Ultracool Dwarfs New Spectral Types L and T Springer-Verlag Berlin Heidelberg GmbH ONLINE LIBRARY Physics and Astronomy http://www.springer.de/phys/ Hugh R. A. Jones Iain A. Steeel (Eds.) Ultracoo l Dwarf s New Spectral Types L and T With 104 Figures and 11 Tables Ä Springer Dr. Hugh R.A. Jones Dr. Iain A. Steele Astrophysics Research Institute Twelve Quays House Egerton Wharf Birkenhead CH411LD United Kingdom E-mail: 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 978-3-642-62682-1 ISBN 978-3-642-56672-1 (eBook) DOI 10.1007/978-3-642-56672-1 ISBN 978-3-642-62682-1 This work is subject to copyright. 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Violations are liable for prosecution under the German Copyright Law. http://www.springer.de © 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 pro tective 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 10844749 55/3141/di 543210 Preface Once you have looked at the 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 took until 1850 to realise that fainter stars were not necessarily further 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 today the measurement of stellar distances is relatively difficult and is gener ally only done using dedicated telescopes.In the early years of the 20th century Hertzsprung and Russell developed a powerful classification diagram which al lowsstars to be distinguished using a plot of their colour versus magnitude. The construction ofthis diagram involved the use ofspectroscopy which has become the cornerstone of modern astronomy. As telescopes become more powerful, de tectors more sensitive and more physics is added to astrophysics, astronomical spectroscopy becomes a more powerful tool. The concern of this book is the spectral classification of stars. With a single spectrum ofa star it is possible to uniquely classify an object and find its place on the Hertzsprung-Russelldiagram.This spectrum is thus equivalent to having the colour and the magnitudeofthe object which can in turn be related to mass and other quantities. More than this a stellar spectrum allows insight into the chemical composition and age of a star.Thus the spectral typing ofstars using spectroscopy is central to astrophysics.Stars are traditionally classified into the main spectraltypes 0,B,A,F,G, K,M,R andS,with eachofthe spectraltypes being broken down into a number of increments,usually 1to 8, and furthermore being allocated a luminosity classI to VIII.Thisrangeofsub-divisions allows all known stars to be given a unique spectral type. 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 objects and the new spectral types and classification systems necessary to understand them. Ultracool dwarfs extend from the coolest M dwarfs into the brown dwarf regime and the new spectraltypes L and T.It is likely that mostofthe ultracool dwarfs are not massive enough to undergo nuclear fusion in their cores to burn hydrogen and are thus brown dwarfs rather than stars. Like any form of classification, spectral typing can easily become bogged down in questions of standardization and nomenclature and there are indeed 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 same philosophies and techniques used by previous generationsofspectroscopists. This traditional approachislikely to provepartic ularly helpful in understanding the properties and the importance of the divide between stars and browndwarfs. On the other hand it will beprobably be neces sary to develop new systemsofspectralclassification,particularly as we attempt to classify objects cooler than 1000 K. The fact that after 70 years new spectral types are required precipitated the International Astronomical Union's Commissions 27 and 34 to call a meeting within the 34th General AssemblyoftheInternational Astronomical Union,dur ing August 2000,to discuss the propertiesand rationaleofultracooldwarfs.This book has arisenfrom thetalksgivenat thismeetingandservesto recordthe state of this new field at the beginning of a new millennium. For convenience we have divided the manuscript into three parts ("Theory", "Observations" and "Spec tral Classification"); however,as for allmodern astrophysics,thedistinctions are blurred, and the reader will certainly find important comments regarding each of these topics in each part. Finally we wish to thank the staff and Commission members of the IAU for their help both before and during the meeting, and especially Prof. Michele Gerbaldi for her assistance in getting the meeting approved and organized. Liverpool, Hugh Jones, July 2001 Jain Steele Liverpool John Moores University Contents Part I Theory Introduction: Theoretical Models ofBrown Dwarfs an Observer's Perspective J. Liebert ......................................................... 3 Unified Model Photospheres for Ultracool Dwarfs of Types Land T T. Tsuji 9 Alkali Metals and the Colour ofBrown Dwarfs A. Burrows 26 Formation ofthe Optical Spectra ofL Dwarfs Y. Pavlenko ....................................................... 33 Part II Observations Introduction: The Coolest Dwarfs - a Brief History R.F. Jameson ..................................................... 53 Imaging and Spectroscopy of Hot (Young) "Ultracool" Companions G. Schneider, P.J. Lowrance, E.E. Becklin, J.D. Kirkpatrick, P. Plait, S.R. Heap, E. Malumuth, R.J. Terille, C. Dumas, A.B. Schultz, B.A. Smith, A.J. Weinberger, D.C. Hines. ............................ 56 Activity and Kinematics of M and L Dwarfs J.E. Gizis. ... .... ..... ......... .... .. ............................. 71 Infrared Spectroscopy ofBrown Dwarfs: the Onset ofCH Absorption in L Dwarfs and the LIT Transition 4 T.R. Geballe, K.S. Noll, S.K. Leggett, G.R. Knapp, X. Fan, and D. Golimowski................................................. 83 VIII Contents Surface Features, Rotation and Atmospheric Variability of Ultra Cool Dwarfs C.A.L. Bailer-Jones................................................ 92 Low-Mass Stellar and Brown Dwarf Binary Systems I.N. Reid, D.W. Koerner, J.E. Gizis, J.D. Kirkpatrick 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. Kirkpatrick, 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 ofL Dwarfs J.D. Kirkpatrick 139 Spectroscopy ofYoung Brown Dwarfs and Isolated Planetary Mass Objects E.L. Martin 153 The Classification of T Dwarfs A.J. Burgasser, J.D. Kirkpatrick, M.E. Brown 169 L-Band Photometry and Spectroscopy of Land T Dwarfs: Exploring Infrared Spectral Typing D.C. Stephens, M.S. Marley, K.S. Noll 183 Index 195 Part I Theory Introduction: Theoretical Models of Brown Dwarfs - an Observer's Perspective J. Liebert Department of Astronomy and Steward Observatory, The University of Arizona, Tucson, AZ 85721, USA Abstract. The three papers presented in thissection on model atmospheres ofbrown dwarfs raise several common themes. I discuss three of these from an observational viewpoint - the temperaturescale for Ldwarfs,the role ofthe alkalielements,and the likely "weather" in brown dwarf atmospheres. 1 Introduction Theresultsreportedin thissectionbyTsuji,Burrows,and Pavlenkodemonstrate the rapid progresstheorists are making in addressingthe many new observations of L and T dwarfs of the last few years. I thought it would be helpful just to givean observer'sperspectiveofsomemain issueswhich may be emerging. I will discuss three of these. 2 The L Dwarf Temperature Scale Ibegin byshowing (Fig.1)anHertzsprung-RussellDiagram- MJ vs.I-Jcolour - of K through T dwarfs with trigonometric parallaxes (or brighter companions with same). This figure is similar to Fig. 2 of Reid et al. [14]. The caption explains the symbols for two samples of K and M dwarfs. The impressive new parallaxes of L dwarfs (filled circles) all come from our colleagues at the U.S. Naval Observatory, Flagstaff Station, a program directed by Conard C. Dahn, with David G. Monet responsible (mostly) for the reduction procedures. USNO participates in both the 2MASS and SDSS projects, and the L dwarf targets come from these two sources and the DENIS project. The T dwarfs have been too faint and recently-discovered for any parallaxes up to now, but the two important companion T dwarfs, Gl 22gB and Gl 570D, haveexcellent parallaxes from the bright primaries. Neill Reid chose the I and J bands because this colour appears to change in a monotonicway with decreasing luminosity and (apparently) Teff.In contrast, the Hand K bands are both strongly affected by the onset of CH absorption 4 and other opacities at the LIT transition. The monotonic relationship between the absolute (MJ) magnitude and colour for K through L dwarfs is indeed im pressive.The M's end where the L'sstart and the latter end where the T'sstart. Kirkpatricket al. [6,7]find that this correlation is strong between the Lsubtype and variousabsolute magnitudesand colours.That is,the Lspectraltyping does H. R. A. Jones et al. (eds.), Ultracool Dwarfs © Springer-Verlag Berlin Heidelberg 2001