Planet formation around stars of various masses: Hot super-Earths

We consider trends resulting from two formation mechanisms for short-period super-Earths: planet-planet scattering and migration. We model scenarios where these planets originate near the snow line in ``cold finger'' circumstellar disks. Low-mass planet-planet scattering excites planets to low periastron orbits only for lower mass stars. With long circularisation times, these planets reside on long-period eccentric orbits. Closer formation regions mean planets that reach short-period orbits by migration are most common around low-mass stars. Above ~1 Solar mass, planets massive enough to migrate to close-in orbits before the gas disk dissipates are above the critical mass for gas giant formation. Thus, there is an upper stellar mass limit for short-period super-Earths that form by migration. If disk masses are distributed as a power law, planet frequency increases with metallicity because most disks have low masses. For disk masses distributed around a relatively high mass, planet frequency decreases with increasing metallicity. As icy planets migrate, they shepherd interior objects toward the star, which grow to ~1 Earth mass. In contrast to icy migrators, surviving shepherded planets are rocky. Upon reaching short-period orbits, planets are subject to evaporation processes. The closest planets may be reduced to rocky or icy cores. Low-mass stars have lower EUV luminosities, so the level of evaporation decreases with decreasing stellar mass.Comment: Accepted to ApJ. 13 pages of emulateap

Local Statistics of Realizable Vertex Models

We study planar "vertex" models, which are probability measures on edge subsets of a planar graph, satisfying certain constraints at each vertex, examples including dimer model, and 1-2 model, which we will define. We express the local statistics of a large class of vertex models on a finite hexagonal lattice as a linear combination of the local statistics of dimers on the corresponding Fisher graph, with the help of a generalized holographic algorithm. Using an $n\times n$ torus to approximate the periodic infinite graph, we give an explicit integral formula for the free energy and local statistics for configurations of the vertex model on an infinite bi-periodic graph. As an example, we simulate the 1-2 model by the technique of Glauber dynamics

Low Mass Stars and Brown Dwarfs around Sigma Orionis

We present optical spectroscopy of 71 photometric candidate low-mass members of the cluster associated with Sigma Orionis. Thirty-five of these are found to pass the lithium test and hence are confirmed as true cluster members, covering a mass range of <0.055-0.3M_{sun}, assuming a mean cluster age of <5 Myr. We find evidence for an age spread on the (I, I-J) colour magnitude diagram, members appearing to lie in the range 1-7 Myr. There are, however, a significant fraction of candidates that are non-members, including some previously identified as members based on photometry alone. We see some evidence that the ratio of spectroscopically confirmed members to photometric candidates decreases with brightness and mass. This highlights the importance of spectroscopy in determining the true initial mass-function.Comment: To appear in the 12th Cambridge Workshop on Cool Stars Stellar Systems and the Su

The discovery of a low mass, pre-main-sequence stellar association around gamma Velorum

We report the serendipitous discovery of a population of low mass, pre-main sequence stars (PMS) in the direction of the Wolf-Rayet/O-star binary system gamma^{2} Vel and the Vela OB2 association. We argue that gamma^{2} Vel and the low mass stars are truly associated, are approximately coeval and that both are at distances between 360-490 pc, disagreeing at the 2 sigma level with the recent Hipparcos parallax of gamma^{2} Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the gamma^{2} Vel system, but also offer an exciting opportunity to investigate the influence of high mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings.Comment: Monthly Notices of the Royal Astronomical Society, Letters - in pres

Thermonuclear runaways in thick hydrogen rich envelopes of neutron stars

A Lagrangian, fully implicit, one dimensional hydrodynamic computer code was used to evolve thermonuclear runaways in the accreted hydrogen rich envelopes of 1.0 Msub solar neutron stars with radii of 10 km and 20 km. Simulations produce outbursts which last from about 750 seconds to about one week. Peak effective temeratures and luninosities were 26 million K and 80 thousand Lsub solar for the 10 km study and 5.3 millison and 600 Lsub solar for the 20 km study. Hydrodynamic expansion on the 10 km neutron star produced a precursor lasting about one ten thousandth seconds

Numerical simulation of the magnetospheric gate model for X-ray bursters

A Lagrangian, fully implicit, one dimensional hydrodynamic computer code was used to investigate the evolution of a gas cloud impacting the surface of a 20 km, 1 Msub solar neutron star. This gas is initially at rest with respect to the surface of the neutron star, extends to 185 km above the surface, and is optically thick. The infall results in a burst which lasts about 0.1 seconds and reached a peak luminosity and effective temperature of 240,000 Lsub solar and 9 million; respectively. The burst was followed by a phase of oscillations with a period 0.2 seconds

The TAOS Project: Upper Bounds on the Population of Small KBOs and Tests of Models of Formation and Evolution of the Outer Solar System

We have analyzed the first 3.75 years of data from TAOS, the Taiwanese American Occultation Survey. TAOS monitors bright stars to search for occultations by Kuiper Belt Objects (KBOs). This dataset comprises 5e5 star-hours of multi-telescope photometric data taken at 4 or 5 Hz. No events consistent with KBO occultations were found in this dataset. We compute the number of events expected for the Kuiper Belt formation and evolution models of Pan & Sari (2005), Kenyon & Bromley (2004), Benavidez & Campo Bagatin (2009), and Fraser (2009). A comparison with the upper limits we derive from our data constrains the parameter space of these models. This is the first detailed comparison of models of the KBO size distribution with data from an occultation survey. Our results suggest that the KBO population is comprised of objects with low internal strength and that planetary migration played a role in the shaping of the size distribution.Comment: 18 pages, 16 figures, Aj submitte