Table Of ContentUltracool Dwarfs
New Spectral Types L and T
Springer-Verlag Berlin Heidelberg GmbH
ONLINE LIBRARY
Physics and Astronomy
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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
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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
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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
