Stephen Lubow - STScI

Stephen Lubow
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Stephen Lubow
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STScI
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Earth and Planetary Astrophysics (16)
 
Solar and Stellar Astrophysics (13)
 
Astrophysics (6)
 
High Energy Astrophysical Phenomena (4)
 
Instrumentation and Methods for Astrophysics (2)
 
Astrophysics of Galaxies (1)

Publications Authored By Stephen Lubow

We use three-dimensional hydrodynamical simulations to show that an initially mildly misaligned circumbinary accretion disk around an eccentric binary can evolve to an orientation that is perpendicular to the orbital plane of the binary (polar alignment). As the disk evolves to the perpendicular state, it undergoes nodal libration oscillations of the tilt angle and the longitude of the ascending node. Dissipation within the disk causes the oscillations to damp. Read More

We analyze the gravitational instability (GI) of a locally isothermal inclined disk around one component of a binary system. Such a disk can undergo global Kozai-Lidov (KL) cycles if the initial disk tilt is above the critical KL angle (of about 40 degrees). During these cycles, an initially circular disk exchanges its inclination for eccentricity, and vice versa. Read More

We use local numerical simulations to study a vertically stratified accretion disk with a resistive mid-plane that damps magnetohydrodynamic (MHD) turbulence. This is an idealized model for the dead zones that may be present at some radii in protoplanetary and dwarf novae disks. We vary the relative thickness of the dead and active zones to quantify how forced fluid motions in the dead zone change. Read More

With hydrodynamical simulations we determine the conditions under which an initially coplanar planet-disc system that orbits a member of a misaligned binary star evolves to form a planet that undergoes Kozai-Lidov (KL) oscillations once the disc disperses. These oscillations may explain the large orbital eccentricities, as well as the large misalignments with respect to the spin of the central star, observed for some exoplanets. The planet is assumed to be massive enough to open a gap in the disc. Read More

2016Feb
Affiliations: 1STScI, 2STScI, Fermi National Accelerator Laboratory, 3JHU, 4STScI, 5STScI, 6STScI, 7STScI, 8STScI, 9STScI, 10STScI, 11STScI, 12STScI

The Hubble Source Catalog is designed to help optimize science from the Hubble Space Telescope by combining the tens of thousands of visit-based source lists in the Hubble Legacy Archive into a single master catalog. Version 1 of the Hubble Source Catalog includes WFPC2, ACS/WFC, WFC3/UVIS, and WFC3/IR photometric data generated using SExtractor software to produce the individual source lists. The catalog includes roughly 80 million detections of 30 million objects involving 112 different detector/filter combinations, and about 160 thousand HST exposures. Read More

We determine the evolution of a giant planet-disk system that orbits a member of a binary star system and is mildly inclined with respect to the binary orbital plane. The planet orbit and disk are initially mutually coplanar. We analyze the evolution of the planet and the disk by analytic means and hydrodynamic simulations. Read More

Previously we showed that a substantially misaligned viscous accretion disk with pressure that orbits around one component of a binary system can undergo global damped Kozai-Lidov (KL) oscillations. These oscillations produce periodic exchanges of the disk eccentricity with inclination. The disk KL mechanism is quite robust and operates over a wide range of binary and disk parameters. Read More

Martin et al. (2014b) showed that a substantially misaligned accretion disk around one component of a binary system can undergo global damped Kozai-Lidov oscillations. During these oscillations, the inclination and eccentricity of the disk are periodically exchanged. Read More

We analyse the interaction of an eccentric binary with a circular coplanar circumbinary disc that rotates in a retrograde sense with respect to the binary. In the circular binary case, no Lindblad resonances lie within the disc and no Lindblad resonant torques are produced, as was previously known. By analytic means, we show that when the binary orbit is eccentric, there exist components of the gravitational potential of the binary which rotate in a retrograde sense to the binary orbit and so rotate progradely with respect to this disc, allowing a resonant interaction to occur between the binary and the disc. Read More

We present deep HST/STIS coronagraphic images of the Beta Pic debris disk obtained at two epochs separated by 15 years. The new images and the re-reduction of the 1997 data provide the most sensitive and detailed views of the disk at optical wavelengths as well as the yet smallest inner working angle optical coronagraphic image of the disk. Our observations characterize the large-scale and inner-disk asymmetries and we identify multiple breaks in the disk radial surface brightness profile. Read More

We extend previous studies of the tidal truncation of coplanar disks in binary systems to the more general case of noncoplanar disks. As in the prograde coplanar case, Lindblad resonances play a key role in tidal truncation. We analyze the tidal torque acting on a misaligned nearly circular disk in a circular orbit binary system. Read More

We carried out two-dimensional high-resolution simulations to study the effect of dust feedback on the evolution of vortices induced by massive planets in protoplanetary disks. Various initial dust to gas disk surface density ratios ($0.001$ -- $0. Read More

We use three dimensional hydrodynamical simulations to show that a highly misaligned accretion disk around one component of a binary system can exhibit global Kozai-Lidov cycles, where the inclination and eccentricity of the disk are interchanged periodically. This has important implications for accreting systems on all scales, for example, the formation of planets and satellites in circumstellar and circumplanetary disks, outbursts in X-ray binary systems and accretion on to supermassive black holes. Read More

Be/X-ray binary systems exhibit both periodic (Type I) X-ray outbursts and giant (Type II) outbursts, whose origin has remained elusive. We suggest that Type II X-ray outbursts occur when a highly misaligned decretion disk around the Be star becomes eccentric, allowing the compact object companion to capture a large amount of material at periastron. Using 3D smoothed particle hydrodynamics simulations we model the long term evolution of a representative Be/X-ray binary system. Read More

Recent observations of large-scale asymmetric features in protoplanetary disks suggest that large-scale vortices exist in such disks. Massive planets are known to be able to produce deep gaps in protoplanetary disks. The gap edges could become hydrodynamically unstable to the Rossby wave/vortex instability and form large-scale vortices. Read More

Zhu, Stone, and Rafikov (2012) found in 3D shearing box simulations a new form of planet-disk interaction that they attributed to a vertical buoyancy resonance in the disk. We describe an analytic linear model for this interaction. We adopt a simplified model involving azimuthal forcing that produces the resonance and permits an analytic description of its structure. Read More

We consider the evolution of accretion discs that contain some turbulence within a disc dead zone, a region about the disc midplane of a disc that is not sufficiently ionised for the magneto-rotational instability (MRI) to drive turbulence. In particular, we determine whether additional sources of turbulence within a dead zone are capable of suppressing gravo-magneto (GM) disc outbursts that arise from a rapid transition from gravitationally produced to MRI produced turbulence. With viscous $\alpha$ disc models we consider two mechanisms that may drive turbulence within the dead zone. Read More

Discs that contain dead zones are subject to the Gravo-Magneto (GM) instability that arises when the turbulence shifts from gravitational to magnetic. We have previously described this instability through a local analysis at some radius in the disc in terms of a limit cycle. A disc may be locally unstable over a radial interval. Read More

Regular satellites in the solar system are thought to form within circumplanetary discs. We consider a model of a layered circumplanetary disc that consists of a nonturbulent midplane layer and and strongly turbulent disc surface layers. The dead zone provides a favorable site for satellite formation. Read More

Object cross-identification in multiple observations is often complicated by the uncertainties in their astrometric calibration. Due to the lack of standard reference objects, an image with a small field of view can have significantly larger errors in its absolute positioning than the relative precision of the detected sources within. We present a new general solution for the relative astrometry that quickly refines the World Coordinate System of overlapping fields. Read More

We perform global time-dependent simulations of an accretion disc around a young stellar object with a dead zone (a region where the magneto-rotational instability cannot drive turbulence because the material is not sufficiently ionised). For infall accretion rates on to the disc of around 10^-7 Msun/yr, dead zones occur if the critical magnetic Reynolds number is larger than about 10^4. We model the collapse of a molecular gas cloud. Read More

Angular momentum is transported outwards through an accretion disc by magnetohydrodynamical (MHD) turbulence thus allowing material to accrete on to the central object. The magneto-rotational instability (MRI) requires a minimum ionisation fraction to drive turbulence in a disc. The inner parts of the disc around a young stellar object are sufficiently hot to be thermally ionised. Read More

We present the first three-dimensional magnetohydrodynamic (MHD) simulations of a circumbinary disk surrounding an equal mass binary. The binary maintains a fixed circular orbit of separation $a$. As in previous hydrodynamical simulations, strong torques by the binary can maintain a gap of radius $\simeq 2a$. Read More

Previous theoretical studies have found that repeating outbursts can occur in certain regions of an accretion disk, due to sudden transitions in time from gravitationally produced turbulence to magnetically produced turbulence. We analyze the disk evolution in a state diagram that plots the mass accretion rate versus disk surface density. We determine steady state accretion branches that involve gravitational and magnetic sources of turbulence. Read More

Rapidly rotating Be stars are observed as shell stars when the decretion disc is viewed edge on. Transitions between the two implies that the discs may be warped and precessing. Type II X-ray outbursts are thought to occur when the warped disc interacts with the fast stellar wind. Read More

We analyse some properties of circumplanetary discs. Flow through such discs may provide most of the mass to gas giant planets, and such discs are likely sites for the formation of regular satellites. We model these discs as accretion discs subject to the tidal forces of the central star. Read More

We determine an expression for the Type I planet migration torque involving a locally isothermal disk, with moderate turbulent viscosity (~0.0005 < alpha < ~0.05), based on three-dimensional nonlinear hydrodynamical simulations. Read More

Planet migration is the process by which a planet's orbital radius changes in time. The main agent for causing gas giant planet migration is the gravitational interaction of the young planet with the gaseous disk from which it forms. We describe the migration rates resulting from these interactions based on a simple model for disk properties. Read More

We consider discs that orbit a central object and are tidally perturbed by a circular orbit companion. Such discs are sometimes subject to an eccentric instability due to the effects of certain resonances. Eccentric instabilities may be present in planetary rings perturbed by satellites, protostellar discs perturbed by planets, and discs in binary star systems. Read More

We analyze the orbital and mass evolution of planets that undergo run-away gas accretion by means of 2D and 3D hydrodynamic simulations. The disk torque distribution per unit disk mass as a function of radius provides an important diagnostic for the nature of the disk-planet interactions. We first consider torque distributions for nonmigrating planets of fixed mass and show that there is general agreement with the expectations of resonance theory. Read More

The exoplanets discovered so far have been mostly around relatively nearby and bright stars. As a result, the host stars are mostly (i) in the Galactic disk, (ii) relatively massive, and (iii) relatively metal rich. The aim of the SWEEPS project is to extend our knowledge to stars which (i) are in a different part of the Galaxy, (ii) have lower masses, and (iii) have a large range of metallicities. Read More

More than 200 extrasolar planets have been discovered around relatively nearby stars, primarily through the Doppler line shifts owing to the reflex motions of their host stars, and more recently through transits of some planets across the face of the host stars. The detection of planets with the shortest known periods, 1.2 to 2. Read More

We investigate the interaction between a giant planet and a viscous circumstellar disk by means of high-resolution, two-dimensional hydrodynamical simulations. We consider planet masses that range from 1 to 3 Jupiter masses (Mjup) and initial orbital eccentricities that range from 0 to 0.4. Read More

We present three-dimensional numerical simulations of the interaction of a circular-orbit planet with a protoplanetary disk. We calculate the flow pattern, the accretion rate, and torques on the planet. We consider planet masses ranging from 1 Earth mass to 1 Jupiter mass. Read More

Doppler spectroscopy of rho1 Cnc has detected evidence of a companion with an orbital period of 14.65 days and a minimum mass of 0.88 Jupiter masses. Read More