No abstract

We investigate the plausibility of a cometary source of the unusual transits observed in the KIC 8462852 light curve. A single comet of similar size to those in our solar system produces a transit depth of the order of 10 −3 lasting less than a day which is much smaller and shorter than the largest dip observed (∼ 20% for ∼ 3 days), but a large, closely traveling cluster of comets can fit the observed depths and durations. We find that a series of large comet swarms, with all but one on the same orbit, provides a good fit for the KIC 8462852 data during Quarters 16 and 17, but does not explain the large dip observed during Quarter 8. However, the transit dips only loosely constrain the orbits and can be fit by swarms with periastrons differing by a factor of 10. To reach a transit depth of ∼ 0.2, the comets need to be in a close group of ∼ 30, if they are ∼ 100 km in radius or in a group of ∼ 300 if they are ∼ 10 km in radius. The total number of comets required to fit all of the dips is ∼ 70 ∼100 km or ∼ 700 ∼ 10 km comets.A single comet family from a tidally disrupted Ceres-sized progenitor or the start of a Late Heavy Bombardment period explains the last ∼ 60 days of the unusual KIC 8462852 light curve.

We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in 2015 October, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1%-2.5% dips, named "Elsie," "Celeste," "Skara Brae," and "Angkor," which persist on timescales from several days to weeks. Our main results so far are as follows: (i) there are no apparent changes of the stellar spectrum or polarization during the dips and (ii) the multiband photometry of the dips shows differential reddening favoring non-gray extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale =1 μm, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term "secular" dimming, which may be caused by independent processes, or probe different regimes of a single process.

We explore scenarios for the origin of two different density planets in the Kepler 36 system in adjacent orbits near the 7:6 mean motion resonance. We find that fine tuning is required in the stochastic forcing amplitude, the migration rate and planet eccentricities to allow two convergently migrating planets to bypass mean motion resonances such as the 4:3, 5:4 and 6:5, and yet allow capture into the 7:6 resonance. Stochastic forcing can eject the system from resonance causing a collision between the planets, unless the disk causing migration and the stochastic forcing is depleted soon after resonance capture.We explore a scenario with approximately Mars mass embryos originating exterior to the two planets and migrating inwards toward two planets. We find that gravitational interactions with embryos can nudge the system out of resonances. Numerical integrations with about a half dozen embryos can leave the two planets in the 7:6 resonance. Collisions between planets and embryos have a wide distribution of impact angles and velocities ranging from accretionary to disruptive. We find that impacts can occur at sufficiently high impact angle and velocity that the envelope of a planet could have been stripped, leaving behind a dense core. Some of our integrations show the two planets exchanging locations, allowing the outer planet that had experienced multiple collisions with embryos to become the innermost planet. A scenario involving gravitational interactions and collisions with embryos may account for both the proximity of the Kepler 36 planets and their large density contrast. 1 The mutual Hill radius r mH ≡ m b +mc 3M * a b +ac 2 .c 0000 RAS

Disintegrating planets allow for the unique opportunity to study the composition of the interiors of small, hot, rocky exoplanets because the interior is evaporating and that material is condensing into dust, which is being blown away and then transiting the star. Their transit signal is dominated by dusty effluents forming a comet-like tail trailing the host planet (or leading it, in the case of K2-22b), making these good candidates for transmission spectroscopy. To assess the ability of such observations to diagnose the dust composition, we simulate the transmission spectra from 5-14 µm for the planet tail assuming an optically-thin dust cloud comprising a single dust species with a constant column density scaled to yield a chosen visible transit depth. We find that silicate resonant features near 10 µm can produce transit depths that are at least as large as those in the visible. For the average transit depth of 0.55% in the Kepler band for K2-22b, the features in the transmission spectra can be as large as 1%, which is detectable with the JWST MIRI low-resolution spectrograph in a single transit. The detectability of compositional features is easier with an average grain size of 1 µm despite features being more prominent with smaller grain sizes. We find most features are still detectable for transit depths of ∼ 0.3% in the visible range. If more disintegrating planets are found with future missions such as the space telescope TESS, follow-up observations with JWST can explore the range of planetary compositions.

Exocomets are small bodies releasing gas and dust which orbit stars other than the Sun. Their existence was first inferred from the detection of variable absorption features in stellar spectra in the late 1980s using spectroscopy. More recently, they have been detected through photometric transits from space, and through far-IR/mm gas emission within debris disks. As (exo)comets are considered to contain the most pristine material accessible in stellar systems, they hold the potential to give us information about early stage formation and evolution conditions of extra solar systems. In the solar system, comets carry the physical and chemical memory of the protoplanetary disk environment where they formed, providing relevant information on processes in the primordial solar nebula. The aim of this paper is to compare essential compositional properties between solar system comets and exocomets to allow for the development of new observational methods and techniques. The paper aims to highlight commonalities and to discuss differences which may aid the communication between the involved research communities and perhaps also avoid misconceptions. The compositional properties of solar system comets and exocomets are summarized before providing an observational comparison between them. Exocomets likely vary in their composition depending on their formation environment like solar system comets do, and since exocomets are not resolved spatially, they pose a challenge when comparing them to high fidelity observations of solar system comets. Observations of gas around main sequence stars, spectroscopic observations of “polluted” white dwarf atmospheres and spectroscopic observations of transiting exocomets suggest that exocomets may show compositional similarities with solar system comets. The recent interstellar visitor 2I/Borisov showed gas, dust and nuclear properties similar to that of solar system comets. This raises the tantalising prospect that observations of interstellar comets may help bridge the fields of exocomet and solar system comets.

We explore the photometrically variable central stars of the planetary nebulae HaTr 4 and Hf 2-2. Both have been classified as close binary star systems previously based on their light curves alone. Here, we present additional arguments and data confirming the identification of both as close binaries with an irradiated cool companion to the hot central star. We include updated light curves, orbital periods, and preliminary binary modeling for both systems. We also identify for the first time the central star of HaTr 4 as an eclipsing binary. Neither system has been well studied in the past, but we utilize the small amount of existing data to limit possible binary parameters, including system inclination. These parameters are then compared to nebular parameters to further our knowledge of the relationship between binary central stars of planetary nebulae and nebular shaping and ejection.

Convergent migration allows pairs of planet to become trapped into mean motion resonances. Once in resonance, the planets' eccentricities grow to an equilibrium value that depends on the ratio of migration time scale to the eccentricity damping timescale, K = τ a /τ e , with higher values of equilibrium eccentricity for lower values of K. For low equilibrium eccentricities, e eq ∝ K −1/2 . Equilibrium eccentricities also depend on the distance between the planets. Resonances near the planet have lower equilibrium eccentricity. The stability of a planet pair depends on eccentricity so the system can become unstable before it reaches its equilibrium eccentricity.Using a resonant overlap criterion that takes into account the role of first and second order resonances and depends on eccentricity, we find a function K min (µ p , j) that defines the lowest value for K, as a function of the ratio of total planet mass to stellar mass (µ p ) and the period ratio of the resonance defined as P 1 /P 2 = j/(j + k), that allows two convergently migrating planets to remain stable in resonance at their equilibrium eccentricities. We scaled the functions K min for each resonance of the same order into a single function K c . The function K c for planet pairs in first order resonances is linear with increasing planet mass and quadratic for pairs in second order resonances with a coefficient depending on the relative migration rate and strongly on the planet to planet mass ratio. The linear relation continues until the mass approaches a critical mass defined by the 2/7 resonance overlap instability law and K c → ∞.We compared our analytic boundary with an observed sample of resonant two planet systems. All but one of the first order resonant planet pair systems found by radial velocity measurements are well inside the stability region estimated by this model. The one system in the instability region is well below K c but is also in the 4:3 resonance which is not explained well with smooth migration (Rein et al. 2012). We calculated K c for Kepler systems without well-constrained eccentricities and found only weak constraints on K. The Kepler systems have all have lower bounds less than K = 10 with most systems with K min < 1. c 0000 RAS