ABSTRACT
For stars with unresolved companions, motions of the centre of light and that of mass decouple, causing a single-source astrometric model to perform poorly. We show that such stars can be ...easily detected with the reduced χ2 statistic, or renormalized unit weight error (RUWE), provided as part of Gaia DR2. We convert RUWE into the amplitude of the image centroid wobble, which, if scaled by the source distance, is proportional to the physical separation between companions (for periods up to several years). We test this idea on a sample of known spectroscopic binaries and demonstrate that the amplitude of the centroid perturbation scales with the binary period and the mass ratio as expected. We apply this technique to the Gaia DR2 data and show how the binary fraction evolves across the Hertzsprung–Russell diagram. The observed incidence of unresolved companions is high for massive young stars and drops steadily with stellar mass, reaching its lowest levels for white dwarfs. We highlight the elevated binary fraction for the nearby blue stragglers and blue horizontal branch stars. We also illustrate how unresolved hierarchical triples inflate the relative velocity signal in wide binaries. Finally, we point out a hint of evidence for the existence of additional companions to the hosts of extrasolar hot Jupiters.
ABSTRACT
We model the gas and dust dynamics in a turbulent protoplanetary disc undergoing extreme-UV photoevaporation in order to better characterize the dust properties in thermal winds (e.g. size ...distribution, flux rate, trajectories). Our semi-analytic approach allows us to rapidly calculate these dust properties without resorting to expensive hydrodynamic simulations. We find that photoevaporation creates a vertical gas flow within the disc that assists turbulence in supplying dust to the ionization front. We examine both the delivery of dust to the ionization front and its subsequent entrainment in the overlying wind. We derive a simple analytic criterion for the maximum grain size that can be entrained and show that this is in good agreement with the results of previous simulations where photoevaporation is driven by a range of radiation types. We show that, in contrast to the case for magnetically driven winds, we do not expect large-scale dust transport within the disc to be effected by photoevaporation. We also show that the maximum size of grains that can be entrained in the wind (smax) is around an order of magnitude larger than the maximum size of grains that can be delivered to the front by advection alone ($s_{\mathrm{crit}}\lesssim 1 \,\, \mu {\mathrm{m}}$ for Herbig Ae/Be stars and $\lesssim 0.01 \,\, \mu {\mathrm{m}}$ for T Tauri stars). We further investigate how larger grains, up to a limiting size slimit, can be delivered to the front by turbulent diffusion alone. In all cases, we find smax ≳ slimit so that we expect that any dust that is delivered to the front can be entrained in the wind and that most entrained dust follows trajectories close to that of the gas.
Abstract
Chemical compositions of giant planets provide a means to constrain how and where they form. Traditionally, super-stellar elemental abundances in giant planets were thought to be possible ...due to accretion of metal-rich solids. Such enrichments are accompanied by oxygen-rich compositions (i.e. C/O below the disc's value, assumed to be solar, C/O = 0.54). Without solid accretion, the planets are expected to have sub-solar metallicity, but high C/O ratios. This arises because the solids are dominated by oxygen-rich species, e.g. H2O and CO2, which freeze out in the disc earlier than CO, leaving the gas metal poor but carbon rich. Here we demonstrate that super-solar metallicities can be achieved by gas accretion alone when growth and radial drift of pebbles are considered in protoplanetary discs. Through this mechanism, planets may simultaneously acquire super-solar metallicities and super-solar C/O ratios. This happens because the pebbles transport volatile species inwards as they migrate through the disc, enriching the gas at snow lines where the volatiles sublimate. Furthermore, the planet's composition can be used to constrain where it formed. Since high C/H and C/O ratios cannot be created by accreting solids, it may be possible to distinguish between formation via pebble accretion and planetesimal accretion by the level of solid enrichment. Finally, we expect that Jupiter's C/O ratio should be near or above solar if its enhanced carbon abundance came through accreting metal-rich gas. Thus, Juno's measurement of Jupiter's C/O ratio should determine whether Jupiter accreted its metals from carbon-rich gas or oxygen-rich solids.
We estimate the mass-loss rates of photoevaporative winds launched from the outer edge of protoplanetary discs impinged by an ambient radiation field. We focus on mild/moderate environments (the ...number of stars in the group/cluster is N ≳ 50), and explore disc sizes ranging between 20 and 250 au. We evaluate the steady-state structures of the photoevaporative winds by coupling temperature estimates obtained with a photodissociation region code with 1D radial hydrodynamical equations. We also consider the impact of dust dragging and grain growth on the final mass-loss rates. We find that these winds are much more significant than have been appreciated hitherto when grain growth is included in the modelling: in particular, mass-loss rates ≳10−8 M⊙ yr−1 are predicted even for modest background field strengths (≳30 G
0) in the case of discs that extend to R > 150 au. Grain growth significantly affects the final mass-loss rates by reducing the average cross-section at far-ultraviolet wavelengths, and thus allowing a much more vigorous flow. The radial profiles of observable quantities (in particular surface density, temperature and velocity patterns) indicate that these winds have characteristic features that are now potentially observable with ALMA. In particular, such discs should have extended gaseous emission that is dust depleted in the outer regions, characterized by a non-Keplerian rotation curve, and with a radially increasing temperature gradient.
We investigate the minimum planet mass that produces observable signatures in infrared scattered light and submillimetre (submm) continuum images and demonstrate how these images can be used to ...measure planet masses to within a factor of about 2. To this end, we perform multi-fluid gas and dust simulations of discs containing low-mass planets, generating simulated observations at 1.65, 10 and 850 μm. We show that the minimum planet mass that produces a detectable signature is ∼15 M⊕: this value is strongly dependent on disc temperature and changes slightly with wavelength (favouring the submm). We also confirm previous results that there is a minimum planet mass of ∼20 M⊕ that produces a pressure maximum in the disc: only planets above this threshold mass generate a dust trap that can eventually create a hole in the submm dust. Below this mass, planets produce annular enhancements in dust outwards of the planet and a reduction in the vicinity of the planet. These features are in steady state and can be understood in terms of variations in the dust radial velocity, imposed by the perturbed gas pressure radial profile, analogous to a traffic jam. We also show how planet masses can be derived from structure in scattered light and submm images. We emphasize that simulations with dust need to be run over thousands of planetary orbits so as to allow the gas profile to achieve a steady state and caution against the estimation of planet masses using gas-only simulations.
ABSTRACT
Protoplanetary discs form and evolve in a wide variety of stellar environments and are accordingly exposed to a wide range of ambient far-ultraviolet (FUV) field strengths. Strong FUV fields ...are known to drive vigorous gaseous flows from the outer disc. In this paper we conduct the first systematic exploration of the evolution of the solid component of discs subject to external photoevaporation. We find that the main effect of photoevaporation is to reduce the reservoir of dust at large radii and this leads to more efficient subsequent depletion of the disc dust due to radial drift. Efficient radial drift means that photoevaporation causes no significant increase of the dust-to-gas ratio in the disc. We show that the disc lifetime in both dust and gas is strongly dependent on the level of the FUV background and that the relationship between these two lifetimes just depends on the Shakura–Sunyaev α parameter, with the similar lifetimes observed for gas and dust in discs pointing to higher α values (∼10−2). On the other hand, the distribution of observed discs in the plane of disc size versus flux at 850 μm is better reproduced by lower α (∼10−3). We find that photoevaporation does not assist rocky planet formation but need not inhibit mechanisms (such as pebble accretion at the water snow line) which can be effective sufficiently early in the disc’s lifetime (i.e. well within a Myr).
On the theory of disc photoevaporation Owen, James E; Clarke, Cathie J; Ercolano, Barbara
Monthly notices of the Royal Astronomical Society,
20/May , Letnik:
422, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We discuss a hydrodynamical model for the dispersal of protoplanetary discs around young, low-mass (<1.5 M⊙) stars by photoevaporation from the central object's energetic radiation, which considers ...the far-ultraviolet (FUV) as well as the X-ray component of the radiation field. We present analytical scaling relations and derive estimates for the total mass-loss rates, as well as discussing the existence of similarity solutions for flows from primordial discs and discs with inner holes. Furthermore, we perform numerical calculations which span a wide range of parameter space and allow us to provide accurate scalings of the mass-loss rates with the physical parameters of the systems (X-ray and FUV luminosity, stellar mass, disc mass, disc temperature and inner hole radius).
The model suggests that the X-ray component dominates the photoevaporative mass-loss rates from the inner disc. The mass-loss rates have values in the range from 10−11 to 10−7 M⊙ yr−1 and scale linearly with X-ray luminosity, with only a weak dependence on the other parameters explored. However, in the case of high FUV-to-X-ray luminosity ratios (L
FUV/L
X > 100), the FUV constricts the X-ray flow and may dominate the mass loss.
Simulations of low-mass discs with inner holes demonstrate a further stage of disc clearing, which we call 'thermal sweeping'. This process occurs when the mid-plane pressure drops to sufficiently low values. At this stage, a bound, warm, X-ray heated region becomes sufficiently large and unstable, such that the remaining disc material is cleared on approximately dynamical time-scales. This process significantly reduces the time taken to clear the outer regions of the disc, resulting in an expected transition disc population that will be dominated by accreting objects, as indicated by recent observations.
Where can a Trappist-1 planetary system be produced? Haworth, Thomas J; Facchini, Stefano; Clarke, Cathie J ...
Monthly notices of the Royal Astronomical Society,
04/2018, Letnik:
475, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Abstract
We study the evolution of protoplanetary discs that would have been precursors of a Trappist-1-like system under the action of accretion and external photoevaporation in different radiation ...environments. Dust grains swiftly grow above the critical size below which they are entrained in the photoevaporative wind, so although gas is continually depleted, dust is resilient to photoevaporation after only a short time. This means that the ratio of the mass in solids (dust plus planetary) to the mass in gas rises steadily over time. Dust is still stripped early on, and the initial disc mass required to produce the observed 4 M⊕ of Trappist-1 planets is high. For example, assuming a Fatuzzo & Adams distribution of UV fields, typical initial disc masses have to be >30 per cent the stellar (which are still Toomre Q stable) for the majority of similar mass M dwarfs to be viable hosts of the Trappist-1 planets. Even in the case of the lowest UV environments observed, there is a strong loss of dust due to photoevaporation at early times from the weakly bound outer regions of the disc. This minimum level of dust loss is a factor of 2 higher than that which would be lost by accretion on to the star during 10 Myr of evolution. Consequently, even in these least irradiated environments, discs that are viable Trappist-1 precursors need to be initially massive (>10 per cent of the stellar mass).
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