Matthew J. Payne

Constraining the Planetary System of Fomalhaut Using High-resolution ALMA Observations

The dynamical evolution of planetary systems leaves observable signatures in debris disks. Optical images trace micron-sized grains, which are strongly affected by stellar radiation and need not coincide with their parent body population. Observations of millimeter-sized grains accurately trace parent bodies, but previous images lack the resolution and sensitivity needed to characterize the rings morphology. Here we present ALMA 350 GHz observations of the Fomalhaut debris ring. These observations demonstrate that the parent body population is 13-19 AU wide with a sharp inner and outer boundary. We discuss three possible origins for the ring and suggest that debris confined by shepherd planets is the most consistent with the rings morphology.

FIRST-ORDER RESONANCE OVERLAP AND THE STABILITY OF CLOSE TWO-PLANET SYSTEMS

Motivated by the population of observed multi-planet systems with orbital period ratios 1 < P 2/P 1 2, we study the long-term stability of packed two-planet systems. The Hamiltonian for two massive planets on nearly circular and nearly coplanar orbits near a first-order mean motion resonance can be reduced to a one-degree-of-freedom problem. Using this analytically tractable Hamiltonian, we apply the resonance overlap criterion to predict the onset of large-scale chaotic motion in close two-planet systems. The reduced Hamiltonian has only a weak dependence on the planetary mass ratio m 1/m 2, and hence the overlap criterion is independent of the planetary mass ratio at lowest order. Numerical integrations confirm that the planetary mass ratio has little effect on the structure of the chaotic phase space for close orbits in the low-eccentricity (e 0.1) regime. We show numerically that orbits in the chaotic web produced primarily by first-order resonance overlap eventually experience large-scale erratic variation in semimajor axes and are therefore Lagrange unstable. This is also true of the orbits in this overlap region which satisfy the Hill criterion. As a result, we can use the first-order resonance overlap criterion as an effective stability criterion for pairs of observed planets. We show that for low-mass ( 10 M ?) planetary systems with initially circular orbits the period ratio at which complete overlap occurs and widespread chaos results lies in a region of parameter space which is Hill stable. Our work indicates that a resonance overlap criterion which would apply for initially eccentric orbits likely needs to take into account second-order resonances. Finally, we address the connection found in previous work between the Hill stability criterion and numerically determined Lagrange instability boundaries in the context of resonance overlap.

Retired A Stars and Their Companions. VI. A Pair of Interacting Exoplanet Pairs Around the Subgiants 24 Sextanis and HD?200964

We report radial velocity (RV) measurements of the G-type subgiants 24 Sextanis (= HD 90043) and HD 200964. Both are massive, evolved stars that exhibit periodic variations due to the presence of a pair of Jovian planets. Photometric monitoring with the T12 0.80 m APT at Fairborn Observatory demonstrates both stars to be constant in brightness to ≤ 0.002 mag, thus strengthening the planetary interpretation of the RV variations. Based on our dynamical analysis of the RV time series, 24 Sex b, c have orbital periods of 452.8 days and 883.0 days, corresponding to semimajor axes 1.333 AU and 2.08 AU, and minimum masses 1.99 M_(Jup) and 0.86 M_(Jup), assuming a stellar mass M_⋆ =1.54 M_⊙. HD 200964 b, c have orbital periods of 613.8 days and 825.0 days, corresponding to semimajor axes 1.601 AU and 1.95 AU, and minimum masses 1.99 M_(Jup) and 0.90 M_(Jup), assuming M_⋆ = 1.44 M_⊙. We also carry out dynamical simulations to properly account for gravitational interactions between the planets. Most, if not all, of the dynamically stable solutions include crossing orbits, suggesting that each system is locked in a mean-motion resonance that prevents close encounters and provides long-term stability. The planets in the 24 Sex system likely have a period ratio near 2:1, while the HD 200964 system is even more tightly packed with a period ratio close to 4:3. However, we caution that further RV observations and more detailed dynamical modeling will be required to provide definitive and unique orbital solutions for both cases, and to determine whether the two systems are truly resonant.

THE DISCOVERY OF HD 37605c AND A DISPOSITIVE NULL DETECTION OF TRANSITS OF HD 37605b

We report the radial velocity discovery of a second planetary mass companion to the K0 V star HD 37605, which was already known to host an eccentric, P ~ 55 days Jovian planet, HD 37605b. This second planet, HD 37605c, has a period of ~7.5 years with a low eccentricity and an Msin i of ~3.4 M_(Jup). Our discovery was made with the nearly 8 years of radial velocity follow-up at the Hobby-Eberly Telescope and Keck Observatory, including observations made as part of the Transit Ephemeris Refinement and Monitoring Survey effort to provide precise ephemerides to long-period planets for transit follow-up. With a total of 137 radial velocity observations covering almost 8 years, we provide a good orbital solution of the HD 37605 system, and a precise transit ephemeris for HD 37605b. Our dynamic analysis reveals very minimal planet-planet interaction and an insignificant transit time variation. Using the predicted ephemeris, we performed a transit search for HD 37605b with the photometric data taken by the T12 0.8 m Automatic Photoelectric Telescope (APT) and the MOST satellite. Though the APT photometry did not capture the transit window, it characterized the stellar activity of HD 37605, which is consistent of it being an old, inactive star, with a tentative rotation period of 57.67 days. The MOST photometry enabled us to report a dispositive null detection of a non-grazing transit for this planet. Within the predicted transit window, we exclude an edge-on predicted depth of 1.9% at the »10σ level, and exclude any transit with an impact parameter b > 0.951 at greater than 5σ. We present the BOOTTRAN package for calculating Keplerian orbital parameter uncertainties via bootstrapping. We made a comparison and found consistency between our orbital fit parameters calculated by the RVLIN package and error bars by BOOTTRAN with those produced by a Bayesian analysis using MCMC.

Collisional evolution of eccentric planetesimal swarms

Models for the steady state collisional evolution of low eccentricity planetesimal belts identify debris disks with hot dust at 1AU, likeCorvi and HD69830, as anomalous since collisional processing should have removed most of the planetesimal mass over their > 1 Gyr lifetimes. This paper looks at the effect of large planetesimal eccentr icities (e � 0:3) on their col- lisional lifetime and the amount of mass that can remain at late times Mlate. Assuming an axisymmetric planetesimal disk with common pericentre distances and eccentricities e, we find that Mlate / e 5/3 (1 + e) 4/3 (1 − e) 3 . For a scattered disk-like population (i.e., with common pericentre distances but range of eccentricities), in the absence of dynamical evo- lution, the mass evolution at late times would be as if only planetesimals with the largest eccentricity were present in the disk. Despite the increase d remaining mass, higher eccentric- ities do not increase the amount of hot emission from the collisional cascade until e > 0:99, partly because most collisions occur near pericentre thus i ncreasing the dust blow-out diam- eter. However, at high eccentricities (e > 0:97) the blow-out population extending outwards from pericentre may be detectable above the collisional cascade; higher eccentricities also increase the probability of witnessing a recent collision. All of the imaging and spectroscopic constraints forCorvi can be explained with a single planetesimal population with pericentre at 0.75AU, apocentre at 150AU, and mass 5M�; however, the origin of such a high eccentric- ity population remains challenging. The mid-infrared excess to HD69830 can be explained by the ongoing destruction of a debris belt produced in a recent collision in an eccentric plan- etesimal belt, but the lack of far-infrared emission would r equire small bound grains to be absent from the parent planetesimal belt, possibly due to sublimation. The model presented here is applicable wherever non-negligible planetesimal eccentricities are implicated and can be readily incorporated into N-body simulations.

The potential for Earth-mass planet formation around brown dwarfs

Recent observations point to the presence of structured dust grains in the discs surrounding young brown dwarfs, thus implying that the first stages of planet formation take place also in the sub-stellar regime. Here, we investigate the pot ential for planet formation around brown dwarfs and very low mass stars according to the sequential core accretion model of planet formation. We find that, for a brown dwarfs mass 0.05M⊙, our models predict a maximum planetary mass of∼ 5M⊕, orbiting with semi-major axis∼ 1AU. However, we note that the predictions for the mass - semi-major axis distribu tion are strongly dependent upon the models chosen for the disc surface density profiles and th e assumed distribution of disc masses. In particular, if brown dwarf disc masses are of the order of a few Jupiter masses, Earth-mass planets might be relatively frequent, while if t ypical disc masses are only a fraction of Jupiter mass, we predict that planet formation would be extremely rare in the substellar regime. As the observational constraints on disc profiles, m ass dependencies and their distributions are poor in the brown dwarf regime, we advise caution in validating theoretical models only on stars similar to the Sun and emphasise the need for observational data on planetary systems around a wide range of stellar masses. We also find tha t, unlike the situation around solar-like stars, Type-II migration is totally absent from the planet formation process around brown dwarfs, suggesting that any future observations of planets around brown dwarfs would provide a direct measure of the role of other types of migration.

The 55 Cancri planetary system: fully self-consistent N-body constraints and a dynamical analysis

We present an updated study of the planets known to orbit 55 Cancri A using 1 418 high-precision radial velocity observations from four observatories (Lick, Keck, Hobby-Eberly Telescope, Harlan J. Smith Telescope) and transit time/durations for the inner-most planet, 55 Cancri ‘e’ (Winn et al. 2011). We provide the first posterior sample for the masses and orbital parameters based on self-consistent N-body orbital solutions for the 55 Cancri planets, all of which are dynamically stable (for at least 10^8 yr). We apply a GPU version of Radial velocity Using N-body Differential evolution Markov Chain Monte Carlo (RUN DMC; Nelson, Ford & Payne) to perform a Bayesian analysis of the radial velocity and transit observations. Each of the planets in this remarkable system has unique characteristics. Our investigation of high-cadence radial velocities and priors based on space-based photometry yields an updated mass estimate for planet ‘e’ (8.09 ± 0.26 M⊕), which affects its density (5.51±^(1.32)_(1.00)g cm^(−3)) and inferred bulk composition. Dynamical stability dictates that the orbital plane of planet ‘e’ must be aligned to within 60° of the orbital plane of the outer planets (which we assume to be coplanar). The mutual interactions between the planets ‘b’ and ‘c’ may develop an apsidal lock about 180°. We find 36–45 per cent of all our model systems librate about the anti-aligned configuration with an amplitude of 51∘±^(6∘)_(10∘). Other cases showed short-term perturbations in the libration of ϖb − ϖc, circulation, and nodding, but we find the planets are not in a 3:1 mean-motion resonance. A revised orbital period and eccentricity for planet ‘d’ pushes it further towards the closest known Jupiter analogue in the exoplanet population.

THE HEAVY-ELEMENT COMPOSITION OF DISK INSTABILITY PLANETS CAN RANGE FROM SUB- TO SUPER-NEBULAR

Transit surveys combined with Doppler data have revealed a class of gas giant planets that are massive and highly enriched in heavy-elements (e.g., HD 149026b, GJ436b, and HAT-P-20b). It is tempting to consider these planets as validation of core accretion plus gas capture because it is often assumed that disk instability planets should be of nebular composition. We show in this paper, to the contrary, that gas giants that form by disk instability can have a variety of heavy-element compositions, ranging from sub- to super-nebular values. High levels of enrichment can be achieved through one or multiple mechanisms, including enrichment at birth, planetesimal capture, and differentiation plus tidal stripping. As a result, the metallicity of an individual gas giant cannot be used to discriminate between gas giant formation modes.

Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys – II. The unique LMC cluster ESO 121−SC03

We present the results of photometric measurements from images of the Large Magellanic Cloud (LMC) cluster ESO 121-SC03 taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. Our resulting colour-magnitude diagram (CMD) reaches 3 mag below the main-sequence turn-off, and represents by far the deepest observation of this cluster to date. We also present similar photometry from ACS imaging of the accreted Sagittarius dSph cluster Palomar 12, used in this work as a comparison cluster. From analysis of its CMD, we obtain estimates for the metallicity and reddening of ESO 121-SC03: [Fe/H] = - 0.97 ± 0.10 and E(V - 1) = 0.04 ± 0.02, in excellent agreement with previous studies. The observed horizontal branch (HB) level in ESO 121-SC03 suggests this cluster may lie 20 per cent closer to us than does the centre of the LMC. ESO 121 - SC03 also possesses a significant population of blue stragglers, which we briefly discuss. Our new photometry allows us to undertake a detailed study of the age of ESO 121-SC03 relative to Palomar 12 and the Galactic globular cluster 47 Tuc. We employ both vertical and horizontal differential indicators on the CMD, calibrated against isochrones from the Victoria-Regina stellar models. These models allow us to account for the different α-element abundances in Palomar 12 and 47 Tuc, as well as the unknown run of α-elements in ESO 121-SC03. Taking a straight error-weighted mean of our set of age measurements yields ESO 121-SC03 to be 73 ± 4 per cent the age of 47 Tuc, and 91 ± 5 per cent the age of Palomar 12. Palomar 12 is 79 ± 6 per cent as old as 47 Tuc, consistent with previous work. Our result corresponds to an absolute age for ESO 121-SC03 in the range 8.3-9.8 Gyr, depending on the age assumed for 47 Tuc, therefore confirming ESO 121-SC03 as the only known cluster to lie squarely within the LMC age gap. We briefly discuss a suggestion from earlier work that ESO 121-SC03 may have been accreted into the LMC system.

Liberating exomoons in white dwarf planetary systems

Previous studies indicate that more than a quarter of all white dwarf (WD) atmospheres are polluted by remnant planetary material, with some WDs being observed to accrete the mass of Pluto in 106 yr. The short sinking time-scale for the pollutants indicates that the material must be frequently replenished. Moons may contribute decisively to this pollution process if they are liberated from their parent planets during the post-main-sequence evolution of the planetary systems. Here, we demonstrate that gravitational scattering events amongst planets in WD systems easily trigger moon ejection. Repeated close encounters within tenths of planetary Hill radii are highly destructive to even the most massive, close-in moons. Consequently, scattering increases both the frequency of perturbing agents in WD systems, as well as the available mass of polluting material in those systems, thereby enhancing opportunities for collision and fragmentation and providing more dynamical pathways for smaller bodies to reach the WD. Moreover, during intense scattering, planets themselves have pericentres with respect to the WD of only a fraction of an astronomical unit, causing extreme Hill-sphere contraction, and the liberation of moons into WD-grazing orbits. Many of our results are directly applicable to exomoons orbiting planets around main-sequence stars.