Department of Astronomy Center for Radiophysics & Space Research

Kassandra Anderson

Professor Dong Lai
Kassandra  Anderson

Research interests:

Extrasolar planetary systems, protoplanetary disks, star and planet formation

Research interests: Extrasolar planetary systems, stellar binaries/triples, protoplanetary disks and planet formation

I am a fifth-year graduate student working with Professor Dong Lai on the dynamics of exoplanets and stellar binaries/triples. A major subject of my research is the formation of short-period planets and binaries via "high-eccentricity migration," in which a planet or stellar binary with an initially large separation is perturbed into a highly eccentric orbit, with subsequent orbital decay at pericenter due to tidal dissipation.  Operating over long timescales, such processes can shrink the orbit of an exoplanet or binary system by a large factor and result in a final orbital period of a few days or less. A major application of this work is the formation of hot Jupiters (giant planets with orbital periods of several days). Despite the fact that hot Jupiters were the first class of exoplanet to be discovered around a main-sequence star, their existence continues to be a puzzle, as it is widely thought that such planets cannot form at their observed orbital locations, and must instead have migrated from farther out.  Competing theories of orbital migration for hot Jupiters exist, and a primary goal of my research is to help distinguish between various proposed migration theories. Our work has focused on planets in stellar binaries, in which extreme eccentricity and inclination variations in a planet can arise from perturbations due a stellar companion (through Lidov-Kozai cycles).  When the planet passes close to the star, the stellar spin axis can tilt due to spin-orbit coupling.  Lidov-Kozai cycles, coupled with tidal dissipation can result in a hot Jupiter with an orbital plane significant misaligned with the stellar equator. Such spin-orbit misalignment is observed in many hot Jupiter system, and can thus serve as a clue of the planet's migration history.  We have conducted a population synthesis of hot Jupiter formation in stellar binaries, and present distributions of spin-orbit misalignment angles for various properties of the planet, host star, and stellar companion.

I have also worked on spin-orbit dynamics in stellar triple systems. Similar to hot Jupiters, many stellar binaries also exhibit spin-orbit misalignment, and it remains unclear whether such misalignments are primordial, or arose after formation due to dynamical interactions. My work focuses on the latter scenario, in which spin-orbit misalignment arises due to perturbations from a tertiary stellar companion, and identifies the properties required by such a companion to tilt the spin axes of the inner binary members.

My most recent work focuses on "warm Jupiters", which are similar to hot Jupiters, but with slightly longer orbital periods of 10-100 days.  Similar to hot Jupiters, the formation histories of warm Jupiters is also unknown, and it unclear whether warm Jupiters arrive at their observed orbits via one of several proposed migration mechanisms, or whether they simply form in-situ. Substantial eccentricities are observed in many warm Jupiters, which are difficult to explain with some proposed formation theories.  We investigate whether the eccentricities of warm Jupiters may arise from perturbations due to external giant planet companions. This research sheds insight into the dynamical role of observed companions to warm Jupiters, as well as helping to identify possible undetected companions to seemingly solitary eccentric warm Jupiters.

I am currently supported by an NSF Graduate Research Fellowship. In 2016-2017 I served as a TA for the introductory courses Astronomy 1101 & 1102.  I received my B.S. in Physics from the University of Michigan, where I worked with Profs. Fred Adams and Nuria Calvet on exoplanetary dynamics and protoplanetary disk evolution. Outside of astronomy, my hobbies include sewing/knitting/other crafts, cats, and cooking.

Selected Publications:  

Kassandra R. Anderson & Dong Lai, "Moderately Eccentric Warm Jupiters from Secular Interactions with Exterior Companions", 2017, MNRAS, in press, https://arxiv.org/abs/1706.00084

Kassandra R. Anderson, Dong Lai, & Natalia I. Storch, "Eccentricity and spin-orbit misalignment in short-period stellar binaries as a signpost of hidden tertiary companions", 2017, MNRAS, 467, 3066, https://arxiv.org/abs/1610.02626

Kassandra R. Anderson, Natalia I. Storch, & Dong Lai,"Formation and stellar spin-orbit misalignment of hot Jupiters from Lidov-Kozai oscillations in stellar binaries", 2016 MNRAS, 456, 3671, https://arxiv.org/abs/1510.08918

Natalia I. Storch, Kassandra R. Anderson, & Dong Lai, "Chaotic dynamics of stellar spin in binaries and the production of misaligned hot Jupiters", 2014, Science, 345, 1317

Kassandra R. Anderson, Fred C. Adams, & Nuria Calvet, 2013 ApJ, 776, 9
"Viscous Evolution and Photoevaporation of Circumstellar Disks due to External Far-Ultraviolet Radiation Fields"
http://arxiv.org/abs/1307.4368

Fred C. Adams, Kassandra R. Anderson, & Anthony M. Bloch, 2013 MNRAS, 432, 438 "Evolution of Planetary Systems with Time Dependent Stellar Mass Loss" http://arxiv.org/abs/1303.3841

Kassandra R. Anderson & Fred C. Adams, 2012 PASP, 124, 809
"Effects of Collisions with Rocky Planets on the Properties of Hot Jupiters" http://arxiv.org/abs/1206.5857