Gilhool, Steven

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  • Publication
    Spectroscopic Analysis Of Stellar Rotational Velocity At The Bottom Of The Main Sequence
    (2018-01-01) Gilhool, Steven
    This thesis presents analyses aimed at understanding the rotational properties of stars at the bottom of the main sequence. The evolution of stellar angular momentum is intertwined with magnetic field generation, mass outflows, convective motions, and many other stellar properties and processes. This complex interplay has made a comprehensive understanding of stellar angular momentum evolution elusive. This is particularly true for low-mass stars due to the observational challenges they present. At the very bottom of the main sequence (spectral type < M4), stars become fully convective. While this ‘Transition to Complete Convection’ presents mysteries of its own, observing the rotation of stars across this boundary can provide insight into stellar structure and magnetic fields, as well as their role in driving the evolution of stellar angular momentum. I present here a review of our understanding of rotational evolution in stars of roughly solar mass down to the end of the main sequence. I detail our efforts to determine large numbers of rotational velocity measurements for M dwarfs observed by the Apache Point Galactic Evolution Experiment (APOGEE). We analyzed the 714 M dwarfs as late as spectral type ~ M7, the largest sample of M dwarfs to date. Consistent with the hypothesis that fully-convective M dwarfs spin down more slowly than solar-type stars, we found that the fraction of detectably rotating stars jumped from about 10% for early to mid M dwarfs, to about 35% for late M dwarfs. We also found some interesting tension between the rotation fractions from spectroscopic studies of vsini like ours, and those expected from rotation periods derived from photometric surveys. Finally, I describe our novel data-driven technique for rapidly estimating vsini in survey data. Rather than directly measuring the broadening of spectral lines, we leveraged the large information content of high-resolution spectral data to empirically estimate vsini. This computationally efficient technique provides a means of rapidly estimating vsini for large numbers of stars in spectroscopic survey data. Indeed, we were able to estimate vsini up to 15 km s-1 for 27,000 APOGEE spectra, in fractions of a second per spectrum.