Ultracool Dwarfs New Spectral Types L and T

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1 Ultracool Dwarfs New Spectral Types L and T

2 Springer-Verlag Berlin Heidelberg GmbH Physics and Astronomy ONLINE LIBRARY

3 Hugh R. A. Jones Iain A. Steele (Eds.) Ultracoo l Dwarf s New Spectral Types L and T Wit h 104 Figures and 1 Tables Ä Springer

4 Dr. Hugh R.A. Jones Dr. Iain A. Steele Astrophysics Research Institute Twelve Quays House Egerton Wharf Birkenhead CH411LD United Kingdom hrajoastro.livjm.ac.uk iasoastro.livjm.ac.uk Library of Congress Cataloging-in-Publication Data applied for Die deutsche Bibliothek - CIP-Einheitsaufnahme Ultracool dwarfs : new spectral types L and T ; with 11 tables / Hugh R. A. Jones ; Iain A. Steele (ed.). - Berlin ; Heidelberg ; New York ; Barcelona; Hong Kong; London ; Milan ; Paris ; Tokyo : Springer, 2001 (Physics and astronomy online library) ISBN ISBN (ebook) DOI / ISBN This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9,1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. Springer-Verlag Berlin Heidelberg 2001 Originally published by Springer-Verlag Berlin Heidelberg in 2001 Softcover reprint of the hardcover ist edition 2001 The use of general descriptive names, registered names, trademarks, 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. Typesetting: Camera-ready copy from the authors using a Springer TjtX macro package Cover design: design & production GmbH, Heidelberg Printed on acid-free paper SPIN /3141/di

5 Preface On ce you have looked at t he night sky on a moonless night it is not hard to realise why so much of our science and religion has its roots in the stars. Yet it too k until 1850 to realise that fainter stars were not necessaril y fur ther away, nor the brighter ones closer. In fact within the magnitude range observable to the naked eye it is probable that the brighter star is in fact further away. Even tod ay t he measurement of stellar distances is relatively difficult and is generally only done using dedicated telescopes. In the early years of the 20th cent ury Hertzsprung and Russell developed a powerful classificat ion diagram which allows stars to be distinguished using a plot of their colour versus magnit ude. The construction of this diagram involved the use of spectroscopy which has become the cornerstone of modern astronomy. As te lescopes become more powerful, detectors more sensit ive and more physics is added to astrophysics, astronomical spectrosco py becomes a more powerful tool. The concern of this book is t he spectral classi fication of stars. With a single spectrum of a star it is possible to uniqu ely class ify an obj ect and find its place on the Hertzsprung-Russell diagram. T his spectrum is thus equivalent to having the colour an d the magnitude of the object which can in t urn be related to mass and ot her quantit ies. More than this a stellar spectrum allows insight into the chemical composition and age of a star. Thus t he spectral typing of stars using spec trosco py is central to astrophysics. Star s are traditionally class ified into the main spectral types 0, B, A, F, G, K, M, R and S, with each of the spectral ty pes being broken down into a number of increments, usually 1 to 8, and furthermore being allocated a luminosity class I to VIII. This range of sub-divisions allows all known stars to be given a unique spectral ty pe. This system has been tweaked over the last 100 years but has altered little since the 1930s. This book arises because since the mid-1990s a number of objects have been discovered which do not fit within the existing spectral classification scheme. Here we are interested in the properties of these obj ects and the new spectral ty pes and classification systems necessar y to understand them. Ult racool dwarfs extend from the coolest M dwarfs into the brown dwarf regime and t he new spect ral ty pes L and T. It is likely that most of the ultracool dwarfs are not massive enough to undergo nuclear fusion in their cores to burn hydrogen and are thus brown dwarfs rat her t ha n stars. Like any form of class ificatio n, spectral ty ping can easily become bogg ed down in questions of standard ization and nomenclature and there are indeed

6 VI Preface different methods presented within this book; however the proposed systems of spectral classification for ultracool dwarfs are trying to avoid this by as far as possible continuing the sa me philosophies and techniques used by pr eviou s generat ions of spec troscopists. This traditional approach is likely to prove particularly helpful in understanding the properties and the import an ce of the divide between stars and brown dwarfs. On the other hand it will be probabl y be necessary to develop new systems of spectral classification, particularly as we atte mpt to classify objects cooler than 1000 K. The fact that after 70 years new spectral typ es are required precipitated the Internation al Astronomical Union 's Commissions 27 and 34 to call a meeting within the 34th General Assembly of the International Astronomical Union, during August 2000, to discuss the properties and rat ion ale of ultracool dwarfs. This book has arisen from the talks given at this meeting and serves to record the state of this new field at the beginning of a new millennium. For convenience we have divided the manuscript into three parts ("Theory", "Observat ions" and "Spectral Classification"); however, as for all modern ast rophysics, the distinctions are blurred, and the reader will cert ainly find important comments regarding each of these topics in each par t. Finally we wish to thank t he staff and Commi ssion members of the IAU for their help both before and during the meeting, and especially P rof. Michele Gerbaldi for her assist ance in gett ing the meeting approved and organized. Liverpool, July 2001 Hugh Jones, Jain Steele Liverpool John Moores University

7 Contents Part I Theory Introduction: Theoretical Models of Brown Dwarfs an Observer's Perspective J. Liebert Unified Model Photospheres for Ultracool Dwarfs of Types Land T T. Tsuji 9 Alkali Metals and the Colour of Brown Dwarfs A. Burrows 26 Formation of the Optical Spectra of L Dwarfs Y. Pavlenko Part II Observations Introduction: The Coolest Dwarfs - a Brief History R.F. Jameson Imaging and Spectroscopy of Hot (Young) "Ultracool" Companions G. Schneid er, P.J. Lowrance, E.E. Becklin, J.D. Kirkpatrick, P. Plait, S.R. Heap, E. Malumuth, R.J. Terille, C. Duma s, A.B. Schultz, B.A. Smith, A.J. Weinberger, D.C. Hines Activity and Kinematics of M and L Dwarfs J.E. Gizis Infrared Spectroscopy of Brown Dwarfs: the Onset of CH 4 Absorption in L Dwarfs and the LIT Transition T.R. Geballe, K.S. Noll, S.K. Leggett, G.R. Knapp, X. Fan, and D. Golimowski

8 VIII Conte nts Surface Features, Rotation and Atmospheric Variability of Ultra Cool Dwarfs C.A.L. Bailer-Jones Low-Mass Stellar and Brown Dwarf Binary Systems I.N. Reid, D. W. Koern er, J.E. Gizis, J.D. K irkpatrick 111 The Second Guide Star Catalogue and Cool Stars R.L. Smart, D. Carollo, M. G. Lattanzi, B. McLean, A. Spagna 119 Low-Luminosity Companions to Nearby Stars: Status of the 2MASS Data Search J.D. K irkpatrick, J.E. Gizis, A.J. Burgasser, J.C. Wilson, C.C. Dahn, D. G. Monet, I.N Reid, J. Liebert 125 Part III Spectral Classification Introduction: The Spectral Types of the Ultracool Dwarfs M.S. Bessell 135 The Classification of L Dwarfs J.D. K irkpatrick 139 Spectroscopy of Young Brown Dwarfs and Isolated Planetary Mass Objects E.L. M artin 153 The Classification of T Dwarfs A. J. Burgasser, J.D. K irkpatrick, M.E. Brown 169 L-Band Photometry and Spectroscopy of Land T Dwarfs: Exploring Infrared Spectral Typing D. C. Step hens, M.S. Marley, K.S. Noll 183 Index 195

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