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Dr Stefan M Lines

(Former)

Stefan Lines

Dr Stefan M Lines

(Former)

Member of

External positions

Research Fellow, University of Exeter, Exeter, UK.

1 Apr 20161 Apr 2019

Research interests

I completed my PhD in Physics in 2016 at the University of Bristol, UK under the supervision of Dr Zoë Malka Leinhardt. My project involved understanding planet formation through both N-body and hydrodynamic simulations. Specifically, I looked into how planets form around stellar binary systems. A binary is a star system that features two stars, rather than one, orbiting a mutual center of mass. Binary stars cause complex perturbations that force solid material in the protoplanetary disk into high velocity and eccentric orbits that hinder planetesimal growth and hence planet formation; a process called orbital crossing can occur causing high velocity encounters between rocky material which are often erosive in nature (Thébault et al , 2006). Planetesimals in the disk must be able to accrete material in order to increase in mass and become protoplanets and planetary embryos.

My first paper focussed on running 3-Dimensional N-body simulations of one million planetesimals in a circumbinary protoplanetary disk using a newly implemented collision model (Leinhardt & Stewart, 2012). This analytical model reads in encounter velocity, impact parameter and mass ratio to calculate catastrophic disruption theshold and detemine the outcome of the collision. Pre-existing models only allow for a fragmentation scheme where material smaller than the resolution limit, 'dust', is deposited into azimuthally homogenous radial bins used in the following timestep. Further model simplifications include perfect merging where colliding particles are always merged together with no mass loss. The N-Body code 'PKDGRAV' is used (Parallel K-D tree GRAVity code). Our study of the feasibility of planetesimal growth in protoplanetary disks around the observed circumbinary system Kepler-34 can be found in the ApJL letter here (Lines et al 2014). In short we found that erosive collisions were dominant across almost the full extent of our disk (ainner = 0.8, aouter = 1.5) making planetesimal growth through accretion based collisions at the location of the observed planet Kepler-34(AB)b difficult. We find that it is likely that almost all closely orbiting (ap < 1.2 au) circumbinary planets discovered to date formed further out in the disk where the secular and dynamical forcing from the binary is lessened. The planet then migrates inwards to its present, observed location. This is support by existing work on planetary migration around binaries (Pierens & Nelson 2013).

My second paper looked at the response of the circumbinary gas disk. For this we used the numerical code FARGO-ADSG to simulate such a disk subject to varying stellar binary type, disk aspect ratio, alpha-viscosity, surface density, boundary condition and relevance of self gravity. Through a series of complex processes such as mode coupling between the binary potential and the gas disk, the disk becomes eccentric and truncated. While changing the fluid and stellar parameters changes the quasi-steady-state disk structure (some more than others) and evolution, all simulations produce a non-axisymmetric precessing disk. Understanding the structure of the gas disk can be important for hybrid models since planetesimal dynamics will change depending on whether they feel a static or dynamic disk. Our work has been accepted by A&A and can be found here (Lines et al 2015). An interesting discovery from this work is the non-importance of a self-gravitating disk. This is contrasting to the relevance of self-gravity in the circumprimary case where Marzari et al. (2009) found including it modified the disk structure and evolution significantly, even at disk masses which were classically stable.

My final paper considered the combined effect of these two mediums on the ability for the Kepler circumbinary planets to form. This involved breaking down the gravitational potential from the FARGO hydrocode output into base modes via Fourier decomposition, and providing an analytical solution to these in a PKDGRAV module that applies the gas potential onto the solid planatesimal disk. Our results can be found in Lines et al 2016 where we show that the gravitational influence of a time-dependent asymmetric gas disk on the planetesimals is even more dynamically provocative than the pure dynamical forcing from the binary itself. The eccentricities of the planetesimals are increased to values high enough to lead to a system of orbit crossings with high encounter velocities. The outcome is an increase in erosive collisions, leading to a disk that is grinding down into dust rather than up into larger bodies such as protoplanets and planetary embryos. We determine that the Kepler circumbinary planets could not have formed in-situ but instead more likely formed further out in a less hostile, dynamically inactive region and then migrated inwards at a later time. This conclusion is supported by work on migration in binary systems such as Pierens & Nelson 2007, 2008 and 2013.

Research interests

I am now an honourary research fellow at the University of Bristol, while working as a full time postdoctoral research fellow on cloud formation in Brown Dwarfs and giant exoplanets at the University of Exeter. My work at Bristol continues to elaborate on the subject of planet formation in binary systems.

I aim to perform simulatios to determine the feasibility of planetesimal growth and planet stability in tightening binary systems. This questions will address the topic of planet formation in S-type binaries and will also consider the creation of P-type systems from initially widely separated binaries.

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