In this project, Dr. Stassun, along with collaborators and students, will execute a multi-faceted, observational research program, with the goal of addressing three fundamental questions relating to the physics of low-mass stars during the planet-building era (1-10 Myr): (1) What governs the angular momentum evolution of low-mass pre-main sequence stars? (2) By which physical mechanisms are X-rays in low-mass pre-main sequence stars generated? (3) How do X-rays influence the properties and evolution of protoplanetary disks?
By harnessing several large optical, infrared, and X-ray databases of hundreds of low-mass pre-main sequence stars in the Orion Nebula Cluster (1 Myr old) and the Orion OB1 association (10 Myr old), they will:
1) Explore mechanisms for angular momentum loss in young, low-mass stars: It is empirically known that angular momentum is not conserved during the pre-main sequence phase of stellar evolution. Rotation data show that low-mass stars deplete their angular momentum content by an order of magnitude or more between ~1 Myr and the main sequence (~100 Myr). What is not known is how this happens. Dr. Stassun will examine two possible mechanisms for angular momentum evolution of pre-main sequence stars. First, the key predictions of so-called "disk locking" theory, in which it is thought that magnetic coupling of stars to their circumstellar disks regulates the star's angular momentum, will be directly tested. Second, motivated by the discovery of extremely powerful X-ray "super-flares" in the Orion Nebula Cluster stars, a new model of angular momentum loss via scaled-up solar-type coronal mass ejections will be developed and refined.
2) Elucidate the origins of X-ray production in young, low-mass stars: It is now well established that low-mass pre-main sequence stars produce X-rays at up to ~104 times that of the present-day Sun. Yet, it is still unclear how they do this. While X-ray production on the main sequence is well understood in terms of a rotation-driven dynamo, X-ray observations of pre-main sequence stars have so far failed to find a clear "rotation-activity relationship" such as that found on the main sequence. An underlying rotation-activity relation for pre-main sequence stars may indeed exist, but it has been missed due to an astrophysical bias that makes stars with known rotation periods systematically more X-ray luminous. To test this hypothesis, and to explore the evolution of the rotation-activity connection, Dr. Stassun will make sensitive v sin i and rotation-period measurements of Orion Nebula Cluster and Orion OB1 stars, and correlate these measures with the stars' X-ray luminosities.
3) Investigate the influence of stellar X-rays on the circumstellar environment: X-rays are thought to play a central role in much of the microphysics that governs magnetic star-disk interaction, accretion processes, outflows of material, and angular momentum evolution of young stars. However, to date there has been no direct observational proof of stellar X-rays interacting with and heating circumstellar gas. Here, Dr. Stassun will conduct an experiment to establish for the first time a direct link between stellar X-rays and the heating of circumstellar gas in a large sample of pre-main sequence stars. In particular, he will search for time-correlated variability in H-alpha and X-rays among 800 pre-main sequence stars for which we have a unique database of simultaneous H-alpha and X-ray light curves.
This project will continue and expand upon the successful undergraduate and graduate programs developed in collaboration with Fisk University: The Fisk Astronomy and Space Science Training (FASST) program, and the Fisk-Vanderbilt Masters-to-PhD Bridge program. This research program will form the basis of a PhD thesis for a Vanderbilt PhD student, with active participation of a Bridge graduate student and one or more FASST undergraduates. The project will also result in an excellent professional development opportunity for a postdoctoral researcher seeking experience in integrating research, teaching, and student mentoring.
This project has centered around a multi-faceted, observational research program, with the goal of addressing three fundamental questions relating to the physics of low-mass stars during the planet-building era (1-10 Myr). These questions are: (1) What governs the angular momentum evolution of low-mass PMS stars? (2) By which physical mechanisms are X-rays in low-mass PMS stars generated? (3) How do X-rays influence the properties and evolution of protoplanetary disks? At the same time, the project has focused on broadening the participation of underrepresented minorities in astronomy at the Master's and PhD levels through direct involvement in the project's research goals. Harnessing several large optical, infrared, and X-ray databases of hundreds of low-mass PMS stars in the Orion Nebula Cluster (ONC; 1 Myr) and the Orion OB1 association (10 Myr), the project pursued these broad interrelated research activities: 1. Explore mechanisms for angular momentum loss in young, low-mass stars. Empirically, we know that angular momentum is not conserved during the PMS phase of stellar evolution. Rotation data show that low-mass stars deplete their angular momentum content by an order of magnitude or more between ~1 Myr and the main sequence (~100 Myr). What we don't know is how this happens. We examined two possible mechanisms for angular momentum evolution of PMS stars. First, directly tested the key predictions of so-called 'disk locking' theory, in which it is thought that magnetic coupling of stars to their circumstellar disks regulates the star's angular momentum. Second, motivated by the discovery of extremely powerful X-ray 'superflares' in the ONC stars, we developedand refined a new model of angular momentum loss via scaled-up solar-type coronal mass ejections. 2. Investigated the influence of stellar X-rays on the circumstellar environment. X-rays are thought to play a central role in much of the microphysics that governs magnetic star-disk interaction, accretionprocesses, outflows of material, and angular momentum evolution of young stars. However, to date there has been no direct observational proof of stellar X-rays interacting with and heating circumstellar gas. We conducted an experiment to establish for the first time a direct link between stellar X-rays and the heating of circumstellar gas in a large sample of PMS stars. In particular, we searched for time-correlated variability in Halpha and X-rays among ~800 PMS stars for which we have a unique database of simultaneous Halpha and X-ray light curves. Major outcomes include the following: 1. Rigorous testing of a dominant theory (so called 'disk locking') for the angular momentum evolution of young stars, leading to important new constraints on the limits of its applicability. 2. Development of an alternative/complementary theory of stellar angular momentum evolution -- Extreme coronal mass ejections -- leading to a new paradigm for one of the longest standing mysteries of star formation (the so called 'angular momentum conundrum' of star formation). 3. Deepened understanding of the origins of X-ray production in young stars, leading to an improved understanding of the energetic environs of the Sun when the Earth and other planets were first forming. 4. Accurate measurement of stellar rotation periods and angular momenta for hundreds of young low-mass stars in the Orion OB1 star forming region, one of the most important of the fundamental physical properties of stars. 5. Establishment of the relationships between flare energy and the mass loss from associated coronal mass ejections in the Sun, leading to a new understanding of the role of these energetic events in governing the evolution of Sun-like stars. Finally, we continued and expanded upon our successful graduate program aimed at broadening participation of underrepresented minorities in astronomy: The Fisk-Vanderbilt Masters-to-PhD Bridge program. The Fisk-Vanderbilt Masters-to-PhD Bridge program enabled the participation of five underrepresented minority Fisk graduate students (three female African-American, one female Native Hawaiian, one male African American) to directly participate in the research, leading to five Masters degree theses, and paving the way to advanced PhD study, including two PhDs completed.
