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The Evolution of Rotation and Magnetism in Small Stars Near the Sun The Evolution of Rotation and Magnetism in Small Stars Near the Sun The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Newton, Elisabeth R. 2016. The Evolution of Rotation and Magnetism in Small Stars Near the Sun. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences. Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:33493355 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA c 2016 — Elisabeth R. Newton All rights reserved. Dissertation Advisor: Professor David Charbonneau Elisabeth R. Newton The Evolution of Rotation and Magnetism in Small Stars Near the Sun Abstract Despite the prevalence of M dwarfs, the smallest and most common type of main sequence star, their sizes, compositions, and ages are not well-constrained. Empirical determination of these properties is important for gaining insight into their stellar structure, magnetic field generation, and angular momentum evolution. I obtained low-resolution (R = 2000) near-infrared spectra of 447 nearby mid-to-late M dwarfs. I measured their absolute radial velocities with an accuracy of 4.4 km/s by exploiting telluric lines to establish an absolute wavelength calibration. I estimated their metallicities from the equivalent width of the sodium absorption feature at 2.2 µm to a precision of 0.12 dex, and from 2MASS colors to a precision of 0.15 dex. Using stars with radii measured from interferometry, I showed that the equivalent widths of aluminum and magnesium absorption features can be used to infer K and M dwarf temperatures to a precision of 69 K, and radii to 0.027 R . I applied these relations to planet-hosting stars from Kepler, showing that the typical planet is 15% larger than inferred when adopting stellar parameters from other recent catalogs. Using photometry from the MEarth-North Observatory, I measured rotation periods from 0.1 to 140 days for 387 M dwarfs. I found a prevalence of stable spot patterns, and no correlation between period and amplitude for fully-convective stars. Using galactic kinematics as a proxy for age, I found that rapid rotators (P < 10 days) are < 2 Gyr, iii and that the slowest are on average 5 3 Gyr old. I then showed that for early M ± dwarfs the typical stellar rotation period at 5 Gyr coincides with the orbital period at which habitable planets are found, and I suggest that mid-to-late M dwarfs are optimal targets around which to search for habitable-zone planets. I obtained optical spectra of 247 nearby M dwarfs, and measured the strength of the chromospheric Ha emission line. I identified a well-defined boundary in the mass–period plane that separates active and inactive M dwarfs. Ha activity is therefore a simple, accessible diagnostic for stellar rotation period, and I present a mass–period relation for inactive 4 M dwarfs. I found a significant (p value < 10− ) positive correlation between Ha emission strength and photometric variability amplitude, which implies that stars with stronger magnetic fields have both higher levels of chromospheric activity and larger or more abundant spots. I suggest that fully convective stars maintain rapid rotation rates and saturated magnetic activity for about 2 Gyr. They then undergo rapid angular momentum evolution upon reaching some critical threshold. Only upon reaching long rotation periods (around 70 days for a 0.2 M star) do their magnetic activity levels drop below what is required for Ha to be seen in emission. The stars I have observed in pursuit of this work are the nearest low-mass stars. As such they are the best targets around which to search for habitable, rocky worlds, and my work provides the means to constrain the sizes, temperatures, and ages of those planets. iv Contents Abstract iii Acknowledgments x 1 Introduction 1 1.1 Observations of M Dwarf Stars . 2 1.2 The Structure of Cool Stars . 5 1.3 The Physics of Cool Stars . 10 1.4 M Dwarfs as Exoplanet Hosts . 18 1.5 New Observations of Mid-to-late M Dwarfs in the Solar Neighborhood 21 1.5.1 Chapter 2 — Fundamental Stellar Parameters: A Spectroscopic Survey in the NIR . 21 1.5.2 Chapter 3 — The Properties of Exoplanet Hosts: Determining the Sizes of Cool Stars . 24 1.5.3 Chapter 4 — Angular Momentum Evolution: Stellar Rotation at the Bottom of the Main Sequence . 27 1.5.4 Chapter 5 — The Hunt for Habitable Planets: The Impact of Stellar Rotation on Radial Velocity Surveys . 29 1.5.5 Chapter 6 — The Origin of Magnetism: Magnetic Activity in Nearby M Dwarfs . 30 1.6 Looking Back, Looking Forward: Insight into the Structure and Physics of M Dwarf Stars . 32 v CONTENTS 2 Near-infrared Metallicities, Radial Velocities and Spectral Types for 447 Nearby M Dwarfs 37 2.1 Introduction . 37 2.2 Sample . 44 2.2.1 MEarth M dwarfs . 49 2.2.2 Spectroscopy targets . 49 2.3 Observations . 51 2.4 Estimation of uncertainty . 54 2.5 NIR spectral types . 56 2.5.1 Spectral typing routine . 59 2.5.2 IRTF spectral standards . 65 2.5.3 Comparing measures of spectral type . 66 2.5.4 Spectroscopic distances . 67 2.6 Metallicity calibration . 69 2.6.1 Metallicities of the primary stars . 71 2.6.2 Identification of calibrators . 73 2.6.3 Empirical metallicity calibration . 76 2.6.4 Influence of effective temperature and surface gravity on the metallicity calibration . 89 2.6.5 Tests of our metallicity relation . 97 2.6.6 Inclusion of previous metallicity estimates . 99 2.7 Photometric metallicity calibrations . 100 2.8 Radial velocities from NIR spectroscopy . 104 2.8.1 Radial velocity method . 107 2.8.2 Using precise RVs to investigate errors and systematics . 110 2.8.3 Validating the use of SpeX for radial velocities . 114 2.9 Conclusions . 116 vi CONTENTS 3 An Empirical Calibration to Estimate Cool Dwarf Fundamental Parameters from H-band Spectra 119 3.1 Introduction . 119 3.2 Observations and measurements . 124 3.2.1 Stars with interferometric measurements . 125 3.2.2 MEarth M dwarfs . 129 3.2.3 Cool KOIs . 130 3.3 Inferring stellar parameters . 131 3.3.1 Behavior of H-band spectral features as a function of effective temperature . 133 3.3.2 Empirical calibrations for stellar parameters using spectral fea- tures . 135 3.3.3 Systematic uncertainties in the calibrations . 141 3.4 Application to M dwarfs from MEarth . 142 3.4.1 Using luminosities to revisit the metallicities of the MEarth sample 147 3.4.2 Comparison to the Mann et al. temperature and radius calibrations 152 3.4.3 Trends between absolute MK and inferred stellar parameters . 154 3.4.4 Identifying over-luminous objects . 156 3.5 Applications to Kepler Objects of Interest . 158 3.5.1 Comparison to previous work . 163 3.5.2 Updated stellar and planetary parameters . 165 3.5.3 Comments on individual systems . 169 3.6 Summary . 172 4 The Rotation and Galactic Kinematics of Mid M dwarfs in the Solar Neigh- borhood 178 4.1 Introduction . 178 4.2 Photometry from MEarth . 183 vii CONTENTS 4.3 Determining rotation periods . 188 4.3.1 Period detection . 188 4.3.2 Identifying multiples . 193 4.3.3 Defining a statistical sample . 197 4.4 Comparison to previous period measurements . 198 4.4.1 Comparison to ground-based photometry . 201 4.4.2 Comparison to Kepler ......................... 202 4.4.3 Comparison with vsini measurements . 205 4.5 Spot characteristics . 215 4.5.1 Amplitude of variability . 217 4.5.2 Spot patterns and stability . 220 4.5.3 Recovery fractions . 221 4.6 Kinematics and metallicities of the rotators . 224 4.6.1 De-biasing the kinematics . 225 4.6.2 General kinematic properties of the sample . 226 4.6.3 Disk membership . 228 4.6.4 Metallicities of the rotators . 229 4.7 The age-rotation relation . 231 4.8 The mass-period relation . 240 4.9 Summary . 244 4.10 Conclusions . 247 5 The Impact of Stellar Rotation on the Detectability of Habitable Planets Around M Dwarfs 251 5.1 Introduction . 251 5.2 Stellar rotation periods and masses across the main sequence . 254 5.3 Stellar rotation and the habitable zone . 257 viii CONTENTS 5.4 M dwarfs with photometric monitoring as targets for radial velocity surveys...................................... 263 5.5 Discussion . 265 6Ha Emission in Nearby M dwarfs and its Relation to Stellar Rotation 270 6.1 Introduction . 270 6.2 Data . 273 6.2.1 Our nearby M dwarf sample . 273 6.2.2 Literature compilation . 277 6.2.3 New optical spectra from FAST . 282 6.3 Analysis . 283 6.3.1 Radial velocities . 283 6.3.2 Ha EWs ................................. 285 6.3.3 Ha luminosity and c .......................... 287 6.4 Results . 289 6.4.1 Activity versus rotation period . 289 6.4.2 The active/inactive boundary . 294 6.4.3 Stars with unusual activity levels . 298 6.4.4 Activity versus photometric amplitude . 299 6.5 Discussion . 302 References 307 ix Acknowledgments Though not quite comparable to the challenge of finishing my Ph.D., I found it very difficult to write these acknowledgements. The road here has been a long one, and on the way I am fortunate to have had more supportive and caring individuals in my life than I could possibly.
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