Sanaea Cooper Rose

I am a Lindheimer Fellow at the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University. I study interactions between stars and stellar remnants in dense star clusters. In particular, I explore how these interactions lead to unique populations of stars, binaries, and black holes and drive extreme events like collisions, mergers, and tidal disruptions. I enjoy leveraging simple physical models to build intuition about complex astrophysical environments. For more details, please see the project descriptions below. I completed my PhD in Astronomy & Astrophysics in 2023 at the University of California, Los Angeles, where I was advised by Smadar Naoz. I received my B.A. in astrophysics from Wellesley College in 2017.

In addition to my research pursuits, I am passionate about serving and building community in both my public and professional spheres. As a member of UCLA's Physics & Astronomy Department Diversity, Equity, and Inclusion Committee, I helped establish DiversiTea, a department-wide diversity journal club. I was also a coordinator for UCLA's Astronomy Live! outreach program and Astronomy on Tap West LA from 2019-2022 and an active member of the astronomy graduate program's peer mentoring program. At CIERA, I serve on the Mentoring Action Team, which connects mentors and mentees across different career stages and organizes monthly events such as mentorship training.

For my publications, please see here

Email: sanaea.rose@northwestern.edu

Companion-driven evolution of massive stellar binaries

Most massive stars reside in binary systems. Additionally, many of these massive binaries may have yet another companion, making them triple star systems. For these triples to remain stable over long periods of time, they must have a hierarchical configuration: two of the stars form a tight inner binary, whose center of mass is orbited more distantly by the third companion. In this configuration, the third star gravitationally perturbs the inner binary, altering its orbital properties. In particular, these perturbations cause the eccentricity and inclination of the inner binary to oscillate over time, a process known as the Eccentric Kozai-Lidov (EKL) Mechanism (see Naoz 2016 for a review). Depending on the efficiency of the stellar tides, this mechanism can drive the inner binary to either merge or shrink and circularize.

In Rose et al. 2019, we examine the impact of EKL-driven evolution on the orbital properties of massive binaries that are embedded in triples. Our objective is to determine how initial conditions inform the final eccentricity and period distributions of these stellar systems. What can observations of massive binary populations tell us about their birth configurations? Simulating large sets of stellar systems with a variety of initial conditions, we find that the period distribution is preserved for wide orbits despite 10 millions years of evolution. We also demonstrate that the final eccentricity distribution is an excellent indicator of the birth distribution regardless of other initial conditions.

A cartoon depiction of a hierarchical triple star system. The third star orbits the center of mass of the inner orbit on a much wider outer orbit. Flip through the images to see an illustration of the EKL oscillations. As the inclination between the orbits increases, the eccentricity decreases and the inner orbit becomes more circular.

Binaries to probe the density of objects in the Galactic Center

Binary systems are expected to reside in the nuclear star cluster at the very center of our galaxy. In this densely populated environment, a binary system frequently encounters neighboring objects, including other stars. With each encounter, a weak gravitational interaction between the binary and passing object, the binary's orbit widens slightly. The effects of these interactions accumulate over time, eventually pulling the binary completely apart. Known as evaporation, this process has an associated timescale, which depends on the surrounding stellar density. A higher the density results in more frequent interactions, which reduces the time needed to unbind the binary.

Evaporation is not the only process that can destroy a binary in the Galactic Center. One of the stars in the binary can also experience a direct collision with a neighboring star, causing the binary to unbind. Like evaporation, collisions occur over a characteristic timescale that depends on the density of the environment. The denser an environment, the sooner a direct collision will occur.

Binaries can tell us about the environments in which they live. Because the collision and evaporation timescales both depend on the surrounding density, the detection of a binary with known properties can constrain the density of objects in the Galactic Center. We demonstrate this procedure in Rose et al. 2020. Building upon the framework in Alexander & Pfuhl (2014), we generalize the evaporation timescale to an eccentric orbit and consider several dynamical processes in tandem. The age of a detected binary sets a lower limit on the evaporation and collision timescales. A lower limit on the timescale leads to an upper limit on the density in the binary's vicinity: if the surroundings are too dense, the binary would have already unbound.

In a dense environment, different dynamical processes can cause a binary star system to break apart, separating the stars. The amount of time that these processes take depends on the density of the environment. Therefore, an existing binary can provide constraints on the density of objects in its neighborhood. Flip through the images to the right to see an illustration.

The formation of intermediate-mass black holes in galactic centers

Recent technological advances have launched us into a new era of gravitational wave detections. These observations are already challenging our understanding of compact objects like black holes. One open question concerns the formation of black holes above about 50 solar masses, found in many gravitational wave detections (for example, GW190521). Most theoretical models predict that these black holes cannot form from the deaths of massive stars. One solution is that these black holes instead form through dynamical interactions in dense star clusters. In Rose et al. (2022), we explore whether these more massive black holes can form in the nuclear star cluster at the center of a galaxy. In this dense star cluster, a black hole frequently collides with surrounding stars. During a single collision, a black hole can accrete a small amount of mass from the star. Over many collisions, a lower-mass black hole can grow to exceed 50 solar masses. We show that this channel can be quite efficient, in some cases producing intermediate-mass black holes over 100 solar masses. Our results suggest that more massive black holes may be ubiquitous in galactic centers.

As a black hole in the nuclear star cluster orbits the central supermassive black hole, represented by the google maps symbol above, it can colllide with stars in its path.

Every time a black hole collides with a star, it can capture some material and potentially grow in mass.

Stellar Collisions in Galactic Nuclei

The centers of other galaxies are too far away for us to observe in detail, but we can learn about these environments by studying the center of the Milky Way. Observations have revealed a dense cluster of stars surrounding our central supermassive black hole. How, where, and when these stars formed and how they behave and interact under the influence of the supermassive black hole remain active areas of research. Additionally, observations of this region have presented many unexplained phenomena, including young, massive stars very near the supermassive black hole and mysterious puffy balls of dust and gas. Using theoretical models, we try to explain these unusual findings with stellar interactions in the dense cluster.

The central cluster is over a million times denser than our Sun's neighborhood. On top of that, the strong gravitational pull of the central supermassive black hole causes stars to move at orbital speeds between hundreds to thousands of km/s. This is much higher than the speed that the Earth orbits the Sun, 30 km/s. In this extreme environment, a dense cluster of stars surrounding a supermassive black hole, close interactions between stars and their neighbors become possible. Events that are unheard of in our neighborhood of the Galaxy, like collisions between stars, become common. In Rose et al. (2023) and Rose & MacLeod (2024), we characterize the outcomes of these collisions in the star cluster. Very near the supermassive black hole, for example, within a distance of just 0.01 parsecs, destructive collisions can halve the stellar population within one billion years. These destructive collisions can also result in a population of strange, low-mass stars throughout the Galactic center: stars lose their outer layers in a destructive collision near the supermassive black hole before migrating to the outer region of the cluster, where they can survive and continue to evolve . Collisions outside of 0.01 parsecs lead to mergers. In certain cases, stars like our Sun can experience many collisions and mergers and go on to become massive stars, including those ten times the Sun’s mass. These merger products can masquerade as rejuvenated, young-looking stars, when in fact they formed through collisions from an older population.

This work was picked up by numerous popular science outlets. You can read about it in the Northwestern press release, ZME Science, Space.com, Popular Science, Discover Magazine, and Space Daily, amongst others.

Miscellaneous Undergraduate Research

I completed a senior thesis with Prof. James Battat at Wellesley College entitled Modeling the Earth-Moon Distance for Different Theories of Gravitation.

I spent a summer at the Maria Mitchell Observatory, where I studied the peculiar cataclysmic variable star system QQ Vulpecula with Dr. Stella Kafka.

I worked with Prof. Meredith Hughes at Wesleyan University on the protoplanetary disk HD 163296 (see Flaherty et al. 2017).

Personal

Interview for The Tempest

I danced pre-professionally as a member of the Commonwealth Ballet Company.