Abstract
Current planet formation theories provide successful frameworks with which to interpret the array of new observational data in this field. However, each of the two main theories (core ...accretion, gravitational instability) is unable to explain some key aspects. In many planet formation calculations, it is usual to treat the initial properties of the planet-forming disc (mass, radius, etc.) as free parameters. In this paper, we stress the importance of setting the formation of planet-forming discs within the context of the formation of the central stars. By exploring the early stages of disc formation, we introduce the concept of the Maximum Mass Solar Nebula, as opposed to the oft-used minimum mass solar nebula. It is evident that almost all protoplanetary discs start their evolution in a strongly self-gravitating state. In agreement with almost all previous work in this area, we conclude that on the scales relevant to planet formation these discs are not gravitationally unstable to gas fragmentation, but instead form strong, transient spiral arms. These spiral arms can act as efficient dust traps allowing the accumulation and subsequent fragmentation of the dust (but not the gas). This phase is likely to populate the disc with relatively large planetesimals on short time-scales while the disc is still veiled by a dusty-gas envelope. Crucially, the early formation of large planetesimals overcomes the main barriers remaining within the core accretion model. A prediction of this picture is that essentially all observable protoplanetary discs are already planet hosting.
Simulations and analytic arguments suggest that the turbulence driven by magnetorotational instability (MRI) in accretion discs can amplify the toroidal (azimuthal) component of the magnetic field to ...a point at which magnetic pressure exceeds the combined gas + radiation pressure in the disc. Arguing from the recent analysis by Pessah & Psaltis, and other MRI results in the literature, we conjecture that the limiting field strength for a thin disc is such that the Alfvén speed roughly equals the geometric mean of the Keplerian speed and the speed of sound in gas. We examine the properties of such magnetically dominated discs, and show that they resolve a number of outstanding problems in accretion disc theory. The discs would be thicker than standard (Shakura–Sunyaev) discs at the same radius and accretion rate, and would tend to have higher colour temperatures. If they transport angular momentum according to an α prescription, they would be stable against the thermal and viscous instabilities that are found in standard disc models. In discs fuelling active galactic nuclei, magnetic pressure support could also alleviate the restriction on accretion rate imposed by disc self-gravity.
We present the discovery of an unusual set of flares in the nova-like variable V704 And. Using data from AAVSO, ASAS-SN, and ZTF for the nova-like variable V704 And, we discovered a trio of ...brightening events that occurred during the high state. These events elevate the optical brightness of the source from ∼13.5 to ∼12.5 mag. The events last for roughly a month, and exhibit the unusual shape of a slow rise and faster decay. Immediately after the third event, we obtained data from regular monitoring with
Swift
, although by this time the flares had ceased and the source returned to its pre-flare level of activity in the high state. The
Swift
observations confirm that during the high state, the source is detectable in the X-rays, and provide simultaneous UV and optical fluxes. As the source is already in the high state prior to the flares, and therefore the disc is expected to already be in the high-viscosity state, we conclude that the driver of the variations must be changes in the mass transfer rate from the companion star and we discuss mechanisms that could lead to such short-timescale mass-transfer variations.
In this paper we consider the evolution of small planetesimals (radii ∼1–10 m) in marginally stable, self-gravitating protoplanetary discs. The drag force between the disc gas and the embedded ...planetesimals generally causes the planetesimals to drift inwards through the disc at a rate that depends on the particle size. In a marginally stable, self-gravitating disc, however, the planetesimals are significantly influenced by the non-axisymmetric spiral structures resulting from the growth of the gravitational instability. The drag force now causes the planetesimals to drift towards the peaks of the spiral arms where the density and pressure are highest. For small particles that are strongly coupled to the disc gas, and for large particles that have essentially decoupled from the disc gas, the effect is not particularly significant. Intermediate-sized particles, which would generally have the largest radial drift rates, do, however, become significantly concentrated at the peaks of the spiral arms. These high-density regions may persist for, of order, an orbital period and may attain densities comparable to that of the disc gas. Although at the end of the simulation only ∼25 per cent of the planetesimal particles lie in regions of enhanced density, during the course of the simulation at least 75 per cent of the planetesimal particles have at some stage been in a such a region. We find that the concentration of particles in the spiral arms results in an increased collision rate, an effect that could significantly accelerate planetesimal growth. The density enhancements may also be sufficient for the growth of planetesimals through direct gravitational collapse. The interaction between small planetesimals and self-gravitating spiral structures may therefore play an important role in the formation of large planetesimals that will ultimately coagulate to form terrestrial planets or the cores of gas/ice giant planets.
Abstract The rapidly evolving dust and gas extinction observed towards WD 1145+017 has opened a real-time window on to the mechanisms for destruction-accretion of planetary bodies on to white dwarf ...stars, and has served to underline the importance of considering the dynamics of dust particles around such objects. Here it is argued that the interaction between (charged) dust grains and the stellar magnetic field is an important ingredient in understanding the physical distribution of infrared emitting particles in the vicinity of such white dwarfs. These ideas are used to suggest a possible model for WD 1145+017 in which the unusual transit shapes are caused by opaque clouds of dust trapped in the stellar magnetosphere. The model can account for the observed transit periodicities if the stellar rotation is near 4.5 h, as the clouds of trapped dust are then located near or within the co-rotation radius. The model requires the surface magnetic field to be at least around some tens of kG. In contrast to the eccentric orbits expected for large planetesimals undergoing tidal disintegration, the orbits of magnetospherically-trapped dust clouds are essentially circular, consistent with the observations.
We present smoothed particle hydrodynamics simulations of molecular cloud formation in spiral galaxies. These simulations model the response of a non-self-gravitating gaseous disc to a galactic ...potential. The spiral shock induces high densities in the gas, and considerable structure in the spiral arms, which we identify as molecular clouds. We regard the formation of these structures as due to the dynamics of clumpy shocks, which perturb the flow of gas through the spiral arms. In addition, the spiral shocks induce a large velocity dispersion in the spiral arms, comparable with the magnitude of the velocity dispersion observed in molecular clouds. We estimate the formation of molecular hydrogen, by post-processing our results and assuming the gas is isothermal. Provided the gas is cold (T≤ 100 K), the gas is compressed sufficiently in the spiral shock for molecular hydrogen formation to occur in the dense spiral arm clumps. These molecular clouds are largely confined to the spiral arms, since most molecular gas is photodissociated to atomic hydrogen upon leaving the arms.
Aligning spinning black holes and accretion discs King, A. R.; Lubow, S. H.; Ogilvie, G. I. ...
Monthly notices of the Royal Astronomical Society,
10/2005, Letnik:
363, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We consider the alignment torque between a spinning black hole and an accretion disc whose angular momenta are misaligned. This situation must hold initially in almost all gas accretion events on to ...supermassive black holes, and may occur in binaries where the black hole receives a natal supernova kick. We show that the torque always acts to align the hole's spin with the total angular momentum without changing its magnitude. The torque acts dissipatively on the disc, reducing its angular momentum, and aligning it with the hole if and only if the angle θ between the angular momenta Jd of the disc and Jh of the hole satisfy the inequality cos θ > −Jd/2Jh. If this condition fails, which requires both θ > π/2 and Jd < 2Jh, the disc counteraligns.
Supernova kicks and misaligned Be star binaries Martin, Rebecca G.; Tout, Christopher A.; Pringle, J. E.
Monthly notices of the Royal Astronomical Society,
08/2009, Letnik:
397, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Be stars are rapidly spinning B stars surrounded by an outflowing disc of gas in Keplerian rotation. Be star/X-ray binary systems contain a Be star and a neutron star. They are found to have non-zero ...eccentricities and there is evidence that some systems have a misalignment between the spin axis of the star and the spin axis of the binary orbit. The eccentricities in these systems are caused by a kick to the neutron star during the supernova that formed it. Such kicks would also give rise to misalignments. In this paper, we investigate the extent to which the same kick distribution can give rise to both the observed eccentricity distribution and the observed misalignments. We find that a Maxwellian distribution of velocity kicks with a low velocity dispersion, σk≈ 15 km s−1, is consistent with the observed eccentricity distribution but is hard to reconcile with the observed misalignments, typically i≥ 25°. Alternatively, a higher velocity kick distribution, σk= 265 km s−1, is consistent with the observed misalignments but not with the observed eccentricities, unless post-supernova circularization of the binary orbits has taken place. We discuss briefly how this might be achieved.
We perform global time-dependent simulations of an accretion disc around a young stellar object with a dead zone (a region where the magneto-rotational instability cannot drive turbulence because the ...material is not sufficiently ionized). For infall accretion rates on to the disc of around 10−7 M⊙ yr−1, dead zones occur if the critical magnetic Reynolds number is larger than about 104. We model the collapse of a molecular gas cloud. At early times when the infall accretion rate is high, the disc is thermally ionized and fully turbulent. However, as the infall accretion rate drops, a dead zone may form if the critical magnetic Reynolds number is sufficiently large; otherwise the disc remains fully turbulent. With a dead zone the disc can become unstable to the gravo-magneto instability. The mass of the star grows in large accretion outbursts that may explain FU Orionis events. At late times there is not sufficient mass in the disc for outbursts to occur but the dead zone becomes even more prominent as the disc cools. Large inner dead zones in the later stages of disc evolution may help to explain observations of transition discs with an inner hole.
We show that for young stars which are still accreting and for which measurements of stellar age t
*, disc mass M
disc and accretion rate
are available, nominal disc age
is approximately equal to ...the stellar age t
*, at least within the considerable observational scatter. We then consider theoretical models of protostellar discs through analytic and numerical models. A variety of viscosity prescriptions including empirical power laws, magnetohydrodynamic turbulence and gravitational instability were considered within models describing the disc phenomena of dead zones, photoevaporation and planet formation. These models are generally poor fits to the observational data, showing values of t
disc which are too high by factors of 3-10. We then ask whether a systematic error in the measurement of one of the observational quantities might provide a reasonable explanation for this discrepancy. We show that for the observed systems only disc mass shows a systematic dependence on the value of t
disc/t
*, and we note that a systematic underestimate of the value of disc mass by a factor of around 3-5 would account for the discrepancy between theory and observations.
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