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Register a new userPredicting the kinematic evidence of gravitational instability
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Observations with the Atacama Large Millimeter/Submillimeter array (ALMA) have dramatically improved our understanding of the site of exoplanet formation: protoplanetary discs. However, many basic properties of these discs are not well-understood. The most fundamental of these is the total disc mass, which sets the mass budget for planet formation. Discs with sufficiently high masses can excite gravitational instability and drive spiral arms that are detectable with ALMA . Although spirals have been detected in ALMA observations of the dust , their association with gravitational instability, and high disc masses, is far from clear. Here we report a prediction for kinematic evidence of gravitational instability. Using hydrodynamics simulations coupled with radiative transfer calculations, we show that a disc undergoing such instability has clear kinematic signatures in molecular line observations across the entire disc azimuth and radius which are independent of viewing angle. If these signatures are detected, it will provide the clearest evidence for the occurrence of gravitational instability in planet-forming discs, and provide a crucial way to measure disc masses.
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We present evidence for localised deviations from Keplerian rotation, i.e., velocity kinks, in 8 of 18 circumstellar disks observed by the DSHARP program: DoAr 25, Elias 2-27, GW Lup, HD 143006, HD 163296, IM Lup, Sz 129 and WaOph 6. Most of the kinks are detected over a small range in both radial extent and velocity, suggesting a planetary origin, but for some of them foreground contamination prevents us from measuring their spatial and velocity extent. Because of the DSHARP limited spectral resolution and signal-to-noise in the 12CO J=2-1 line, as well as cloud contamination, the kinks are usually detected in only one spectral channel, and will require confirmation. The strongest circumstantial evidence for protoplanets in the absence of higher spectral resolution data and additional tracers is that, upon deprojection, we find that all of the candidate planets lie within a gap and/or at the end of a spiral detected in dust continuum emission. This suggests that a significant fraction of the dust gaps and spirals observed by ALMA in disks are caused by embedded protoplanets.
Discs of gas and dust surrounding young stars are the birthplace of planets. However, direct detection of protoplanets forming within discs has proved elusive to date. We present the detection of a large, localized deviation from Keplerian velocity in the protoplanetary disc surrounding the young star HD163296. The observed velocity pattern is consistent with the dynamical effect of a two Jupiter-mass planet orbiting at a radius $approx$ 260au from the star.
We present models of the inner region of the circumstellar disk of RY Tau which aim to explain our near-infrared ($K$-band: $2.1,mu$m) interferometric observations while remaining consistent with the optical to near-infrared portions of the spectral energy distribution. Our sub-milliarcsecond resolution CHARA Array observations are supplemented with shorter baseline, archival data from PTI, KI and VLTI/GRAVITY and modeled using an axisymmetric Monte Carlo radiative transfer code. The $K$-band visibilities are well-fit by models incorporating a central star illuminating a disk with an inner edge shaped by dust sublimation at $0.210pm0.005,$au, assuming a viewing geometry adopted from millimeter interferometry ($65^{circ}$ inclined with a disk major axis position angle of $23^{circ}$). This sublimation radius is consistent with that expected of Silicate grains with a maximum size of $0.36-0.40,mu$m contributing to the opacity and is an order of magnitude further from the star than the theoretical magnetospheric truncation radius. The visibilities on the longest baselines probed by CHARA indicate that we lack a clear line-of-sight to the stellar photosphere. Instead, our analysis shows that the central star is occulted by the disk surface layers close to the sublimation rim. While we do not see direct evidence of temporal variability in our multi-epoch CHARA observations, we suggest the aperiodic photometric variability of RY~Tau is likely related temporal and/or azimuthal variations in the structure of the disk surface layers.
There are different methods for finding exoplanets such as radial spectral shifts, astrometrical measurements, transits, timing etc. Gravitational microlensing (including pixel-lensing) is among the most promising techniques with the potentiality of detecting Earth-like planets at distances about a few astronomical units from their host star or near the so-called snow line with a temperature in the range $0-100^0$ C on a solid surface of an exoplanet. We emphasize the importance of polarization measurements which can help to resolve degeneracies in theoretical models. In particular, the polarization angle could give additional information about the relative position of the lens with respect to the source.
Context: Around 30 per cent of the observed exoplanets that orbit M dwarf stars are gas giants that are more massive than Jupiter. These planets are prime candidates for formation by disc instability. Aims: We want to determine the conditions for disc fragmentation around M dwarfs and the properties of the planets that are formed by disc instability. Methods: We performed hydrodynamic simulations of M dwarf protostellar discs in order to determine the minimum disc mass required for gravitational fragmentation to occur. Different stellar masses, disc radii, and metallicities were considered. The mass of each protostellar disc was steadily increased until the disc fragmented and a protoplanet was formed. Results: We find that a disc-to-star mass ratio between $sim 0.3$ and $sim 0.6$ is required for fragmentation to happen. The minimum mass at which a disc fragments increases with the stellar mass and the disc size. Metallicity does not significantly affect the minimum disc fragmentation mass but high metallicity may suppress fragmentation. Protoplanets form quickly (within a few thousand years) at distances around $sim50$ AU from the host star, and they are initially very hot; their centres have temperatures similar to the ones expected at the accretion shocks around planets formed by core accretion (up to 12,000K). The final properties of these planets (e.g. mass and orbital radius) are determined through long-term disc-planet or planet-planet interactions. Conclusions: Disc instability is a plausible way to form gas giant planets around M dwarfs provided that discs have at least 30% the mass of their host stars during the initial stages of their formation. Future observations of massive M dwarf discs or planets around very young M dwarfs are required to establish the importance of disc instability for planet formation around low-mass stars.
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