Differentuial rotation in sun like stars from surface variability and asteroseim

Springer Theses Recognizing Outstanding Ph. Research Martin Bo Nielsen Differential Rotation in Sun-like Stars from Surface Variability and Asteroseismology Springer Theses Recognizing Outstanding Ph. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph. theses from around the world and across the physical sciences.

Nominated and endorsed by two recognized specialists, each published volume has been selected for its scientific excellence and the high impact of its contents for the pertinent field of research. For greater accessibility to non-specialists, the published versions include an extended introduction, as well as a foreword by the student’s supervisor explaining the special relevance of the work for the field. As a whole, the series will provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists.

Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria • They must be written in good English. • The topic should fall within the confines of Chemistry, Physics, Earth Sciences, Engineering and related interdisciplinary fields such as Materials, Nanoscience, Chemical Engineering, Complex Systems and Biophysics. • The work reported in the thesis must represent a significant scientific advance. • If the thesis includes previously published material, permission to reproduce this must be gained from the respective copyright holder.

• They must have been examined and passed during the 12 months prior to nomination. • Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. • The theses should have a clearly defined structure including an introduction accessible to scientists not expert in that particular field. More information about this series at http://www.com/series/8790 Martin Bo Nielsen Differential Rotation in Sun-like Stars from Surface Variability and Asteroseismology Doctoral Thesis accepted by The University of Göttingen, Göttingen, Germany 123 Author Supervisor Dr.

Martin Bo Nielsen Prof. Laurent Gizon Institut für Astrophysik Institut für Astrophysik Georg-August-Universität Georg-August-Universität Göttingen Göttingen Germany Germany and Max-Planck-Institut für Sonnensystemforschung Göttingen Germany ISSN 2190-5053 ISSN 2190-5061 (electronic) Springer Theses ISBN 978-3-319-50988-4 ISBN 978-3-319-50989-1 (eBook) DOI 10.1007/978-3-319-50989-1 Library of Congress Control Number: 2016960711 © Springer International Publishing Switzerland 2017 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc.

in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer International Publishing AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Supervisor’s Foreword The study of rotation in Sun-like stars has been the main topic of Martin Bo Nielsen’s Ph.

Martin started his doctoral studies in November 2012 at the University of Göttingen within the framework of the DFG Collaborative Research Center “Astrophysical Flow Instabilities and Turbulence”. Martin’s work is a cumulative dissertation based on three peer-reviewed publications, each of high international standard that constitute a very consistent piece of work. He success- fully defended his dissertation in April 2016, with the highest grade summa cum laude. He was subsequently awarded the 2016 Berliner-Ungewitter Prize for an outstanding thesis from the Göttingen Faculty of Physics.

Magnetic activity in stars other than the Sun is not understood, in part due to the lack of information about rotation in stars. Rotation provides the means by which magnetic fields are generated and maintained in stellar convection zones. In par- ticular, regions of rotational shear play a key role in dynamo theories. Physically, differential rotation is a consequence of the interaction of convection with rotation, but this relationship is poorly understood.

The main purpose of Martin’s thesis was to measure rotation in solar-like stars using two independent methods, asteroseis- mology and photometric variability (starspots), which were both applied to Kepler time series. By identifying a handful of Sun-like stars for which individual mode frequency splittings and photometric rotation periods can be measured, he placed new constraints on stellar differential rotation in these stars. Martin’s work has demonstrated the diagnostic potential of asteroseismology when measuring stellar internal rotation in combination with classical methods of investigation. These exciting results open new opportunities for the study of stellar activity and the solar-stellar connections, which will be fully realized when the space missions TESS and PLATO provide access to a much larger and diverse sample of bright stars.

I fully expect Martin to contribute to the future progress of this field. Göttingen, Germany Prof. Laurent Gizon August 2016 v Preface The Sun and other stars are known to oscillate. Through the study of small perturbations to the frequencies of these oscillations the rotation of the deep interior can be inferred.

Thanks to helioseismology, we know that the Sun rotates as a solid body in the radiative interior and that the convective envelope rotates differentially, with a shear layer in between. Such a shear is thought to be one of the ways in which the large-scale magnetic field of the Sun can be generated. However, thus far the internal rotation of other stars like the Sun is unknown, and placing constraints on models of the relationship between stellar rotation and dynamos is difficult. In this sense the study of rotation in other stars will help further our understanding of magnetic activity on the Sun.

The NASA Kepler mission observed a multitude of Sun-like stars over a period of four years. This has provided high-quality photometric data that can be used to study the rotation of stars with two different techniques: asteroseismology and surface activity. Using asteroseismology it is possible to measure the perturbations to the oscillation frequencies of a star which are caused by rotation. This provides a means of measuring rotation in the stellar interior.

In addition to this, the photo- metric observations are modulated by the presence of magnetically active regions on the stellar surface. These features trace the movement of the outermost layers of the star and the stellar rotation period can therefore be inferred by this variability. The combination of these two methods can be used to put constraints on the radial differential rotation in Sun-like stars. First, we developed an automated method for measuring the rotation of stars using surface variability.

This method was initially applied to the entire Kepler catalog, out of which we detected signatures of rotation in  12,000 stars across the main sequence, providing robust estimates of the surface rotation rates and the associated errors. We compared these measurements to spectroscopic v sin i values and found good agreement for F-, G- and K-type stars, showing that this method is suitable for measuring the surface rotation rates of Sun-like stars. Second, we performed an asteroseismic analysis of six Sun-like stars, where we were able to measure the rotational splitting as a function of frequency in the p-mode envelope. This was done by dividing the oscillation spectrum into vii viii Preface individual segments, and fitting a model independently to each segment.

Any potential difference in the splittings between each segment could be an indication of strong differential rotation. We found however, that the measured splittings were all consistent with a constant value, indicating little differential rotation; similar to what could be expected if the Sun was observed as a star by the Kepler satellite. Third, we compared the asteroseismic rotation rates of five Sun-like stars to their surface rotation rates. We found that the values were in good agreement, indicating little differential rotation between the regions where the two methods are most sensitive.

The asteroseismic measurements are primarily sensitive to rotation in the convective envelope. Because of the high degree of correlation the surface rotation periods can therefore be used as an indicator of the rotation in the convective zone, and the remaining contribution to the rotational splitting from rotation in the radiative interior can estimated. Finally, we discuss how the surface rotation rates may be used as a prior on the seismic envelope rotation rate in a double-zone model, consisting of an indepen- dently rotating radiative interior and convective envelope. This allows us to find the upper limits on the radial differential rotation in Sun-like stars.

We find that the rotation rates of the radiative interior and convective envelope likely do not differ by more than  50%. This further supports the idea that Sun-like stars likely show a rotation pattern similar to that of the Sun, potentially indicating that solar-like dynamo mechanisms are present in these stars. These results are the latest step toward being able to accurately measure the internal dynamics of stars other than the Sun, thereby improving stellar dynamo models. Although the Kepler data are the best quality observations currently available, we are still limited by its intrinsic systematic and random noise; pre- venting us from making more precise measurements of differential rotation.

Results from the analysis presented herein do, however, provide physical limits on the internal differential rotation of Sun-like stars, and show that this method may be easily applied to a wider variety of stars. This means that we now have the potential for analyzing many more stars, advancing our understanding of stellar rotation and magnetic dynamos. Göttingen, Germany Dr. Martin Bo Nielsen Publications • H.

Asteroseismic for radial differential rotation of Sun-like stars: ensemble fits. Rotation periods of 12 000 main sequence Kepler stars: Dependence on stellar spectral type and comparison with v sin i observations. Rotational splitting as a of mode frequency for six Sun-like stars. Constraining dierential of Sun-like stars from asteroseismic and starspot rotation periods.

Asteroseismic inference on the spin-orbit misalignment and stellar parameters of HAT-P-7. de Oliveira Fialho, P. Gomes da Silva, T. Quirrenbach, ix x Publications R.

Experimental Astronomy, 38:249-330, November 2014. do Nascimento, Jr. Rotation and magnetism of Kepler pulsating solar-like stars. Towards asteroseismically calibrated age-rotation relations.

Testing the recovery of stellar rotation signals from Kepler light curves using a blind hare-and-hounds exercise. Sounding stellar cycles with Kepler—II. Ground-based observations. Acknowledgements I would like to especially thank my supervisors Laurent and Hannah.

Thank you Laurent, for keeping me on track and seeing the bigger picture during this project and the writing of this thesis. Thank you Hannah, for listening to, and sorting through all my sometimes not quite simply posed and random questions. There are of course also a multitude of people who were not directly related to this project but still provided much advice, and many answers to my questions when other help was not available. Thank you Jesper, for always having an answer to my most obscure and detailed questions.

Thank you Warrick, for my often repeated and sudden interruptions of your work with questions that were likely quite trivial (and also the philosocoffee). Thank you Timo for all the useful dis- cussions and for being an excellent office-mate. I have also found many friends during my Ph., thank you all for listening to my complaints and worries. Thank you Jan, thank you Björn, for helping me find my way through the forest of paperwork and translation that is associated with doing a Ph.