This project is a systematic study, from radio to X-ray wavelengths, of the origin and properties of magnetic dynamos in ultracool dwarf stars. These are low-mass, unevolved stars and sub-stellar objects that bridge the star-planet gap and dominate the Galactic stellar population by number. Ultracool dwarfs have fully convective interiors and were long assumed to be magnetically inactive. However, recent studies of these objects uncovered unexpectedly vigorous magnetic activity. Taking advantage of the increase in sensitivity and bandwidth of the Expanded Very Large Array, a new implementation of time-resolved spectroscopy for 60,000 Sloan Digital Sky Survey ultracool dwarfs, and simultaneous observations of magnetic plasma heating and particle acceleration, the project will address fundamental open questions: (i) the distribution of magnetic field strengths; (ii) the scale, geometry, and coherence timescale of the magnetic fields; and (iii) whether there is a correlation between the dynamo mechanism efficiency and properties such as rotation, age, or binarity.
Undergraduate and graduate students will gain experience with the acquisition, analysis, and publication of data across the electromagnetic spectrum, as well as expertise in the areas of low mass stars, brown dwarfs, extrasolar planets, and stellar magnetic fields. The project will also integrate this experience into undergraduate education, and narrow the gap between classroom learning and hands-on research, through the active participation of students in observing with Harvard-Smithsonian telescopes.
Intellectual Merit: The program focused on the study of magnetic field properties in low mass stars and brown dwarfs using multi-wavelength observations from ground- and space-based observatories. Specifically, the main goals were to carry out a systematic study of the role of rotation in convective magnetic dynamos (through radio and X-ray observations); to characterize the H-alpha variability of ultracool dwarfs using time-resolved spectroscopy from the Sloan Digital Sky Survey; and to determine the utility of radio very long baseline interferometry for astrometric companion searches. We addressed and accomplished all key goals of the project, leading to 11 refereed publications. Key results included a determination that: (i) rotation plays a role in convective dynamos, at least to spectral type L3; (ii) convective dynamos exhibit a bimodal state with distinct radio and X-ray emission properties; (iii) some ultracool dwarfs exhibit large-scale dipole fields, leading to rotationally-modulated emission; (iv) that H-alpha varaibility is more pronounced in objects wtih stronger H-alpha emission and in late-M dwarfs, suggesting that chromospheric heating is more stochastic in these sources; and (v) very long baseline interferometry radio observations can probe the presence of exoplanets around ultracool dwarfs down to a mass below that of Jupiter. Broader Impacts: The program involved four undergraduate students, one graduate student, and three post-doctoral fellows. Public outreach activities included public lectures in the Cambridge/Boston area, volunteer work as part of the Cambridge Public Library Literacy Project, and volunteer work in Cambridge schools.
