Gas has been detected in many exoplanetary systems ($>$10 Myr), thought to be released in the destruction of volatile-rich planetesimals orbiting in exo-Kuiper belts. In this letter, we aim to explore whether gas is also expected in the Kuiper belt (
KB) in our Solar System. To quantify the gas release in our Solar System, we use models for gas release that have been applied to extrasolar planetary systems, as well as a physical model that accounts for gas released due to the progressive internal warming of large planetesimals. We find that only bodies larger than about 4 km can still contain CO ice after 4.6 Gyr of evolution. This finding may provide a clue as to why Jupiter-family comets, thought to originate in the Kuiper belt, are deficient in CO compared to Oort-clouds comets. We predict that gas is still produced in the KB right now at a rate of $2 times 10^{-8}$ M$_oplus$/Myr for CO and orders of magnitude more when the Sun was younger. Once released, the gas is quickly pushed out by the Solar wind. Therefore, we predict a gas wind in our Solar System starting at the KB location and extending far beyond with regards to the heliosphere with a current total CO mass of $sim 2 times 10^{-12}$ M$_oplus$. We also predict the existence of a slightly more massive atomic gas wind made of carbon and oxygen (neutral and ionized) with a mass of $sim 10^{-11}$ M$_oplus$. We predict that gas is currently present in our Solar System beyond the Kuiper belt and that although it cannot be detected with current instrumentation, it could be observed in the future with an in situ mission using an instrument similar to Alice on New Horizons with larger detectors. Our model of gas release due to slow heating may also work for exoplanetary systems and provide the first real physical mechanism for the gas observations.
White dwarfs are routinely observed to have polluted atmospheres, and sometimes significant infrared excesses, that indicate ongoing accretion of circumstellar dust and rocky debris. Typically this debris is assumed to be in the form of a (circular)
disc, and to originate from asteroids that passed close enough to the white dwarf to be pulled apart by tides. However, theoretical considerations suggest that the circularisation of the debris, which initially occupies highly eccentric orbits, is very slow. We therefore hypothesise that the observations may be readily explained by the debris remaining on highly eccentric orbits, and we explore the properties of such debris. For the generic case of an asteroid originating at several au from the white dwarf, we find that all of the tidal debris is always bound to the white dwarf and that the orbital energy distribution of the debris is narrow enough that it executes similar elliptical orbits with only a narrow spread. Assuming that the tidal field of the white dwarf is sufficient to minimise the effects of self-gravity and collisions within the debris, we estimate the time over which the debris spreads into a single elliptical ring, and we generate toy spectra and lightcurves from the initial disruption to late times when the debris distribution is essentially time steady. Finally we speculate on the connection between these simple considerations and the observed properties of these systems, and on additional physical processes that may change this simple picture.
We have made high precision polarimetric observations of the polluted white dwarf G29-38 with the HIgh Precision Polarimetric Instrument 2. The observations were made at two different observatories -- using the 8.1-m Gemini North Telescope and the 3.
9-m Anglo AustralianTelescope -- and are consistent with each other. After allowing for a small amount of interstellar polarization, the intrinsic linear polarization of the system is found to be 275.3 +/- 31.9 parts-per-million at a position angle of 90.8 +/- 3.8 degrees in the SDSS g band. We compare the observed polarization with the predictions of circumstellar disc models. The measured polarization is small in the context of the models we develop which only allows us to place limits on disc inclination and Bond albedo for optically thin disc geometries. In this case either the inclination is near face-on or the albedo is small -- likely in the range 0.05 to 0.15 -- which is in line with other debris disc measurements. A preliminary search for the effects of G29-38s pulsations in the polarization signal produced inconsistent results. This may be caused by beating effects, indicate a clumpy dust distribution, or be a consequence of measurement systematics.
In spiral galaxies, the pitch angle, $alpha$, of the spiral arms is often proposed as a discriminator between theories for the formation of the spiral structure. In Lin-Shu density wave theory, $alpha$ stays constant in time, being simply a property
of the underlying galaxy. In other theories (e.g tidal interaction, self-gravity) it is expected that the arms wind up in time, so that to a first approximation $cot alpha propto t$. For these theories, it would be expected that a sample of galaxies observed at random times should show a uniform distribution of $cot alpha$. We show that a recent set of measurements of spiral pitch angles (Yu & Ho 2018) is broadly consistent with this expectation.
The manner in which warps in accretion disks evolve depends on the magnitude of the viscosity. For small viscosity $(alpha < H/R)$ the warp evolves in a wave-like manner; for large viscosity $H/R<alpha ll 1$ it evolves diffusively. Here $alpha$ is th
e viscosity parameter and $H/R$ the disk aspect ratio. Currently there is no simple set of equations which describes the evolution in both regimes. In this paper we describe a possible solution to this problem and introduce a set of one-dimensional equations that describe the evolution of a warped disk that are applicable in both high and low viscosity regimes for arbitrary tilts, but small warps.
The rapidly evolving dust and gas extinction observed towards WD 1145+017 has opened a real-time window onto the mechanisms for destruction-accretion of planetary bodies onto white dwarf stars, and has served to underline the importance of considerin
g 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 investigate cloud-cloud collisions, and GMC evolution, in hydrodynamic simulations of isolated galaxies. The simulations include heating and cooling of the ISM, self--gravity and stellar feedback. Over timescales $<5$ Myr most clouds undergo no ch
ange, and mergers and splits are found to be typically two body processes, but evolution over longer timescales is more complex and involves a greater fraction of intercloud material. We find that mergers, or collisions, occur every 8-10 Myr (1/15th of an orbit) in a simulation with spiral arms, and once every 28 Myr (1/5th of an orbit) with no imposed spiral arms. Both figures are higher than expected from analytic estimates, as clouds are not uniformly distributed in the galaxy. Thus clouds can be expected to undergo between zero and a few collisions over their lifetime. We present specific examples of cloud--cloud interactions in our results, including synthetic CO maps. We would expect cloud--cloud interactions to be observable, but find they appear to have little or no impact on the ISM. Due to a combination of the clouds typical geometries, and moderate velocity dispersions, cloud--cloud interactions often better resemble a smaller cloud nudging a larger cloud. Our findings are consistent with the view that spiral arms make little difference to overall star formation rates in galaxies, and we see no evidence that collisions likely produce massive clusters. However, to confirm the outcome of such massive cloud collisions we ideally need higher resolution simulations.
In this Letter, we analyse the distributions of stellar ages in Giant Molecular Clouds (GMCs) in spiral arms, inter-arm spurs, and at large galactic radii, where the spiral arms are relatively weak. We use the results of numerical simulations of gala
xies, which follow the evolution of GMCs and include star particles where star formation events occur. We find that GMCs in spiral arms tend to have predominantly young (< 10 Myr) stars. By contrast, clouds which are the remainders of spiral arm GMAs that have been sheared into inter-arm GMCs, contain fewer young (< 10 Myr) stars, and more ~20 Myr stars. We also show that clouds which form in the absence of spiral arms, due to local gravitational and thermal instabilities, contain preferentially young stars. We propose the age distributions of stars in GMCs will be a useful diagnostic to test different cloud evolution scenarios, the origin of spiral arms, and the success of numerical models of galactic star formation. We discuss the implications of our results in the context of Galactic and extragalactic molecular clouds.
In this paper we revisit the arguments for the basis of the time evolution of the flares expected to arise when a star is disrupted by a supermassive black hole. We present a simple analytic model relating the lightcurve to the internal density struc
ture of the star. We thus show that the standard lightcurve proportional to $t^{-5/3}$ only holds at late times. Close to the peak luminosity the lightcurve is shallower, deviating more strongly from $t^{-5/3}$ for more centrally concentrated (e.g. solar--type) stars. We test our model numerically by simulating the tidal disruption of several stellar models, described by simple polytropic spheres with index $gamma$. The simulations agree with the analytical model given two considerations. First, the stars are somewhat inflated on reaching pericentre because of the effective reduction of gravity in the tidal field of the black hole. This is well described by a homologous expansion by a factor which becomes smaller as the polytropic index becomes larger. Second, for large polytropic indices wings appear in the tails of the energy distribution, indicating that some material is pushed further away from parabolic orbits by shocks in the tidal tails. In all our simulations, the $t^{-5/3}$ lightcurve is achieved only at late stages. In particular we predict that for solar type stars, this happens only after the luminosity has dropped by at least two magnitudes from the peak. We discuss our results in the light of recent observations of flares in otherwise quiescent galaxies and note the dependence of these results on further parameters, such as the star/hole mass ratio and the stellar orbit.
In the microquasar V4641 Sgr the spin of the black hole is thought to be misaligned with the binary orbital axis. The accretion disc aligns with the black hole spin by the Lense-Thirring effect near to the black hole and further out becomes aligned w
ith the binary orbital axis. The inclination of the radio jets and the Fe$Kalpha$ line profile have both been used to determine the inclination of the inner accretion disc but the measurements are inconsistent. Using a steady state analytical warped disc model for V4641 Sgr we find that the inner disc region is flat and aligned with the black hole up to about $900 R_{rm g}$. Thus if both the radio jet and fluorescent emission originates in the same inner region then the measurements of the inner disc inclination should be the same.
