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Register a new userEvolutionary constraints on the planetary hypothesis for transition discs
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We assume a scenario in which transition discs (i.e. discs around young stars that have signatures of cool dust but lack significant near infra-red emission from warm dust) are associated with the presence of planets (or brown dwarfs). These are assumed to filter the dust content of any gas flow within the planetary orbit and produce an inner `opacity hole. In order to match the properties of transition discs with the largest (~50 A.U. scale) holes, we place such `planets at large radii in massive discs and then follow the evolution of the tidally coupled disc-planet system, comparing the systems evolution in the plane of mm flux against hole radius with the properties of observed transition discs. We find that, on account of the high disc masses in these systems, all but the most massive `planets (100 Jupiter masses) are conveyed to small radii by Type II migration without significant fading at millimetre wavelengths. Such behaviour would contradict the observed lack of mm bright transition discs with small (<10 A.U.) holes. On the other hand, imaging surveys clearly rule out the presence of such massive companions in transition discs. We conclude that this is a serious problem for models that seek to explain transition discs in terms of planetary companions unless some mechanism can be found to halt inward migration and/or suppress mm flux production. We suggest that the dynamical effects of substantial accretion on to the planet/through the gap may offer the best prospect for halting such migration but that further long term simulations are required to clarify this issue.
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Transitional disks, protoplanetary disks with deep and wide central gaps, may be the result of planetary sculpting. By comparing numerical planet-opening-gap models with observed gaps, we find systems of 3-6 giant planets are needed in order to open gaps with the observed depths and widths. We explore the dynamical stability of such multi-planet systems using N-body simulations that incorporate prescriptions for gas effects. We find they can be stable over a typical disk lifetime, with the help of eccentricity damping from the residual gap gas that facilitates planets locking into mean motion resonances. However, in order to account for the occurrence rate of transitional disks, the planet sculpting scenario demands gap-opening-friendly disk conditions, in particular, a disk viscosity $alphalesssim0.001$. In addition, the demography of giant planets at $sim 3-30$ AU separations, poorly constrained by current data, has to largely follow occurrence rates extrapolated outward from radial velocity surveys, not the lower occurrence rates extrapolated inward from direct imaging surveys. Even with the most optimistic occurrence rates, transitional disks cannot be a common phase that most gas disks experience at the end of their life, as popularly assumed, simply because there are not enough planets to open these gaps. Finally, as consequences of demanding almost all giant planets at large separations participate in transitional disk sculpting, the majority of such planets must form early and end up in a chain of mean motion resonances at the end of disk lifetime.
At about 70 solar masses, the recently-discovered dark object orbited by a B-type star in the system LB-1 is difficult to understand as the end point of standard stellar evolution, except as a binary black hole (BBH). LB-1 shows a strong, broad H-alpha emission line that is best attributed to a gaseous disk surrounding the dark mass. We use the observed H-alpha line shape, particularly its wing extension, to constrain the inner radius of the disk and thereby the separation of a putative BBH. The hypothesis of a current BBH is effectively ruled out on the grounds that its merger time must be a small fraction of the current age of the B star. The hypothesis of a previous BBH that merged to create the current dark mass is also effectively ruled out by the low orbital eccentricity, due to the combination of mass loss and kick resulted from gravitational wave emission in any past merger. We conclude that the current dark mass is a single black hole produced by the highly mass-conserving, monolithic collapse of a massive star.
This paper presents new observations of the planet-hosting, visual binary GJ 86 (HR 637) using the Hubble Space Telescope. Ultraviolet and optical imaging with WFC3 confirms the stellar companion is a degenerate star and indicates the binary semimajor axis is larger than previous estimates, with a > 28 AU. Optical STIS spectroscopy of the secondary reveals a helium-rich white dwarf with C2 absorption bands and Teff = 8180 K, thus making the binary system rather similar to Procyon. Based on the 10.8 pc distance, the companion has 0.59 Msun and descended from a main-sequence A star of 1.9 Msun with an original orbital separation a > 14 AU. If the giant planet is coplanar with the binary, the mass of GJ 86Ab is between 4.4 and 4.7 MJup. The similarity of GJ 86 and Procyon prompted a re-analysis of the white dwarf in the latter system, with the tentative conclusion that Procyon hosts a planetesimal population. The periastron distance in Procyon is 20% smaller than in alpha Cen AB, but the metal-enriched atmosphere of Procyon B indicates that the planet formation process minimally attained 25 km bodies, if not small planets as in alpha Cen.
In protoplanetary discs, planetary cores must be at least 0.1 earth mass at 1 au for migration to be significant; this mass rises to 1 earth mass at 5 au. Planet formation models indicate that these cores form on million year timescales. We report here a study of the evolution of 0.1 earth mass and 1 earth mass cores, migrating from about 2 and 5 au respectively, in million year old photoevaporating discs. In such a disc, a gap opens up at around 2 au after a few million years. The inner region subsequently accrete onto the star on a smaller timescale. We find that, typically, the smallest cores form systems of non-resonant planets beyond 0.5 au with masses up to about 1.5 earth mass. In low mass discs, the same cores may evolve in situ. More massive cores form systems of a few earth masses planets. They migrate within the inner edge of the disc gap only in the most massive discs. Delivery of material to the inner parts of the disc ceases with opening of the gap. Interestingly, when the heavy cores do not migrate significantly, the type of systems that are produced resembles our solar system. This study suggests that low mm flux transition discs may not form systems of planets on short orbits but may instead harbour earth mass planets in the habitable zone.
Future direct observations of extrasolar Earth-sized planets in the habitable zone could be hampered by a worrisome source of noise, starlight-reflecting exozodiacal dust. Mid-infrared surveys are currently underway to constrain the amount of exozodiacal dust in the habitable zones around nearby stars. However, at visible wavelengths another source of dust, invisible to these surveys, may dominate over exozodiacal dust. For systems observed near edge-on, a cloud of dust with face-on optical depth 10^-7 beyond ~5 AU can mimic the surface brightness of a cloud of exozodiacal dust with equal optical depth if the dust grains are sufficiently forward-scattering. We posit that dust migrating inward from cold debris belts via Poynting-Robertson drag could produce this pseudo-zodiacal effect, potentially making it ~50% as common as exozodiacal clouds. We place constraints on the disk radii and scattering phase function required to produce the effect.
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