Jonathan Horner

Simulations of the population of Centaurs – I. The bulk statistics

Large-scale simulations of the Centaur population are carried out. The evolution of 23 328 particles based on the orbits of 32 well-known Centaurs is followed for up to 3 Myr in the forward and backward direction under the influence of the four massive planets. The objects exhibit a rich variety of dynamical behaviour with half-lives ranging from 540 kyr (1996 AR20) to 32 Myr (2000 FZ53). The mean half-life of the entire sample of Centaurs is 2.7 Myr. The data are analysed using a classification scheme based on the controlling planets at perihelion and aphelion, previously given in Horner et al. Transfer probabilities are computed and show the main dynamical pathways of the Centaur population. The total number of Centaurs with diameters larger than 1 km is estimated as ∼44 300, assuming an inward flux of one new short-period comet every 200 yr. The flux into the Centaur region from the Edgeworth‐Kuiper Belt is estimated to be one new object every 125 yr. Finally, the flux from the Centaur region to Earth-crossing orbits is one new Earth-crosser every 880 yr. Ke yw ords: stellar dynamics ‐ celestial mechanics ‐ Kuiper belt ‐ minor planets, asteroids ‐ planets and satellites: general.

Resolving debris discs in the far-infrared: Early highlights from the DEBRIS survey

We present results from the earliest observations of DEBRIS, a Herschel Key Programme to conduct a volume- and flux-limited survey fo r debris discs in A-type through M-type stars. PACS images (from chop/nod or scan-mode observations) at 100 and 160� m are presented toward two

A dynamical analysis of the proposed HU Aquarii planetary system

It has recently been suggested that the eclipsing polar HU Aquarii is a host to at least two giant planets. We have performed highly detailed dynamical analyses of the orbits of those planets and showed that the proposed system is highly unstable on time-scales of <5 × 10 3 yr. For the coplanar orbits suggested in the discovery Letter, we find stable orbital solutions for the planetary system only if the outer body moves on an orbit that brings it no closer to the host star than ~6 au. The required periastron distance for the outer planet lies approximately 5 Hill radii beyond the orbit of the inner planet, and well beyond the 1σ error bars placed on the orbit of the outer planet in the discovery Letter. If the orbits of the proposed planets are significantly inclined with respect to one another, the median stability increases slightly, but such systems still become destabilized on astronomically minute time-scales (typically within a few 10 4 yr). Only in the highly improbable scenario where the outer planet follows a retrograde but coplanar orbit (i.e. inclined by 180° to the orbit of the inner planet) is there any significant region of stability within the original 1σ orbital uncertainties. Our results suggest that, if there is a second (and potentially, a third planet) in the HU Aquarii system, its orbit is dramatically different from that suggested in the discovery Letter, and that more observations are critically required in order to constrain the nature of the suggested orbital bodies.

Revisiting the proposed planetary system orbiting the eclipsing polar HU Aquarii

It has recently been proposed, on the basis of eclipse-timing data, that the eclipsing polar cataclysmic variable HU Aquarii is host to at least two giant planets. However, that result has been called into question based upon the dynamical stability of the proposed planets. In this work, we present a detailed re-analysis of all eclipse-timing data available for the HU Aquarii system, making use of standard techniques used to fit orbits to radial-velocity data. We find that the eclipse timings can be used to obtain a two-planet solution that does not require the presence of additional bodies within the system. We then perform a highly detailed dynamical analysis of the proposed planetary system. We show that the improved orbital parameters we have derived correspond to planets that are dynamically unstable on unfeasibly short time-scales (of the order of 10 4 yr or less). Given these results, we discuss briefly how the observed signal might in fact be the result of the intrinsic properties of the eclipsing polar, rather than being evidence of dynamically improbable planets. Taken in concert, our results highlight the need for caution in interpreting such timing variations as being planetary in nature.

The populations of comet-like bodies in the Solar system

A new classification scheme is introduced for comet-like bodies in the Solar system. It covers the traditional comets as well as the Centaurs and Edgeworth–Kuiper belt objects. At low inclinations, close encounters with planets often result in near-constant perihelion or aphelion distances, or in perihelion–aphelion interchanges, so the minor bodies can be labelled according to the planets predominantly controlling them at perihelion and aphelion. For example, a JN object has a perihelion under the control of Jupiter and aphelion under the control of Neptune, and so on. This provides 20 dynamically distinct categories of outer Solar system objects in the Jovian and trans-Jovian regions. The Tisserand parameter with respect to the planet controlling perihelion is also often roughly constant under orbital evolution. So, each category can be further subdivided according to the Tisserand parameter. The dynamical evolution of comets, however, is dominated not by the planets nearest at perihelion or aphelion, but by the more massive Jupiter. The comets are separated into four categories – Encke-type, short-period, intermediate and long-period – according to aphelion distance. The Tisserand parameter categories now roughly correspond to the well-known Jupiter-family comets, transition types and Halley types. In this way, the nomenclature for the Centaurs and Edgeworth–Kuiper belt objects is based on, and consistent with, that for comets. Given the perihelion and aphelion distances together with the Tisserand parameter, our classification scheme provides a description for any comet-like body in the Solar system. The usefulness of the scheme is illustrated with examples drawn from numerical simulations and from the present-day Solar system.

Simulations of the population of Centaurs – II. Individual objects

Detailed orbit integrations of clones of five Centaurs ‐ namely, 1996 AR20, 2060 Chiron, 1995 SN55, 2000 FZ53 and 2002 FY36 ‐ for durations of ∼3 Myr are presented. One of our Centaur sample starts with perihelion initially under the control of Jupiter (1996 AR20), two start under the control of Saturn (Chiron and 1995 SN55) and one each starts under the control of Uranus (2000 FZ53) and Neptune (2002 FY36), respectively. A variety of interesting pathways are illustrated with detailed examples including: capture into the Jovian Trojans, repeated bursts of short-period comet behaviour, capture into mean-motion resonances with the giant planets and into Kozai resonances, as well as traversals of the entire Solar system. For each of the Centaurs, we provide statistics on the numbers (i) ejected, (ii) showing short-period comet behaviour and (iii) becoming Earth- and Mars-crossing. For example, Chiron has over 60 per cent of its clones becoming short-period objects, while 1995 SN55 has over 35 per cent. Clones of these two Centaurs typically make numerous close approaches to Jupiter. At the other extreme, 2000 FZ53 has ∼2 per cent of its clones becoming short-period objects. In our simulations, typically 20 per cent of the clones which become short-period comets subsequently evolve into Earth-crossers.

The GALAH survey: observational overview and Gaia DR1 companion

The Galactic Archaeology with HERMES (GALAH) Survey is a massive observational project to trace the Milky Ways history of star formation, chemical enrichment, stellar migration and minor mergers. Using high-resolution (R

The Mcdonald Observatory Planet Search: New Long-Period Giant Planets And Two Interacting Jupiters In The HD 155358 System

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A DEBRIS Disk Around The Planet Hosting M-star GJ 581 Spatially Resolved with Herschel ⋆

28,000) spectra taken with the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) instrument at the Anglo-Australian Telescope (AAT), GALAH will determine stellar parameters and abundances of up to 29 elements for up to one million stars. Selecting targets from a colour-unbiased catalogue built from 2MASS, APASS and UCAC4 data, we expect to observe dwarfs at 0.3 to 3 kpc and giants at 1 to 10 kpc. This enables a thorough local chemical inventory of the Galactic thin and thick disks, and also captures smaller samples of the bulge and halo. In this paper we present the plan, process and progress as of early 2016 for GALAH survey observations. In our first two years of survey observing we have accumulated the largest high-quality spectroscopic data set at this resolution, over 200,000 stars. We also present the first public GALAH data catalogue: stellar parameters (Teff, log(g), [Fe/H], [alpha/Fe]), radial velocity, distance modulus and reddening for 10680 observations of 9860 Tycho-2 stars that may be included in the first Gaia data release.

A detailed investigation of the proposed NN Serpentis planetary system

We present high-precision radial velocity (RV) observations of four solar-type (F7-G5) stars—HD 79498, HD 155358, HD 197037, and HD 220773—taken as part of the McDonald Observatory Planet Search Program. For each of these stars, we see evidence of Keplerian motion caused by the presence of one or more gas giant planets in long-period orbits. We derive orbital parameters for each system and note the properties (composition, activity, etc.) of the host stars. While we have previously announced the two-gas-giant HD 155358 system, we now report a shorter period for planet c. This new period is consistent with the planets being trapped in mutual 2:1 mean-motion resonance. We therefore perform an in-depth stability analysis, placing additional constraints on the orbital parameters of the planets. These results demonstrate the excellent long-term RV stability of the spectrometers on both the Harlan J. Smith 2.7 m telescope and the Hobby-Eberly telescope.