritic composition. The data showed a decrease in surface energy by a factor ~5 with increasing temperature. Here, we focus on the effect of surface water on grains. Under ambient conditions in the laboratory, multiple water layers are present. At the low pressure of protoplanetary discs and for moderate temperatures, grains likely only hold a monolayer. As dust drifts inwards, even this monolayer eventually evaporates completely in higher temperature regions. To account for this, we measured the tensile strength for the same chondritic material as was prepared and measured under normal laboratory conditions in our previous work, but now introducing two new preparation methods: drying dust cylinders in air (dry samples), and heating dust pressed into cylinders in vacuum (super-dry samples). For all temperatures up to 1000 K, the data of the dry samples are consistent with a simple increase in the sticking force by a factor of ~10 over wet samples. Up to 900 K super-dry samples behave like dry samples. However, the sticking forces then exponentially increase up to another factor ~100 at about 1200 K. The increase in sticking from wet to dry extends a trend that is known for amorphous silicates to multimineral mixtures. The findings for super-dry dust imply that aggregate growth is boosted in a small spatial high-temperature region around 1200 K, which might be a sweet spot for planetesimal formation.
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Sticking properties rule the early phases of pebble growth in protoplanetary discs in which grains regularly travel from cold, water-rich regions to the warm inner part. This drift affects composition, grain size, morphology, and water content as grains experience ever higher temperatures. In this study we tempered chondritic dust under vacuum up to 1400 K. Afterwards, we measured the splitting tensile strength of millimetre-sized dust aggregates. The deduced effective surface energy starts out as $gamma_e = 0.07,rm J/m^2$. This value is dominated by abundant iron-oxides as measured by Mossbauer spectroscopy. Up to 1250 K, $gamma_e$ continuously decreases by up to a factor five. Olivines dominate at higher temperature. Beyond 1300 K dust grains significantly grow in size. The $gamma_e$ no longer decreases but the large grain size restricts the capability of growing aggregates. Beyond 1400 K aggregation is no longer possible. Overall, under the conditions probed, the stability of dust pebbles would decrease towards the star. In view of a minimum aggregate size required to trigger drag instabilities it becomes increasingly harder to seed planetesimal formation closer to a star.
We perform a population synthesis of protoplanetary discs including infall with a total of $50,000$ simulations using a 1D vertically integrated viscous evolution code, studying a large parameter space in final stellar mass. Initial conditions and infall locations are chosen based on the results from a radiation-hydrodynamic population synthesis of circumstellar discs. We also consider a different infall prescription based on a magnetohydrodynamic (MHD) collapse simulation in order to assess the influence of magnetic fields on disc formation. The duration of the infall phase is chosen to produce a stellar mass distribution in agreement with the observationally determined stellar initial mass function. We find that protoplanetary discs are very massive early in their lives. When averaged over the entire stellar population, the discs have masses of $sim 0.3$ and $0.1,mathrm{M_odot}$ for systems based on hydrodynamic or MHD initial conditions, respectively. In systems with final stellar mass $sim 1,mathrm{M_odot}$, we find disc masses of $sim 0.7,mathrm{M_odot}$ for the `hydro case and $sim 0.2,mathrm{M_odot}$ for the `MHD case at the end of the infall phase. Furthermore, the inferred total disc lifetimes are long, $approx 5-7,mathrm{Myr}$ on average, despite our choice of a high value of $10^{-2}$ for the background viscosity $alpha$-parameter. In addition, fragmentation is common in systems that are simulated using hydrodynamic cloud collapse, with more fragments of larger mass formed in more massive systems. In contrast, if disc formation is limited by magnetic fields, fragmentation is suppressed entirely.
Tidal encounters in star clusters perturb discs around young protostars. In Cuello et al. (2019a, Paper I) we detailed the dynamical signatures of a stellar flyby in both gas and dust. Flybys produce warped discs, spirals with evolving pitch angles, increasing accretion rates, and disc truncation. Here we present the corresponding observational signatures of these features in optical/near-infrared scattered light and (sub-) millimeter continuum and CO line emission. Using representative prograde and retrograde encounters for direct comparison, we post-process hydrodynamical simulations with radiative transfer methods to generate a catalogue of multi-wavelength observations. This provides a reference to identify flybys in recent near-infrared and sub-millimetre observations (e.g., RW Aur, AS 205, HV Tau & DO Tau, FU Ori, V2775 Ori, and Z CMa).
We present results from a near-infrared (NIR) adaptive optics (AO) survey of pre-main-sequence stars in the Lupus Molecular Cloud with VLT/NACO to identify (sub)stellar companions down to $sim$20 au separation and investigate the effects of multiplicity on circumstellar disc properties. We observe for the first time in the NIR with AO a total of 47 targets and complement our observations with archival data for another 58 objects previously observed with the same instrument. All 105 targets have millimetre ALMA data available, which provide constraints on disc masses and sizes. We identify a total of 13 multiple systems, including 11 doubles and 2 triples. In agreement with previous studies, we find that the most massive (M$_{rm dust}$ $>$ 50 M$_{oplus}$) and largest ($R_{rm dust}>$ 70 au) discs are only seen around stars lacking visual companions (with separations of 20-4800 au) and that primaries tend to host more massive discs than secondaries. However, as recently shown in a very similar study of $>$200 PMS stars in the Ophiuchus Molecular Cloud, the distribution of disc masses and sizes are similar for single and multiple systems for M$_{rm dust} < 50$ M$_{oplus}$ and radii $R_{rm dust}<$ 70 au. Such discs correspond to $sim $80-90% of the sample. This result can be seen in the combined sample of Lupus and Ophiuchus objects, which now includes more than 300 targets with ALMA imaging and NIR AO data, and implies that stellar companions with separations $>$20 au mostly affect discs in the upper 10$%$ of the disc mass and size distributions.
It is expected that a pressure bump can be formed at the inner edge of a dead-zone, and where vortices can develop through the Rossby Wave Instability (RWI). It has been suggested that self-gravity can significantly affect the evolution of such vortices. We present the results of 2D hydrodynamical simulations of the evolution of vortices forming at a pressure bump in self-gravitating discs with Toomre parameter in the range $4-30$. We consider isothermal plus non-isothermal disc models that employ either the classical $beta$ prescription or a more realistic treatment for cooling. The main aim is to investigate whether the condensating effect of self-gravity can stabilize vortices in sufficiently massive discs. We confirm that in isothermal disc models with ${cal Q} gtrsim 15$, vortex decay occurs due to the vortex self-gravitational torque. For discs with $3lesssim {cal Q} lesssim 7$, the vortex develops gravitational instabilities within its core and undergoes gravitational collapse, whereas more massive discs give rise to the formation of global eccentric modes. In non-isothermal discs with $beta$ cooling, the vortex maintains a turbulent core prior to undergoing gravitational collapse for $beta lesssim 0.1$, whereas it decays if $beta ge 1$. In models that incorpore both self-gravity and a better treatment for cooling, however, a stable vortex is formed with aspect ratio $chi sim 3-4$. Our results indicate that self-gravity significantly impacts the evolution of vortices forming in protoplanetary discs, although the thermodynamical structure of the vortex is equally important for determining its long-term dynamics.
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