arXiv subjects

Yansong Qian

Publications and source records attributed to Yansong Qian.

6 recordsLinked to original sources

arXiv2026

Geometry of dust rings in protoplanetary disks: the case of LkCa 15

Dust properties in proto-planetary disks shape the pathways for planet formation. Here, we present a method to measure these properties in moderately inclined dust rings. Our method exploits the simple geometric fact that, for such a ring, its ansae appear brighter because our line of sight traverses a longer path through the ring material, and appear broader because the minor axis are foreshortened by projection. The resultant patterns of apparent brightness and width can used to constrain three parameters: the intrinsic ring width, its vertical thickness and its optical depth. We apply this method to ALMA archival images of the LkCa 15 disk, in Bands 7, 6 and 3. We find that the optical depth of its main ring drops from 1.6 at 0.89mm to 0.4 at 3mm. Simultaneously, both the ring width and the ring height decrease from about two to one gas scale heights. Such wavelength-dependent morphology can only be explained by the presence of multiple grain populations. If we adopt a simple two-size model, we infer that the ring contains a massive population of small grains (size < 20 micron; total mass ~ 100 earth masses) that are broadly distributed, and a less massive population of large grains (size > 200 micron) that are more spatially concentrated. This large surplus of small grains is not predicted by models of dust coagulation, but it naturally explains the fluffy ring in LkCa 15, and possibly rings in other disks.

astro-ph.EP↗

arXiv2025

Bouncing Grains Keep Protoplanetary Disks Bright

Proto-planetary disks display the so-called size-luminosity relation, where their mm-wavelength fluxes scale linearly with their emitting areas. This suggests that these disks are optically thick in mm-band, an interpretation further supported by their near-black-body spectral indexes. Such characteristics are seen not only among disks in very young star-forming regions like Lupus (1-3 Myrs), but, as we demonstrate here, also among disks in the much older Upper Scorpius region (5-11 Myrs). How can disks shine brightly for so long, when grain growth and subsequent radial drift should have quickly depleted their solid reservoir? Here, we suggest that the "bouncing barrier" provides the answer. Even colliding at very low speeds (below 1cm/s), grains already fail to stick to each other but instead bounce off in-elastically. This barrier stalls grain growth at a near-universal size of 100 micron. These small grains experience much reduced radial drift, and so are able to keep the disks bright for millions of years. They are also tightly coupled to gas, offering poor prospects for processes like streaming instability or pebble accretion. We speculate briefly on how planetesimals can arise in such a bath of 100-micron grains.

astro-ph.EP↗

arXiv2024

Shadows Wreak Havocs in Transition Disks

We demonstrate that shadows cast on a proto-planetary disk can drive it eccentric. Stellar irradiation dominates heating across much of these disks, so an uneven illumination can have interesting dynamical effects. Here, we focus on transition disks. We carry out 3D Athena++ simulations, using a constant thermal relaxation time to describe the disk's response to changing stellar illumination. We find that an asymmetric shadow, a feature commonly observed in real disks, perturbs the radial pressure gradient and distorts the fluid streamlines into a set of twisted ellipses. Interactions between these streamlines have a range of consequences. For a narrow ring, an asymmetric shadow can sharply truncate its inner edge, possibly explaining the steep density drop-offs observed in some disks and obviating the need for massive perturbers. For a wide ring, such a shadow can dismantle it into two (or possibly more) eccentric rings. These rings continuously exert torque on each other and drive gas accretion at a healthy rate, even in the absence of disk viscosity. Signatures of such twisted eccentric rings may have already been observed as, e.g., twisted velocity maps inside gas cavities. We advocate for more targeted observations, and for a better understanding on the origin of such shadows.

astro-ph.EP↗

arXiv2022

The structure of accreted stellar streams

Many of the Milky Way's globular clusters are likely accreted from satellite galaxies that have long since merged with the Milky Way. When these globular clusters are susceptible to tidal disruption, this process likely starts already inside the parent satellite leading to an early stellar stream within the satellite. When the parent satellite merges with the Milky Way, the globular cluster and its pre-merger stellar stream are accreted in a somewhat chaotic process. Here, we investigate the properties of the accreted stream after the merger as we would see it today using a suite of simulations of accretion events. We find that the accretion process leads to a wide range of behaviors, but generally scatters the accreted stream over a wide, two-dimensional area of the sky. The behavior ranges from a set of a few or more well-defined "sub-streams" extending out from the post-merger thin stream by tens of degrees, to more widely dispersed debris over much of the sky, depending on how close to the center of the MilkyWay the merger happened. Using mock Gaia-like observations of the simulated streams, we demonstrate that an accreted-stream component can explain the off-track features observed in the GD-1 stream. Sub-streams can appear like thin tidal streams themselves that are seemingly unassociated with the post-merger stream, raising the possibility that some of the progenitor-less streams observed in the Milky Way are part of a single or a few accreted streams created in an ancient merger event.

astro-ph.GA↗

arXiv2021

Super-Earths and sub-Neptunes are Insensitive to Stellar Metallicity

Kepler planets (including super-Earths and sub-Neptunes, from 1 to 4 Earth radii) are likely formed before the gaseous proto-planetary disks have dissipated, as are the Jovian planets. If the metal content in these disks resembles that in the host stars, one might expect Kepler planets to occur more frequently, and to be more massive, around metal-rich stars. Contrary to these expectations, we find that the radii of Kepler planets (a proxy for mass) are independent of host metallicity. Previous claims that larger planets prefer more metal-rich stars can be adequately explained by the combined facts that more massive stars tend to host bigger planets, and that more massive stars are also more metal rich in the Kepler sample. We interpret this independence as that the mass of a Kepler planet is not determined by the availability of solids, but is instead regulated by an as yet unknown process. Moreover, we find that the occurrence rates of Kepler planets rise only weakly with stellar metallicity, a trend that is further flattened when the influence of close stellar binaries is accounted for. We explain this weak dependence, in contrast to the strong dependence exhibited by Jovian planets, using a phenomenological model, wherein the masses of proto-planetary disks have a much larger spread than the spread in stellar metallicity, and wherein the formation of Jovian planets requires disks that contain some $5$ times more solid than that needed to form Kepler planets. This model predicts that stars more metal-poor than half-solar should rarely host any Kepler planets.

astro-ph.EP↗

arXiv2021

A Distinct Population of Small Planets: Sub-Earths

The sizes of small planets have been known to be bi-modal, with a gap separating planets that have lost their primordial atmospheres (super-Earths), and the ones that retain them (mini-Neptunes). Here, we report evidences for another distinct population at smaller sizes. By focussing on planets orbiting around GK-dwarfs inward of 16 days, and correcting for observational completeness, we find that the number of super-Earths peak around 1.4 Earth radii and disappear shortly below this size. Instead, a new population of planets (sub-Earths) appear to dominate at sizes below ~ 1 Earth radius, with an occurrence that rises with decreasing size. This pattern is also observed in ultra-short-period planets. The end of super-Earths supports earlier claims that super-Earths and mini-Neptunes, planets that likely form in gaseous proto-planetary disks, have a narrow mass distribution. The sub-Earths, in contrast, can be described by a power-law mass distribution and may be explained by the theory of terrestrial planet formation. We therefore speculate that they are formed well after the gaseous disks have dissipated. The extension of these sub-Earths towards longer orbital periods, currently invisible, may be the true terrestrial analogues. This strongly motivates new searches.

astro-ph.EP↗